What is the difference between asynchronous and synchronous execution?

Asynchronous vs synchronous execution, what is the main difference?

I have a Node.js/Express app that queries a MySQL db within the route and displays the result to the user.  My problem is how do I run the queries and block until both queries are done before redirecting the user to the page they requested?

In my example I have 2 queries that need to finish before I render the page.  I can get the queries to run synchronously if i nest query 2 inside the &#39;result&#39; callback of query 1.  This however will become very convoluted when the number of queries increase.

How do I go about running multiple (in this case 2) database queries synchronously without nesting the subsequent query in the prior query&#39;s &#39;result&#39; callback?

I&#39;ve looked at the &#39;Flow control / Async goodies&#39; in the Node modules and tried flow-js but I can&#39;t get it to work with the async queries.

Listed below are the 2 queries that I&#39;m attempting to execute from the &#39;/home&#39; route.  Can the Node experts explain the &#39;right&#39; way to do this.

    app.get(&#39;/home&#39;, function (req,res) {
        var user_array = [];
        var title_array = [];

        // first query
        var sql = &#39;select user_name from users&#39;;
        db.execute(sql)
            .addListener(&#39;row&#39;, function(r) {
                user_array.push( { user_name: r.user_name } );
            })
            .addListener(&#39;result&#39;, function(r) {
                req.session.user_array = user_array;
            });

        // second query
        var sql = &#39;select title from code_samples&#39;;
        db.execute(sql)
            .addListener(&#39;row&#39;, function(r) {
                title_array.push( { title: r.title } );
            })
            .addListener(&#39;result&#39;, function(r) {
                req.session.title_array = title_array;
            });
            
            // because the queries are async no data is returned to the user
            res.render(&#39;home.ejs&#39;, {layout: false, locals: { user_name: user_array, title: title_array }});
    });

Synchronous database queries with Node.js

The use and effects of the O_SYNC and O_DIRECT flags is very confusing and appears to vary somewhat among platforms.  From the Linux man page (see an example [here][1]), O_DIRECT provides synchronous I/O, minimizes cache effects and requires you to handle block size alignment yourself.  O_SYNC just guarantees synchronous I/O.  Although both guarantee that data is written into the hard disk&#39;s cache, I believe that direct I/O operations are supposed to be faster than plain synchronous I/O since they bypass the page cache (Though FreeBSD&#39;s man page for open(2) states that the cache is bypassed when O_SYNC is used.  See [here][2]).

What exactly are the differences between the O_DIRECT and O_SYNC flags?  Some implementations suggest using O_SYNC | O_DIRECT.  Why?  


  [1]: http://linux.die.net/man/2/open
  [2]: http://www.freebsd.org/cgi/man.cgi?query=open&amp;apropos=0&amp;sektion=0&amp;manpath=FreeBSD%208.2-RELEASE&amp;format=html

How are the O_SYNC and O_DIRECT flags in open(2) different/alike?

Is there a way to load and execute a javascript file in a synchronous way just like a synchronous XMLHttpRequest?

I&#39;m currently using a sync XMLHttpRequest and then eval for this, but debugging that code is very difficult...

Thanks for your help!


**Update**

I tried this now:

test.html

    &lt;html&gt;
    	&lt;head&gt;
    		&lt;script type=&quot;text/javascript&quot;&gt;
    			var s = document.createElement(&quot;script&quot;);
    			s.setAttribute(&quot;src&quot;,&quot;script.js&quot;);
    			document.head.appendChild(s);
    			console.log(&quot;done&quot;);
    		&lt;/script&gt;
    	&lt;/head&gt;
    	&lt;body&gt;
    	&lt;/body&gt;
    &lt;/html&gt;

script.js

    console.log(&quot;Hi&quot;);

Output:
done
Hi

So it was not executed synchronously. Any idea to make &quot;Hi&quot; appear first?


**Update 2**
Other example

test.html (code inside a script tag)

    var s = document.createElement(&quot;script&quot;);
    s.setAttribute(&quot;src&quot;,&quot;script.js&quot;);
    document.head.appendChild(s);
    SayHi();

script.js

    function SayHi(){
    	console.log(&quot;hi&quot;);
    }

Output: Uncaught ReferenceError: SayHi is not defined

Load and execute javascript code SYNCHRONOUSLY

I am trying to load parsed data in cells, but the problem is that it is happening synchronously and UitableView doesn&#39;t show until the data has finished loading. I tried to solve the problem by using performSelectorInBackground, but now data isn&#39;t loaded in the cells until I start scrolling. Here is my code:

    - (void)viewDidLoad
    {
        [super viewDidLoad];
    	// Do any additional setup after loading the view, typically from a nib.
        
        [self performSelectorInBackground:@selector(fethchData) withObject:nil];
            
    
    }
    
    
    - (void)viewDidUnload
    {
        [super viewDidUnload];
        // Release any retained subviews of the main view.
    self.listData = nil;
        self.plot=nil;
    }
    
    
    -(void) fethchData
    
    {
        NSError *error = nil;
        NSURL *url=[[NSURL alloc] initWithString:@&quot;http://www.website.com/&quot;];
        NSString *strin=[[NSString alloc] initWithContentsOfURL:url encoding:NSUTF8StringEncoding error:nil];
        
        HTMLParser *parser = [[HTMLParser alloc] initWithString:strin error:&amp;error];
        
        if (error) {
            NSLog(@&quot;Error: %@&quot;, error);
            return;
        }
        
        listData =[[NSMutableArray alloc] init];
        plot=[[NSMutableArray alloc] init];
        
        HTMLNode *bodyNode = [parser body];
        NSArray *contentNodes = [bodyNode findChildTags:@&quot;p&quot;];
        
        
        for (HTMLNode *inputNode in contentNodes) {
                [plot addObject:[[inputNode allContents] stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]]];    
            }
          
        
        NSArray *divNodes = [bodyNode findChildTags:@&quot;h2&quot;];
        
        for (HTMLNode *inputNode in divNodes) {
           
                [listData addObject:[[inputNode allContents] stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]]];          
                
            }
        }
        
    
    - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath 
    {
        
        static NSString *CellIdentifier = @&quot;Cell&quot;;
        //here you check for PreCreated cell.
        UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
        if (cell == nil) {
            cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:CellIdentifier];
            cell.accessoryType = UITableViewCellAccessoryDisclosureIndicator;
           
        }
        
        //Fill the cells...  
        
       
        cell.textLabel.text = [listData objectAtIndex:indexPath.row];
        cell.textLabel.font = [UIFont boldSystemFontOfSize:14];
        cell.textLabel.numberOfLines=6; 
        cell.textLabel.textColor=[UIColor colorWithHue:0.7 saturation:1 brightness:0.4 alpha:1];
     
        
        cell.detailTextLabel.text=[plot objectAtIndex:indexPath.row];
        cell.detailTextLabel.font=[UIFont systemFontOfSize:11];
        cell.detailTextLabel.numberOfLines=6;
       
        return cell;
        
    
    }



Data doesn&#39;t load in UITableView until I scroll

Besides the asynchronous/synchronous nature of a particular problem and taking into account that MOMs (in this case having chosen JMS) offer additional features for free like load balancing and others, what else can one consider when choosing JMS rather than REST or vice-versa?

Thanks

When to use JMS and when to use REST?

I&#39;ve been reading [nodebeginner](http://www.nodebeginner.org/)
And I came across the following two pieces of code.

The first one:

        var result = database.query(&quot;SELECT * FROM hugetable&quot;);
        console.log(&quot;Hello World&quot;);

The second one:

        database.query(&quot;SELECT * FROM hugetable&quot;, function(rows) {
           var result = rows;
        });
        console.log(&quot;Hello World&quot;);

I get what they&#39;re supposed to do, they query the database to retrieve the answer to the query. And then `console.log(&#39;Hello world&#39;)`.

The first one is supposedly synchronous code. 
And the second one is asynchronous code.

The difference between the two pieces is very vague to me. What would the output be?

Googling on asynchronous programming didn&#39;t help me either. 

What is the difference between synchronous and asynchronous programming (in node.js)

I&#39;ve done some jQuery in the past, but I am completely stuck on this. I know about the pros and cons of using synchronous ajax  calls, but here it will be required. 

The remote page is loaded (controlled with firebug), but no return is shown.

**What should I do different to make my function to return properly?**

    function getRemote() {
    	
    	var remote;
    	
    	$.ajax({
    		type: &quot;GET&quot;,
    		url: remote_url,
    		async: false,
            success : function(data) {
          		remote = data;
            }
        });
    	
    	return remote;
    
    }

jQuery: Performing synchronous AJAX requests

Suppose you maintain a library that exposes a function &lt;code&gt;getData&lt;/code&gt;. Your users call it to get actual data:   
&lt;code&gt;var output = getData();&lt;/code&gt;  
Under the hood data is saved in a file so you implemented &lt;code&gt;getData&lt;/code&gt; using Node.js built-in &lt;code&gt;fs.readFileSync&lt;/code&gt;. It&#39;s obvious both &lt;code&gt;getData&lt;/code&gt; and &lt;code&gt;fs.readFileSync&lt;/code&gt; are sync functions. One day you were told to switch the underlying data source to a repo such as MongoDB which can only be accessed asynchronously. You were also told to avoid pissing off your users, &lt;code&gt;getData&lt;/code&gt; API cannot be changed to return merely a promise or demand a callback parameter. How do you meet both requirements?

Asynchronous function using callback/promise is the DNA of JavasSript and Node.js. Any non-trivial JS app is probably permeated with this coding style. But this practice can easily lead to so called callback pyramid of doom. Even worse, if any code in any caller in the call chain depends on the result of the async function, those code has to be wrapped in callback function as well, imposing a coding style constraint on caller. From time to time I find the need to encapsulate an async function (often provided in a 3rd party library) into a sync function in order to avoid massive global re-factoring. Searching for a solution on this subject usually ended up with [Node Fibers](https://github.com/laverdet/node-fibers) or npm packages derived from it. But Fibers just cannot solve the problem I am facing. Even the example provided by Fibers&#39; author illustrated the deficiency:

    ...
    Fiber(function() {
        console.log(&#39;wait... &#39; + new Date);
        sleep(1000);
        console.log(&#39;ok... &#39; + new Date);
    }).run();
    console.log(&#39;back in main&#39;);
Actual output:

    wait... Fri Jan 21 2011 22:42:04 GMT+0900 (JST)
    back in main
    ok... Fri Jan 21 2011 22:42:05 GMT+0900 (JST)

If function Fiber really turns async function sleep into sync, the output should be:

    wait... Fri Jan 21 2011 22:42:04 GMT+0900 (JST)
    ok... Fri Jan 21 2011 22:42:05 GMT+0900 (JST)
    back in main
I have created another simple example in [JSFiddle](http://jsfiddle.net/4YN35/) and looking for code to yield expected output. I&#39;ll accept a solution that only works in Node.js so you are free to require any npm package despite not working in JSFiddle.

How to wrap async function calls into a sync function in Node.js or Javascript?

I want to drop some mongodb collections, but that&#39;s an asynchronous task. The code will be:

    var mongoose = require(&#39;mongoose&#39;);

    mongoose.connect(&#39;mongo://localhost/xxx&#39;);

    var conn = mongoose.connection;

    [&#39;aaa&#39;,&#39;bbb&#39;,&#39;ccc&#39;].forEach(function(name){
    	conn.collection(name).drop(function(err) {
            console.log(&#39;dropped&#39;);
        });
    });
    console.log(&#39;all dropped&#39;);

The console displays:

    all dropped
    dropped
    dropped
    dropped

What is the simplest way to make sure `all dropped` will be printed after all collections has been dropped? Any 3rd-party can be used to simplify the code.





Simplest way to wait some asynchronous tasks complete, in Javascript?

What is the difference between asynchronous and non-blocking calls? Also between blocking and synchronous calls (with examples please)?

asynchronous and non-blocking calls? also between blocking and synchronous

Is it possible to call in a `.js` file synchronously and then use it immediately afterward?

    &lt;script type=&quot;text/javascript&quot;&gt;
        var head = document.getElementsByTagName(&#39;head&#39;).item(0);
        var script = document.createElement(&#39;script&#39;);
        script.setAttribute(&#39;type&#39;, &#39;text/javascript&#39;);
        script.setAttribute(&#39;src&#39;, &#39;http://mysite/my.js&#39;);
        head.appendChild(script);

        myFunction(); // Fails because it hasn&#39;t loaded from my.js yet.

        window.onload = function() {
            // Works most of the time but not all of the time.
            // Especially if my.js injects another script that contains myFunction().
            myFunction();
        };
    &lt;/script&gt;

This is simplified. In my implementation the createElement stuff is in a function. I thought about adding something to the function that could check to see if a certain variable was instantiated before returning control. But then there is still the problem of what to do when including js from another site that I have no control over.

Thoughts?

Edit:

I&#39;ve accepted the best answer for now because it gives a good explanation for what&#39;s going on. But if anyone has any suggestions for how to improve this I&#39;m open to them. Here&#39;s an example of what I&#39;d like to do.

    // Include() is a custom function to import js.
    Include(&#39;my1.js&#39;);
    Include(&#39;my2.js&#39;);

    myFunc1(&#39;blarg&#39;);
    myFunc2(&#39;bleet&#39;);

I just want to keep from having to know the internals too much and just be able to say, &quot;I wish to use this module, and now I will use some code from it.&quot;

document.createElement(&quot;script&quot;) synchronously

I&#39;ve only found rather complicated answers involving classes, event handlers and callbacks (which seem to me to be a somewhat sledgehammer approach). I think callbacks may be useful but I cant seem to apply these in the simplest context. See this example:

    &lt;html&gt;
      &lt;head&gt;
        &lt;script type=&quot;text/javascript&quot;&gt;
          function myfunction()  {
            longfunctionfirst();
            shortfunctionsecond();
          }
      
          function longfunctionfirst() {
            setTimeout(&#39;alert(&quot;first function finished&quot;);&#39;,3000);
          }

          function shortfunctionsecond() {
            setTimeout(&#39;alert(&quot;second function finished&quot;);&#39;,200);
          }
        &lt;/script&gt;
      &lt;/head&gt;
      &lt;body&gt;
        &lt;a href=&quot;#&quot; onclick=&quot;javascript:myfunction();return false;&quot;&gt;Call my function&lt;/a&gt;
      &lt;/body&gt;
    &lt;/html&gt;

In this, the second function completes before the first function; what is the simplest way (or is there one?) to force the second function to delay execution until the first function is complete?

---Edit---

So that was a rubbish example but thanks to David Hedlund I see with this new example that it is indeed synchronous (along with crashing my browser in the test process!):

    &lt;html&gt;
    &lt;head&gt;
    
    &lt;script type=&quot;text/javascript&quot;&gt;
    function myfunction() {
        longfunctionfirst();
        shortfunctionsecond();
    }

    function longfunctionfirst() {
        var j = 10000;
        for (var i=0; i&lt;j; i++) {
            document.body.innerHTML += i;
        }
        alert(&quot;first function finished&quot;);
    }

    function shortfunctionsecond() {
        var j = 10;
        for (var i=0; i&lt;j; i++) {
            document.body.innerHTML += i;
        }
        alert(&quot;second function finished&quot;);
    }
    &lt;/script&gt;
    
    &lt;/head&gt;
    
    &lt;body&gt;
      &lt;a href=&quot;#&quot; onclick=&quot;javascript:myfunction();return false;&quot;&gt;Call my function&lt;/a&gt;
    &lt;/body&gt;
    &lt;/html&gt;

As my ACTUAL issue was with jQuery and IE I will have to post a separate question about that if I can&#39;t get anywhere myself!

How to force Sequential Javascript Execution?

Is it possible to synchronously read from stdin in node.js? Because I&#39;m writing a brainfuck to JavaScript compiler in JavaScript (just for fun). Brainfuck supports a read operation which needs to be implemented synchronously.

I tried this:

    const fs = require(&#39;fs&#39;);
    var c = fs.readSync(0,1,null,&#39;utf-8&#39;);
    console.log(&#39;character: &#39;+c+&#39; (&#39;+c.charCodeAt(0)+&#39;)&#39;);

But this only produces this output:

    fs:189
      var r = binding.read(fd, buffer, offset, length, position);
                  ^
    Error: EAGAIN, Resource temporarily unavailable
        at Object.readSync (fs:189:19)
        at Object.&lt;anonymous&gt; (/home/.../stdin.js:3:12)
        at Module._compile (module:426:23)
        at Module._loadScriptSync (module:436:8)
        at Module.loadSync (module:306:10)
        at Object.runMain (module:490:22)
        at node.js:254:10





node.js: readSync from stdin?

I&#39;m writing a program in Node.js that (in some situations) wants to act as a simple filter: read everything from stdin (up to end of file), do some processing, write the result to stdout.

How do you do the &#39;read everything from stdin&#39; part? The closest solutions I&#39;ve found so far, seem to work either for one line at a time from the console, or else only work when stdin is a file not a pipe.

Read all text from stdin to a string

the question is fairly simple and technical:

    var it_works = false;
    
    $.post(&quot;some_file.php&quot;, &#39;&#39;, function(data) {
    										
         it_works = true;
    
    });

    alert(it_works); # false (yes, that &#39;alert&#39; has to be here and not inside $.post itself)

What I want to achieve is:

    alert(it_works); # true

Is there a way to do that? If not can `$.post()` return a value to be applied to `it_works`?

JavaScript: Global variables after Ajax requests

Nobody has actually asked this (from all the &#39;suggestions&#39; I&#39;m getting and also from searching before I asked here).

So why is node.js asynchronous?

From what I have deduced after some research:

Languages like PHP and Python are scripting languages (I could be wrong about the actual languages that are scripting languages) whilst JavaScript isn&#39;t. (I suppose this derives from the fact that JS doesn&#39;t compile?)

Node.js runs on a single thread whilst scripting languages use multiple threads.

Asynchronous means stateless and that the connection is persistent whilst synchronous is the (almost) opposite.

Maybe the answer is found somewhere stated above, but I&#39;m still not sure.

My second and last question related to this topic is this:

Could JavaScript be made into a synchronous language?

PS. I know some of you will ask &quot;why would you want to make JS synchronous?&quot; in your answers, but the truth is that I don&#39;t. I&#39;m just asking these types of questions because I&#39;m sure there are more people out there than just myself that have thought about such questions.

All Synchronous Solutions on Gang of Coders

Total of 17 Synchronous Solutions