I want to call from a parent rest service a child rest service. The number of times child service is called depends on parameters to parent rest services. Once I call all child service instance concurrently with different parameters. I want to combine the responses from all instances of child service. I am using below snippet. But I don't want to use timeout. It should either be timeout or when all calls of child service are over which ever is lesser.
for( i=0; i<length; i++)
{
url=accountID[i] +'+'+sortcode[i] +'+' +accountHolderName[i];
micro(url ,filter[i],function(resp)
{
this.resutlObject[count]=resp;
console.log("count"+count);
count=count+1;
}.bind( {resutlObject: resutlObject} ));
}//end of for
setTimeout(function () {
console.log("in time out");
res.end(JSON.stringify(resutlObject || {}, null, 2));
},500);
Also you could use Promises. Suppose service call returns promise, then you wait while all of them are fulfilled. Node.js supports promises starting from v4. If you have earlier version of node, just use some library.
//Instead of
function micro(url, filter, cb) {
var resp = "result of async job";//do some async work
cb(resp)
}
//Modify your service to return a promise
function micro(url, filter) {
return new Promise(function(resolve, reject) {
var resp = "result of async job using `url` and `filter`";
if (resp) {
resolve(resp);
} else {
reject("reason");
}
});
}
//Create a list of service calls.
var promises = [];
for( i=0; i<length; i++)
{
url=accountID[i] +'+'+sortcode[i] +'+' +accountHolderName[i];
promises.push(micro(url, filter[i]));
}
//Wait for all off them to fulfill
Promise.all(promises)
.then(function(resultObject) {
//Response
res.end(JSON.stringify(resultObject || {}, null, 2));
}, function(reason) {
res.sendStatus(500);
console.error(reason);
});
you can use async module async. It provides the parallel foreach loop.
var obj = {dev: "/dev.json", test: "/test.json", prod: "/prod.json"};
var configs = {};
async.forEachOf(obj, function (value, key, callback) {
fs.readFile(__dirname + value, "utf8", function (err, data) {
if (err) return callback(err);
try {
configs[key] = JSON.parse(data);
} catch (e) {
return callback(e);
}
callback();
})
}, function (err) {
if (err) console.error(err.message);
// configs is now a map of JSON data
doSomethingWith(configs);
})
here in the example it is reading files listed in parameters.
similarly you can do for your task
You could use async module. It's designed to do the stuff you're after. Something like this:
var async = require('async');
var collection = [];
for(i=0;i<length;i++) {
collection.push(
(function(i) {
return function(callback) {
url=accountID[i] +'+'+sortcode[i] +'+' +accountHolderName[i];
micro(url ,filter[i],function(resp) {
callback(null, resp);
});
}
})(i)
);
}//end of for
async.parallel(collection, function(err, results) {
console.log(results) // array of results from all requests
})
What happens
async.parallel takes an array of functions as an argument. Each function receives callback as an argument. Callback is a function, which takes error and result as an argument.
After all callback are executed async calls the final callback which receives the array of results from all other callbacks.
In the loop we are creating just that, a collection of functions. In this example the code is a bit more complex because we use closure in order to preserve the value of i for each of these functions.
Related
I'm new to NodeJS and I'm currently working on node-soap module that can be found from https://github.com/vpulim/node-soap.
I'm calling a Web Service by using the referenced module. I need to return the SOAP client's response to user's web browser, however, the problem is that the response gets returned before it is fetched to an array. This is related to asynchronous way of working.
The second problem is that I need to call the Web Service again until I get specific amount of results. Each result will be pushed to the same array. How to do this? I need to return this array to user but as described before, it is always empty.
How would you use soap.createClientAsync and client.methodAsync in this case?
I have already tried writing a while-loop that continues until I get specific amount of results. I tried wrapping soap.createClient to a promise as well as soap.method. Those promises are in different functions and I tried to call them in async function which returns the array.
function createSoapClient() {
return new Promise(function(resolve, reject) {
var url = '...';
soap.createClient(url, function(err, client) {
if (err) {
reject(err);
}
resolve(client);
});
});
}
function fetchServiceCustomers(client) {
return new Promise(function(resolve, reject) {
var args = {...};
client.method(args, function(error, result, rawResponse, soapHeader, rawRequest) {
if (error) {
reject(error);
}
resolve(result);
}, {timeout: 60 * 1000});
});
}
exports.getServiceCustomers = async function() {
let client = await createSoapClient();
var results = 0,
completeResult = [];
while (results <= 0 || results >= 10000) {
completeResult.push(await fetchServiceCustomers(client);
results = completeResult[completeResult.length - 1];
console.log(results);
}
return completeResult;
}
I'm trying my damndest to avoid callback hell with my Node JS. But I'm trying to make a large number of api-requests and insert these into my database.
My issue here (of course) is that my for-loop runs and increments i before I finish my request and database insertion.
for(var i = 0; i <= 1 ; i++){
apiRequest = data[i];
apicall(apiRequest);
}
function apicall(urlApi){
request((urlApi), function(error, response, body){
if(error){
console.log("error");
} else if(!error && response.statusCode == 200){
var myobj = JSON.parse(body);
dbInsert(myobj);
}
});
}
function dbInsert(obj) {
//insert into database
}
If someone else would come by this question I can truly recommend this blogpost which I found after reading the response by joshvermaire:
http://www.sebastianseilund.com/nodejs-async-in-practice
There are a number of ways to approach this type of problem. Firstly, if you can run all the API calls in parallel (all in flight at the same time) and it doesn't matter what order they are inserted in your database, then you can get a result a lot faster by doing that (vs. serializing them in order).
In all the options below, you would use this code:
const rp = require('request-promise');
function apicall(urlApi){
return rp({url: urlApi, json: true}).then(function(obj){
return dbInsert(obj);
});
}
function dbInsert(obj) {
//insert into database
// return a promise that resolves when the database insertion is done
}
Parallel Using ES6 Standard Promises
let promises = [];
for (let i = 0; i <= data.length; i++) {
promises.push(apicall(data[i]));
}
Promise.all(promises).then(() => {
// all done here
}).catch(err => {
// error here
});
Parallel using Bluebird Promise Library
With the Bluebird Promise library, you can use Promise.map() to iterate your array and you can pass it the concurrency option to control how many async calls are in flight at the same time which might keep from overwhelming either the database or the target API host and might help control max memory usage.
Promise.map(data, apiCall, {concurrency: 10}).then(() => {
// all done here
}).catch(err => {
// error here
});
In Series using Standard ES6 Promises
If you have to serialize them for some reason such as inserting into the database in order, then you can do that like this. The .reduce() pattern shown below is a classic way to serialize promise operations on an array using standard ES6:
data.reduce(data, (p, item) => {
return p.then(() => {
return apicall(item);
});
}, Promise.resolve()).then(() => {
// all done here
}).catch(err => {
// error here
});
In Series Using Bluebird Promises
Bluebird has a Promise.mapSeries() that iterates an array in series, calling a function that returns a promise on each item in the array which is a little simpler than doing it manually.
Promise.mapSeries(data, apiCall).then(() => {
// all done here
}).catch(err => {
// error here
});
I'd recommend using something like async.each. Then you could do:
async.each(data, function(apiRequest, cb) {
apicall(apiRequest, cb);
}, function(err) {
// do something after all api requests have been made
});
function apicall(urlApi, cb){
request((urlApi), function(error, response, body){
if(error){
console.log("error");
cb(error);
} else if(!error && response.statusCode == 200){
var myobj = JSON.parse(body);
dbInsert(myobj, cb);
}
});
}
function dbInsert(obj, cb) {
doDBInsert(obj, cb);
}
When the dbInsert method completes, make sure the cb callback is called. If you need to do this in a series, look at async.eachSeries.
I am trying to control the flow of the execution in my code below, meaning I want it to be serial.
I am reading and updating data from and to my DB, and ofc I want that to happen in the correct order. Below is the function I am calling my DB from, the queries functions are wrapped in callbacks.
I am pretty new to promises so perhaps the error might be something silly I am overlooking. If you need anything to ask please do so.
function my_function(array, array2)
{
var array3 = [];
return Promise.resolve(true)
.then(function()
{
console.log("1")
for(var i=0; i< array.length; i++)
{
get(array[i], function(results){
console.log("2")
array3.push(..);
});
}
return array3;
}).then(function()
{
console.log("3")
for(var i=0; i< array2.length; i+=2)
{
//...
get(array2[i], function(results){
console.log("4")
return array3.push(...);
});
}
return array3;
}).then(function(array3)
{
console.log("5")
for(var i=0; i<array3.length; i++)
{
get(array3[i], function(results){
console.log("6")
update(.., function(callb_result){
return;
});
});
}
});
}
And here is the way I am calling the queries.
function get(array, callback)
{
db.get(`SELECT .. FROM .. WHERE ..;`, function(error, row) {
...
return callback(something);
});
}
function update(.., callback)
{
db.run(`UPDATE .. SET ...`);
return callback("updated"); //I dont want to return anything
}
Whats printed in the log
1
3
5
2
4
6
I was thinking perhaps the way I ma calling the queries is async and that's messing up everything.
You're using for loops to run asynchronous tasks and return an array that is modified by them. But because they are asynchronous the return happens before they are finished. Instead you can create an array of promises where each promise is one of the asynchronous tasks that resolves once the task is done. To wait until every task is done you can call Promise.all with the array of promises, which returns a promise that resolves with an array of the resolved results.
For the first .then you can use Array.prototype.map to easily create an array of promises. Each item in the array needs to return a new Promise that resolves with the result from the callback of get.
.then(function() {
console.log("1");
const promiseArray = array.map(function(item) {
return new Promise(function(resolve) {
get(item, function(result) {
console.log("2");
resolve(result);
});
});
});
return Promise.all(promiseArray);
})
As you return Promise.all the next .then call be executed once all the promises in the promiseArray are fulfilled. It will receive the array of results as the first parameter to the function. That means you can use them there. The second .then is similar to the first one, except that you don't want to call get on every item. In this case map is not applicable, so the for loop will just create a promise and add it to the array of promises. Before you have used array3 to store the results that you want to update, but with promises you don't really need that. In this case you can simply concat the results of both arrays.
.then(function(resultsArray) {
console.log("3");
const promiseArray2 = [];
for (var i = 0; i < array2.length; i += 2) {
const promise = new Promise(function(resolve) {
get(array2[i], function(results) {
console.log("4");
resolve(results);
});
});
promiseArray2.push(promise);
}
// Wait for all promises to be resolved
// Then concatenate both arrays of results
return Promise.all(promiseArray2).then(function(resultsArray2) {
return resultsArray.concat(resultsArray2);
});
})
This returns a promise that resolves with the concatenated array, so you will have all the results (from both .then calls) as an array, which is passed to the next .then function. In the third and final .then you simply call update on each element of the array. You don't need to call get again, as you've already done this and you passed on the results.
.then(function(finalResults) {
console.log("5");
for (var i = 0; i < finalResults.length; i++) {
console.log("6");
update(finalResults[i], function(result) {
console.log(result);
});
}
});
Full runnable code (get uses a timeout to simulate asynchronous calls)
function myFunction(array, array2) {
return Promise.resolve(true)
.then(function() {
console.log("1");
const promiseArray = array.map(function(item) {
return new Promise(function(resolve) {
get(item, function(results) {
console.log("2");
resolve(results);
});
});
});
return Promise.all(promiseArray);
})
.then(function(resultsArray) {
console.log("3");
const promiseArray2 = [];
for (var i = 0; i < array2.length; i += 2) {
const promise = new Promise(function(resolve) {
get(array2[i], function(results) {
console.log("4");
resolve(results);
});
});
promiseArray2.push(promise);
}
return Promise.all(promiseArray2).then(function(resultsArray2) {
return resultsArray.concat(resultsArray2);
});
})
.then(function(finalResults) {
console.log("5");
for (var i = 0; i < finalResults.length; i++) {
console.log("6");
update(finalResults[i]);
}
});
}
function get(item, cb) {
// Simply call the callback with the item after 1 second
setTimeout(() => cb(item), 1000);
}
function update(item) {
// Log what item is being updated
console.log(`Updated ${item}`);
}
// Test data
const array = ["arr1item1", "arr1item2", "arr1item3"];
const array2 = ["arr2item1", "arr2item2", "arr2item3"];
myFunction(array, array2);
Improving the code
The code now works as expected, but there are many improvements that make it a lot easier to understand and conveniently also shorter.
To simplify the code you can change your get function to return a promise. This makes it a lot easier, since you don't need to create a promise in every step. And update doesn't need to be a promise, neither does it need a callback as it's synchronous.
function get(array) {
return new Promise(function(resolve, reject) {
db.get(`SELECT .. FROM .. WHERE ..;`, function(error, row) {
if (err) {
return reject(error);
}
resolve(something);
});
});
}
Now you can use get everywhere you used to create a new promise. Note: I added the reject case when there is an error, and you'll have to take care of them with a .catch on the promise.
There are still too many unnecessary .then calls. First of all Promise.resolve(true) is useless since you can just return the promise of the first .then call directly. All it did in your example was to automatically wrap the result of it in a promise.
You're also using two .then calls to create an array of the results. Not only that, but they perform exactly the same call, namely get. Currently you also wait until the first set has finished until you execute the second set, but they can be all executed at the same time. Instead you can create an array of all the get promises and then wait for all of them to finish.
function myFunction(array, array2) {
// array.map(get) is equivalent to array.map(item => get(item))
// which in turn is equivalent to:
// array.map(function(item) {
// return get(item);
// })
const promiseArray = array.map(get);
for (let i = 0; i < array2.length; i += 2) {
promiseArray.push(get(array2[i]));
}
return Promise.all(promiseArray).then(results => results.forEach(update));
}
The myFunction body has been reduced from 32 lines of code (not counting the console.log("1") etc.) to 5.
Runnable Snippet
function myFunction(array, array2) {
const promiseArray = array.map(get);
for (let i = 0; i < array2.length; i += 2) {
promiseArray.push(get(array2[i]));
}
return Promise.all(promiseArray).then(results => results.forEach(update));
}
function get(item) {
console.log(`Starting get of ${item}`);
return new Promise((resolve, reject) => {
// Simply call the callback with the item after 1 second
setTimeout(() => resolve(item), 1000);
});
}
function update(item) {
// Log what item is being updated
console.log(`Updated ${item}`);
}
// Test data
const testArr1 = ["arr1item1", "arr1item2", "arr1item3"];
const testArr2 = ["arr2item1", "arr2item2", "arr2item3"];
myFunction(testArr1, testArr2).then(() => console.log("Updated all items"));
I have an array of URLs and I want to loop through them and fetch thr content. After I have looped through them and fetched thr content I want a callback function to be called.
I know I can do this via async library but I want to do this without using any library.
Sample of what kind of code I want is below
['yahoo.com', 'gmail.com'].each(function(item){
//code to fetch URL content
},someCallbackFunctionToBeExecutedAtTheEndOfLoop);
This is typically the type of thing you do using promises (But you would need a library), with a code like:
var ops = [];
urls.forEach(function(url) {
ops.push(fetchUrl(url));
});
P.all(ops).then(callback);
function fetchUrl(url) {
var defer = P.defer();
//do stuff
// call defer.resolve(result);
return defer.promise;
}
If you don't want to use promises, you can use a counter of operations, like:
var ops = urls.length;
urls.forEach(function(url) {
// do stuff
ops--;
if (ops === 0) {
callback();
}
});
If you chose the promises, I advice to use p-promise module, which is far more optimized than Q.
If you want to do it without any sort of library like async, then you have to write your own counter to keep track of when all the async responses have been completed:
var request = require('request');
function loadAll(list, fn) {
var cnt = list.length;
var responses = [];
list.forEach(function(url, index) {
request(url, function(error, response, body) {
if (error) {
fn(error);
} else {
responses[index] = response;
--cnt;
if (cnt === 0) {
fn(0, responses);
}
}
});
})
}
loadAll(['http://www.yahoo.com', 'http://www.gmail.com'], function(err, results) {
if (!err) {
// process results array here
}
});
If you're going to be doing many async operations in node.js, then getting a promise library like Bluebird will save you a lot of time. For example, I think you could do the above in something like this (untested):
var Promise = require("bluebird");
var requestP = Promise.promisfy(require("request"));
Promise.map(['http://www.yahoo.com', 'http://www.gmail.com'], requestP).then(function(results) {
// process the array of results here
});
I have a loop that I need to run inside of a callback, unfortunately accessing the array outside of the callback leaves me with a blank array. I know why this happens, but I want to know the best solution to tackle this.
Gallery.prototype.getGallery = function(cb) {
self = this;
var cos = new pb.CustomObjectService();
var ms = new pb.MediaService();
var s = [];
cos.loadTypeByName('Gallery Image', function(err, gallery){
cos.findByType(gallery._id.toString(), function(err, rpy){
for(var i = 0; i < rpy.length; i++){
ms.loadById(rpy[i].Image, function(e,r){
s.push(r.location);
console.log(r.location); /* <-- logs expected data */
});
}
console.log(s[0]); /* <-- this is undefined */
});
});
};
Replace your for loop with a call to async.*; in this case async.map seems right. Pass a callback to async.map; it will be invoked when all the individual calls to ms.loadById are done, with the array of results.
async.map(
rpy,
function(elt, callback) {
ms.loadById(elt.Image, callback);
},
function(err, data) {
// comes here after all individual async calls have completed
// check errors; array of results is in data
}
);
If you want to go into the promises world, then wrap the calls to ms.loadById in a promise. Here's a roll-your-own version, but various versions of what is usually called promisify are also out there.
function loadByIdPromise(elt) {
return new Promise(function(resolve, reject) {
ms.loadById(elt.image, function(err, data) {
if (err) return reject(err);
resolve(data);
});
});
}
Then do a Promise.all on the resulting promises:
Promise.all(rpy.map(loadByIdPromise))
.then(function(data) {
// comes here when all individual async calls complete successfully
// data is your array of results
});
Using the promises style, your entire code would look like:
loadTypeByNamePromise('Gallery Image') .
then(function(gallery) { return findByTypePromise(gallery._id.toString(); }) .
then(function(rpy) { return Promise.all(rpy.map(loadByIdPromise)); }) .
then(function(results) { /* do something with [results] */ });