Summary

This RFC describes an architecture for asynchronous script verification and its interaction with the state layer. This architecture imposes constraints on the ordering of operations in the state layer.

Motivation

As in the rest of Zebra, we want to express our work as a collection of work-items with explicit dependencies, then execute these items concurrently and in parallel on a thread pool.

Definitions

UTXO: unspent transaction output. Transaction outputs are modeled in zebra-chain by the transparent::Output structure.
Transaction input: an output of a previous transaction consumed by a later transaction (the one it is an input to). Modeled in zebra-chain by the transparent::Input structure.
lock script: the script that defines the conditions under which some UTXO can be spent. Stored in the transparent::Output::lock_script field.
unlock script: a script satisfying the conditions of the lock script, allowing a UTXO to be spent. Stored in the transparent::Input::PrevOut::lock_script field.

Guide-level explanation

Zcash's transparent address system is inherited from Bitcoin. Transactions spend unspent transaction outputs (UTXOs) from previous transactions. These UTXOs are encumbered by locking scripts that define the conditions under which they can be spent, e.g., requiring a signature from a certain key. Transactions wishing to spend UTXOs supply an unlocking script that should satisfy the conditions of the locking script for each input they wish to spend.

This means that script verification requires access to data about previous UTXOs, in order to determine the conditions under which those UTXOs can be spent. In Zebra, we aim to run operations asychronously and out-of-order to the greatest extent possible. For instance, we may begin verification of a block before all of its ancestors have been verified or even downloaded. So we need to design a mechanism that allows script verification to declare its data dependencies and execute as soon as all required data is available.

It's not necessary for this mechanism to ensure that the transaction outputs remain unspent, only to give enough information to perform script verification. Checking that all transaction inputs are actually unspent is done later, at the point that its containing block is committed to the chain.

At a high level, this adds a new request/response pair to the state service:

Request::AwaitUtxo(OutPoint) requests a transparent::Output specified by OutPoint from the state layer;
Response::Utxo(transparent::Output) supplies requested the transparent::Output.

Note that this request is named differently from the other requests, AwaitUtxo rather than GetUtxo or similar. This is because the request has rather different behavior: the request does not complete until the state service learns about a UTXO matching the request, which could be never. For instance, if the transaction output was already spent, the service is not required to return a response. The caller is responsible for using a timeout layer or some other mechanism.

This allows a script verifier to asynchronously obtain information about previous transaction outputs and start verifying scripts as soon as the data is available. For instance, if we begin parallel download and verification of 500 blocks, we should be able to begin script verification of all scripts referencing outputs from existing blocks in parallel, and begin verification of scripts referencing outputs from new blocks as soon as they are committed to the chain.

Because spending outputs from older blocks is more common than spending outputs from recent blocks, this should allow a significant amount of parallelism.

Reference-level explanation

We add a Request::AwaitUtxo(OutPoint) and Response::Utxo(transparent::Output) to the state protocol. As described above, the request name is intended to indicate the request's behavior: the request does not resolve until the state layer learns of a UTXO described by the request.

To verify scripts, a script verifier requests the relevant UTXOs from the state service and waits for all of them to resolve, or fails verification with a timeout error. Currently, we outsource script verification to zcash_consensus, which does FFI into the same C++ code as zcashd uses. We need to ensure this code is thread-safe.

Implementing the state request correctly requires considering two sets of behaviors:

behaviors related to the state's external API (a Buffered tower::Service);
behaviors related to the state's internal implementation (using rocksdb).

Making this distinction helps us to ensure we don't accidentally leak "internal" behaviors into "external" behaviors, which would violate encapsulation and make it more difficult to replace rocksdb.

In the first category, our state is presented to the rest of the application as a Buffered tower::Service. The Buffer wrapper allows shared access to a service using an actor model, moving the service to be shared into a worker task and passing messages to it over an multi-producer single-consumer (mpsc) channel. The worker task receives messages and makes Service::calls. The Service::call method returns a Future, and the service is allowed to decide how much work it wants to do synchronously (in call) and how much work it wants to do asynchronously (in the Future it returns).

This means that our external API ensures that the state service sees a linearized sequence of state requests, although the exact ordering is unpredictable when there are multiple senders making requests.

Because the state service has exclusive access to the rocksdb database, and the state service sees a linearized sequence of state requests, we have an easy way to opt in to asynchronous database access. We can perform rocksdb operations synchronously in the Service::call, waiting for them to complete, and be sure that all future requests will see the resulting rocksdb state. Or, we can perform rocksdb operations asynchronously in the future returned by Service::call.

If we perform all writes synchronously and allow reads to be either synchronous or asynchronous, we ensure that writes cannot race each other. Asynchronous reads are guaranteed to read at least the state present at the time the request was processed, or a later state.

Now, returning to the UTXO lookup problem, we can map out the possible states with this restriction in mind. This description assumes that UTXO storage is split into disjoint sets, one in-memory (e.g., blocks after the reorg limit) and the other in rocksdb (e.g., blocks after the reorg limit). The details of this storage are not important for this design, only that the two sets are disjoint.

When the state service processes a Request::AwaitUtxo(OutPoint) referencing some UTXO u, there are three disjoint possibilities:

u is already contained in an in-memory block storage;
u is already contained in the rocksdb UTXO set;
u is not yet known to the state service.

In case 3, we need to queue u and scan all future blocks to see whether they contain u. However, if we have a mechanism to queue u, we can perform check 2 asynchronously, because restricting to synchronous writes means that any async read will return the current or later state. If u was in the rocksdb UTXO set when the request was processed, the only way that an async read would not return u is if the UTXO were spent, in which case the service is not required to return a response.

This behavior can be encapsulated into a PendingUtxos structure described below.


#![allow(unused)]
fn main() {
// sketch
#[derive(Default, Debug)]
struct PendingUtxos(HashMap<OutPoint, oneshot::Sender<transparent::Output>>);

impl PendingUtxos {
    // adds the outpoint and returns (wrapped) rx end of oneshot
    // return can be converted to `Service::Future`
    pub fn queue(&mut self, outpoint: OutPoint) -> impl Future<Output=Result<Response, ...>>;

    // if outpoint is a hashmap key, remove the entry and send output on the channel
    pub fn respond(&mut self, outpoint: OutPoint, output: transparent::Output);


    // scans the hashmap and removes any entries with closed senders
    pub fn prune(&mut self);
}
}

The state service should maintain an Arc<Mutex<PendingUtxos>>, used as follows:

In Service::call(Request::AwaitUtxo(u)), the service should:

call PendingUtxos::queue(u) to get a future f to return to the caller; spawn a task that does a rocksdb lookup for u, calling PendingUtxos::respond(u, output) if present;
check the in-memory storage for u, calling PendingUtxos::respond(u, output) if present;
return f to the caller (it may already be ready). The common case is that u references an old UTXO, so spawning the lookup task first means that we don't wait to check in-memory storage for u before starting the rocksdb lookup.

In Service::call(Request::CommitBlock(block, ..)), the service should:

call PendingUtxos::check_block(block.as_ref());
do any other transactional checks before committing a block as normal. Because the AwaitUtxo request is informational, there's no need to do the transactional checks before matching against pending UTXO requests, and doing so upfront potentially notifies other tasks earlier.

In Service::poll_ready(), the service should call PendingUtxos::prune() at least some of the time. This is required because when a consumer uses a timeout layer, the cancelled requests should be flushed from the queue to avoid a resource leak. However, doing this on every call will result in us spending a bunch of time iterating over the hashmap.

Drawbacks

One drawback of this design is that we may have to wait on a lock. However, the critical section basically amounts to a hash lookup and a channel send, so I don't think that we're likely to run into problems with long contended periods, and it's unlikely that we would get a deadlock.

Rationale and alternatives

High-level design rationale is inline with the design sketch. One low-level option would be to avoid encapsulating behavior in the PendingUtxos and just have an Arc<Hashmap<..>>, so that the lock only protects the hashmap lookup and not sending through the channel. But I think the current design is cleaner and the cost is probably not too large.

Unresolved questions

We need to pick a timeout for UTXO lookup. This should be long enough to account for the fact that we may start verifying blocks before all of their ancestors are downloaded.

The Zebra Book