node.js how to handle fast producer and slow consumer with backpressure

问题

I'm very novice in node.js and don't understand the documentation about streams. Hoping to get some tips.

I'm reading a very large file line, and then for each line I'm calling an async network api.

Obviously the local file is read much faster than the async calls are completed:

var lineReader = require('readline').createInterface({
  input: require('fs').createReadStream(program.input)
});

lineReader.on('line', function (line) {
    client.execute(query, [line], function(err, result) {
        // needs to pressure the line reader here
        var myJSON = JSON.stringify(result);
        console.log("line=%s json=%s",myJSON);
    });
});

What is the way to add back pressure in the "execute" method?

回答1:

The solution is to wrap the async behavior in a stream writer and throttle the async reader from within the writer:

val count = 0;
var writable = new stream.Writable({
    write: function (line, encoding, next) {
        count++;
        if (count < concurrent) {
            next();
        }

        asyncFunctionToCall(...) {
            // completion callback
            // reduce the count and release back pressure
            count--;
            next();
            ...
      }
});

var stream = fs.createReadStream(program.input, {encoding: 'utf8'});
stream = byline.createStream(stream);
stream.pipe(writable);

来源：https://stackoverflow.com/questions/50114753/node-js-how-to-handle-fast-producer-and-slow-consumer-with-backpressure