Memory optimized promise blocking queue with concurrency control
npm install promise-blocking-queue
Useful for rate-limiting async (or sync) operations that consume large data sets.
For example, when interacting with a REST API or when doing CPU/memory intensive tasks.
Bluebird.map() for example, we are forced to load all the data in memory, If we use p-queue (by the amazing sindresorhus)
for example, we can utilize streams to avoid memory bloat, but we have no (easy) way to control
the stream flow without hitting that Out Of Memory Exception.
The solution - a blocking queue that returns a promise that will be resolved when the added item gains an available slot in the
queue, thus, allowing us to pause the stream consumption, until there is a _real_ need to consume the next item - keeping us
memory smart while maintaining concurrency level of data handling.
``shell`
npm install promise-blocking-queue
Let's assume we have a very large (a couple of GBs) file called users.json which contains a long list of users we want to add to our DB.
Also, let's assume that our DB instance it very cheap, and as such we don't want to load it too much, so we only want to handle
2 concurrent DB insert operations.
We can achieve a short scalable solution like so:
`typescript
import * as JSONStream from 'JSONStream';
import * as fs from 'fs';
import * as es from 'event-stream';
import * as sleep from 'sleep-promise';
import { BlockingQueue } from 'promise-blocking-queue';
const queue = new BlockingQueue({ concurrency: 2 });
let handled = 0;
let failed = 0;
let awaitDrain: Promise
const readStream = fs.createReadStream('./users.json', { flags: 'r', encoding: 'utf-8' });
const jsonReadStream = JSONStream.parse('*');
const jsonWriteStream = JSONStream.stringify();
const writeStream = fs.createWriteStream('./results.json');
const addUserToDB = async (user) => {
try {
console.log(adding ${user.username});added ${user.username} #${++handled}
// Simulate long running task
await sleep((handled + 1) * 100);
console.log();failed ${++failed}
const writePaused = !jsonWriteStream.write(user.username);
if (writePaused && !awaitDrain) {
// Down stream asked to pause the writes for now
awaitDrain = new Promise((resolve) => {
jsonWriteStream.once('drain', resolve);
});
}
} catch (err) {
console.log(, err);
}
};
const handleUser = async (user) => {
// Wait until the down stream is ready to receive more data without increasing the memory footprint
if (awaitDrain) {
await awaitDrain;
awaitDrain = undefined;
}
return queue.enqueue(addUserToDB, user).enqueuePromise;
};
// Do not use async!
const mapper = (user, cb) => {
console.log(streamed ${user.username});
handleUser(user)
.then(() => {
cb();
});
// Pause the read stream until we are ready to handle more data
return false;
};
const onReadEnd = () => {
console.log('done read streaming');
// If nothing was written, idle event will not be fired
if (queue.pendingCount === 0 && queue.activeCount === 0) {
jsonWriteStream.end();
} else {
// Wait until all work is done
queue.on('idle', () => {
jsonWriteStream.end();
});
}
};
const onWriteEnd = () => {
console.log(done processing - ${handled} handled, ${failed} failed);
process.exit(0);
};
jsonWriteStream
.pipe(writeStream)
.on('error', (err) => {
console.log('error wrtie streaming', err);
process.exit(1);
})
.on('end', onWriteEnd)
.on('finish', onWriteEnd);
readStream
.pipe(jsonReadStream)
.pipe(es.map(mapper))
.on('data', () => {
// Do nothing
})
.on('error', (err) => {
console.log('error read streaming', err);
process.exit(1);
})
.on('finish', onReadEnd)
.on('end', onReadEnd);
`
If users.json is like:
`json`
[
{
"username": "a"
},
{
"username": "b"
},
{
"username": "c"
},
{
"username": "d"
}
]
Output will be:
`bash`
streamed a
adding a
streamed b
adding b
streamed c // c now waits in line to start and streaming is paused until then
added a #1
adding c // c only gets handled after a is done
streamed d // d only get streamed after c has a spot in the queue
added b #2
adding d // d only gets handled after b is done
done read streaming
added c #3
added d #4
done processing - 4 handled, 0 failed
results.json will be:
`json`
[
"a"
,
"b"
,
"c"
,
"d"
]
Returns a new queue instance, which is an EventEmitter subclass.
#### options
Type: object
##### concurrency
Type: number Infinity
Default: 1
Minimum:
Concurrency limit.
BlockingQueue instance.
#### .enqueue(fn, ...args)
Adds a sync or async task to the queue
##### Return value
Type: object
###### enqueuePromise
Type: Promise
A promise that will be resolved when the queue has an available slot to run the task.
Used to realize that it is a good time to add another task to the queue.
###### fnPromise
Type: Promise
A promise that will be resolved with the result of fn.
###### started
Type: boolean
Indicates if the task has already started to run
##### fn
Type: Function
Promise/Value returning function.
##### args
Type: any[]
The arguments to pass to the function
#### activeCount
The number of promises that are currently running.
#### pendingCount
The number of promises that are waiting to run.
#### empty
Emitted when the queue becomes empty.
Useful if, for example, you add additional items at a later time.
#### idle
Emitted when the queue becomes empty, and all promises have completed: queue.activeCount === 0 && queue.pendingCount === 0.
The difference with empty is that idle guarantees that all work from the queue has finished.empty merely signals that the queue is empty, but it could mean that some promises haven't completed yet.
Promise Blocking Queue supports Node 6 LTS and higher.
All contributions are happily welcomed!
Please make all pull requests to the master` branch from your fork and ensure tests pass locally.