Iterate large number of async calls / results in node.js (using ES6 / async / bluebird / generators)?

Question

I am writing a utility in node.js that has to process and concatenate a large number of files every night. In synchronous pseudocode it would look like that (omitting try / catch for clarity):

while (true) {
    var next = db.popNext();
    if (!next) return;

    out.append(next);
}

However, in the library I am using popNext() is actually a node-style asynchronous method and rather looks like this: popNext(callback).

Since I am writing the middleware from scratch I could use --harmony (e.g., generators), async or bluebird.

Ideally I would prefer something like:

forEachOrdered(db.popNext, (error, next, ok, fail) => {
   if(error) return; // skip

   // If there was an internal error, terminate the whole loop.
   if(out.append(next)) ok();
   else fail();
}).then(() => {
   // All went fine.
}).catch(e => {
   // Fail was called.
});

However, I am open to other 'standard' solutions. I was wondering what would be the most concise solution to this problem?

Edit Just spawning all (in a regular for loop) at the same time would probably not solve my problem since we're talking about 100k's and for every item I have to open and read a file, so I would probably run out of file descriptors.

Answer 1

Here is a solution using bluebird coroutines using your "ideal" code:

var db = Promise.promisifyAll(db);

var processAll = Promise.coroutine(function*(){
  while(true){
    var next = yield db.popNextAsync(); // promisify gives Async suffix
    if(!next) return;
    out.append(next); // some processing
  }       
});

In ES2016 (ES7) this becomes:

var db = Promise.promisifyAll(db); // still need to promisify

async function processAll(){
  let next;
  while(next = await db.popNextAsync()){
     // whatever
     out.append(next);
  }
}

Although, I'd argue the output collection should be an iterable (and lazy) too, so using ES2016 async iterators:

var db = Promise.promisifyAll(db);
async function* process(){
    while(true){
       var val = await db.popNextAsync();
       if(!val) return;
       // process val;
       yield process(val); // yield it forward
    }
}

Although if we really want to go all out here, after converting db.popNext into an async iterator, this becomes in ES2016 for await notation:

async function* processAll(){
    for await(let next of db.asAsyncIterator()){ // need to write this like above
       yield process(next); // do some processing
    }
}

Leveraging the whole ES2016 async iteration API. If you can't, or don't want to use generators you can always convert while loops to recursion:

function processAll(){ // works on netscape 7
   return db.popNextAsync().then(function next(value){
      if(!value) return;
      out.push(process(value));
      return db.popNextAsync().then(next); // after bluebird promisify
   });
}