Runtime triggers #516
Comments
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Thanks for writing up this proposal Bowen, it appears to me to be very sensible and solve our problem. I've got a few suggestions of how I think we can make this simpler and potentially more powerful using some well understood primitives. If we consider an external action of type Or rendered slightly closer to the WASM API Each yield emits a new receipt type YieldedReciept. Resume continues the execution in the Resume calls can be executed from the first To sketch out how this would be used in the sign endpoint: More generally, when including the in contract state, this is an untyped effect system. Potential use cases are:
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@DavidM-D sorry I don't think I fully understand your proposal. A couple of questions:
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Correct, the complication arises from the fact that WASM execution state is quite intertwined with the native state, so we can’t just save some explicitly defined data structures and save them, we would need to save things like stack, machine registers and such as well. Fortunately since the spots where the saving and restoring might happen are well defined (by virtue of them being specific user-invoked functions,) some of the concerns about adapting codegen to deal with being suspended and relocated at a further date aren’t as prominent. |
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There is some prior work where it is possible to save some execution context. Essentially it relies on the async system provided by rust to enable this support. https://internetcomputer.org/docs/current/developer-docs/backend/rust/intercanister is a simple example of how something like this can be used to call another smart contract; yield execution till the response comes back; and then continuing execution. https://github.com/dfinity/cdk-rs/blob/main/src/ic-cdk/src/futures.rs is the implementation on the SDK of this feature. |
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I don't think we need to save the execution context for this feature, although this is an interesting topic in and of itself that potentially deserves its own NEP (I remember briefly discussing how this would look like in Rust SDK last year). Let me rewrite @DavidM-D's code in a way that would be representable with the existing tooling: pub struct Signer {
...
}
impl Signer {
/// Caller wants to sign payload using a key derived with the given path
pub fn sign_request(payload: Vec<u8>, derivation_path: String) -> Promise<Signature> {
let sign_request_id = generate_request_id();
store.insert(sign_request_id, (payload, derivation, env::signer_account_id()));
// Yield to self (current account id), meaning only this contract can resume. Pre-allocates gas for the resume
// transaction.
YieldPromise::new(env::current_account_id(), {sign_request_id, payload, derivation, caller}, 30k TGas)
.then(Promise::new(env::current_account_id(), "sign_on_finish"))
}
/// Caller wants to fulfill a signature request and resume the chain of promises by the given receipt id
pub fn sign_response(signature: Signature, sign_request_id: RequestId, action_receipt_id: ActionReceiptId) -> Promise<()> {
assert_is_allowed_to_respond!(env::signer_account_id());
let (payload, derivation, caller) = store.get(sign_request_id);
assert_valid_sig!(signature, payload, caller);
// Resumes a yielded promise from the corresponding `sign_request` call and consumes gas preallocated for
// the resume transaction, thus refunding caller the gas they have spent so that ideally calling `sign_response`
// did not cost anything.
ResumePromise::new(env::current_account_id(), action_reciept_id, {sign_request_id, signature})
}
#[private]
pub fn sign_on_finish(sign_request_id: RequestId, signature: Signature) -> Signature {
store.remove(sign_request_id);
signature
}
}The way I see it runtime triggers are essentially polling the contract's state to see if it has changed in a specific way, but that can very straightforwardly be replaced by scheduling a |
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I just had a call with @ itegulov to understand their proposal above a bit better. I like this it very much, I think it is much simpler than what was proposed originally. In the original proposal, we need a way to call into the contract regularly to allow it to implement polling to decide if the desired condition is met. However, as @ itegulov points out, if the polling function is inspecting some state in the smart contract that is going to be updated over time by some other calls, then instead of having the polling, the contract can realise that the condition is met from these other calls and then resume execution. The biggest challenge though is to figure out how to delay execution while the condition is met. More specifically, we have the following case:
There is some related work that we can explore. This work is trying to come up with a clean way to implement what are effectively asynchronous host functions. It works as following:
The above solves the problem of having multiple call trees that need to be conflated and solves the problem of delaying responding to a request till some condition is met. The problem with this solution as I see it is that it will move many bits of the signature aggregation into the protocol. |
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@akhi3030 on your proposal with virtual smart contract, how exactly does it solve the issue mentioned in the previous approach that is related to the ability to trigger a callback? You still need a way for the virtual smart contract to signal that the action is complete and the callback can be executed. |
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@bowenwang1996: I chatted with @DavidM-D earlier and am in line with what @itegulov is proposing above. I think it is simpler to implement in the protocol and also a more general framework. Current situationWhen a contract ProposalIntroduce a When How this will work for the fastauth project?A smart contract calls the signer contract to get a payload signed. The signer contract initiates the signing process and calls I think @itegulov's proposed API above shows how this would look like. Changes needed in the protocolI imagine that the changes needed in the protocol is that when We will have to figure out how to charge for the additional state created when |
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@bowenwang1996, @itegulov, @DavidM-D, @saketh-are, @walnut-the-cat: just so we are all on the same page, we are now planning on moving forward with the proposal in the comment above. There are still a couple of open questions for me for the API that I will make subsequent posts about to clarify. Saketh and I will then write up a draft NEP so that we are all agreed on the precise API and then start working on an implementation in nearcore. |
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@itegulov, @DavidM-D, @saketh-are: I have a question about the API for In order for the API to be generic enough, it will be possible that there are multiple outstanding yielded "executions" in a given contract that can later be resumed. It will also be possible that they will be ready to be resumed in different order than in which they were yielded. As such, the contract needs to specify which "execution" to resume. Does this concern make sense? So what I am thinking is that @itegulov: Looking at your comment, the closest thing I see resembling this identifier is |
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@akhi3030 yes, so this is what I meant by |
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@itegulov: cool, thanks, then we are on agreement on this. A follow on question is that we need to have some sort of "time limit" for how long the yielded execution can stay alive for. This is because each yielded execution will create some state in the protocol that needs to be paid for. Otherwise, if a contract keeps forgetting to call resume on yielded execution, then that will accumulate state in the protocol. My high level idea is that when some execution is yielded and it is not resumed within There are two options for an API here. First is that the An additional API we could offer is that when |
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Do we have to have an opaque reference type. Is it not possible to do it
all on the application level? Surely if the contract has to verify the
response it can also associate it with the correct callback?
…On Thu, 16 Nov 2023, 21:56 Akhilesh Singhania, ***@***.***> wrote:
@itegulov <https://github.com/itegulov>: cool, thanks, then we are on
agreement on this.
A follow on question is that we need to have some sort of "time limit" for
how long the yielded execution can stay alive for. This is because each
yielded execution will create some state in the protocol that needs to be
paid for. Otherwise, if a contract keeps forgetting to call resume on
yielded execution, then that will accumulate state in the protocol.
My high level idea is that when some execution is yielded and it is not
resumed within N blocks, then the protocol will resume it with a timeout
error. Something like this could then also be used a feature if a contract
just wants a callback sometime in the future.
There are two options for an API here. First is that the N blocks
specified above is constant of the protocol that cannot be changed and then
the gas fees are calculated based on that. But we could consider
generalising this a bit more and say that the contract can specify N
(which some upper bounds if needed) and then the gas fee will be calculated
accordingly. Does anyone have opinions on which approach to take here?
An additional API we could offer is that when N is about to run out, the
contract could request to extend the time that the execution is yielded
for. I think that we should keep this as future work. I think we can have
the simpler API for now and build this in future if needed.
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@DavidM-D: I am not sure how that will work. For each yielded execution, the protocol has to create and some state. And when the application wants to resume some execution, it has to tell the application which yielded execution to continue. |
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@akhi3030: having a timeout mechanism seems very reasonable to me. We can have a retry mechanism on the application level and even potentially subsidize it (e.g., set up a relayer that, given proof that you sent a transaction that resulted in timeout, funds a new retry transaction for you).
Let me think this through, in the meantime I have a couple of counter-questions. Let's say the execution got resumed in
Agreed, this can be kept as a potential future extension. I don't see this being particularly useful for our use case, but a need might arise from somewhere else. |
Good point. Yes, it would make sense to refund the remaining gas for
Hmm.... I don't have a good intuition for this yet. It will depend on how much state we need to store in the protocol. I imagine that we are probably talking storing less than around 500 bytes per yielded execution. So we will have to do some gas estimations to figure out how much gas we should charge per block for that much storage. Maybe you have some intuition for this? The other thought I had on this just now is that we probably do not need to specify an upper limit in the protocol on how big |
This sounds very much like the previous discussion on charging delayed receipts for their storage usage..
What's realistic feasibility of this? Currently, we do not guarantee when delayed receipts will be executed in the future and it seems for this type of timeout to work, we need to provide some guarantee such as 'resume() will be executed one block after condition is met'. |
I don't think we need to provide any guarantee on when something will execute. Once the timeout passes, we just need to mark the receipt ready to be executed. It doesn't matter when it actually executes. |
Yes. This is like the single trigger model which should work well with the current gas model. |
But doesn't that mean users cannot control or estimate how much gas will be refunded or what 'N (which some upper bounds if needed)' should be? I am afraid this resulting in the similar situation we have with gas attachment for txn call(where users always use highest/largest value) |
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No... that is not how I envision the API working. If the contract requests that the execution be yielded for up to |
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I have created a draft NEP for this work. |
I agree that Protocol doesn't have to provide such guarantee, but in the case where callback is yielded for N blocks and timeout as it couldn't get executed within the time limit, will contract end up wasting gas for nothing?
Just to be clear, N used here is different from N used in the past sentence, as we are talking about 'minimum' delay, instead of 'maximum' delay? |
Suppose that a contract requests that execution be yielded for up to
This guarantee is referring to the fact that the timeout case won't be triggered by the protocol within |
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Following a discussion with @bowenwang1996, I propose removing the timeout feature for a couple of reasons:
Without a timeout, we would take a storage deposit to pay for storing the yielded computation indefinitely. |
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Will storage deposits be denominated in gas or NEAR? NEAR storage deposits make for complex interactions with relayers. |
Normally it should be in NEAR. However, I agree that the interactions would be more complex. Given that such a receipt will be quite small (a few hundred bytes), I think we can also consider burning gas (similar to zero-balance accounts). |
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I have some more questions about how things would work if we do not have timeouts. I mentioned them on a slack thread but also mentioning here in case someone else is following the conversation here. If we do not have timeouts, then the following situation can happen:
This can create some problems such as:
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Summary of a meeting with @DavidM-D @itegulov @saketh-are:
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Re: storage costs. Lightweight Yields Couple of questions: is it reasonable to assume that a First, it obviates the need to specify a resuming contract ID, and it could eliminate the need to specify a resuming function name, if a standard one is agreed-upon (not required for this optimization, and it leads to slightly worse devx imo). However, it would also, and more importantly, eliminate the need to support promise chaining ( An API similar to that described by @itegulov above: // Yield to self (current account id), meaning only this contract can resume. Pre-allocates gas for the resume
// transaction.
YieldPromise::new(env::current_account_id(), {sign_request_id, payload, derivation, caller}, 30k TGas)
.then(Promise::new(env::current_account_id(), "sign_on_finish"))Can therefore be simplified to have an upper-bound in storage cost. The arguments struct in particular can be replaced in favor of a contract-generated resume promise ID (e.g.
The Of course, this is not a complete picture (e.g. I assume that keeping an action receipt around, unresolved, like the one referenced in the |
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thanks for the feedback encody. The plan is indeed that only the contract that yielded a receipt can then resume it. |
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Here's how the proposed yield_create/yield_resume API could fit into the MPC contract. Thanks @itegulov for some helpful input on this already. pub struct MpcContract {
protocol_state: ProtocolContractState,
// Maps yielded promise id to the requested payload
pending_requests: LookupMap<PromiseId, [u8; 32]>,
}
impl MpcContract {
// Called by the end user; accepts a payload and returns a signature.
pub fn sign(&mut self, payload: [u8; 32]) -> Promise {
let promise = yield_create(
// Callback with return type Option<Signature>
Self::ext(env::current_account_id()).sign_on_finish(),
YIELD_NUM_BLOCKS,
STATIC_GAS_ALLOTMENT,
GAS_WEIGHT
);
self.pending_requests.insert(env::promise_id(promise), payload);
promise
}
// Called by an MPC node to submit a completed signature
pub fn sign_respond(&mut self, request_promise_id: PromiseId, signature: Signature) {
assert_is_allowed_to_respond!(env::current_account_id());
let Some(payload) = self.pending_requests.get(&request_promise_id) {
assert_valid_sig!(payload, signature);
// The arg tuple passed here needs to match the type signature of the callback function
yield_resume(request_promise_id, (request_promise_id, Ok(signature),));
} else {
env::panic_str("Unexpected response");
}
}
// Callback made after the yield has completed, whether due to resumption or due to timeout
pub fn sign_on_finish(
&mut self,
request_promise_id: PromiseId,
signature: Result<Signature, PromiseError>
) -> Option<Signature> {
self.pending_requests.remove(request_promise_id);
signature.ok()
}
} |
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Thank you @saketh-are! Some questions:
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Thanks, I have edited above:
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Thanks for making the change. I think the following change might still be needed to make the types match up: Otherwise, this looks good to me. |
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After some tinkering on implementation I think it makes more sense to design the host functions in the following way: promise_await_data(account_id, yield_num_blocks) -> (Promise, DataId);
promise_submit_data(data_id, data);Simply, We can rely on the composability of promises to attach a callback and consume the data as desired. impl MpcContract {
pub fn sign(&mut self, payload: [u8; 32]) -> Promise {
let (promise, data_id) = promise_await_data(
env::current_account_id(),
YIELD_NUM_BLOCKS
);
self.pending_requests.insert(data_id, payload);
promise.then(Self::ext(env::current_account_id()).sign_on_finish())
}
} |
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@saketh-are: I like this simplification very much! Question: why do need to include the |
It is sometimes useful for smart contract to subscribe to some event and perform an action based on the result of an event. One simple example is cronjob. Currently smart contracts have no way of scheduling an action that is periodically triggered (in this case the event that triggers the action is time) and as a result, if someone wants to achieve this, they need to rely on offchain infrastructure that periodically calls the smart contract, which is quite cumbersome. A more complex use case is chain signatures, which needs the execution to resume once the signature is generated by validators.
In terms of effect, this should roughly be equivalent to if the smart contract has a method that constantly calls itself
However, a function like this is not practical as is because of gas limit associated with each specific function call. However, we can extend the mechanism of callbacks to allow them be not triggered immediately, but rather when a specific condition is met. More specifically, we can introduce a new type of Promise
DelayedPromisethat generates a postponed receipt, similar to what callbacks do today. However, the postponed receipt is stored globally in the shard, instead of under a specific account (at least conceptually, in practice a separate index could be created if that helps), along with the condition that triggers the delayed promise. Then, during the execution of every chunk, the runtime first checks whether any of the delayed promise should be triggered and execute them if the triggering condition is met. Roughly this would allow us to rewrite the example above intoFor this idea to work, a few issues need to be resolved:
VMContext). One idea is that we could introduce a special type of function that a smart contract can implement to specify the behavior of a trigger it plans to use. The function must return a boolean value and must consume little gas (the exact threshold needs to be defined). Otherwise a malicious attacker could run an infinite loop to cause validators to do a lot more work and slow down the network as a result. The cost of running the trigger should be priced into the cost of a delayed promise so that this cost is accounted for.The text was updated successfully, but these errors were encountered: