Node async parallel inside waterfall

Question

I'm building a server in Nodejs to retrieve data from some database. I've worked with the async library for some time now and figured out some things like putting a waterfall inside a parallel function.

I stumbled upon a problem where I first need to execute a query and then use that query's result in other queries that can be executed concurrently. The code looks something like this:

async.waterfall([
    function(callback) {
        connection.query(   query,
                            function(err, rows, fields) {
                                if (!err) {
                                    callback(null,rows);
                                } else {
                                    callback(null,"SORRY");
                                }
                            }
        );
    },
    async.parallel([
        function(resultFromWaterfall,callback) {
            connection.query(query,
                            function(err, rows, fields) {
                                if (!err) {
                                    callback(null,rows);
                                } else {
                                    callback(null,"SORRY");
                                }
                            }
           );
        },
        function(resultFromWaterfall,callback) {
            connection.query(query,
                            function(err, rows, fields) {
                                if (!err) {
                                    callback(null,rows);
                                } else {
                                    callback(null,"SORRY");
                                }
                            }
           );
        }
    ])
], finalCallback
);

Now my problem is to access the result from the waterfall function and to use it in the parallel functions.

vkstack · Accepted Answer · 2016-08-31T10:00:04.823

async.waterfall([
    function(callback) {
        connection.query(query,
            function(err, rows, fields) {
                if (!err) {
                    callback(null, rows);
                } else {
                    callback(null, "SORRY");
                }
            }
        );
    },
    function(prevData,callback){
      console.log(prevData);//Use it whereever you want.
      async.parallel([
          function(callbackOfAsyncParallel) {
              connection.query(query1,
                  function(err, rows1, fields1) {
                      if (!err) {
                          callbackOfAsyncParallel(null, rows1);
                      } else {
                          callbackOfAsyncParallel(null, "SORRY1");
                      }
                  }
              );
          },
          function(callback) {
              connection.query(query2,
                  function(err, rows2, fields2) {
                    if (!err) {
                        callbackOfAsyncParallel(null, rows2);
                    } else {
                        callbackOfAsyncParallel(null, "SORRY2");
                    }
                  }
              );
          }
      ],function mainCBOfParallel(err,reuslts){
        if(!err){
          //this will be done after tasks in async.parallel are finished.
          callback(null,results);
          //results[0]===>rows1
          //results[1]===>rows2
        }
      });
    }
], finalCallback);

You have two mistakes in your code,

tasks should be a function to execute.
async.parallel has only callbacks in its task functions.

Update

There are callbacks (callbackOfAsyncParallel) which will be called when a task in async.parallel is finished.

It should not invoke callback (callback) of async.waterfall.If done probably there may be error/unexpected results.

So there's really no way to pass the argument to the async.parallel task functions? :( — Hans, Aug 31 '16 at 09:40
No!! It is available everywhere in async.parallel ...!why u need to pass ?? — vkstack, Aug 31 '16 at 09:51
Yes, I noticed now it is available in the functions and I don't need to pass it. It works. — Hans, Aug 31 '16 at 09:58

abdulbarik · Answer 2 · 2016-08-31T09:37:28.743

1

Try this..

    async.waterfall([

  function(callback) {
    connection.query(query,
      function(err, rows, fields) {
        if (!err) {
          callback(null, rows);
        } else {
          callback(null, "SORRY");
        }
      }
    );
  },
  function(resultFromWaterfall, callback) {
    async.parallel([

      function() {
        connection.query(query,
          function(err, rows, fields) {
            if (!err) {
              callback(null, rows);
            } else {
              callback(null, "SORRY");
            }
          }
        );
      },
      function() {
        connection.query(query,
          function(err, rows, fields) {
            if (!err) {
              callback(null, rows);
            } else {
              callback(null, "SORRY");
            }
          }
        );
      }
    ]);
  }

], finalCallback);

edited Aug 31 '16 at 09:37

answered Aug 31 '16 at 09:04

abdulbarik

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Doesn't work either, resultFromWaterfall holds the callback function in that case – Hans Aug 31 '16 at 09:18
what are you getting in `callback`? – abdulbarik Aug 31 '16 at 09:23
No need to pass another `callback` in `parallel`, I just updated answer, it must be work – abdulbarik Aug 31 '16 at 09:38

score 1 · Answer 3 · answered Nov 14 '18 at 09:19

  doWhatever(callback) {
    async.waterfall([
      (waterfallCallback) => {
        connection.query(query, waterfallCallback);
      },
      (rows, fields, waterfallCallback) => {
        async.parallel([
          (parallelCallback) => {
            connection.query(query, parallelCallback);
          },
          (parallelCallback) => {
            connection.query(query, parallelCallback);
          }
        ], waterfallCallback);
      }
    ], callback);
  },

That's just me trying to make it clearer…

Anyway, my exemple is not taking into account parameters and arguments of query. It's just a scaffold

Node async parallel inside waterfall

3 Answers3

Update