如何组织async.waterfall调用循环

Question

How to organize a loop of async.waterfall calls so that every iteration starts only after the previous one is fully finished .

So far I have the following code:

var hasMoreData = true;
async.whilst(function () {
  // * has more data?
  return hasMoreData;
},
function processData(callback1) {
  async.waterfall([
    function readDataFromSource(callback2) {
      // * read data
      readData(function readFinished(data, hasMore) {
        // * got data 
        hasMoreData = hasMore;
        callback2(null, data);
      });
    }, 
    function writeDataToDest(data, callback2) {
      // * write data
      writeData(data, function writeFinished() {
        callback2();
      });
    },
  ], function (err) {
    callback1(err);
  });
},
function finished(err) {

});

It works like this:

• has more data ? (1)
• read data (1)
• has more data ? (2)
• read data (2)
• got data (1)
• write data (1)
• ...

I understand why it works in this order , and this may be totally correct in some cases. But in my specific case I need to start a new iteration only after the previous one is finished:

• has more data ? (1)
• read data (1)
• got data (1)
• write data (1)
• has more data ? (2)
• read data (2)
• ...

PS: I can't read all the data into array and then process the array with async.each, for example. Neither I can predict how much data the external source has.

Answer 1

I don't use async , but just looking at the code and description and API docs, it seems like this does what you want, not using waterfall at all:

var hasMoreData = true;
async.whilst(function () {
  // * has more data?
  return hasMoreData;
},
function processData(callback1) {
  // * read data
  // blocking i/o operation
  readData(function readFinished(data, hasMore) {
    // * got data
    hasMoreData = hasMore;
    // * write data
    // blocking i/o operation
    writeData(data, function writeFinished() {
      callback1();
    });
  });
},
function finished(err) {

});

Live Example (using shims for readData and writeData ):

 // Shims var datacount = 0; function readData(callback) { // since you said "blocking", busy-wait for a quarter second var done = Date.now() + 250; while (done > Date.now()) { } // but calls like this are almost always async, so we'll complete async ++datacount; setTimeout(function() { callback(datacount, datacount < 3); }, 0); } function writeData(data, callback) { // since you said "blocking", busy-wait for a quarter second var done = Date.now() + 250; while (done > Date.now()) { } // but calls like this are almost always async, so we'll complete async setTimeout(function() { callback(); }, 0); } // The code var hasMoreData = true; async.whilst(function() { // * has more data? snippet.log("has more data?"); return hasMoreData; }, function processData(callback1) { // * read data snippet.log("read data"); // blocking i/o operation readData(function readFinished(data, hasMore) { // * got data snippet.log("got data: " + data + " (more? " + hasMore + ")"); hasMoreData = hasMore; // * write data snippet.log("write data"); // blocking i/o operation writeData(data, function writeFinished() { callback1(); }); }); }, function finished(err) { snippet.log("finished"); }); 

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