wait.for

Sequential programming for node.js, end of callback hell

Wait.for

Sequential programming for node.js, end of callback hell.

Simple, straightforward abstraction over Fibers.

By using wait.for, you can call any nodejs standard async function in sequential/Sync mode, waiting for result data, without blocking node's event loop (thanks to fibers)

A nodejs standard async function is a function in which the last parameter is a callback: function(err,data)

Advantages:

  • Avoid callback hell / pyramid of doom
  • Simpler, sequential programming when required, without blocking node's event loop (thanks to fibers)
  • Simpler, try-catch exception programming. (default callback handler is: if (err) throw err; else return data)
  • You can also launch multiple parallel non-concurrent fibers.
  • No multi-threaded debugging nightmares, only one fiber running at a given time (thanks to fibers)
  • Can use any node-standard async function with callback(err,data) as last parameter.
  • Plays along with node programming style. Write your async functions with callback(err,data), but use them in sequential/SYNC mode when required.
  • Plays along with node cluster. You design for one thread/processor, then scale with cluster on multicores.

##NEWS


####March-2014 - LiteScript

I've ported this functionality to LiteScript.

LiteScript is a higly readable, compile to js language. LiteScript has type annotations, a compile-time validation phase, and catch common js errors and typos in object property names, speeding up development (you code faster) and saving hours of debugging over a mistyped property name. Try LiteScript online

Here it is a sample of LiteScript Code, showing "yield until" (wait for async to complete) and "yield parallel" (launch in parallel, wait until all asyncs complete)

#####get google.com IPs, then reverse DNS (in parallel) global import dns, nicegen nice function resolveAndParallelReverse try var addresses:array = yield until dns.resolve "google.com" var results = yield parallel map addresses dns.reverse for each index,addr in addresses print "#{addr} reverse: #{results[index]}" catch err print "caught:", err.stack end nice function


###Aug-2013 - Wait.for-ES6 based on ES6-generators

I've developed a version based on JavaScript upcoming ES6-Harmony generators. It's not based on node-fibers. Surprisingly, ES6 based implementation of wait.for(asyncFn) is almost a no-op, you can even completely omit it. Warning: Bleeding edge. Check [Wait.for-ES6] (https://github.com/luciotato/waitfor-ES6)


    npm install wait.for

You need to be in a Fiber to be able to use wait.for. The ideal place to launch a fiber is when a request arrives, to handle it:

var server = http.createServer(
  function(reqres){
    console.log('req!');
    wait.launchFiber(handler,req,res); //handle in a fiber, keep node spinning 
  }).listen(8000);

then,at function handler(req,res) and every function you call from there, you'll be able to use wait.for(ayncFn...

DNS testing, using pure node.js (a little of callback hell):

var dns = require("dns");
    
function test(){ 
    dns.resolve4("google.com", function(erraddresses) {
        if (err) throw err;
        for (var i = 0; i < addresses.length; i++) {
            var a = addresses[i];
            dns.reverse(a, function (errdata) {
                if (err) throw err;
                console.log("reverse for " + a + "" + JSON.stringify(data));
            });
        };
    });
}
 
test();

THE SAME CODE, using wait.for (sequential):

var dns = require("dns"), wait=require('wait.for');
 
function test(){
    var addresses = wait.for(dns.resolve4,"google.com");
    for (var i = 0; i < addresses.length; i++) {
        var a = addresses[i];
        console.log("reverse for " + a + "" + JSON.stringify(wait.for(dns.reverse,a)));
    }
}
 
wait.launchFiber(test); 

using pure node.js (a callback hell):

var db = require("some-db-abstraction");
 
function handleWithdrawal(req,res){  
    try {
        var amount=req.param("amount");
        db.select("* from sessions where session_id=?",req.param("session_id"),function(err,sessiondata) {
            if (err) throw err;
            db.select("* from accounts where user_id=?",sessiondata.user_ID),function(err,accountdata) {
                if (err) throw err;
                    if (accountdata.balance < amount) throw new Error('insufficient funds');
                    db.execute("withdrawal(?,?)",accountdata.ID,req.param("amount"), function(err,data) {
                        if (err) throw err;
                        res.write("withdrawal OK, amount: "+ req.param("amount"));
                        db.select("balance from accounts where account_id=?", accountdata.ID,function(err,balance) {
                            if (err) throw err;
                            res.end("your current balance is "  + balance.amount);
                        });
                    });
                });
            });
        }
        catch(err) {
            res.end("Withdrawal error: "  + err.message);
    }
}

Note: The above code, although it looks like it will catch the exceptions, it will not. Catching exceptions with callback hell adds a lot of pain, and i'm not sure if you will have the 'res' parameter to respond to the user. If somebody like to fix this example... be my guest.

THE SAME CODE, using wait.for (sequential logic - sequential programming):

var db = require("some-db-abstraction"), wait=require('wait.for');
 
function handleWithdrawal(req,res){  
    try {
        var amount=req.param("amount");
        sessiondata = wait.forMethod(db,"select","* from session where session_id=?",req.param("session_id"));
        accountdata = wait.forMethod(db,"select","* from accounts where user_id=?",sessiondata.user_ID);
        if (accountdata.balance < amount) throw new Error('insufficient funds');
        wait.forMethod(db,"execute","withdrawal(?,?)",accountdata.ID,req.param("amount"));
        res.write("withdrawal OK, amount: "+ req.param("amount"));
        balance = wait.forMethod(db,"select","balance from accounts where account_id=?", accountdata.ID);
        res.end("your current balance is "  + balance.amount);
        }
    catch(err) {
        res.end("Withdrawal error: "  + err.message);
}  

Note: Exceptions will be catched as expected. db methods (db.select, db.execute) will be called with this=db

var wait = require('wait.for');
var express = require('express');
var app = express();
 
// in  a Fiber
function handleGet(req, res){
  res.send( wait.for(fs.readFile,'largeFile.html') );
}
 
app.get('/', function(req,res){
      wait.launchFiber(handleGet, req, res); //handle in a fiber, keep node spinning
});
 
app.listen(3000);
 
var wait=require('wait.for');
 
// launch a new fiber 
wait.launchFiber(my_seq_function, arg,arg,...)
 
// fiber 
function my_seq_function(arg,arg...){
    // call async_function(arg1), wait for result, return data 
    var myObj = wait.for(async_function, arg1); 
    // call myObj.querydata(arg1,arg2), wait for result, return data 
    var myObjData = wait.forMethod(myObj,'queryData', arg1, arg2);
    console.log(myObjData.toString());
}

###wait.parallel.launch(functions:Array)

Note: must be in a Fiber

####input:

  • functions: Array = [func,arg,arg],[func,arg,arg],...

####actions: -launchs a fiber for each func -the fiber does resultArray[index] = func.apply(undefined,args)

####returns:

  • array with a result for each function
  • do not "returns" until all fibers complete
  • throws if error

###wait.parallel.map(arr:Array, mappedFn:function)

Note: must be in a Fiber

####input:

  • arr: Array
  • mappedFn = function(item,index,arr) -- mappedFn should return converted item. Since we're in a fiber -- mappedFn can use wait.for and also throw/try/catch

####returns:

  • array with converted items
  • do not "returns" until all fibers complete
  • throws if error

###wait.parallel.filter(arr:Array, itemTestFn:function)

Note: must be in a Fiber

####input:

  • arr: Array
  • itemTestFn = function(item,index,arr) -- itemTestFn should return true|false. Since we're in a fiber -- itemTestFn can use wait.for and also throw/try/catch

####returns

  • array with items where itemTestFn() returned true
  • do not "returns" until all fibers complete
  • throws if error

Parallel Usage Example: see:


##Notes on usage on non-standard callbacks. e.g.: connection.query from mysql

wait.for expects standardized callbacks. A standardized callback always returns (err,data) in that order.

A solution for the sql.query method and other non-standard callbacks is to create a wrapper function standardizing the callback, e.g.:

 connection.prototype.q = function(sql, params, stdCallback){ 
             this.query(sql,params, function(err,rows,columns){ 
                                 return stdCallback(err,{rows:rows,columns:columns}); 
                         });
 }

usage:

try {
  var result = wait.forMethod(connection, "q", options.sql, options.params); 
  console.log(result.rows);
  console.log(result.columns);
} 
catch(err) {
   console.log(err);
}