Currently try to learn Node JS and getting my head around Async/Sync workflow.
Try to the follow:
Step 1:
- Get data 1 with function 1
- Get data 2 with function 2
- Get data 3 with function 3
Step2:
- Work out logic with data 1,2,3
Step 3
- Do final call
I been looking at Q and Async packages but still havent really find an example.
Can some one show me how they will go about this issue in Node JS?
Thanks
Not entirely clear on your implementation, but depending on how specific your ordering needs to be you could try something like this:
var data1 = null;
var data2 = null;
var data3 = null;
async.series([
function(httpDoneCallback){
async.parallel([
function(data1Callback){
$http(...).then(function(response){
// some logic here
data1 = response;
data1Callback();
})
},
function(data2Callback){
$http(...).then(function(response){
// some logic here
data2 = response;
data2Callback();
})
},
function(data3Callback){
$http(...).then(function(response){
// some logic here
data3 = response;
data3Callback();
})
}
], function(){
//all requests dome, move onto logic
httpDoneCallback();
})
},
function(logicDoneCallback){
// do some logic, maybe more asynchronous calls with the newly acquired data
logicDoneCallback();
}
], function(){
console.log('all done');
})
Do you want function 1, 2, and 3 to trigger at the same time? If so then this should help:
var async = require('async');
async.parallel([
function(cb1) {
cb1(null, "one")
},
function(cb2){
cb2(null, "two")
},
function(cb3){
cb3(null, "three")
}
], function(err, results) {
console.log(results); // Logs ["one", "two", "three"]
finalCall();
});
To explain, every function in the array submitted as the first param to the parallel method will also receive a callback function. Activating the callback function signifies that you're done fetching your data or doing whatever you need to do in said function. All three functions will trigger at the same time, and once all three callbacks are called, the final function is called. The callback accepts two parameters: "error", and "result." If everything's successful, pass "null" as the error parameter. The results will be given to the final function as an array containing each of the results for your individual functions.
You can setup a chain of Promises to do things sequentially:
var funcA = () => {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve('some data from A')
}, 1000)
});
}
var funcB = (dataFromA) => {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve(dataFromA + ' data from B')
}, 2000)
})
}
var funcC = (dataFromB) => {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve(dataFromB + ' data from C')
}, 500)
})
}
// Doing the functions on after another
funcA().then(funcB).then(funcC).then((data) => {
console.log(data);
})
Or if you want to do them all at the same time you can use Promise.all():
var promises = [];
promises.push(new Promise((resolve, reject) => {
setTimeout(() => {
resolve('some data from A')
}, 1000)
}));
promises.push(new Promise((resolve, reject) => {
setTimeout(() => {
resolve('some data from B')
}, 1000)
}));
promises.push(new Promise((resolve, reject) => {
setTimeout(() => {
resolve('some data from C')
}, 1000)
}));
// Execute the array of promises at the same time, and wait for them all to complete
Promise.all(promises).then((data) => {
console.log(data);
})
Probably the best thing to do is use Promises like #Tyler here states. However, for conceptual understanding it is best to first understand the node callback pattern.
Because some tasks take time, we give the task a function and say 'When you are done, put the data you retrieved into this function'. These functions that we give to other functions are called callbacks. They must be constructed to accept the data, and also an error in case there is a problem while fetching the data. In Node the error is the first callback parameter and the data is the second.
fs.readFile('/file/to/read.txt', function callback(error, data) {
if (error) console.log(error);
else console.log(data);
});
In this example, once node reads the file, it will feed that data into the callback function. In the callback we must account for the case that there was a problem and handle the error.
In your question you want to do multiple async tasks and use their results. Therefore you must take this pattern and nest several of them. So, continuing this example, if there is no error you will begin another async task.
fs.readFile('/file/to/read.txt', function callback(error, data) {
if (error) console.log(error);
else {
someOtherAsyncThing(function callback(error, data2) {
if (error) console.log(error);
else {
console.log(data + data2)
}
});
}
});
Related
I want to return results from two database queries in one object.
function route(start, end) {
return new Promise((resolve, reject) => {
const queries = routeQuery(start, end);
var empty_obj = new Array();
for (i=0; i<queries.length; i++) {
query(queries[i], (err, res) => {
if (err) {
reject('query error', err);
console.log(err);
return;
} else {
empty_obj.push(res.rows);
}});
}
console.log(empty_obj);
resolve({coords: empty_obj});
});
}
This is my code right now, the queries are working fine but for some reason, pushing each result into an empty array does not work. When I console log that empty object, it stays empty. The goal is to resolve the promise with the generated object containing the two query results. I'm using node-postgres for the queries.
Output of res is an object:
{
command: 'SELECT',
rowCount: 18,
oid: null,
rows: [
{ ...
I suggest you turn your query function into a Promise so that you can use Promise.all:
// turn the callback-style asynchronous function into a `Promise`
function queryAsPromise(arg) {
return new Promise((resolve, reject) => {
query(arg, (err, res) => {
if (err) {
console.error(err);
reject(err);
return;
}
resolve(res);
});
});
}
Then, you could do the following in your route function:
function route(start, end) {
const queries = routeQuery(start, end);
// use `Promise.all` to resolve with
// an array of results from queries
return Promise.all(
queries.map(query => queryAsPromise(query))
)
// use `Array.reduce` w/ destructing assignment
// to combine results from queries into a single array
.then(results => results.reduce(
(acc, item) => [...acc, ...item.rows],
[]
))
// return an object with the `coords` property
// that contains the final array
.then(coords => {
return { coords };
});
}
route(1, 10)
.then(result => {
// { coords: [...] }
})
.catch(error => {
// handle errors appropriately
console.error(error);
});
References:
Promise.all - MDN
Array.reduce - MDN
Destructing assignment - MDN
Hope this helps.
The issue you currently face is due to the fact that:
resolve({coords: empty_obj});
Is not inside the callback. So the promise resolves before the query callbacks are called and the rows are pushed to empty_obj. You could move this into the query callback in the following manner:
empty_obj.push(res.rows); // already present
if (empty_obj.length == queries.length) resolve({coords: empty_obj});
This would resolve the promises when all rows are pushed, but leaves you with another issue. Callbacks might not be called in order. Meaning that the resulting order might not match the queries order.
The easiest way to solve this issue is to convert each individual callback to a promise. Then use Promise.all to wait until all promises are resolved. The resulting array will have the data in the same order.
function route(start, end)
const toPromise = queryText => new Promise((resolve, reject) => {
query(queryText, (error, response) => error ? reject(error) : resolve(response));
});
return Promise.all(routeQuery(start, end).map(toPromise))
.then(responses => ({coords: responses.map(response => response.rows)}))
.catch(error => {
console.error(error);
throw error;
});
}
I am executing multiple promises with the following snippet:
await Promise.all([promise1, promise2, promise3]);
What I would like to achieve is to rollback the effects of the successful promises on the case of a failure from Promise.all().
In more specific terms, this means that the above will do some file encryptions, but if one fails, I would like to delete the other two (or one) files that were encrypted successfully so as to have consistent and clean file groups.
From what I've read this means that I would need two steps:
1. Catching the errors for each promise so that Promise.all() won't throw an error.
2. The puzzling part: Having another Promise.all() sort of:
await Promise.all([rollbackPromise1, rollbackPromise2, rollbackPromise3]);
This one seems to be the tricky part: Should I execute all the rollbacks independent of the promise that failed? This means that I should do another catch for every error such that the Promise.all() waits for every rollback to finish.
Is this the best way to do this, I find it pretty inefficient and ugly in terms of code.
You could create your own function implementing the asynchronous call of the functions and performing a rollback if required.
// Function that'll perform a promise.all and rollback if required
async function allWithRollback(promises) {
// using the map we are going to wrap the promise inside of a new one
return Promise.all(promises.map(([
func,
rollbackFunc,
], xi) => ((async() => {
try {
await func;
console.log('One Function succeed', xi);
} catch (err) {
console.log('One Function failed, require rollback', xi);
await rollbackFunc();
}
})())));
}
// Call the custom Promise.all
allWithRollback([
[
// First param is the promise
okPromise(),
// Second param is the rollback function to execute
() => {},
],
[okPromise(), () => {}],
[errPromise(), rollback1],
[errPromise(), rollback2],
[okPromise(), () => {}],
]);
// ---------
async function okPromise() {
return true;
}
async function errPromise() {
throw new Error('no one read this');
}
async function rollback1() {
console.log('Performed the rollback1');
}
async function rollback2() {
console.log('Performed the rollback2');
}
You can create a naive solution as follows:
const errorHandlers = []
function enc1 () {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve('str')
}, 1000)
errorHandlers.push(() => {
console.log('handler 1')
})
})
}
function enc2 () {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve('str')
}, 2000)
errorHandlers.push(() => {
console.log('handler 2')
})
})
}
function enc3 () {
return new Promise((resolve, reject) => {
setTimeout(() => {
reject('str')
}, 3000)
errorHandlers.push(() => {
console.log('handler 3')
})
})
}
Promise.all([enc1(), enc2(), enc3()]).then(() => {
console.log('all resovled')
}).catch((e) => {
errorHandlers.forEach(handler => handler(e))
})
It'd give you option to handle the 'global' error in each promise. Before creating promise all, you can reset the errorHandlers to prevent multiple errorHandler execution
I have a foreach loop, where I call an async function. How can I make sure that all the async functions called the specified callback function, and after that, run something?
Keep a counter.
Example:
const table = [1, 2, 3];
const counter = 0;
const done = () => {
console.log('foreach is done');
}
table.forEach((el) => {
doSomeAsync((err, result) => {
counter++;
if (counter === 3) {
done();
}
});
});
As the other answer says, you can use the async package which is really good. But for the sake of it I recommend using Promises and use the Vanila Promise.all(). Example:
const table = [1, 2, 3];
Promise.all(table.map((el) => {
return new Promise((resolve, reject) => {
doSomeAsync((err, result) => {
return err ? reject(err) : resolve(result);
});
});
}))
.then((result) => {
// when all calls are resolved
})
.catch((error) => {
// if one call encounters an error
});
You can use Async library for this. It has various useful utility functions.
There is a Queue function in it which can be used to execute a set of tasks and you get a callback when all tasks are executed where you can do whatever you want. You can also control the concurrency of your queue(how many tasks are executed at a time in parallel).
Here is a sample code-
// create a queue object with concurrency 2
var q = async.queue(function(task, callback) {
console.log('hello ' + task.name);
callback();
}, 2);
// The callback function which is called after all tasks are processed
q.drain = function() {
console.log('all tasks have been processed');
};
// add some tasks to the queue
q.push({name: 'foo'}, function(err) {
console.log('finished processing foo');
});
q.push({name: 'bar'}, function (err) {
console.log('finished processing bar');
});
In the project, I have a loop going through a list of urls. It downloads file from every url and do some post process over the downloaded file.
After the all the process done (both download process and post process), I want to execute a callback function. Because post process includes some streaming task, it has close event. If the last item can be identified, I can pass the callback function to the close event. However, since the loop is async, I can't track which item is done at last.
For now, I use a 5 second timeout to make sure the callback is executed after the whole process. Obviously, this is not sustainable. What's a good way to handle this?
loop code:
exports.processArray = (items, process, callback) => {
var todo = items.concat();
setTimeout(function() {
process(todo.shift());
if(todo.length > 0) {
// execute download and post process each second
// however it doesn't guarantee one start after previous one done
setTimeout(arguments.callee, 1000);
} else {
setTimeout(() => {callback();}, 5000);
}
}, 1000);
};
processArray(
// First param, the array
urlList,
// Second param, download and post process
(url) => {
if(url.startsWith('http')) {
getDataReg(url, uid);
}
else if(url.startsWith('ftp')) {
getDataFtp(url, uid);
}
else {
console.log('not a valid resource');
}
},
// Third param, callback to be executed after all done
() => {
Request.get(`${config.demouri}bound=${request.query.boundary};uid=${uid}`, {
method: 'GET',
auth: auth
})
.on('response', (response) => {
console.log('response event emmits');
zipFiles(uid)
.then((path) => {
reply.file(path, { confine: false, filename: uid + '.zip', mode: 'inline'}).header('Content-Disposition');
});
});
}
);
Download and post process:
exports.getDataFtp = (url, uid) => {
console.log('get into ftp');
var usefulUrl = url.split('//')[1];
var spliter = usefulUrl.indexOf('/');
var host = usefulUrl.substring(0, spliter);
var dir = usefulUrl.substring(spliter+1, usefulUrl.length);
var client = new ftp();
var connection = {
host: host
};
var fileNameStart = dir.lastIndexOf('/') + 1;
var fileNameEnd = dir.length;
var fileName = dir.substring(fileNameStart, fileNameEnd);
console.log('filename: ', fileName);
client.on('ready', () => {
console.log('get into ftp ready');
client.get(dir, (err, stream) => {
if (err) {
console.log('get file err:', err);
return;
} else{
console.log('get into ftp get');
stream.pipe(fs.createWriteStream(datadir + `download/${uid}/${fileName}`));
stream.on('end', () => {
console.log('get into ftp close');
unzipData(datadir + `download/${uid}/`, fileName, uid);
client.end();
});
}
});
});
client.connect(connection);
};
exports.getDataReg = (url, uid) => {
console.log('get into http');
var fileNameStart = url.lastIndexOf('/') + 1;
var fileNameEnd = url.length;
var fileName = url.substring(fileNameStart, fileNameEnd);
var file = fs.createWriteStream(datadir + `download/${uid}/${fileName}`);
if (url.startsWith('https')) {
https.get(url, (response) => {
console.log('start piping file');
response.pipe(file);
file.on('finish', () => {
console.log('get into http finish');
unzipData(datadir + `download/${uid}/`, fileName, uid);
});
}).on('error', (err) => { // Handle errors
fs.unlink(datadir + `download/${uid}/${fileName}`);
console.log('download file err: ', err);
});
} else {
http.get(url, (response) => {
console.log('start piping file');
response.pipe(file);
file.on('finish', () => {
unzipData(datadir + `download/${uid}/`, fileName, uid);
});
}).on('error', (err) => {
fs.unlink(datadir + `download/${uid}/${fileName}`);
console.log('download file err: ', err);
});
}
};
function unzipData(path, fileName, uid) {
console.log('get into unzip');
console.log('creating: ', path + fileName);
fs.createReadStream(path + fileName)
.pipe(unzip.Extract({path: path}))
.on('close', () => {
console.log('get into unzip close');
var filelist = listFile(path);
filelist.forEach((filePath) => {
if (!filePath.endsWith('.zip')) {
var components = filePath.split('/');
var component = components[components.length-1];
mv(filePath, datadir + `processing/${uid}/${component}`, (err) => {
if(err) {
console.log('move file err: ');
} else {
console.log('move file done');
}
});
}
});
fs.unlink(path + fileName, (err) => {});
});
}
After the all the process done (both download process and post process), I want to execute a callback function.
The interesting thing about a series of asynchronous processes is that you can never know when exactly all processes will complete. So setting a timeout for the callback is quick&dirty way to do it, but it's not reliable for sure.
You can instead use a counter to solve this problem.
Let's say you have 10 operations to perform. At the beginning you set your counter to ten counter = 10 And after each process is completed, regardless how (it can either succeed or fail), you can decrement the counter by 1 like counter -= 1 and right after it you can check if the counter is 0, if so that means all processes are completed and we reached the end. You can now safely run your callback function, like if(counter === 0) callback();
If I were you, I would do something like this:
*Notice that the called process should return a promise, so that I can know when it finishes (again regardless how)
*If you need help about promises, this useful article might help you: https://howtonode.org/promises
*Oh and one more thing, you should avoid using arguments.callee, because it's deprecated. Here is why Why was the arguments.callee.caller property deprecated in JavaScript?
exports.processArray = (items, process, callback) => {
var todo = [].concat(items);
var counter = todo.length;
runProcess();
function runProcess() {
// Check if the counter already reached 0
if(checkCounter() === false) {
// Nope. Counter is still > 0, which means we got work to do.
var processPromise = process(todo.shift());
processPromise
.then(function() {
// success
})
.catch(function() {
// failure
})
.finally(function() {
// The previous process is done.
// Now we can go with the next one.
--counter;
runProcess();
})
}
};
function checkCounter() {
if(counter === 0) {
callback();
return true;
} else {
return false;
}
}
};
What you want to do is to make all your asynchronous processes converge into a single promise that you can use to execute the callback at the correct moment.
Lets start at the point each process is complete, which I assume is in the callback passed to the mv() function in unzipData(). You want to wrap each of these asynchronous actions in a Promise that resolves in the callback and you also want to use these promises later and for that you use the .map() method to collect the promises in an array (instead of .forEach()).
Here's the code:
var promises = filelist.map((filePath) => {
if (!filePath.endsWith('.zip')) {
var components = filePath.split('/');
var component = components[components.length-1];
return new Promise((resolve, reject) =>
mv(filePath, datadir + `processing/${uid}/${component}`, (err) => {
if(err) {
console.log('move file err: ');
reject(); // Or resolve() if you want to ignore the error and not cause it to prevent the callback from executing later
} else {
console.log('move file done');
resolve();
}
}));
}
return Promise.resolve();
});
(if the asynchronous action is not to be executed, a Promise that resolves immediately is returned instead)
Now, we can turn this list of Promises into a single Promise that resolves when all of the promises in the list has resolved:
var allPromise = Promise.all(promises);
Next, we need to look further up in the code. We can see that the code we've just been looking at is itself part of an event handler of an asynchronous action, i.e. fs.createReadStream(). You need to wrap that in a promise that gets resolved when the inner promises resolve and this is the promise that the unzipData() function shall return:
function unzipData(path, fileName, uid) {
console.log('get into unzip');
console.log('creating: ', path + fileName);
return new Promise((outerResolve) =>
fs.createReadStream(path + fileName)
.pipe(unzip.Extract({path: path}))
.on('close', () => {
console.log('get into unzip close');
var filelist = listFile(path);
// Code from previous examples
allPromise.then(outerResolve);
}));
}
Next, we look at the functions that use unzipData(): getDataReg() and getDataFtp(). They only perform one asynchronous action so all you need to do is to make them return a promise that resolves when the promise returned by unzipData() resolves.
Simplified example:
exports.getDataReg = (url, uid) => {
return new Promise((resolve, reject) => {
// ...
https.get(url, (response) => {
response.pipe(file);
file.on('finish', () => {
unzipData(datadir + `download/${uid}/`, fileName, uid)
.then(resolve);
});
}).on('error', (err) => { // Handle errors
fs.unlink(datadir + `download/${uid}/${fileName}`);
reject(); // Or resolve() if you want to ignore the error and not cause it to prevent the callback from executing later
});
// ...
});
}
Finally, we get to the processArray() function and here you need to do the same thing we did to begin with: map the processes into a list of promises. First, the process function passed needs to return the promises returned by getDataReg() and getDataFtp():
// Second param, download and post process
(url) => {
if(url.startsWith('http')) {
return getDataReg(url, uid);
}
else if(url.startsWith('ftp')) {
return getDataFtp(url, uid);
}
else {
console.log('not a valid resource');
}
return Promise.reject(); // or Promise.resolve() if you want invalid resources to be ignored and not prevent the callback from executing later
}
Now, your processArray() function can look like this:
exports.processArray = (items, process, callback) =>
Promise.all(items.map(process))
.then(callback)
.catch(() => console.log('Something went wrong somewhere'));
Your callback will get invoked when all asynchronous actions have completed, regardless of in which order they do. If any one of the promises rejects, the callback will never be executed so manage your promise rejections accordingly.
Here's a JSFiddle with the complete code: https://jsfiddle.net/upn4yqsw/
In general, since nodejs does not appear to have implemented Streams Standard to be Promise based, at least from what can gather; but rather, uses an event based or callback mechanism, you can use Promise constructor within function call, to return a fulfilled Promise object when a specific event has been dispatched
const doStuff = (...args) => new Promise((resolve, reject)) => {
/* define and do stream stuff */
doStreamStuff.on(/* "close", "end" */, => {
// do stuff
resolve(/* value */)
})
});
doStuff(/* args */)
.then(data => {})
.catch(err => {})
I'm confronted to a situation which drives me a bit mad.
So The situation is as below :
module.exports = {
generation: function (req, res) {
// Let's firstly fetch all the products from the productTmp Table
function fetchProductsTmp (){
ProductsTmp.find().then(function (products) {
return Promise.all(products.map (function (row){
Service.importProcess(row);
}));
});
}
fetchProductsTmp();
}
Here I simply call my model ProductsTmp to fetch my datas and iterate through my rows calling importProcess.
importProcess :
importProcess: function (product) {
async.series([
function (callback) {
return SousFamille.findOne({name: product.sous_famille}).then(function (sf) {
console.log('1');
if (!sf) {
return SousFamille.create({name: product.sous_famille}).then(function (_sf) {
console.log('2');
callback(null, _sf.sf_id);
});
} else {
callback(null, sf.sf_id);
}
});
},
function (callback){
console.log('3');
},
], function(err, results){
if(err) return res.send({message: "Error"});
});
}
So I got with my console log :
1
1
1
2
3
2
3
2
3
What I want to Obtain is 1 2 3 1 2 3 1 2 3 So that each function wait for the promise to finish before calling the next one.
In the generation function of the first section, replace
return Promise.all(products.map (function (row){
Service.importProcess(row);
}));
with
var results = [],
pushResult = id => results.push(id);
return products.reduce(function(prev, row){//Go through all the products
//Take the previous promise, and schedule next call to Service.importProcess to be
//made after the previous promise has been resolved
return prev.then(function(){
return Service.importProcess(row).then(pushResult);
});
}, Promise.resolve())
.then(() => results);
You also need to return a promise from importProcess for this to work. Just ditch the whole async.series thingy and do something like
return new Promise(function(resolve, reject){
...
resolve(sf.sf_id); //instead of the callback(null, sf.sf_id)
...
});
Update: This forces the calls to Service.importProcess to be sequential instead of concurrent, which does affect the overall performance of calls to generation. But I guess you have more solid reasons to do so than sequential console.logs.
Sorry, can't help the urge to do it in ES6, basically things can be reduced to single line, like Bergi said, async is redundant( using Bluebird Promise Library):
importProcess: product =>
SousFamille.findOne({name: product.sous_famille})
.then(sf => sf? sf.sf_id : SousFamille.create({name: product.sous_famille}).then(_sf => _sf.sf_id))
// the other module
module.exports = {
generation: (req, res) => ProductsTmp.find()
.then(products => Promise.mapSeries(products, Service.importProcess.bind(Service)) )
.then(ids => res.send({ids}))
.catch(error => res.send({message: 'Error'}))
}
also like noppa said, your problem is the missing return in Service.importProcess(row), same code in ES5:
module.exports = {
generation: function (req, res) {
ProductsTmp.find()
.then(function (products) {
return Promise.mapSeries(products, Service.importProcess.bind(Service)) );
}).then(function(ids){
res.send({ids: ids});
}).catch(function(error){
res.send({message: 'Error'});
})
}
importProcess: function (product) {
return SousFamille.findOne({name: product.sous_famille})
.then(function (sf) {
if (sf) return sf.sf_id;
return SousFamille.create({name: product.sous_famille})
.then(function (_sf){ return _sf.sf_id});
});
}