Add middleware support

[emr]

Hello, thank you for this useful library. We were using ethers.js to listen to new logs on blockchain, but due to occasional websocket disconnection, we made research for solutions and found this library and decided to use it. Now we want to add our functionalities to the alchemy-web3 provider. Functionalities like monitoring, caching, I mean, middleware things. To do so, we added middleware support to the library. So now we can monitor our outgoing/incoming ws/http messages and cache them.

Example middleware for monitoring:

  ...
  createJsonRpcSenderMiddleware = (): JsonRpcSenderMiddleware => {
    return (request, next) => {
      const startedTime = Date.now();
      this.emit('request', request);
      return next().then((response) => {
        const executionTime = Date.now() - startedTime;
        this.emit('response', response, executionTime);
        return res;
      });
    };
  };
  ...

For caching:

const methodsNotToCache = ['eth_blockNumber', 'net_version', 'eth_call', 'eth_subscribe'];
const createAlchemyCacheMiddleware = async (cache: CacheInterface) => {
  return (request, next) => {
    if (methodsNotToCache.includes(request.method)) {
      return next();
    }
    const cached = await cache.get([request.method, request.params]);
    if (cached) {
      return {
        ...cached,
        id: request.id, // change id with requested id to treat as if we really went to the provider
                        // and to prevent incompatibility between requested id and responded id
      };
    }
    const response = next();
    await cache.set([request.method, request.params], response);
    return response;
  }
}

I removed some boring logics like checking if a request/response is batch, if it contains ids, etc to keep examples simple.

Usage:

const web3 = createAlchemyWeb3(wsUrl, {
  jsonRpcSenderMiddlewares: [
    monitor.createJsonRpcSenderMiddleware(),
    createAlchemyCacheMiddleware(new InMemoryCache()),
    // other middlewares
    // ...
  ],
});

We tested the feature with these 2 functionalities, it works perfectly. If you accept this PR, we can update the docs with your help.

In the future, maybe you can think of implementing internal caching which can be enabled with the library options.

Thanks!

[thebrianchen]

Thanks for your contribution! It's super slick, and I can definitely see how this opens a lot of possibilities!

With the current implementation, calling next() twice in a JsonRpcSenderMiddleware function would result in skipped middlewares functions or an array out of bounds error. Can you update the sendJsonRpcPayload function to include a boolean check that makes sure each middleware function can only call next() a single time?

Thanks!

[emr]

@thebrianchen nice catch. I created a closure and added a condition to ensure that middlewares can call next only once. But if a middleware calls next twice while sending net_version call, the websocket provider catches the error and reconnects:
https://github.com/alchemyplatform/alchemy-web3/blob/master/src/web3-adapter/webSocketProvider.ts#L223=
To prevent this, we need to catch only network errors and let others be thrown in this catch block.

[thebrianchen]

thanks for the quick turn around, looks good to me! just adding @dphilipson as a reviewer for another check

[emr]

@thebrianchen I was thinking of a case where if we want to implement our custom retry logic with middleware, in that case, we would not be able to call next twice. So, removing the called condition may cover that case. Please don't merge this PR now and let me test this case and update the PR when I'm free.

[emr]

@thebrianchen I made the change that allows calling next multiple times as well as not to cause any error.

[dphilipson]

@emr Great work on this- it's a fantastic addition. I'll merge this then publish.

[dphilipson]

Published in v1.4.0.