Add 0.1.0 remote log initial spec

assets/remotelog/remote-log.msc

@@ -0,0 +1,21 @@
+# Alice and Bob: remote log data sync
+msc {
+  hscale="2", wordwraparcs=on;
+
+  alice [label="Alice"],
+  cas [label="CAS"],
+  ns [label="NS"],
+  bob [label="Bob"];
+
+  --- [label="Alice replicates data to a remote log"];
+  alice => cas [label="Add content"];
+  cas => alice [label="Address"];
+  alice => ns [label="Update NameUpdate"];
+  ns => alice [label="Response"];
+
+  --- [label="Bob comes online"];
+  bob => ns [label="Fetch"];
+  ns => bob [label="Content"];
+  bob => cas [label="Fetch Query"];
+  cas => bob [label="Content"];
+}

remote-log.md

@@ -0,0 +1,224 @@
+# Remote log specification
+
+> Version: 0.1.1 (Draft)
+>
+> Authors: Oskar Thorén oskar@status.im, Dean Eigenmann dean@status.im
+
+## Table of Contents
+
+- [Abstract](#abstract)
+- [Definitions](#definitions)
+- [Wire Protocol](#wire-protocol)
+    - [Secure Transport, storage, and name system](#secure-transport-storage-and-name-system)
+    - [Payloads](#payloads)
+- [Synchronization](#synchronization)
+    - [Roles](#roles)
+    - [Flow](#flow)
+    - [Remote log](#remote-log)
+    - [Next page semantics](#next-page-semantics)
+    - [Interaction with MVDS](#interaction-with-mvds)
+- [Footnotes](#footnotes)
+- [Acknowledgements](#acknowledgements)
+
+## Abstract
+
+A remote log is a replication of a local log. This means a node can read data from a node that is offline.
+
+This specification is complemented by a proof of concept implementation <sup>1</sup>.
+
+## Definitions
+
+| Term        | Definition                                                                                   |
+| ----------- | --------------------------------------------------------------------------------------       |
+| CAS         | Content-addressed storage. Stores data that can be addressed by its hash.                    |
+| NS          | Name system. Associates mutable data to a name.                                              |
+| Remote log  | Replication of a local log at a different location.                                          |
+
+## Wire Protocol
+
+### Secure Transport, storage, and name system
+
+This specification does not define anything related to: secure transport,
+content addressed storage, or the name system. It is assumed these capabilities
+are abstracted away in such a way that any such protocol can easily be
+implemented.
+
+<!-- TODO: Elaborate on properties required here. -->
+
+### Payloads
+
+Payloads are implemented using [protocol buffers v3](https://developers.google.com/protocol-buffers/).
+
+**CAS service**:
+
+```protobuf
+syntax = "proto3";
+
+package vac.cas;
+
+service CAS {
+  rpc Add(Content) returns (Address) {}
+  rpc Get(Address) returns (Content) {}
+}
+
+message Address {
+  bytes id = 1;
+}
+
+message Content {
+  bytes data = 1;
+}
+```
+
+<!-- XXX/TODO: Can we get rid of the id/data complication and just use bytes? -->
+
+**NS service**:
+
+```protobuf
+syntax = "proto3";
+
+package vac.cas;
+
+service NS {
+  rpc Update(NameUpdate) returns (Response) {}
+  rpc Fetch(Query) returns (Content) {}
+}
+
+message NameUpdate {
+  string name = 1;
+  bytes content = 2;
+}
+
+message Query {
+  string name = 1;
+}
+
+message Content {
+  bytes data = 1;
+}
+
+message Response {
+  bytes data = 1;
+}
+```
+
+<!-- XXX: Response and data type a bit weird, Ok/Err enum? -->
+<!-- TODO: Do we want NameInit here? -->
+
+**Remote log:**
+
+```protobuf
+syntax = "proto3";
+
+package vac.cas;
+
+message RemoteLog {
+  repeated Pair pair = 1;
+  bytes tail = 2;
+
+  message Pair {
+    bytes remoteHash = 1;
+    bytes localHash = 2;
+    bytes data = 3;
+  }
+}
+```
+
+<!-- TODO: Better name for Pair, Mapping? -->
+
+<!-- TODO: Extend pair with (optional) data -->
+
+## Synchronization
+
+<!-- TODO: Elaborate on interaction with MVDS, especially with what messages are synced, etc -->
+
+### Roles
+
+There are four fundamental roles:
+
+1. Alice
+2. Bob
+2. Name system (NS)
+3. Content-addressed storage (CAS)
+
+The *remote log* protobuf is what is stored in the name system.
+
+"Bob" can represent anything from 0 to N participants. Unlike Alice, Bob only needs read-only access to NS and CAS.
+
+### Flow
+
+<!-- diagram -->
+
+<p align="center">
+    <img src="./assets/remotelog/remote-log.png" />
+    <br />
+    Figure 1: Remote log data synchronization.
+</p>
+
+### Remote log
+
+The remote log lets receiving nodes know what data they are missing. Depending
+on the specific requirements and capabilities of the nodes and name system, the
+information can be referred to differently. We distinguish between three rough
+modes:
+
+1. Fully replicated log
+2. Normal sized page with CAS mapping
+3. "Linked list" mode - minimally sized page with CAS mapping
+
+**Data format:**
+
+```
+| H1_3 | H2_3 |
+| H1_2 | H2_2 |
+| H1_1 | H2_1 |
+| ------------|
+| next_page   |
+```
+
+Here the upper section indicates a list of ordered pairs, and the lower section
+contains the address for the next page chunk. `H1` is the native hash function,
+and `H2` is the one used by the CAS. The numbers corresponds to the messages.
+
+To indicate which CAS is used, a remote log SHOULD use a multiaddr.
+
+**Embedded data:**
+
+A remote log MAY also choose to embed the wire payloads that corresponds to the
+native hash. This bypasses the need for a dedicated CAS and additional
+round-trips, with a trade-off in bandwidth usage.
+
+```
+| H1_3 | | C_3 |
+| H1_2 | | C_2 |
+| H1_1 | | C_1 |
+| -------------|
+| next_page    |
+```
+
+Here `C` stands for the content that would be stored at the CAS.
+
+Both patterns can be used in parallel, e,g. by storing the last `k` messages
+directly and use CAS pointers for the rest. Together with the `next_page` page
+semantics, this gives users flexibility in terms of bandwidth and
+latency/indirection, all the way from a simple linked list to a fully replicated
+log. The latter is useful for things like backups on durable storage.
+
+### Next page semantics
+
+The pointer to the 'next page' is another remote log entry, at a previous point
+in time.
+
+<!-- TODO: Determine requirement re overlapping, adjacent, and/or missing entries -->
+
+### Interaction with MVDS
+
+[vac.mvds.Message](./mvds.md#payloads) payloads are the only payloads that MUST be uploaded. Other messages types MAY be uploaded, depending on the implementation.
+
+## Footnotes
+
+1. <https://github.com/vacp2p/research/tree/master/remote_log>
+
+## Acknowledgements
+
+TBD.