CODEWITHSUNDEEP
node.jstypescriptecmascript-6es6-promisenodejs-stream
Spencer Pollock
Spencer Pollock

Reputation: 447

NodeJS streams not awaiting async

I have run into an issue when testing NodeJS streams. I can't seem to get my project to wait for the output from the Duplex and Transform streams after running a stream.pipeline, even though it is returning a promise. Perhaps I'm missing something, but I believe that the script should wait for the function to return before continuing. The most important part of the project I'm trying to get working is:

// Message system is a duplex (read/write) stream
export class MessageSystem extends Duplex {
    constructor() {
        super({highWaterMark: 100, readableObjectMode: true, writableObjectMode: true});
    }
    public _read(size: number): void {
        var chunk = this.read();
        console.log(`Recieved ${chunk}`);
        this.push(chunk);
    }
    public _write(chunk: Message, encoding: string, 
        callback: (error?: Error | null | undefined, chunk?: Message) => any): void {
        if (chunk.data === null) {
            callback(new Error("Message.Data is null"));
        } else {
            callback();
        }
    }
}

export class SystemStream extends Transform {
    public type: MessageType = MessageType.Global;
    public data: Array<Message> = new Array<Message>();
    constructor() {
        super({highWaterMark: 100, readableObjectMode: true, writableObjectMode: true});
    }
    public _transform(chunk: Message, encoding: string, 
        callback: TransformCallback): void {
        if (chunk.single && (chunk.type === this.type || chunk.type === MessageType.Global)) {
            console.log(`Adding ${chunk}`);
            this.data.push(chunk);
            chunk = new Message(chunk.data, MessageType.Removed, true);
            callback(undefined, chunk); // TODO: Is this correct?
        } else if (chunk.type === this.type || chunk.type === MessageType.Global) { // Ours and global
            this.data.push(chunk);
            callback(undefined, chunk);
        } else { // Not ours
            callback(undefined, chunk);
        }
    }
}

export class EngineStream extends SystemStream {
    public type: MessageType = MessageType.Engine;
}

export class IOStream extends SystemStream {
    public type: MessageType = MessageType.IO;
}

let ms = new MessageSystem();
let es = new EngineStream();
let io = new IOStream();

let pipeline = promisify(Stream.pipeline);

async function start() {
    console.log("Running Message System");
    console.log("Writing new messages");
    ms.write(new Message("Hello"));
    ms.write(new Message("world!"));
    ms.write(new Message("Engine data", MessageType.Engine));
    ms.write(new Message("IO data", MessageType.IO));
    ms.write(new Message("Order matters in the pipe, even if Global", MessageType.Global, true));
    ms.end(new Message("Final message in the stream"));
    console.log("Piping data");
    await pipeline(
        ms,
        es,
        io
    );
}

Promise.all([start()]).then(() => {
    console.log(`Engine Messages to parse: ${es.data.toString()}`);
    console.log(`IO Messages to parse: ${io.data.toString()}`);
});

Output should look something like:

Running message system
Writing new messages
Hello
world!
Engine Data
IO Data
Order Matters in the pipe, even if Global
Engine messages to parse: Engine Data
IO messages to parse: IO Data

Any help would be greatly appreciated. Thanks!

Note: I posted this with my other account, and not this one that is my actual account. Apologies for the duplicate.

Edit: I initially had the repo private, but have made it public to help clarify the answer. More usage can be found on the feature/inital_system branch. It can be run with npm start when checked out.

Edit: I've put my custom streams here for verbosity. I think I'm on a better track than before, but now getting a "null" object recieved down the pipeline.

Upvotes: 1

Views: 3531

Answers (2)

Spencer Pollock
Spencer Pollock

Reputation: 447

After some work of the past couple of days, I've found my answer. The issue was my implementation of the Duplex stream. I have since changed the MessageSystem to be a Transform stream to be easier to manage and work with.

Here is the product:

export class MessageSystem extends Transform {
    constructor() {
        super({highWaterMark: 100, readableObjectMode: true, writableObjectMode: true});
    }
    public _transform(chunk: Message, encoding: string,
        callback: TransformCallback): void {
            try {
                let output: string = chunk.toString();
                callback(undefined, output);
            } catch (err) {
                callback(err);
            }
        }
}

Thank you to @estus for the quick reply and check. Again, I find my answer in the API all along!

An archived repository of my findings can be found in this repository.

Upvotes: 0

Estus Flask
Estus Flask

Reputation: 222493

As the documentation states, stream.pipeline is callback-based doesn't return a promise.

It has custom promisified version that can be accessed with util.promisify:

const pipeline = util.promisify(stream.pipeline);

...

await pipeline(...);

Upvotes: 2

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