Permalink
| # Copyright (c) 2013-2014 Sandstorm Development Group, Inc. and contributors | |
| # Licensed under the MIT License: | |
| # | |
| # Permission is hereby granted, free of charge, to any person obtaining a copy | |
| # of this software and associated documentation files (the "Software"), to deal | |
| # in the Software without restriction, including without limitation the rights | |
| # to use, copy, modify, merge, publish, distribute, sublicense, and/or sell | |
| # copies of the Software, and to permit persons to whom the Software is | |
| # furnished to do so, subject to the following conditions: | |
| # | |
| # The above copyright notice and this permission notice shall be included in | |
| # all copies or substantial portions of the Software. | |
| # | |
| # THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR | |
| # IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, | |
| # FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE | |
| # AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER | |
| # LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, | |
| # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN | |
| # THE SOFTWARE. | |
| @0xb312981b2552a250; | |
| # Recall that Cap'n Proto RPC allows messages to contain references to remote objects that | |
| # implement interfaces. These references are called "capabilities", because they both designate | |
| # the remote object to use and confer permission to use it. | |
| # | |
| # Recall also that Cap'n Proto RPC has the feature that when a method call itself returns a | |
| # capability, the caller can begin calling methods on that capability _before the first call has | |
| # returned_. The caller essentially sends a message saying "Hey server, as soon as you finish | |
| # that previous call, do this with the result!". Cap'n Proto's RPC protocol makes this possible. | |
| # | |
| # The protocol is significantly more complicated than most RPC protocols. However, this is | |
| # implementation complexity that underlies an easy-to-grasp higher-level model of object oriented | |
| # programming. That is, just like TCP is a surprisingly complicated protocol that implements a | |
| # conceptually-simple byte stream abstraction, Cap'n Proto is a surprisingly complicated protocol | |
| # that implements a conceptually-simple object abstraction. | |
| # | |
| # Cap'n Proto RPC is based heavily on CapTP, the object-capability protocol used by the E | |
| # programming language: | |
| # http://www.erights.org/elib/distrib/captp/index.html | |
| # | |
| # Cap'n Proto RPC takes place between "vats". A vat hosts some set of objects and talks to other | |
| # vats through direct bilateral connections. Typically, there is a 1:1 correspondence between vats | |
| # and processes (in the unix sense of the word), although this is not strictly always true (one | |
| # process could run multiple vats, or a distributed virtual vat might live across many processes). | |
| # | |
| # Cap'n Proto does not distinguish between "clients" and "servers" -- this is up to the application. | |
| # Either end of any connection can potentially hold capabilities pointing to the other end, and | |
| # can call methods on those capabilities. In the doc comments below, we use the words "sender" | |
| # and "receiver". These refer to the sender and receiver of an instance of the struct or field | |
| # being documented. Sometimes we refer to a "third-party" that is neither the sender nor the | |
| # receiver. Documentation is generally written from the point of view of the sender. | |
| # | |
| # It is generally up to the vat network implementation to securely verify that connections are made | |
| # to the intended vat as well as to encrypt transmitted data for privacy and integrity. See the | |
| # `VatNetwork` example interface near the end of this file. | |
| # | |
| # When a new connection is formed, the only interesting things that can be done are to send a | |
| # `Bootstrap` (level 0) or `Accept` (level 3) message. | |
| # | |
| # Unless otherwise specified, messages must be delivered to the receiving application in the same | |
| # order in which they were initiated by the sending application. The goal is to support "E-Order", | |
| # which states that two calls made on the same reference must be delivered in the order which they | |
| # were made: | |
| # http://erights.org/elib/concurrency/partial-order.html | |
| # | |
| # Since the full protocol is complicated, we define multiple levels of support that an | |
| # implementation may target. For many applications, level 1 support will be sufficient. | |
| # Comments in this file indicate which level requires the corresponding feature to be | |
| # implemented. | |
| # | |
| # * **Level 0:** The implementation does not support object references. Only the bootstrap interface | |
| # can be called. At this level, the implementation does not support object-oriented protocols and | |
| # is similar in complexity to JSON-RPC or Protobuf services. This level should be considered only | |
| # a temporary stepping-stone toward level 1 as the lack of object references drastically changes | |
| # how protocols are designed. Applications _should not_ attempt to design their protocols around | |
| # the limitations of level 0 implementations. | |
| # | |
| # * **Level 1:** The implementation supports simple bilateral interaction with object references | |
| # and promise pipelining, but interactions between three or more parties are supported only via | |
| # proxying of objects. E.g. if Alice (in Vat A) wants to send Bob (in Vat B) a capability | |
| # pointing to Carol (in Vat C), Alice must create a proxy of Carol within Vat A and send Bob a | |
| # reference to that; Bob cannot form a direct connection to Carol. Level 1 implementations do | |
| # not support checking if two capabilities received from different vats actually point to the | |
| # same object ("join"), although they should be able to do this check on capabilities received | |
| # from the same vat. | |
| # | |
| # * **Level 2:** The implementation supports saving persistent capabilities -- i.e. capabilities | |
| # that remain valid even after disconnect, and can be restored on a future connection. When a | |
| # capability is saved, the requester receives a `SturdyRef`, which is a token that can be used | |
| # to restore the capability later. | |
| # | |
| # * **Level 3:** The implementation supports three-way interactions. That is, if Alice (in Vat A) | |
| # sends Bob (in Vat B) a capability pointing to Carol (in Vat C), then Vat B will automatically | |
| # form a direct connection to Vat C rather than have requests be proxied through Vat A. | |
| # | |
| # * **Level 4:** The entire protocol is implemented, including joins (checking if two capabilities | |
| # are equivalent). | |
| # | |
| # Note that an implementation must also support specific networks (transports), as described in | |
| # the "Network-specific Parameters" section below. An implementation might have different levels | |
| # depending on the network used. | |
| # | |
| # New implementations of Cap'n Proto should start out targeting the simplistic two-party network | |
| # type as defined in `rpc-twoparty.capnp`. With this network type, level 3 is irrelevant and | |
| # levels 2 and 4 are much easier than usual to implement. When such an implementation is paired | |
| # with a container proxy, the contained app effectively gets to make full use of the proxy's | |
| # network at level 4. And since Cap'n Proto IPC is extremely fast, it may never make sense to | |
| # bother implementing any other vat network protocol -- just use the correct container type and get | |
| # it for free. | |
| using Cxx = import "/capnp/c++.capnp"; | |
| $Cxx.namespace("capnp::rpc"); | |
| # ======================================================================================== | |
| # The Four Tables | |
| # | |
| # Cap'n Proto RPC connections are stateful (although an application built on Cap'n Proto could | |
| # export a stateless interface). As in CapTP, for each open connection, a vat maintains four state | |
| # tables: questions, answers, imports, and exports. See the diagram at: | |
| # http://www.erights.org/elib/distrib/captp/4tables.html | |
| # | |
| # The question table corresponds to the other end's answer table, and the imports table corresponds | |
| # to the other end's exports table. | |
| # | |
| # The entries in each table are identified by ID numbers (defined below as 32-bit integers). These | |
| # numbers are always specific to the connection; a newly-established connection starts with no | |
| # valid IDs. Since low-numbered IDs will pack better, it is suggested that IDs be assigned like | |
| # Unix file descriptors -- prefer the lowest-number ID that is currently available. | |
| # | |
| # IDs in the questions/answers tables are chosen by the questioner and generally represent method | |
| # calls that are in progress. | |
| # | |
| # IDs in the imports/exports tables are chosen by the exporter and generally represent objects on | |
| # which methods may be called. Exports may be "settled", meaning the exported object is an actual | |
| # object living in the exporter's vat, or they may be "promises", meaning the exported object is | |
| # the as-yet-unknown result of an ongoing operation and will eventually be resolved to some other | |
| # object once that operation completes. Calls made to a promise will be forwarded to the eventual | |
| # target once it is known. The eventual replacement object does *not* get the same ID as the | |
| # promise, as it may turn out to be an object that is already exported (so already has an ID) or | |
| # may even live in a completely different vat (and so won't get an ID on the same export table | |
| # at all). | |
| # | |
| # IDs can be reused over time. To make this safe, we carefully define the lifetime of IDs. Since | |
| # messages using the ID could be traveling in both directions simultaneously, we must define the | |
| # end of life of each ID _in each direction_. The ID is only safe to reuse once it has been | |
| # released by both sides. | |
| # | |
| # When a Cap'n Proto connection is lost, everything on the four tables is lost. All questions are | |
| # canceled and throw exceptions. All imports become broken (all future calls to them throw | |
| # exceptions). All exports and answers are implicitly released. The only things not lost are | |
| # persistent capabilities (`SturdyRef`s). The application must plan for this and should respond by | |
| # establishing a new connection and restoring from these persistent capabilities. | |
| using QuestionId = UInt32; | |
| # **(level 0)** | |
| # | |
| # Identifies a question in the sender's question table (which corresponds to the receiver's answer | |
| # table). The questioner (caller) chooses an ID when making a call. The ID remains valid in | |
| # caller -> callee messages until a Finish message is sent, and remains valid in callee -> caller | |
| # messages until a Return message is sent. | |
| using AnswerId = QuestionId; | |
| # **(level 0)** | |
| # | |
| # Identifies an answer in the sender's answer table (which corresponds to the receiver's question | |
| # table). | |
| # | |
| # AnswerId is physically equivalent to QuestionId, since the question and answer tables correspond, | |
| # but we define a separate type for documentation purposes: we always use the type representing | |
| # the sender's point of view. | |
| using ExportId = UInt32; | |
| # **(level 1)** | |
| # | |
| # Identifies an exported capability or promise in the sender's export table (which corresponds | |
| # to the receiver's import table). The exporter chooses an ID before sending a capability over the | |
| # wire. If the capability is already in the table, the exporter should reuse the same ID. If the | |
| # ID is a promise (as opposed to a settled capability), this must be indicated at the time the ID | |
| # is introduced (e.g. by using `senderPromise` instead of `senderHosted` in `CapDescriptor`); in | |
| # this case, the importer shall expect a later `Resolve` message that replaces the promise. | |
| # | |
| # ExportId/ImportIds are subject to reference counting. Whenever an `ExportId` is sent over the | |
| # wire (from the exporter to the importer), the export's reference count is incremented (unless | |
| # otherwise specified). The reference count is later decremented by a `Release` message. Since | |
| # the `Release` message can specify an arbitrary number by which to reduce the reference count, the | |
| # importer should usually batch reference decrements and only send a `Release` when it believes the | |
| # reference count has hit zero. Of course, it is possible that a new reference to the export is | |
| # in-flight at the time that the `Release` message is sent, so it is necessary for the exporter to | |
| # keep track of the reference count on its end as well to avoid race conditions. | |
| # | |
| # When a connection is lost, all exports are implicitly released. It is not possible to restore | |
| # a connection state after disconnect (although a transport layer could implement a concept of | |
| # persistent connections if it is transparent to the RPC layer). | |
| using ImportId = ExportId; | |
| # **(level 1)** | |
| # | |
| # Identifies an imported capability or promise in the sender's import table (which corresponds to | |
| # the receiver's export table). | |
| # | |
| # ImportId is physically equivalent to ExportId, since the export and import tables correspond, | |
| # but we define a separate type for documentation purposes: we always use the type representing | |
| # the sender's point of view. | |
| # | |
| # An `ImportId` remains valid in importer -> exporter messages until the importer has sent | |
| # `Release` messages that (it believes) have reduced the reference count to zero. | |
| # ======================================================================================== | |
| # Messages | |
| struct Message { | |
| # An RPC connection is a bi-directional stream of Messages. | |
| union { | |
| unimplemented @0 :Message; | |
| # The sender previously received this message from the peer but didn't understand it or doesn't | |
| # yet implement the functionality that was requested. So, the sender is echoing the message | |
| # back. In some cases, the receiver may be able to recover from this by pretending the sender | |
| # had taken some appropriate "null" action. | |
| # | |
| # For example, say `resolve` is received by a level 0 implementation (because a previous call | |
| # or return happened to contain a promise). The level 0 implementation will echo it back as | |
| # `unimplemented`. The original sender can then simply release the cap to which the promise | |
| # had resolved, thus avoiding a leak. | |
| # | |
| # For any message type that introduces a question, if the message comes back unimplemented, | |
| # the original sender may simply treat it as if the question failed with an exception. | |
| # | |
| # In cases where there is no sensible way to react to an `unimplemented` message (without | |
| # resource leaks or other serious problems), the connection may need to be aborted. This is | |
| # a gray area; different implementations may take different approaches. | |
| abort @1 :Exception; | |
| # Sent when a connection is being aborted due to an unrecoverable error. This could be e.g. | |
| # because the sender received an invalid or nonsensical message or because the sender had an | |
| # internal error. The sender will shut down the outgoing half of the connection after `abort` | |
| # and will completely close the connection shortly thereafter (it's up to the sender how much | |
| # of a time buffer they want to offer for the client to receive the `abort` before the | |
| # connection is reset). | |
| # Level 0 features ----------------------------------------------- | |
| bootstrap @8 :Bootstrap; # Request the peer's bootstrap interface. | |
| call @2 :Call; # Begin a method call. | |
| return @3 :Return; # Complete a method call. | |
| finish @4 :Finish; # Release a returned answer / cancel a call. | |
| # Level 1 features ----------------------------------------------- | |
| resolve @5 :Resolve; # Resolve a previously-sent promise. | |
| release @6 :Release; # Release a capability so that the remote object can be deallocated. | |
| disembargo @13 :Disembargo; # Lift an embargo used to enforce E-order over promise resolution. | |
| # Level 2 features ----------------------------------------------- | |
| obsoleteSave @7 :AnyPointer; | |
| # Obsolete request to save a capability, resulting in a SturdyRef. This has been replaced | |
| # by the `Persistent` interface defined in `persistent.capnp`. This operation was never | |
| # implemented. | |
| obsoleteDelete @9 :AnyPointer; | |
| # Obsolete way to delete a SturdyRef. This operation was never implemented. | |
| # Level 3 features ----------------------------------------------- | |
| provide @10 :Provide; # Provide a capability to a third party. | |
| accept @11 :Accept; # Accept a capability provided by a third party. | |
| # Level 4 features ----------------------------------------------- | |
| join @12 :Join; # Directly connect to the common root of two or more proxied caps. | |
| } | |
| } | |
| # Level 0 message types ---------------------------------------------- | |
| struct Bootstrap { | |
| # **(level 0)** | |
| # | |
| # Get the "bootstrap" interface exported by the remote vat. | |
| # | |
| # For level 0, 1, and 2 implementations, the "bootstrap" interface is simply the main interface | |
| # exported by a vat. If the vat acts as a server fielding connections from clients, then the | |
| # bootstrap interface defines the basic functionality available to a client when it connects. | |
| # The exact interface definition obviously depends on the application. | |
| # | |
| # We call this a "bootstrap" because in an ideal Cap'n Proto world, bootstrap interfaces would | |
| # never be used. In such a world, any time you connect to a new vat, you do so because you | |
| # received an introduction from some other vat (see `ThirdPartyCapId`). Thus, the first message | |
| # you send is `Accept`, and further communications derive from there. `Bootstrap` is not used. | |
| # | |
| # In such an ideal world, DNS itself would support Cap'n Proto -- performing a DNS lookup would | |
| # actually return a new Cap'n Proto capability, thus introducing you to the target system via | |
| # level 3 RPC. Applications would receive the capability to talk to DNS in the first place as | |
| # an initial endowment or part of a Powerbox interaction. Therefore, an app can form arbitrary | |
| # connections without ever using `Bootstrap`. | |
| # | |
| # Of course, in the real world, DNS is not Cap'n-Proto-based, and we don't want Cap'n Proto to | |
| # require a whole new internet infrastructure to be useful. Therefore, we offer bootstrap | |
| # interfaces as a way to get up and running without a level 3 introduction. Thus, bootstrap | |
| # interfaces are used to "bootstrap" from other, non-Cap'n-Proto-based means of service discovery, | |
| # such as legacy DNS. | |
| # | |
| # Note that a vat need not provide a bootstrap interface, and in fact many vats (especially those | |
| # acting as clients) do not. In this case, the vat should either reply to `Bootstrap` with a | |
| # `Return` indicating an exception, or should return a dummy capability with no methods. | |
| questionId @0 :QuestionId; | |
| # A new question ID identifying this request, which will eventually receive a Return message | |
| # containing the restored capability. | |
| deprecatedObjectId @1 :AnyPointer; | |
| # ** DEPRECATED ** | |
| # | |
| # A Vat may export multiple bootstrap interfaces. In this case, `deprecatedObjectId` specifies | |
| # which one to return. If this pointer is null, then the default bootstrap interface is returned. | |
| # | |
| # As of verison 0.5, use of this field is deprecated. If a service wants to export multiple | |
| # bootstrap interfaces, it should instead define a single bootstarp interface that has methods | |
| # that return each of the other interfaces. | |
| # | |
| # **History** | |
| # | |
| # In the first version of Cap'n Proto RPC (0.4.x) the `Bootstrap` message was called `Restore`. | |
| # At the time, it was thought that this would eventually serve as the way to restore SturdyRefs | |
| # (level 2). Meanwhile, an application could offer its "main" interface on a well-known | |
| # (non-secret) SturdyRef. | |
| # | |
| # Since level 2 RPC was not implemented at the time, the `Restore` message was in practice only | |
| # used to obtain the main interface. Since most applications had only one main interface that | |
| # they wanted to restore, they tended to designate this with a null `objectId`. | |
| # | |
| # Unfortunately, the earliest version of the EZ RPC interfaces set a precedent of exporting | |
| # multiple main interfaces by allowing them to be exported under string names. In this case, | |
| # `objectId` was a Text value specifying the name. | |
| # | |
| # All of this proved problematic for several reasons: | |
| # | |
| # - The arrangement assumed that a client wishing to restore a SturdyRef would know exactly what | |
| # machine to connect to and would be able to immediately restore a SturdyRef on connection. | |
| # However, in practice, the ability to restore SturdyRefs is itself a capability that may | |
| # require going through an authentication process to obtain. Thus, it makes more sense to | |
| # define a "restorer service" as a full Cap'n Proto interface. If this restorer interface is | |
| # offered as the vat's bootstrap interface, then this is equivalent to the old arrangement. | |
| # | |
| # - Overloading "Restore" for the purpose of obtaining well-known capabilities encouraged the | |
| # practice of exporting singleton services with string names. If singleton services are desired, | |
| # it is better to have one main interface that has methods that can be used to obtain each | |
| # service, in order to get all the usual benefits of schemas and type checking. | |
| # | |
| # - Overloading "Restore" also had a security problem: Often, "main" or "well-known" | |
| # capabilities exported by a vat are in fact not public: they are intended to be accessed only | |
| # by clients who are capable of forming a connection to the vat. This can lead to trouble if | |
| # the client itself has other clients and wishes to foward some `Restore` requests from those | |
| # external clients -- it has to be very careful not to allow through `Restore` requests | |
| # addressing the default capability. | |
| # | |
| # For example, consider the case of a sandboxed Sandstorm application and its supervisor. The | |
| # application exports a default capability to its supervisor that provides access to | |
| # functionality that only the supervisor is supposed to access. Meanwhile, though, applications | |
| # may publish other capabilities that may be persistent, in which case the application needs | |
| # to field `Restore` requests that could come from anywhere. These requests of course have to | |
| # pass through the supervisor, as all communications with the outside world must. But, the | |
| # supervisor has to be careful not to honor an external request addressing the application's | |
| # default capability, since this capability is privileged. Unfortunately, the default | |
| # capability cannot be given an unguessable name, because then the supervisor itself would not | |
| # be able to address it! | |
| # | |
| # As of Cap'n Proto 0.5, `Restore` has been renamed to `Bootstrap` and is no longer planned for | |
| # use in restoring SturdyRefs. | |
| # | |
| # Note that 0.4 also defined a message type called `Delete` that, like `Restore`, addressed a | |
| # SturdyRef, but indicated that the client would not restore the ref again in the future. This | |
| # operation was never implemented, so it was removed entirely. If a "delete" operation is desired, | |
| # it should exist as a method on the same interface that handles restoring SturdyRefs. However, | |
| # the utility of such an operation is questionable. You wouldn't be able to rely on it for | |
| # garbage collection since a client could always disappear permanently without remembering to | |
| # delete all its SturdyRefs, thus leaving them dangling forever. Therefore, it is advisable to | |
| # design systems such that SturdyRefs never represent "owned" pointers. | |
| # | |
| # For example, say a SturdyRef points to an image file hosted on some server. That image file | |
| # should also live inside a collection (a gallery, perhaps) hosted on the same server, owned by | |
| # a user who can delete the image at any time. If the user deletes the image, the SturdyRef | |
| # stops working. On the other hand, if the SturdyRef is discarded, this has no effect on the | |
| # existence of the image in its collection. | |
| } | |
| struct Call { | |
| # **(level 0)** | |
| # | |
| # Message type initiating a method call on a capability. | |
| questionId @0 :QuestionId; | |
| # A number, chosen by the caller, that identifies this call in future messages. This number | |
| # must be different from all other calls originating from the same end of the connection (but | |
| # may overlap with question IDs originating from the opposite end). A fine strategy is to use | |
| # sequential question IDs, but the recipient should not assume this. | |
| # | |
| # A question ID can be reused once both: | |
| # - A matching Return has been received from the callee. | |
| # - A matching Finish has been sent from the caller. | |
| target @1 :MessageTarget; | |
| # The object that should receive this call. | |
| interfaceId @2 :UInt64; | |
| # The type ID of the interface being called. Each capability may implement multiple interfaces. | |
| methodId @3 :UInt16; | |
| # The ordinal number of the method to call within the requested interface. | |
| allowThirdPartyTailCall @8 :Bool = false; | |
| # Indicates whether or not the receiver is allowed to send a `Return` containing | |
| # `acceptFromThirdParty`. Level 3 implementations should set this true. Otherwise, the callee | |
| # will have to proxy the return in the case of a tail call to a third-party vat. | |
| params @4 :Payload; | |
| # The call parameters. `params.content` is a struct whose fields correspond to the parameters of | |
| # the method. | |
| sendResultsTo :union { | |
| # Where should the return message be sent? | |
| caller @5 :Void; | |
| # Send the return message back to the caller (the usual). | |
| yourself @6 :Void; | |
| # **(level 1)** | |
| # | |
| # Don't actually return the results to the sender. Instead, hold on to them and await | |
| # instructions from the sender regarding what to do with them. In particular, the sender | |
| # may subsequently send a `Return` for some other call (which the receiver had previously made | |
| # to the sender) with `takeFromOtherQuestion` set. The results from this call are then used | |
| # as the results of the other call. | |
| # | |
| # When `yourself` is used, the receiver must still send a `Return` for the call, but sets the | |
| # field `resultsSentElsewhere` in that `Return` rather than including the results. | |
| # | |
| # This feature can be used to implement tail calls in which a call from Vat A to Vat B ends up | |
| # returning the result of a call from Vat B back to Vat A. | |
| # | |
| # In particular, the most common use case for this feature is when Vat A makes a call to a | |
| # promise in Vat B, and then that promise ends up resolving to a capability back in Vat A. | |
| # Vat B must forward all the queued calls on that promise back to Vat A, but can set `yourself` | |
| # in the calls so that the results need not pass back through Vat B. | |
| # | |
| # For example: | |
| # - Alice, in Vat A, calls foo() on Bob in Vat B. | |
| # - Alice makes a pipelined call bar() on the promise returned by foo(). | |
| # - Later on, Bob resolves the promise from foo() to point at Carol, who lives in Vat A (next | |
| # to Alice). | |
| # - Vat B dutifully forwards the bar() call to Carol. Let us call this forwarded call bar'(). | |
| # Notice that bar() and bar'() are travelling in opposite directions on the same network | |
| # link. | |
| # - The `Call` for bar'() has `sendResultsTo` set to `yourself`. | |
| # - Vat B sends a `Return` for bar() with `takeFromOtherQuestion` set in place of the results, | |
| # with the value set to the question ID of bar'(). Vat B does not wait for bar'() to return, | |
| # as doing so would introduce unnecessary round trip latency. | |
| # - Vat A receives bar'() and delivers it to Carol. | |
| # - When bar'() returns, Vat A sends a `Return` for bar'() to Vat B, with `resultsSentElsewhere` | |
| # set in place of results. | |
| # - Vat A sends a `Finish` for the bar() call to Vat B. | |
| # - Vat B receives the `Finish` for bar() and sends a `Finish` for bar'(). | |
| thirdParty @7 :RecipientId; | |
| # **(level 3)** | |
| # | |
| # The call's result should be returned to a different vat. The receiver (the callee) expects | |
| # to receive an `Accept` message from the indicated vat, and should return the call's result | |
| # to it, rather than to the sender of the `Call`. | |
| # | |
| # This operates much like `yourself`, above, except that Carol is in a separate Vat C. `Call` | |
| # messages are sent from Vat A -> Vat B and Vat B -> Vat C. A `Return` message is sent from | |
| # Vat B -> Vat A that contains `acceptFromThirdParty` in place of results. When Vat A sends | |
| # an `Accept` to Vat C, it receives back a `Return` containing the call's actual result. Vat C | |
| # also sends a `Return` to Vat B with `resultsSentElsewhere`. | |
| } | |
| } | |
| struct Return { | |
| # **(level 0)** | |
| # | |
| # Message type sent from callee to caller indicating that the call has completed. | |
| answerId @0 :AnswerId; | |
| # Equal to the QuestionId of the corresponding `Call` message. | |
| releaseParamCaps @1 :Bool = true; | |
| # If true, all capabilities that were in the params should be considered released. The sender | |
| # must not send separate `Release` messages for them. Level 0 implementations in particular | |
| # should always set this true. This defaults true because if level 0 implementations forget to | |
| # set it they'll never notice (just silently leak caps), but if level >=1 implementations forget | |
| # to set it to false they'll quickly get errors. | |
| union { | |
| results @2 :Payload; | |
| # The result. | |
| # | |
| # For regular method calls, `results.content` points to the result struct. | |
| # | |
| # For a `Return` in response to an `Accept`, `results` contains a single capability (rather | |
| # than a struct), and `results.content` is just a capability pointer with index 0. A `Finish` | |
| # is still required in this case. | |
| exception @3 :Exception; | |
| # Indicates that the call failed and explains why. | |
| canceled @4 :Void; | |
| # Indicates that the call was canceled due to the caller sending a Finish message | |
| # before the call had completed. | |
| resultsSentElsewhere @5 :Void; | |
| # This is set when returning from a `Call` that had `sendResultsTo` set to something other | |
| # than `caller`. | |
| takeFromOtherQuestion @6 :QuestionId; | |
| # The sender has also sent (before this message) a `Call` with the given question ID and with | |
| # `sendResultsTo.yourself` set, and the results of that other call should be used as the | |
| # results here. `takeFromOtherQuestion` can only used once per question. | |
| acceptFromThirdParty @7 :ThirdPartyCapId; | |
| # **(level 3)** | |
| # | |
| # The caller should contact a third-party vat to pick up the results. An `Accept` message | |
| # sent to the vat will return the result. This pairs with `Call.sendResultsTo.thirdParty`. | |
| # It should only be used if the corresponding `Call` had `allowThirdPartyTailCall` set. | |
| } | |
| } | |
| struct Finish { | |
| # **(level 0)** | |
| # | |
| # Message type sent from the caller to the callee to indicate: | |
| # 1) The questionId will no longer be used in any messages sent by the callee (no further | |
| # pipelined requests). | |
| # 2) If the call has not returned yet, the caller no longer cares about the result. If nothing | |
| # else cares about the result either (e.g. there are no other outstanding calls pipelined on | |
| # the result of this one) then the callee may wish to immediately cancel the operation and | |
| # send back a Return message with "canceled" set. However, implementations are not required | |
| # to support premature cancellation -- instead, the implementation may wait until the call | |
| # actually completes and send a normal `Return` message. | |
| # | |
| # TODO(someday): Should we separate (1) and implicitly releasing result capabilities? It would be | |
| # possible and useful to notify the server that it doesn't need to keep around the response to | |
| # service pipeline requests even though the caller still wants to receive it / hasn't yet | |
| # finished processing it. It could also be useful to notify the server that it need not marshal | |
| # the results because the caller doesn't want them anyway, even if the caller is still sending | |
| # pipelined calls, although this seems less useful (just saving some bytes on the wire). | |
| questionId @0 :QuestionId; | |
| # ID of the call whose result is to be released. | |
| releaseResultCaps @1 :Bool = true; | |
| # If true, all capabilities that were in the results should be considered released. The sender | |
| # must not send separate `Release` messages for them. Level 0 implementations in particular | |
| # should always set this true. This defaults true because if level 0 implementations forget to | |
| # set it they'll never notice (just silently leak caps), but if level >=1 implementations forget | |
| # set it false they'll quickly get errors. | |
| } | |
| # Level 1 message types ---------------------------------------------- | |
| struct Resolve { | |
| # **(level 1)** | |
| # | |
| # Message type sent to indicate that a previously-sent promise has now been resolved to some other | |
| # object (possibly another promise) -- or broken, or canceled. | |
| # | |
| # Keep in mind that it's possible for a `Resolve` to be sent to a level 0 implementation that | |
| # doesn't implement it. For example, a method call or return might contain a capability in the | |
| # payload. Normally this is fine even if the receiver is level 0, because they will implicitly | |
| # release all such capabilities on return / finish. But if the cap happens to be a promise, then | |
| # a follow-up `Resolve` may be sent regardless of this release. The level 0 receiver will reply | |
| # with an `unimplemented` message, and the sender (of the `Resolve`) can respond to this as if the | |
| # receiver had immediately released any capability to which the promise resolved. | |
| # | |
| # When implementing promise resolution, it's important to understand how embargos work and the | |
| # tricky case of the Tribble 4-way race condition. See the comments for the Disembargo message, | |
| # below. | |
| promiseId @0 :ExportId; | |
| # The ID of the promise to be resolved. | |
| # | |
| # Unlike all other instances of `ExportId` sent from the exporter, the `Resolve` message does | |
| # _not_ increase the reference count of `promiseId`. In fact, it is expected that the receiver | |
| # will release the export soon after receiving `Resolve`, and the sender will not send this | |
| # `ExportId` again until it has been released and recycled. | |
| # | |
| # When an export ID sent over the wire (e.g. in a `CapDescriptor`) is indicated to be a promise, | |
| # this indicates that the sender will follow up at some point with a `Resolve` message. If the | |
| # same `promiseId` is sent again before `Resolve`, still only one `Resolve` is sent. If the | |
| # same ID is sent again later _after_ a `Resolve`, it can only be because the export's | |
| # reference count hit zero in the meantime and the ID was re-assigned to a new export, therefore | |
| # this later promise does _not_ correspond to the earlier `Resolve`. | |
| # | |
| # If a promise ID's reference count reaches zero before a `Resolve` is sent, the `Resolve` | |
| # message may or may not still be sent (the `Resolve` may have already been in-flight when | |
| # `Release` was sent, but if the `Release` is received before `Resolve` then there is no longer | |
| # any reason to send a `Resolve`). Thus a `Resolve` may be received for a promise of which | |
| # the receiver has no knowledge, because it already released it earlier. In this case, the | |
| # receiver should simply release the capability to which the promise resolved. | |
| union { | |
| cap @1 :CapDescriptor; | |
| # The object to which the promise resolved. | |
| # | |
| # The sender promises that from this point forth, until `promiseId` is released, it shall | |
| # simply forward all messages to the capability designated by `cap`. This is true even if | |
| # `cap` itself happens to desigate another promise, and that other promise later resolves -- | |
| # messages sent to `promiseId` shall still go to that other promise, not to its resolution. | |
| # This is important in the case that the receiver of the `Resolve` ends up sending a | |
| # `Disembargo` message towards `promiseId` in order to control message ordering -- that | |
| # `Disembargo` really needs to reflect back to exactly the object designated by `cap` even | |
| # if that object is itself a promise. | |
| exception @2 :Exception; | |
| # Indicates that the promise was broken. | |
| } | |
| } | |
| struct Release { | |
| # **(level 1)** | |
| # | |
| # Message type sent to indicate that the sender is done with the given capability and the receiver | |
| # can free resources allocated to it. | |
| id @0 :ImportId; | |
| # What to release. | |
| referenceCount @1 :UInt32; | |
| # The amount by which to decrement the reference count. The export is only actually released | |
| # when the reference count reaches zero. | |
| } | |
| struct Disembargo { | |
| # **(level 1)** | |
| # | |
| # Message sent to indicate that an embargo on a recently-resolved promise may now be lifted. | |
| # | |
| # Embargos are used to enforce E-order in the presence of promise resolution. That is, if an | |
| # application makes two calls foo() and bar() on the same capability reference, in that order, | |
| # the calls should be delivered in the order in which they were made. But if foo() is called | |
| # on a promise, and that promise happens to resolve before bar() is called, then the two calls | |
| # may travel different paths over the network, and thus could arrive in the wrong order. In | |
| # this case, the call to `bar()` must be embargoed, and a `Disembargo` message must be sent along | |
| # the same path as `foo()` to ensure that the `Disembargo` arrives after `foo()`. Once the | |
| # `Disembargo` arrives, `bar()` can then be delivered. | |
| # | |
| # There are two particular cases where embargos are important. Consider object Alice, in Vat A, | |
| # who holds a promise P, pointing towards Vat B, that eventually resolves to Carol. The two | |
| # cases are: | |
| # - Carol lives in Vat A, i.e. next to Alice. In this case, Vat A needs to send a `Disembargo` | |
| # message that echos through Vat B and back, to ensure that all pipelined calls on the promise | |
| # have been delivered. | |
| # - Carol lives in a different Vat C. When the promise resolves, a three-party handoff occurs | |
| # (see `Provide` and `Accept`, which constitute level 3 of the protocol). In this case, we | |
| # piggyback on the state that has already been set up to handle the handoff: the `Accept` | |
| # message (from Vat A to Vat C) is embargoed, as are all pipelined messages sent to it, while | |
| # a `Disembargo` message is sent from Vat A through Vat B to Vat C. See `Accept.embargo` for | |
| # an example. | |
| # | |
| # Note that in the case where Carol actually lives in Vat B (i.e., the same vat that the promise | |
| # already pointed at), no embargo is needed, because the pipelined calls are delivered over the | |
| # same path as the later direct calls. | |
| # | |
| # Keep in mind that promise resolution happens both in the form of Resolve messages as well as | |
| # Return messages (which resolve PromisedAnswers). Embargos apply in both cases. | |
| # | |
| # An alternative strategy for enforcing E-order over promise resolution could be for Vat A to | |
| # implement the embargo internally. When Vat A is notified of promise resolution, it could | |
| # send a dummy no-op call to promise P and wait for it to complete. Until that call completes, | |
| # all calls to the capability are queued locally. This strategy works, but is pessimistic: | |
| # in the three-party case, it requires an A -> B -> C -> B -> A round trip before calls can start | |
| # being delivered directly to from Vat A to Vat C. The `Disembargo` message allows latency to be | |
| # reduced. (In the two-party loopback case, the `Disembargo` message is just a more explicit way | |
| # of accomplishing the same thing as a no-op call, but isn't any faster.) | |
| # | |
| # *The Tribble 4-way Race Condition* | |
| # | |
| # Any implementation of promise resolution and embargos must be aware of what we call the | |
| # "Tribble 4-way race condition", after Dean Tribble, who explained the problem in a lively | |
| # Friam meeting. | |
| # | |
| # Embargos are designed to work in the case where a two-hop path is being shortened to one hop. | |
| # But sometimes there are more hops. Imagine that Alice has a reference to a remote promise P1 | |
| # that eventually resolves to _another_ remote promise P2 (in a third vat), which _at the same | |
| # time_ happens to resolve to Bob (in a fourth vat). In this case, we're shortening from a 3-hop | |
| # path (with four parties) to a 1-hop path (Alice -> Bob). | |
| # | |
| # Extending the embargo/disembargo protocol to be able to shorted multiple hops at once seems | |
| # difficult. Instead, we make a rule that prevents this case from coming up: | |
| # | |
| # One a promise P has been resolved to a remove object reference R, then all further messages | |
| # received addressed to P will be forwarded strictly to R. Even if it turns out later that R is | |
| # itself a promise, and has resolved to some other object Q, messages sent to P will still be | |
| # forwarded to R, not directly to Q (R will of course further forward the messages to Q). | |
| # | |
| # This rule does not cause a significant performance burden because once P has resolved to R, it | |
| # is expected that people sending messages to P will shortly start sending them to R instead and | |
| # drop P. P is at end-of-life anyway, so it doesn't matter if it ignores chances to further | |
| # optimize its path. | |
| target @0 :MessageTarget; | |
| # What is to be disembargoed. | |
| using EmbargoId = UInt32; | |
| # Used in `senderLoopback` and `receiverLoopback`, below. | |
| context :union { | |
| senderLoopback @1 :EmbargoId; | |
| # The sender is requesting a disembargo on a promise that is known to resolve back to a | |
| # capability hosted by the sender. As soon as the receiver has echoed back all pipelined calls | |
| # on this promise, it will deliver the Disembargo back to the sender with `receiverLoopback` | |
| # set to the same value as `senderLoopback`. This value is chosen by the sender, and since | |
| # it is also consumed be the sender, the sender can use whatever strategy it wants to make sure | |
| # the value is unambiguous. | |
| # | |
| # The receiver must verify that the target capability actually resolves back to the sender's | |
| # vat. Otherwise, the sender has committed a protocol error and should be disconnected. | |
| receiverLoopback @2 :EmbargoId; | |
| # The receiver previously sent a `senderLoopback` Disembargo towards a promise resolving to | |
| # this capability, and that Disembargo is now being echoed back. | |
| accept @3 :Void; | |
| # **(level 3)** | |
| # | |
| # The sender is requesting a disembargo on a promise that is known to resolve to a third-party | |
| # capability that the sender is currently in the process of accepting (using `Accept`). | |
| # The receiver of this `Disembargo` has an outstanding `Provide` on said capability. The | |
| # receiver should now send a `Disembargo` with `provide` set to the question ID of that | |
| # `Provide` message. | |
| # | |
| # See `Accept.embargo` for an example. | |
| provide @4 :QuestionId; | |
| # **(level 3)** | |
| # | |
| # The sender is requesting a disembargo on a capability currently being provided to a third | |
| # party. The question ID identifies the `Provide` message previously sent by the sender to | |
| # this capability. On receipt, the receiver (the capability host) shall release the embargo | |
| # on the `Accept` message that it has received from the third party. See `Accept.embargo` for | |
| # an example. | |
| } | |
| } | |
| # Level 2 message types ---------------------------------------------- | |
| # See persistent.capnp. | |
| # Level 3 message types ---------------------------------------------- | |
| struct Provide { | |
| # **(level 3)** | |
| # | |
| # Message type sent to indicate that the sender wishes to make a particular capability implemented | |
| # by the receiver available to a third party for direct access (without the need for the third | |
| # party to proxy through the sender). | |
| # | |
| # (In CapTP, `Provide` and `Accept` are methods of the global `NonceLocator` object exported by | |
| # every vat. In Cap'n Proto, we bake this into the core protocol.) | |
| questionId @0 :QuestionId; | |
| # Question ID to be held open until the recipient has received the capability. A result will be | |
| # returned once the third party has successfully received the capability. The sender must at some | |
| # point send a `Finish` message as with any other call, and that message can be used to cancel the | |
| # whole operation. | |
| target @1 :MessageTarget; | |
| # What is to be provided to the third party. | |
| recipient @2 :RecipientId; | |
| # Identity of the third party that is expected to pick up the capability. | |
| } | |
| struct Accept { | |
| # **(level 3)** | |
| # | |
| # Message type sent to pick up a capability hosted by the receiving vat and provided by a third | |
| # party. The third party previously designated the capability using `Provide`. | |
| # | |
| # This message is also used to pick up a redirected return -- see `Return.acceptFromThirdParty`. | |
| questionId @0 :QuestionId; | |
| # A new question ID identifying this accept message, which will eventually receive a Return | |
| # message containing the provided capability (or the call result in the case of a redirected | |
| # return). | |
| provision @1 :ProvisionId; | |
| # Identifies the provided object to be picked up. | |
| embargo @2 :Bool; | |
| # If true, this accept shall be temporarily embargoed. The resulting `Return` will not be sent, | |
| # and any pipelined calls will not be delivered, until the embargo is released. The receiver | |
| # (the capability host) will expect the provider (the vat that sent the `Provide` message) to | |
| # eventually send a `Disembargo` message with the field `context.provide` set to the question ID | |
| # of the original `Provide` message. At that point, the embargo is released and the queued | |
| # messages are delivered. | |
| # | |
| # For example: | |
| # - Alice, in Vat A, holds a promise P, which currently points toward Vat B. | |
| # - Alice calls foo() on P. The `Call` message is sent to Vat B. | |
| # - The promise P in Vat B ends up resolving to Carol, in Vat C. | |
| # - Vat B sends a `Provide` message to Vat C, identifying Vat A as the recipient. | |
| # - Vat B sends a `Resolve` message to Vat A, indicating that the promise has resolved to a | |
| # `ThirdPartyCapId` identifying Carol in Vat C. | |
| # - Vat A sends an `Accept` message to Vat C to pick up the capability. Since Vat A knows that | |
| # it has an outstanding call to the promise, it sets `embargo` to `true` in the `Accept` | |
| # message. | |
| # - Vat A sends a `Disembargo` message to Vat B on promise P, with `context.accept` set. | |
| # - Alice makes a call bar() to promise P, which is now pointing towards Vat C. Alice doesn't | |
| # know anything about the mechanics of promise resolution happening under the hood, but she | |
| # expects that bar() will be delivered after foo() because that is the order in which she | |
| # initiated the calls. | |
| # - Vat A sends the bar() call to Vat C, as a pipelined call on the result of the `Accept` (which | |
| # hasn't returned yet, due to the embargo). Since calls to the newly-accepted capability | |
| # are embargoed, Vat C does not deliver the call yet. | |
| # - At some point, Vat B forwards the foo() call from the beginning of this example on to Vat C. | |
| # - Vat B forwards the `Disembargo` from Vat A on to vat C. It sets `context.provide` to the | |
| # question ID of the `Provide` message it had sent previously. | |
| # - Vat C receives foo() before `Disembargo`, thus allowing it to correctly deliver foo() | |
| # before delivering bar(). | |
| # - Vat C receives `Disembargo` from Vat B. It can now send a `Return` for the `Accept` from | |
| # Vat A, as well as deliver bar(). | |
| } | |
| # Level 4 message types ---------------------------------------------- | |
| struct Join { | |
| # **(level 4)** | |
| # | |
| # Message type sent to implement E.join(), which, given a number of capabilities that are | |
| # expected to be equivalent, finds the underlying object upon which they all agree and forms a | |
| # direct connection to it, skipping any proxies that may have been constructed by other vats | |
| # while transmitting the capability. See: | |
| # http://erights.org/elib/equality/index.html | |
| # | |
| # Note that this should only serve to bypass fully-transparent proxies -- proxies that were | |
| # created merely for convenience, without any intention of hiding the underlying object. | |
| # | |
| # For example, say Bob holds two capabilities hosted by Alice and Carol, but he expects that both | |
| # are simply proxies for a capability hosted elsewhere. He then issues a join request, which | |
| # operates as follows: | |
| # - Bob issues Join requests on both Alice and Carol. Each request contains a different piece | |
| # of the JoinKey. | |
| # - Alice is proxying a capability hosted by Dana, so forwards the request to Dana's cap. | |
| # - Dana receives the first request and sees that the JoinKeyPart is one of two. She notes that | |
| # she doesn't have the other part yet, so she records the request and responds with a | |
| # JoinResult. | |
| # - Alice relays the JoinAswer back to Bob. | |
| # - Carol is also proxying a capability from Dana, and so forwards her Join request to Dana as | |
| # well. | |
| # - Dana receives Carol's request and notes that she now has both parts of a JoinKey. She | |
| # combines them in order to form information needed to form a secure connection to Bob. She | |
| # also responds with another JoinResult. | |
| # - Bob receives the responses from Alice and Carol. He uses the returned JoinResults to | |
| # determine how to connect to Dana and attempts to form the connection. Since Bob and Dana now | |
| # agree on a secret key that neither Alice nor Carol ever saw, this connection can be made | |
| # securely even if Alice or Carol is conspiring against the other. (If Alice and Carol are | |
| # conspiring _together_, they can obviously reproduce the key, but this doesn't matter because | |
| # the whole point of the join is to verify that Alice and Carol agree on what capability they | |
| # are proxying.) | |
| # | |
| # If the two capabilities aren't actually proxies of the same object, then the join requests | |
| # will come back with conflicting `hostId`s and the join will fail before attempting to form any | |
| # connection. | |
| questionId @0 :QuestionId; | |
| # Question ID used to respond to this Join. (Note that this ID only identifies one part of the | |
| # request for one hop; each part has a different ID and relayed copies of the request have | |
| # (probably) different IDs still.) | |
| # | |
| # The receiver will reply with a `Return` whose `results` is a JoinResult. This `JoinResult` | |
| # is relayed from the joined object's host, possibly with transformation applied as needed | |
| # by the network. | |
| # | |
| # Like any return, the result must be released using a `Finish`. However, this release | |
| # should not occur until the joiner has either successfully connected to the joined object. | |
| # Vats relaying a `Join` message similarly must not release the result they receive until the | |
| # return they relayed back towards the joiner has itself been released. This allows the | |
| # joined object's host to detect when the Join operation is canceled before completing -- if | |
| # it receives a `Finish` for one of the join results before the joiner successfully | |
| # connects. It can then free any resources it had allocated as part of the join. | |
| target @1 :MessageTarget; | |
| # The capability to join. | |
| keyPart @2 :JoinKeyPart; | |
| # A part of the join key. These combine to form the complete join key, which is used to establish | |
| # a direct connection. | |
| # TODO(before implementing): Change this so that multiple parts can be sent in a single Join | |
| # message, so that if multiple join parts are going to cross the same connection they can be sent | |
| # together, so that the receive can potentially optimize its handling of them. In the case where | |
| # all parts are bundled together, should the recipient be expected to simply return a cap, so | |
| # that the caller can immediately start pipelining to it? | |
| } | |
| # ======================================================================================== | |
| # Common structures used in messages | |
| struct MessageTarget { | |
| # The target of a `Call` or other messages that target a capability. | |
| union { | |
| importedCap @0 :ImportId; | |
| # This message is to a capability or promise previously imported by the caller (exported by | |
| # the receiver). | |
| promisedAnswer @1 :PromisedAnswer; | |
| # This message is to a capability that is expected to be returned by another call that has not | |
| # yet been completed. | |
| # | |
| # At level 0, this is supported only for addressing the result of a previous `Bootstrap`, so | |
| # that initial startup doesn't require a round trip. | |
| } | |
| } | |
| struct Payload { | |
| # Represents some data structure that might contain capabilities. | |
| content @0 :AnyPointer; | |
| # Some Cap'n Proto data structure. Capability pointers embedded in this structure index into | |
| # `capTable`. | |
| capTable @1 :List(CapDescriptor); | |
| # Descriptors corresponding to the cap pointers in `content`. | |
| } | |
| struct CapDescriptor { | |
| # **(level 1)** | |
| # | |
| # When an application-defined type contains an interface pointer, that pointer contains an index | |
| # into the message's capability table -- i.e. the `capTable` part of the `Payload`. Each | |
| # capability in the table is represented as a `CapDescriptor`. The runtime API should not reveal | |
| # the CapDescriptor directly to the application, but should instead wrap it in some kind of | |
| # callable object with methods corresponding to the interface that the capability implements. | |
| # | |
| # Keep in mind that `ExportIds` in a `CapDescriptor` are subject to reference counting. See the | |
| # description of `ExportId`. | |
| union { | |
| none @0 :Void; | |
| # There is no capability here. This `CapDescriptor` should not appear in the payload content. | |
| # A `none` CapDescriptor can be generated when an application inserts a capability into a | |
| # message and then later changes its mind and removes it -- rewriting all of the other | |
| # capability pointers may be hard, so instead a tombstone is left, similar to the way a removed | |
| # struct or list instance is zeroed out of the message but the space is not reclaimed. | |
| # Hopefully this is unusual. | |
| senderHosted @1 :ExportId; | |
| # The ID of a capability in the sender's export table (receiver's import table). It may be a | |
| # newly allocated table entry, or an existing entry (increments the reference count). | |
| senderPromise @2 :ExportId; | |
| # A promise that the sender will resolve later. The sender will send exactly one Resolve | |
| # message at a future point in time to replace this promise. Note that even if the same | |
| # `senderPromise` is received multiple times, only one `Resolve` is sent to cover all of | |
| # them. If `senderPromise` is released before the `Resolve` is sent, the sender (of this | |
| # `CapDescriptor`) may choose not to send the `Resolve` at all. | |
| receiverHosted @3 :ImportId; | |
| # A capability (or promise) previously exported by the receiver (imported by the sender). | |
| receiverAnswer @4 :PromisedAnswer; | |
| # A capability expected to be returned in the results of a currently-outstanding call posed | |
| # by the sender. | |
| thirdPartyHosted @5 :ThirdPartyCapDescriptor; | |
| # **(level 3)** | |
| # | |
| # A capability that lives in neither the sender's nor the receiver's vat. The sender needs | |
| # to form a direct connection to a third party to pick up the capability. | |
| # | |
| # Level 1 and 2 implementations that receive a `thirdPartyHosted` may simply send calls to its | |
| # `vine` instead. | |
| } | |
| } | |
| struct PromisedAnswer { | |
| # **(mostly level 1)** | |
| # | |
| # Specifies how to derive a promise from an unanswered question, by specifying the path of fields | |
| # to follow from the root of the eventual result struct to get to the desired capability. Used | |
| # to address method calls to a not-yet-returned capability or to pass such a capability as an | |
| # input to some other method call. | |
| # | |
| # Level 0 implementations must support `PromisedAnswer` only for the case where the answer is | |
| # to a `Bootstrap` message. In this case, `path` is always empty since `Bootstrap` always returns | |
| # a raw capability. | |
| questionId @0 :QuestionId; | |
| # ID of the question (in the sender's question table / receiver's answer table) whose answer is | |
| # expected to contain the capability. | |
| transform @1 :List(Op); | |
| # Operations / transformations to apply to the result in order to get the capability actually | |
| # being addressed. E.g. if the result is a struct and you want to call a method on a capability | |
| # pointed to by a field of the struct, you need a `getPointerField` op. | |
| struct Op { | |
| union { | |
| noop @0 :Void; | |
| # Does nothing. This member is mostly defined so that we can make `Op` a union even | |
| # though (as of this writing) only one real operation is defined. | |
| getPointerField @1 :UInt16; | |
| # Get a pointer field within a struct. The number is an index into the pointer section, NOT | |
| # a field ordinal, so that the receiver does not need to understand the schema. | |
| # TODO(someday): We could add: | |
| # - For lists, the ability to address every member of the list, or a slice of the list, the | |
| # result of which would be another list. This is useful for implementing the equivalent of | |
| # a SQL table join (not to be confused with the `Join` message type). | |
| # - Maybe some ability to test a union. | |
| # - Probably not a good idea: the ability to specify an arbitrary script to run on the | |
| # result. We could define a little stack-based language where `Op` specifies one | |
| # "instruction" or transformation to apply. Although this is not a good idea | |
| # (over-engineered), any narrower additions to `Op` should be designed as if this | |
| # were the eventual goal. | |
| } | |
| } | |
| } | |
| struct ThirdPartyCapDescriptor { | |
| # **(level 3)** | |
| # | |
| # Identifies a capability in a third-party vat that the sender wants the receiver to pick up. | |
| id @0 :ThirdPartyCapId; | |
| # Identifies the third-party host and the specific capability to accept from it. | |
| vineId @1 :ExportId; | |
| # A proxy for the third-party object exported by the sender. In CapTP terminology this is called | |
| # a "vine", because it is an indirect reference to the third-party object that snakes through the | |
| # sender vat. This serves two purposes: | |
| # | |
| # * Level 1 and 2 implementations that don't understand how to connect to a third party may | |
| # simply send calls to the vine. Such calls will be forwarded to the third-party by the | |
| # sender. | |
| # | |
| # * Level 3 implementations must release the vine once they have successfully picked up the | |
| # object from the third party. This ensures that the capability is not released by the sender | |
| # prematurely. | |
| # | |
| # The sender will close the `Provide` request that it has sent to the third party as soon as | |
| # it receives either a `Call` or a `Release` message directed at the vine. | |
| } | |
| struct Exception { | |
| # **(level 0)** | |
| # | |
| # Describes an arbitrary error that prevented an operation (e.g. a call) from completing. | |
| # | |
| # Cap'n Proto exceptions always indicate that something went wrong. In other words, in a fantasy | |
| # world where everything always works as expected, no exceptions would ever be thrown. Clients | |
| # should only ever catch exceptions as a means to implement fault-tolerance, where "fault" can | |
| # mean: | |
| # - Bugs. | |
| # - Invalid input. | |
| # - Configuration errors. | |
| # - Network problems. | |
| # - Insufficient resources. | |
| # - Version skew (unimplemented functionality). | |
| # - Other logistical problems. | |
| # | |
| # Exceptions should NOT be used to flag application-specific conditions that a client is expected | |
| # to handle in an application-specific way. Put another way, in the Cap'n Proto world, | |
| # "checked exceptions" (where an interface explicitly defines the exceptions it throws and | |
| # clients are forced by the type system to handle those exceptions) do NOT make sense. | |
| reason @0 :Text; | |
| # Human-readable failure description. | |
| type @3 :Type; | |
| # The type of the error. The purpose of this enum is not to describe the error itself, but | |
| # rather to describe how the client might want to respond to the error. | |
| enum Type { | |
| failed @0; | |
| # A generic problem occurred, and it is believed that if the operation were repeated without | |
| # any change in the state of the world, the problem would occur again. | |
| # | |
| # A client might respond to this error by logging it for investigation by the developer and/or | |
| # displaying it to the user. | |
| overloaded @1; | |
| # The request was rejected due to a temporary lack of resources. | |
| # | |
| # Examples include: | |
| # - There's not enough CPU time to keep up with incoming requests, so some are rejected. | |
| # - The server ran out of RAM or disk space during the request. | |
| # - The operation timed out (took significantly longer than it should have). | |
| # | |
| # A client might respond to this error by scheduling to retry the operation much later. The | |
| # client should NOT retry again immediately since this would likely exacerbate the problem. | |
| disconnected @2; | |
| # The method failed because a connection to some necessary capability was lost. | |
| # | |
| # Examples include: | |
| # - The client introduced the server to a third-party capability, the connection to that third | |
| # party was subsequently lost, and then the client requested that the server use the dead | |
| # capability for something. | |
| # - The client previously requested that the server obtain a capability from some third party. | |
| # The server returned a capability to an object wrapping the third-party capability. Later, | |
| # the server's connection to the third party was lost. | |
| # - The capability has been revoked. Revocation does not necessarily mean that the client is | |
| # no longer authorized to use the capability; it is often used simply as a way to force the | |
| # client to repeat the setup process, perhaps to efficiently move them to a new back-end or | |
| # get them to recognize some other change that has occurred. | |
| # | |
| # A client should normally respond to this error by releasing all capabilities it is currently | |
| # holding related to the one it called and then re-creating them by restoring SturdyRefs and/or | |
| # repeating the method calls used to create them originally. In other words, disconnect and | |
| # start over. This should in turn cause the server to obtain a new copy of the capability that | |
| # it lost, thus making everything work. | |
| # | |
| # If the client receives another `disconnencted` error in the process of rebuilding the | |
| # capability and retrying the call, it should treat this as an `overloaded` error: the network | |
| # is currently unreliable, possibly due to load or other temporary issues. | |
| unimplemented @3; | |
| # The server doesn't implement the requested method. If there is some other method that the | |
| # client could call (perhaps an older and/or slower interface), it should try that instead. | |
| # Otherwise, this should be treated like `failed`. | |
| } | |
| obsoleteIsCallersFault @1 :Bool; | |
| # OBSOLETE. Ignore. | |
| obsoleteDurability @2 :UInt16; | |
| # OBSOLETE. See `type` instead. | |
| } | |
| # ======================================================================================== | |
| # Network-specific Parameters | |
| # | |
| # Some parts of the Cap'n Proto RPC protocol are not specified here because different vat networks | |
| # may wish to use different approaches to solving them. For example, on the public internet, you | |
| # may want to authenticate vats using public-key cryptography, but on a local intranet with trusted | |
| # infrastructure, you may be happy to authenticate based on network address only, or some other | |
| # lightweight mechanism. | |
| # | |
| # To accommodate this, we specify several "parameter" types. Each type is defined here as an | |
| # alias for `AnyPointer`, but a specific network will want to define a specific set of types to use. | |
| # All vats in a vat network must agree on these parameters in order to be able to communicate. | |
| # Inter-network communication can be accomplished through "gateways" that perform translation | |
| # between the primitives used on each network; these gateways may need to be deeply stateful, | |
| # depending on the translations they perform. | |
| # | |
| # For interaction over the global internet between parties with no other prior arrangement, a | |
| # particular set of bindings for these types is defined elsewhere. (TODO(someday): Specify where | |
| # these common definitions live.) | |
| # | |
| # Another common network type is the two-party network, in which one of the parties typically | |
| # interacts with the outside world entirely through the other party. In such a connection between | |
| # Alice and Bob, all objects that exist on Bob's other networks appear to Alice as if they were | |
| # hosted by Bob himself, and similarly all objects on Alice's network (if she even has one) appear | |
| # to Bob as if they were hosted by Alice. This network type is interesting because from the point | |
| # of view of a simple application that communicates with only one other party via the two-party | |
| # protocol, there are no three-party interactions at all, and joins are unusually simple to | |
| # implement, so implementing at level 4 is barely more complicated than implementing at level 1. | |
| # Moreover, if you pair an app implementing the two-party network with a container that implements | |
| # some other network, the app can then participate on the container's network just as if it | |
| # implemented that network directly. The types used by the two-party network are defined in | |
| # `rpc-twoparty.capnp`. | |
| # | |
| # The things that we need to parameterize are: | |
| # - How to store capabilities long-term without holding a connection open (mostly level 2). | |
| # - How to authenticate vats in three-party introductions (level 3). | |
| # - How to implement `Join` (level 4). | |
| # | |
| # Persistent references | |
| # --------------------- | |
| # | |
| # **(mostly level 2)** | |
| # | |
| # We want to allow some capabilities to be stored long-term, even if a connection is lost and later | |
| # recreated. ExportId is a short-term identifier that is specific to a connection, so it doesn't | |
| # help here. We need a way to specify long-term identifiers, as well as a strategy for | |
| # reconnecting to a referenced capability later. | |
| # | |
| # Three-party interactions | |
| # ------------------------ | |
| # | |
| # **(level 3)** | |
| # | |
| # In cases where more than two vats are interacting, we have situations where VatA holds a | |
| # capability hosted by VatB and wants to send that capability to VatC. This can be accomplished | |
| # by VatA proxying requests on the new capability, but doing so has two big problems: | |
| # - It's inefficient, requiring an extra network hop. | |
| # - If VatC receives another capability to the same object from VatD, it is difficult for VatC to | |
| # detect that the two capabilities are really the same and to implement the E "join" operation, | |
| # which is necessary for certain four-or-more-party interactions, such as the escrow pattern. | |
| # See: http://www.erights.org/elib/equality/grant-matcher/index.html | |
| # | |
| # Instead, we want a way for VatC to form a direct, authenticated connection to VatB. | |
| # | |
| # Join | |
| # ---- | |
| # | |
| # **(level 4)** | |
| # | |
| # The `Join` message type and corresponding operation arranges for a direct connection to be formed | |
| # between the joiner and the host of the joined object, and this connection must be authenticated. | |
| # Thus, the details are network-dependent. | |
| using SturdyRef = AnyPointer; | |
| # **(level 2)** | |
| # | |
| # Identifies a persisted capability that can be restored in the future. How exactly a SturdyRef | |
| # is restored to a live object is specified along with the SturdyRef definition (i.e. not by | |
| # rpc.capnp). | |
| # | |
| # Generally a SturdyRef needs to specify three things: | |
| # - How to reach the vat that can restore the ref (e.g. a hostname or IP address). | |
| # - How to authenticate the vat after connecting (e.g. a public key fingerprint). | |
| # - The identity of a specific object hosted by the vat. Generally, this is an opaque pointer whose | |
| # format is defined by the specific vat -- the client has no need to inspect the object ID. | |
| # It is important that the objec ID be unguessable if the object is not public (and objects | |
| # should almost never be public). | |
| # | |
| # The above are only suggestions. Some networks might work differently. For example, a private | |
| # network might employ a special restorer service whose sole purpose is to restore SturdyRefs. | |
| # In this case, the entire contents of SturdyRef might be opaque, because they are intended only | |
| # to be forwarded to the restorer service. | |
| using ProvisionId = AnyPointer; | |
| # **(level 3)** | |
| # | |
| # The information that must be sent in an `Accept` message to identify the object being accepted. | |
| # | |
| # In a network where each vat has a public/private key pair, this could simply be the public key | |
| # fingerprint of the provider vat along with a nonce matching the one in the `RecipientId` used | |
| # in the `Provide` message sent from that provider. | |
| using RecipientId = AnyPointer; | |
| # **(level 3)** | |
| # | |
| # The information that must be sent in a `Provide` message to identify the recipient of the | |
| # capability. | |
| # | |
| # In a network where each vat has a public/private key pair, this could simply be the public key | |
| # fingerprint of the recipient along with a nonce matching the one in the `ProvisionId`. | |
| using ThirdPartyCapId = AnyPointer; | |
| # **(level 3)** | |
| # | |
| # The information needed to connect to a third party and accept a capability from it. | |
| # | |
| # In a network where each vat has a public/private key pair, this could be a combination of the | |
| # third party's public key fingerprint, hints on how to connect to the third party (e.g. an IP | |
| # address), and the nonce used in the corresponding `Provide` message's `RecipientId` as sent | |
| # to that third party (used to identify which capability to pick up). | |
| using JoinKeyPart = AnyPointer; | |
| # **(level 4)** | |
| # | |
| # A piece of a secret key. One piece is sent along each path that is expected to lead to the same | |
| # place. Once the pieces are combined, a direct connection may be formed between the sender and | |
| # the receiver, bypassing any men-in-the-middle along the paths. See the `Join` message type. | |
| # | |
| # The motivation for Joins is discussed under "Supporting Equality" in the "Unibus" protocol | |
| # sketch: http://www.erights.org/elib/distrib/captp/unibus.html | |
| # | |
| # In a network where each vat has a public/private key pair and each vat forms no more than one | |
| # connection to each other vat, Joins will rarely -- perhaps never -- be needed, as objects never | |
| # need to be transparently proxied and references to the same object sent over the same connection | |
| # have the same export ID. Thus, a successful join requires only checking that the two objects | |
| # come from the same connection and have the same ID, and then completes immediately. | |
| # | |
| # However, in networks where two vats may form more than one connection between each other, or | |
| # where proxying of objects occurs, joins are necessary. | |
| # | |
| # Typically, each JoinKeyPart would include a fixed-length data value such that all value parts | |
| # XOR'd together forms a shared secret that can be used to form an encrypted connection between | |
| # the joiner and the joined object's host. Each JoinKeyPart should also include an indication of | |
| # how many parts to expect and a hash of the shared secret (used to match up parts). | |
| using JoinResult = AnyPointer; | |
| # **(level 4)** | |
| # | |
| # Information returned as the result to a `Join` message, needed by the joiner in order to form a | |
| # direct connection to a joined object. This might simply be the address of the joined object's | |
| # host vat, since the `JoinKey` has already been communicated so the two vats already have a shared | |
| # secret to use to authenticate each other. | |
| # | |
| # The `JoinResult` should also contain information that can be used to detect when the Join | |
| # requests ended up reaching different objects, so that this situation can be detected easily. | |
| # This could be a simple matter of including a sequence number -- if the joiner receives two | |
| # `JoinResult`s with sequence number 0, then they must have come from different objects and the | |
| # whole join is a failure. | |
| # ======================================================================================== | |
| # Network interface sketch | |
| # | |
| # The interfaces below are meant to be pseudo-code to illustrate how the details of a particular | |
| # vat network might be abstracted away. They are written like Cap'n Proto interfaces, but in | |
| # practice you'd probably define these interfaces manually in the target programming language. A | |
| # Cap'n Proto RPC implementation should be able to use these interfaces without knowing the | |
| # definitions of the various network-specific parameters defined above. | |
| # interface VatNetwork { | |
| # # Represents a vat network, with the ability to connect to particular vats and receive | |
| # # connections from vats. | |
| # # | |
| # # Note that methods returning a `Connection` may return a pre-existing `Connection`, and the | |
| # # caller is expected to find and share state with existing users of the connection. | |
| # | |
| # # Level 0 features ----------------------------------------------- | |
| # | |
| # connect(vatId :VatId) :Connection; | |
| # # Connect to the given vat. The transport should return a promise that does not | |
| # # resolve until authentication has completed, but allows messages to be pipelined in before | |
| # # that; the transport either queues these messages until authenticated, or sends them encrypted | |
| # # such that only the authentic vat would be able to decrypt them. The latter approach avoids a | |
| # # round trip for authentication. | |
| # | |
| # accept() :Connection; | |
| # # Wait for the next incoming connection and return it. Only connections formed by | |
| # # connect() are returned by this method. | |
| # | |
| # # Level 4 features ----------------------------------------------- | |
| # | |
| # newJoiner(count :UInt32) :NewJoinerResponse; | |
| # # Prepare a new Join operation, which will eventually lead to forming a new direct connection | |
| # # to the host of the joined capability. `count` is the number of capabilities to join. | |
| # | |
| # struct NewJoinerResponse { | |
| # joinKeyParts :List(JoinKeyPart); | |
| # # Key parts to send in Join messages to each capability. | |
| # | |
| # joiner :Joiner; | |
| # # Used to establish the final connection. | |
| # } | |
| # | |
| # interface Joiner { | |
| # addJoinResult(result :JoinResult) :Void; | |
| # # Add a JoinResult received in response to one of the `Join` messages. All `JoinResult`s | |
| # # returned from all paths must be added before trying to connect. | |
| # | |
| # connect() :ConnectionAndProvisionId; | |
| # # Try to form a connection to the joined capability's host, verifying that it has received | |
| # # all of the JoinKeyParts. Once the connection is formed, the caller should send an `Accept` | |
| # # message on it with the specified `ProvisionId` in order to receive the final capability. | |
| # } | |
| # | |
| # acceptConnectionFromJoiner(parts :List(JoinKeyPart), paths :List(VatPath)) | |
| # :ConnectionAndProvisionId; | |
| # # Called on a joined capability's host to receive the connection from the joiner, once all | |
| # # key parts have arrived. The caller should expect to receive an `Accept` message over the | |
| # # connection with the given ProvisionId. | |
| # } | |
| # | |
| # interface Connection { | |
| # # Level 0 features ----------------------------------------------- | |
| # | |
| # send(message :Message) :Void; | |
| # # Send the message. Returns successfully when the message (and all preceding messages) has | |
| # # been acknowledged by the recipient. | |
| # | |
| # receive() :Message; | |
| # # Receive the next message, and acknowledges receipt to the sender. Messages are received in | |
| # # the order in which they are sent. | |
| # | |
| # # Level 3 features ----------------------------------------------- | |
| # | |
| # introduceTo(recipient :Connection) :IntroductionInfo; | |
| # # Call before starting a three-way introduction, assuming a `Provide` message is to be sent on | |
| # # this connection and a `ThirdPartyCapId` is to be sent to `recipient`. | |
| # | |
| # struct IntroductionInfo { | |
| # sendToRecipient :ThirdPartyCapId; | |
| # sendToTarget :RecipientId; | |
| # } | |
| # | |
| # connectToIntroduced(capId :ThirdPartyCapId) :ConnectionAndProvisionId; | |
| # # Given a ThirdPartyCapId received over this connection, connect to the third party. The | |
| # # caller should then send an `Accept` message over the new connection. | |
| # | |
| # acceptIntroducedConnection(recipientId :RecipientId) :Connection; | |
| # # Given a RecipientId received in a `Provide` message on this `Connection`, wait for the | |
| # # recipient to connect, and return the connection formed. Usually, the first message received | |
| # # on the new connection will be an `Accept` message. | |
| # } | |
| # | |
| # struct ConnectionAndProvisionId { | |
| # # **(level 3)** | |
| # | |
| # connection :Connection; | |
| # # Connection on which to issue `Accept` message. | |
| # | |
| # provision :ProvisionId; | |
| # # `ProvisionId` to send in the `Accept` message. | |
| # } |