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How to force sequential execution for asynchronously called functions

I'm new to the idea of asynchronous code, and am still trying to wrap my brain around how everything works.
I'm building a Node Express application which will interface with a database. When running in a development environment I want it to interface with a Sqlite database. (The production database will not use Sqlite. This only applies to creating a small development environment.)
My problem is I'm having trouble controlling the execution order and timing of queries to the database.
I would like to build my SqliteDatabase.js file such that it can only execute queries sequentially, despite the fact that functions in this file will be called by other parts of the program that are running asynchronously.
How can I acheive this?
For reference, here is how I currently have my SqliteDatabase.js file set up:
var debug = require('debug')('app:DATABASE');
var sqlite = require('sqlite3').verbose();
open = function() {
var db = new sqlite.Database('./test-database.db', sqlite.OPEN_READWRITE | sqlite.OPEN_CREATE, function(err) {
if (err) {
debug("We've encountered an error opening the sqlite database.");
debug(err);
} else {
debug("Sqlite database opened successfully.");
}
});
return db;
}
executeSQLUpdate = function(sql, next) {
var db = open();
db.serialize(function() {
console.log("executing " + sql);
db.run(sql);
db.close();
next();
});
}
exports.executeSQLUpdate = executeSQLUpdate;
Is there some way to build a queue, and make it so when the "executeSQLUpdate" function is called, the request is added to a queue, and is not started until all previous requests have been completed?
To give an example, take a look at this code which utilises my SqliteDatabase.js file:
ar database = require('../../bin/data_access/SqliteDatabase.js');
var createTestTableStmt = "CREATE TABLE IF NOT EXISTS Test(\n" +
"Name TEXT PRIMARY KEY NOT NULL UNIQUE,\n" +
"Age INT NOT NULL,\n" +
"Gender TEXT NOT NULL\n" +
");";
var clearTestTableStmt = "DELETE FROM Test;";
var testInsertStmt = "INSERT INTO Test (Name, Age, Gender)\n" +
"VALUES (\"Connor\", 23, \"Male\");";
createTable = function() {
database.executeSQLUpdate(createTestTableStmt, clearTable);
}
clearTable = function() {
database.executeSQLUpdate(clearTestTableStmt, insertRow);
}
insertRow = function() {
database.executeSQLUpdate(testInsertStmt, function() {
console.log("Done!");
});
}
createTable();
9 times out of 10 the above code works fine, but every once in a while, the "insert row" function is called before the "clearTable" function is called, which throws an error because of a violated database constraint.
How can I change my implementation of the SqliteDatabase.js file to avoid this issue?

You can use async to do this using await. This code will wait for each asynchronous database call to complete before executing the next line.
async function createTable() {
await database.executeSQLUpdate(createTestTableStmt);
await database.executeSQLUpdate(clearTestTableStmt);
await database.executeSQLUpdate(testInsertStmt);
console.log("Done!");
}
Your console.log statement will only execute once all three have completed.
I should also mention that you need a try...catch block around the three database calls to trap any errors and provide an alternate exit point if something should go wrong.

I realized why the callback function next() was sometimes being called before db.run(sql)
It turns out that db.run() is itself an asychronous function. I updated my code, and added a callback to the db.run() line to make sure we don't skip ahead until it's done.
Here's what it looks like now:
executeSQLUpdate = function(sql, next) {
var db = open();
db.run(sql, function(err) {
db.close(function() {
if (next) next(err);
});
});
}
Nesting each asynchronous function in the previous function's callback, makes each function execute in order.
Thanks to everyone who gave me hints that helped me figure out what the problem was.

Meteor insert method callback should execute async

Hello Stackoverflow community,
In nuts
I am wondering why the insert callback is not being called async properly as the documentation says, having a code like:
Meteor.methods({
addUpdate: function (text) {
Updates.insert({
text: text,
createdAt: new Date(),
owner_email: Meteor.user().emails[0].address,
owner_username: Meteor.user().username
}, function(e, id) {
debugger; //<-- executed first with 'e' always undefined
});
debugger; //<-- executed after
}
});
the debugger inside the callback function is executed before the debugger afterwards, if the function is async the debugger inside the callback should be called at the end right?
More info
I am very new with meteor, the thing is that I am trying to make an small app, and experimenting, by now I wanted to confirm what I had understood about some concepts in this case the "insert" method. given the following code:
lib/collections/updateCollection.js
Update = function (params, id) {
params = params || {};
// define properties for update model such as text
this._text = params.text;
}
Update.prototype = {
// define some getters and setters, such as doc
get doc() {
return {
createdAt: this.createdAt,
text: this.text,
owner_email: this.owner_email,
owner_username: this.owner_username
};
},
notify: function notify(error, id) {
var client, notification, status;
client = Meteor.isClient ? window.Website : false;
notification = (client && window.Hub.update.addUpdate) || {}
status = (!error && notification.success) || notification.error;
if (client) {
return client.notify(status);
}
}
save: function save(callback) {
var that;
that = this;
callback = callback || this.notify;
Updates.insert(that.doc, function (error, _id) {
that._id = _id;
callback(error, _id); <-- here is the deal
});
}
}
lib/methods/updateService.js
updateService = {
add: function add(text) {
var update;
update = new Update({
text: text,
createdAt: new Date(),
owner_email: Meteor.user().emails[0].address,
owner_username: Meteor.user().username
});
update.save();
},
// methods to interact with the Update object
};
lib/methods/main/methods.js
Meteor.methods({
addUpdate: function (text) {
updateService.add(text);
}
});
My expectations here is when the client do something like Meteor.call('addUpdate', text); and everything is cool, a successful message is shown, otherwise the error is "truth" and an error message is shown. What is actually happening is that the callback is always called with error undefined (like if everything es cool), the callback also is not being called async, it is just called directly.
Even when I turn off the connection the update insertion shows a success message.
Any idea? maybe my app structure is making meteor work wrong? I really do not know. Thanks in advance.

Your code is executing inside a method. On the client, methods are executed simply to simulate what the server will do before the server responds (so that the app seems more responsive). Because the DB changes here are just simulating what the server is already doing, they are not sent to the server, and therefore synchronous. On the server, all code runs inside a Fiber, so it acts synchronous. (However, Fibers run in parallel just like normal callback-soup Node.)

Parse CloudCode order of execution

Im trying to send a push message to everyone with read access every time a new note is saved.
In pseudocode it should get the ACL. Evaluate each member in the ACL and return an array of all users with read access. Then send a push notification to each member.
I've tried running separate task one by one and it works properly. However when I put everything together in the following code I get strange results. Looking at the log I can see it not executing in order as I expect. I first though the getACL call was an asynchronous call so I tried to implement promises with no luck. Then after help from stackoverflow I find out that getACL is not asynchronous therefore the code should just work, right?
This is what I'm trying:
Parse.Cloud.afterSave("Notes", function(request) {
var idsToSend = [];
var i = 0;
console.log("1 start");
var objACL = request.object.getACL();
var ACLinJSON = objACL.toJSON();
console.log("2 ACL = " + ACLinJSON);
for (var key in ACLinJSON) {
if (ACLinJSON[key].read == "true") {
idsToSend[i] = key.id;
console.log("3 i = " + i + " = " + idsToSend[i]);
i++;
}
}
console.log("4 idsToSend = " + idsToSend);
//lookup installations
var query = new Parse.Query(Parse.Installation);
query.containedIn('user', idsToSend);
Parse.Push.send({
where: query,
data: {
alert: "note updated"
}
}, {
success: function() {
console.log("Success sent push");
},
error: function(error) {
console.error("can’t find user"); //error
}
});
});
And this is the response I see from parse log
I2014-08-04T08:08:06.708Z]4 idsToSend =
I2014-08-04T08:08:06.712Z]2 ACL = [object Object]
I2014-08-04T08:08:06.714Z]1 start
I2014-08-04T08:08:06.764Z]Success sent push
Everything is completely out of order??
How can I execute the above function in the way it's written?

I've found the logs are not in order when I run things too, could be a timing issue or something, ignore the order when they're in the same second, I have done other tests to confirm things really do run in order on my own Cloud Code... had me completely confused for a while there.
The issue you're having is that log #3 is never being hit... try tracing ACLinJSON on it's own to see the actual structure. When you append it to a string it outputs [object Object] as you have seen, so do console.log(ACLinJSON); instead.
Here's the structure I've seen:
{
"*":{"read":true},
"Administrator":{"write":true}
}
Based on that I would expect your loop to work, but it may have a different level of wrapping.
UPDATE:
Turns out the issue was looking for the string "true" instead of a boolean true, thus the fix is to replace the following line:
// replace this: if (ACLinJSON[key].read == "true") {
if (ACLinJSON[key].read == true) {

Array is empty outside the function, trying to fetch data from database, in Javascript

I'm trying to build an application with Ionic Framework and Javascript, and I want to fetch all the data from a table in an array, but when I try to access that array outside the function it's empty. I've read something that maybe the array gets populated after the console.log("length "+ $scope.cards.length); but I don't know how to solve this so I can use the array after that.
This is the function for fetching the data from the table(it works):
fetchCards: function(success) {
database.transaction(function(tx) {
tx.executeSql("SELECT cardId, name, chosen, filePath, found FROM Cards", [],
function (tx, results) {
console.log('success select ' + results);
cards = [];
for (i = 0; i < results.rows.length; i++) {
cards.push(results.rows.item(i));
}
success(cards);
},
function () {
console.log('error select');
});
});
}
This is where I try to access the array, in a controller:
.controller('kindergartenController', function($scope, $ionicSideMenuDelegate,service) {
//list with the difficulties
$scope.difficultyList=[
{text:"Easy", value:"easy"},
{text:"Medium", value:"medium"},
{text:"Hard", value:"hard"}];
$scope.data={difficulty:''};
//redirecting if it presses
$scope.goToAnimalsGame = function() {
if($scope.data.difficulty=='easy'){
window.location = "#/menu/animalseasy";
//in this array I want the results
$scope.cards=[];
game = new Game(1,"easyAnimalGame");
console.log("created a new game " + game.getName());
game.createMemoryGame();
console.log("difficulty " + $scope.data.difficulty);
randomNumbers=game.randomNumbers($scope.data.difficulty);
console.log('Random numbers are:');
for(i=0;i<randomNumbers.length;i++){
console.log(randomNumbers[i]);
}
//here is where I call the other function, to get data from database
service.fetchCards(function(cards) {
$scope.cards = cards;
//it shows the good length
console.log("length "+ $scope.cards.length);
});
//it shows it's empty
console.log("length "+ $scope.cards.length);
}
Any suggestions are helpful, thank you.

It will show empty because:
service.fetchCards
is Asynchronous, therefore inside that function where you define your anonymous callback, won't fire till the data is retrieved.
the console.log("length ".... outside of the function will be executed immediately after the function call fetchCards, but possibly before the callback where the array gets populated.
Unfortunately you can only deal with the populated array within the callback or from a function fired within the callback.
Here is a timeline of execution to aid:
service.fetchCards();
->
console.log("length "....) below the above function
->
anonymous callback (function (cards){}) inside the service.fetchCards()
If that makes sense.
The asynchronism of the service means that the callback you defined anonymously could fire at any time.
The solution:
service.fecthCards(function (data) {
// do your code
$scope.cards = cards;
//it shows the good length
console.log("length "+ $scope.cards.length);
// manipulate the array within this function or get it to call another function like so
nowContinue();
});
function nowContinue() {
// continue what ever it was that you wanted to do using the newly populated information
}
Because nowContinue is called with in the anonymous callback it will fire whenever the callback gets fired, and will execute after you have populated the controllers local variables with data.

Why does creating and deleting an Azure Table fail?

I am wondering why trying to run the following test suite fails when I try to delete the table I have stored entities in. The error I get is the following
1) Azure Storage cloud storage operations "after all" hook:
Error: The specified resource does not exist. RequestId:3745d709-fa5e-4a2b-b517-89edad3efdd2
Time:2013-12-03T22:26:39.5532356Z
If I comment out the actual insertion of data it fails every other time, and if I try to do the insertion of data it fails every time with an additional "The table specified does not exist.".
For the first case this seems to indicate that there is some kind of delay in the table creation, so in every other test it is successful, and for the second case it seems to indicate that even though my callbacks are being called after table creation, the table(s) still aren't ready for data insertion.
The test suite and associated code looks like this:
describe('cloud storage operations', function () {
var storage;
before(function (done) {
this.timeout(5000);
storage = AzureStorage.usingTable('TEST', done);
});
after(function (done) {
storage.deleteTable(done);
});
it('should store without trouble', function (done) {
storage.save(factory.createChangeSet()).then(done, done);
});
});
... // snipped from azure.js
var AzureStorage = function (storageClient, tableName, callback) {
assert(storageClient && tableName && partitionKey, "Missing parameters");
this.storageClient = storageClient;
this.tableName = tableName;
var defaultCallback = function (err) { if (err) { throw error; } };
this.storageClient.createTableIfNotExists(this.tableName, function () {
callback();
} || defaultCallback);
};
AzureStorage.usingTable = function (tableName, callback) {
return new AzureStorage(
azure.createTableService(accountName, accountKey)
, tableName
, callback
);
};
AzureStorage.prototype.deleteTable = function (callback) {
this.storageClient.deleteTable(this.tableName, callback);
};

I've hit this using the c# library as well but I'm pretty sure the error message indicated the table could not be created because an operation was still in process for a table of the same name. Thinking of the backend supporting storage, it makes sense that it would not be instant. The table needs to be removed from the 3 local replicas as well as the replicas in the paired data center.
With that kind of async operation, it is going to be challenging to build up an tear them down fast enough for tests.
A workaround might be to increment a value appended to the "TEST" table name that would be unique to that test run.