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I am doing an IO wait operation inside a for loop now the thing is when all of the operations terminates I want to send the response to the server. Now I was just wondering that suppose two IO operation terminates exactly at the same time now can they execute code at the same time(parallel) or will they execute serially?
As far as I know, as Node is Concurrent but not the Parallel language so I don't think they will execute at the same time.
node.js runs Javascript with a single thread. That means that two pieces of Javascript can never be running at the exact same moment.
node.js processes I/O completion using an event queue. That means when an I/O operation completes, it places an event in the event queue and when that event gets to the front of the event queue and the JS interpreter has finished whatever else it was doing, then it will pull that event from the event queue and call the callback associated with it.
Because of this process, even if two I/O operations finish at basically the same moment, one of them will put its completion event into the event queue before the other (access to the event queue internally is likely controlled by a mutex so one will get the mutex before the other) and that one's completion callback will get into the event queue first and then called before the other. The two completion callbacks will not run at the exact same time.
Keep in mind that more than one piece of Javascript can be "in flight" or "in process" at the same time if it contains non-blocking I/O operations or other asynchronous operations. This is because when you "wait" for an asynchronous operation to complete in Javscript, you return control back to the system and you then resume processing only when your completion callback is called. While the JS interpreter is waiting for an asynchronous I/O operation to complete and the associated callback to be called, then other Javascript can run. But, there's still only one piece of Javascript actually ever running at a time.
As far as I know, as Node is Concurrent but not the Parallel language so I don't think they will execute at the same time.
Yes, that's correct. That's not exactly how I'd describe it since "concurrent" and "parallel" don't have strict technical definitions, but based on what I think you mean by them, that is correct.
you can use Promise.all :
let promises = [];
for(...)
{
promises.push(somePromise); // somePromise represents your IO operation
}
Promise.all(promises).then((results) => { // here you send the response }
You don't have to worry about the execution order.
Node.js is designed to be single thread. So basically there is no way that 'two IO operation terminates exactly at the same time' could happen. They will just finish one by one.
Assume makeBurger() will take 10 seconds
In synchronous program,
function serveBurger() {
makeBurger();
makeBurger();
console.log("READY") // Assume takes 5 seconds to log.
}
This will take a total of 25 seconds to execute.
So for NodeJs lets say we make an async version of makeBurgerAsync() which also takes 10 seconds.
function serveBurger() {
makeBurgerAsync(function(count) {
});
makeBurgerAsync(function(count) {
});
console.log("READY") // Assume takes 5 seconds to log.
}
Since it is a single thread. I have troubling imagine what is really going on behind the scene.
So for sure when the function run, both async functions will enter event loops and console.log("READY") will get executed straight away.
But while console.log("READY") is executing, no work is really done for both async function right? Since single thread is hogging console.log for 5 seconds.
After console.log is done. CPU will have time to switch between both async so that it can run a bit of each function each time.
So according to this, the function doesn't necessarily result in faster execution, async is probably slower due to switching between event loop? I imagine that, at the end of the day, everything will be spread on a single thread which will be the same thing as synchronous version?
I am probably missing some very big concept so please let me know. Thanks.
EDIT
It makes sense if the asynchronous operations are like query DB etc. Basically nodejs will just say "Hey DB handle this for me while I'll do something else". However, the case I am not understanding is the self-defined callback function within nodejs itself.
EDIT2
function makeBurger() {
var count = 0;
count++; // 1 time
...
count++; // 999999 times
return count;
}
function makeBurgerAsync(callback) {
var count = 0;
count++; // 1 time
...
count++; // 999999 times
callback(count);
}
In node.js, all asynchronous operations accomplish their tasks outside of the node.js Javascript single thread. They either use a native code thread (such as disk I/O in node.js) or they don't use a thread at all (such as event driven networking or timers).
You can't take a synchronous operation written entirely in node.js Javascript and magically make it asynchronous. An asynchronous operation is asynchronous because it calls some function that is implemented in native code and written in a way to actually be asynchronous. So, to make something asynchronous, it has to be specifically written to use lower level operations that are themselves asynchronous with an asynchronous native code implementation.
These out-of-band operations, then communicate with the main node.js Javascript thread via the event queue. When one of these asynchronous operations completes, it adds an event to the Javascript event queue and then when the single node.js thread finishes what it is currently doing, it grabs the next event from the event queue and calls the callback associated with that event.
Thus, you can have multiple asynchronous operations running in parallel. And running 3 operations in parallel will usually have a shorter end-to-end running time than running those same 3 operations in sequence.
Let's examine a real-world async situation rather than your pseudo-code:
function doSomething() {
fs.readFile(fname, function(err, data) {
console.log("file read");
});
setTimeout(function() {
console.log("timer fired");
}, 100);
http.get(someUrl, function(err, response, body) {
console.log("http get finished");
});
console.log("READY");
}
doSomething();
console.log("AFTER");
Here's what happens step-by-step:
fs.readFile() is initiated. Since node.js implements file I/O using a thread pool, this operation is passed off to a thread in node.js and it will run there in a separate thread.
Without waiting for fs.readFile() to finish, setTimeout() is called. This uses a timer sub-system in libuv (the cross platform library that node.js is built on). This is also non-blocking so the timer is registered and then execution continues.
http.get() is called. This will send the desired http request and then immediately return to further execution.
console.log("READY") will run.
The three asynchronous operations will complete in an indeterminate order (whichever one completes it's operation first will be done first). For purposes of this discussion, let's say the setTimeout() finishes first. When it finishes, some internals in node.js will insert an event in the event queue with the timer event and the registered callback. When the node.js main JS thread is done executing any other JS, it will grab the next event from the event queue and call the callback associated with it.
For purposes of this description, let's say that while that timer callback is executing, the fs.readFile() operation finishes. Using it's own thread, it will insert an event in the node.js event queue.
Now the setTimeout() callback finishes. At that point, the JS interpreter checks to see if there are any other events in the event queue. The fs.readfile() event is in the queue so it grabs that and calls the callback associated with that. That callback executes and finishes.
Some time later, the http.get() operation finishes. Internal to node.js, an event is added to the event queue. Since there is nothing else in the event queue and the JS interpreter is not currently executing, that event can immediately be serviced and the callback for the http.get() can get called.
Per the above sequence of events, you would see this in the console:
READY
AFTER
timer fired
file read
http get finished
Keep in mind that the order of the last three lines here is indeterminate (it's just based on unpredictable execution speed) so that precise order here is just an example. If you needed those to be executed in a specific order or needed to know when all three were done, then you would have to add additional code in order to track that.
Since it appears you are trying to make code run faster by making something asynchronous that isn't currently asynchronous, let me repeat. You can't take a synchronous operation written entirely in Javascript and "make it asynchronous". You'd have to rewrite it from scratch to use fundamentally different asynchronous lower level operations or you'd have to pass it off to some other process to execute and then get notified when it was done (using worker processes or external processes or native code plugins or something like that).
Is the Node.js I/O event loop single- or multithreaded?
If I have several I/O processes, node puts them in an external event loop. Are they processed in a sequence (fastest first) or handles the event loop to process them concurrently (...and in which limitations)?
Event Loop
The Node.js event loop runs under a single thread, this means the application code you write is evaluated on a single thread. Nodejs itself uses many threads underneath through libuv, but you never have to deal with with those when writing nodejs code.
Every call that involves I/O call requires you to register a callback. This call also returns immediately, this allows you to do multiple IO operations in parallel without using threads in your application code. As soon as an I/O operation is completed it's callback will be pushed on the event loop. It will be executed as soon as all the other callbacks that where pushed on the event loop before it are executed.
There are a few methods to do basic manipulation of how callbacks are added to the event loop.
Usually you shouldn't need these, but every now and then they can be useful.
setImmediate
process.nextTick
At no point will there ever be two true parallel paths of execution, so all operations are inherently thread safe. There usually will be several asynchronous concurrent paths of execution that are being managed by the event loop.
Read More about the event loop
Limitations
Because of the event loop, node doesn't have to start a new thread for every incoming tcp connection. This allows node to service hundreds of thousands of requests concurrently , as long as you aren't calculating the first 1000 prime numbers for each request.
This also means it's important to not do CPU intensive operations, as these will keep a lock on the event loop and prevent other asynchronous paths of execution from continuing.
It's also important to not use the sync variant of all the I/O methods, as these will keep a lock on the event loop as well.
If you want to do CPU heavy things you should ether delegate it to a different process that can execute the CPU bound operation more efficiently or you could write it as a node native add on.
Read more about use cases
Control Flow
In order to manage writing many callbacks you will probably want to use a control flow library.
I believe this is currently the most popular callback based library:
https://github.com/caolan/async
I've used callbacks and they pretty much drove me crazy, I've had much better experience using Promises, bluebird is a very popular and fast promise library:
https://github.com/petkaantonov/bluebird
I've found this to be a pretty sensitive topic in the node community (callbacks vs promises), so by all means, use what you feel will work best for you personally. A good control flow library should also give you async stack traces, this is really important for debugging.
The Node.js process will finish when the last callback in the event loop finishes it's path of execution and doesn't register any other callbacks.
This is not a complete explanation, I advice you to check out the following thread, it's pretty up to date:
How do I get started with Node.js
From Willem's answer:
The Node.js event loop runs under a single thread. Every I/O call requires you to register a callback. Every I/O call also returns immediately, this allows you to do multiple IO operations in parallel without using threads.
I would like to start explaining with this above quote, which is one of the common misunderstandings of node js framework that I am seeing everywhere.
Node.js does not magically handle all those asynchronous calls with just one thread and still keep that thread unblocked. It internally uses google's V8 engine and a library called libuv(written in c++) that enables it to delegate some potential asynchronous work to other worker threads (kind of like a pool of threads waiting there for any work to be delegated from the master node thread). Then later when those threads finish their execution they call their callbacks and that is how the event loop is aware of the fact that the execution of a worker thread is completed.
The main point and advantage of nodejs is that you will never need to care about those internal threads and they will stay away from your code!. All the nasty sync stuff that should normally happen in multi threaded environments will be abstracted out by nodejs framework and you can happily work on your single thread (main node thread) in a more programmer friendly environment (while benefiting from all the performance enhancements of multiple threads).
Below is a good post if anyone is interested:
When is the thread pool used?
you have to know first about nodeJs implementaion in order to know event loop.
actually node js core implementation using two components :
v8 javascript runtime engine
libuv for handlign non i/o blocking operation and handling threads and concurrent operations for you;
with the javascript you can actually write code with one thread but this means not that your code execute on the one thread although you can execute on multiple thread s using clusters in node js
now when you want to execute some code like :
let fs = require('fs');
fs.stat('path',(err,stat)=>{
//do something with the stat;
console.log('second');
});
console.log('first');
the execution of this code at high level is like this:
first the v8 engine run this code and then if there is no error
everything is good then it looks for the
it try to run it run line by line when it gets to the fs .stats this is a node js api very similar to the web apis like setTimeout that the browser handle it for us when it encounter to the fs.stats it is pass the code to the libuv components with a flag and pass your callback to the event queue then the libuv you execute your code during the operation and when its done just send some signal and then d the v8 execute your code az a callback you set on the queue but it always check for the stack is empty then go for the your code on the queue # always remember that !
Well, to understand nodejs I/O events in the event, you must understand nodejs event loop properly.
from the name event loop, we understand it's a loop that runs cycle after cycle round-robin basis until there are no events remains in the loop or the app closed.
The event loop is one of the topmost features in nodejs, it is what makes async programming in nodejs.
When the program starts we are in a node process in the single thread where the event loop runs. Now the most importing things we need to know that the event loop is where all the application code that is inside callback functions is executed.
So, basically all code that is not top-level code will run in the event loop. Some part (mostly heavy duties) might get offloaded to the thread pool
(When is the thread pool used?), the event loop will take care of those heavy duties and return the result to the event of the event loop.
It is the heart of the node architecture, and nodejs built around callback functions. so callbacks will triggered as soon as some work is finished sometime in the future because node uses an event-triggered architecture.
When an application receives an HTTP request on a node server or a timer expiring or a file finishing to read all these will emit events as soon as they are done with their work, and our event loop will then pick up these events and call the callback functions that are associated with each event, it's usually said that the event loop does the orchestration, which simply means that it receives events, calls their callback functions, and offloads the more expensive tasks to the thread pool.
Now, how does all this actually work behind the scenes? In what order are these callbacks executed?
Well, when we start our node application, the event loop starts running right away. An event loop has multiple phases, and each phase has a callback queue, where the four most important phases are 1. Expired timer callbacks, 2.I/O polling and callbacks 3. setImmediate callbacks, and 4. Close callbacks. There are other phases that is used internally by Node.
So, the first phase takes care of callbacks of expired timers, for example, from the setTimeout() function. So, if there are callback functions from timers that just expired, these are the first ones to be processed by the event loop.
** The most important thing is, If a timer expires later during the time when one of the other phases is being processed, well then the callback of that timer will only be called as soon as the event loop comes back to this first phase. And it works like this in all four phases.**
So callbacks in each queue are processed one by one until there are no ones left in the queue and only then, the event loop will enter the next phase. for example, suppose there is 1000 setTimeOut callbacks timer expired and the event loop is in the first phase then all these 1000 setTimeOuts callbacks will execute one by one then it will go to the next phase(I/O pooling and callbacks).
Next up, we have I/O pooling and execution of I/O callbacks. Here I/O stands for input/output and polling basically means looking for new I/O events that are ready to be processed and putting theme into the callback queue.
In the context of a Node application, I/O means mainly stuff like networking and file access, so in this phase where probably 99% of general application code gets executed.
The next phase is for setImmediate callbacks, and SetImmediate is a special kind of timer that we can use if we want to process callbacks immediately after the I/O polling and execution phase.
And finally, the fourth phase is the close callbacks, in this phase, all close events are processed, for example when a server or a WebSocket shut down.
These are the four phases in the event loop, but besides these four callbacks queues there are actually also two other queues,
1. nextTick() other
2. microtasks queue(which is mainly for resolved promises)
If there are any callbacks in one of these two queues to be processed, they will be executed right after the current phase of the event loop finishes instead of waiting for the entire loop/cycle to finish.
In other words, after each of these four phases, if there are any callbacks in these two special queues, they will be executed right away. Now imagine that a promise resolves and returns some data from an API call while the callback of an expired timer is running, In this case, the promise callback will be executed right after the one from the timer finish.
The same logic also applies to the nextTick() queue. The nextTick() is a function that we can use when we really, really need to execute a certain callback right after the current event loop phase. It's a bit similar to setImmediate, with the difference that setImmediate only runs after the I/O callback phase.
Will all the above things can happen in one tick/cycle of the event loop, In the meantime their new events could have arisen in a particular phase or old event could be expired, the event loop will handle those events with another new cycle.
So now it's time to decide whether the loop should continue to the next tick or if the program should exit. Node simply checks whether there are any timers or I/O tasks that are still running in the background if there aren't any then it will exit the application. But if there are any pending timers or I/O tasks, then the node will continue running the event loop and go starting to the next cycle.
For example, in node application when we are listening for incoming HTTP requests, we basically running an infinite I/O task, and that is run in the event loop, for that Node.js keep running and keep listening for new HTTP request coming in instead of just exiting the application.
Also when we are writing or reading a file in the background that's also an I/O task and it makes sense that the app doesn't exist while it's working with that file, right?
Now The event loop in practices:
const fs = require('fs');
setTimeout(()=>console.log('Timer 1 finished'), 0);
fs.readFile('test-file.txt', ()=>{
console.log('I/O finished');
});
setImmediate(()=>console.log('Immediate 1 finished'))
console.log('Hello from the top level code');
Output:
Well the first lin is Hello from the top level code, yes it is expected because this is a code that gets executed immediately. Then after we have three output, Timer 1 finished this line is expected because of phase one as we discuess before, but after that I/O finished should be printed, because we discuess that setImmediate runs after the I/O callback phase, but this code is actually not in an I/O cycle, so it is not running inside of the event loop, because it's not runnin inside of any callback function.
Now lets do another test:
const fs = require('fs');
setTimeout(()=>console.log('Timer 1 finished'), 0);
setImmediate(()=>console.log('Immediate 1 finished'));
fs.readFile('test-file.txt', ()=>{
console.log('I/O finished');
setTimeout(()=>console.log('Timer 2 finished'), 0);
setImmediate(()=>console.log('Immediate 2 finished'));
setTimeout(()=>console.log('Timer 3 finished'), 0);
setImmediate(()=>console.log('Immediate 3 finished'));
});
console.log('Hello from the top level code')
Output:
The output is as expected right? Now let's add some delay:
setTimeout(()=>console.log('Timer 1 finished'), 0);
setImmediate(()=>console.log('Immediate 1 finished'));
fs.readFile('test-file.txt', ()=>{
console.log('I/O finished');
setTimeout(()=>console.log('Timer 2 finished'), 3000);
setImmediate(()=>console.log('Immediate 2 finished'));
setTimeout(()=>console.log('Timer 3 finished'), 0);
setImmediate(()=>console.log('Immediate 3 finished'));
});
console.log('Hello from the top level code')
output:
In the first cycle inside I/O everything executed, but because of the dealy Timer-2 executed inside its code in the second cycle.
Now Lets add nextTick(), and see how nodejs behaves:
setTimeout(()=>console.log('Timer 1 finished'), 0);
setImmediate(()=>console.log('Immediate 1 finished'));
fs.readFile('test-file.txt', ()=>{
console.log('I/O finished');
setTimeout(()=>console.log('Timer 2 finished'), 3000);
setImmediate(()=>console.log('Immediate 2 finished'));
setTimeout(()=>console.log('Timer 3 finished'), 0);
setImmediate(()=>console.log('Immediate 3 finished'));
process.nextTick(()=>console.log('Process Next Tick'));
});
console.log('Hello from the top level code')
Output:
Well, the first callback is executed is inside the process.NextTick(), as it is expected right? Because nextTicks callbacks stays in the microtask queue an they executed after each phase.
If you run this simple node code
console.log('starting')
setTimeout(()=>{
console.log('0sec')
}, 0)
setTimeout(()=>{
console.log('2sec')
}, 2000)
console.log('end')
What do you expect output to be?
If its,
starting
0sec
end
2sec
it's is wrong guess, we will get
starting
end
0sec
2sec
because node will never print code in event loop before exiting main()
So basically, First main() will go in stack, then console.log('starting ') so you will see it printed first, after that come setTimeout(()=>{console.log('0sec')}, 0) will go in a stack and then in nodeAPI (node uses multi-threads (lib written in c++) to execute setTimeout to finish, even tho above code is single thread code) after time is up it moves to the event loop, now node can't print it unless stack is not empty. So, next line i.e setTimeout of 2sec will be first pushed to stack,then nodeAPI which will wait for 2 sec to finish, and then to even loop, in mean while next code line will be executed that is console.log('end') and so we see end msg before 0sec, because if nodes non blocking nature. After end code is over and hence main is poped out and its turn of event loop code to be executed that is first 0sec and after that 2sec msg will be printed.
I have a very basic question in node.js programming .
I am finding it interesting as I have started understanding it deeper.
I came across the following code :
Abc.do1(‘operation’,2, function(err,res1)
{
If(err) {
Callback(err);
}
def.do2(‘operation2’,function(l) {
}
}
My question is :
since def.do2 is written in the callback of abc.do1 ,
is it true that def.do2 will be executed after the ‘operation’ of abc.do1 is completed and the callback function gets called . If yes, is this a good programming practice , because we speak only asynchronous and non blocking code in node.js
Yes, you're correct. def.do2() is executed after abc.do1() is done. However, this done on purpose to make sure that do1() is done before do2() can start. If it's meant to be done in parallel, then do2() would have been outside of do1()'s callback. This code is not exactly blocking. after do1() is started, the code is still continuing on to execute everything below do1() functon (outside of do1's callback), just not do2() until do1() is done which is meant to be.
Yes you are absolutely correct and you have given a correct example of callback function.
The main browser process is a single threaded event loop. If you execute a long-running operation within a single-threaded event loop, the process "blocks". This is bad because the process stops processing other events while waiting for your operation to complete. 'alert' is one of the few blocking browser methods: if you call alert('test'), you can no longer click links, perform ajax queries, or interact with the browser UI.
In order to prevent blocking on long-running operations, the XMLHttpRequest provides an asynchronous interface. You pass it a callback to run after the operation is complete, and while it is processing it cedes control back to the main event loop instead of blocking.
There's no reason to use a callback unless you want to bind something to an event handler, or your operation is potentially blocking and therefore requires an asynchronous programming interface.
See this video
http://www.yuiblog.com/blog/2010/08/30/yui-theater-douglas-crockford-crockford-on-javascript-scene-6-loopage-52-min/
Does asynchronous call always create a new thread?
Example:
If JavaScript is single threaded then how can it do an async postback? Is it actually blocking until it gets a callback? If so, is this really an async call?
This is an interesting question.
Asynchronous programming is a paradigm of programming that is principally single threaded, i.e. "following one thread of continuous execution".
You refer to javascript, so lets discuss that language, in the environment of a web browser. A web browser runs a single thread of javascript execution in each window, it handles events (such as onclick="someFunction()") and network connections (such as xmlhttprequest calls).
<script>
function performRequest() {
xmlhttp.open("GET", "someurl", true);
xmlhttp.onreadystatechange = function() {
if (xmlhttp.readyState == 4) {
alert(xmlhttp.responseText);
}
}
xmlhttp.send(sometext);
}
</script>
<span onclick="performRequest()">perform request</span>
(This is a nonworking example, for demonstration of concepts only).
In order to do everything in an asynchronous manner, the controlling thread has what is known as a 'main loop'. A main loop looks kind of like this:
while (true) {
event = nextEvent(all_event_sources);
handler = findEventHandler(event);
handler(event);
}
It is important to note that this is not a 'busy loop'. This is kind of like a sleeping thread, waiting for activity to occur. Activity could be input from the user (Mouse Movement, a Button Click, Typing), or it could be network activity (The response from the server).
So in the example above,
When the user clicks on the span, a ButtonClicked event would be generated, findEventHandler() would find the onclick event on the span tag, and then that handler would be called with the event.
When the xmlhttp request is created, it is added to the all_event_sources list of event sources.
After the performRequest() function returns, the mainloop is waiting at the nextEvent() step waiting for a response. At this point there is nothing 'blocking' further events from being handled.
The data comes back from the remote server, nextEvent() returns the network event, the event handler is found to be the onreadystatechange() method, that method is called, and an alert() dialog fires up.
It is worth noting that alert() is a blocking dialog. While that dialog is up, no further events can be processed. It's an eccentricity of the javascript model of web pages that we have a readily available method that will block further execution within the context of that page.
The Javascript model is single-threaded. An asynchronous call is not a new thread, but rather interrupts an existing thread. It's analogous to interrupts in a kernel.
Yes it makes sense to have asynchronous calls with a single thread. Here's how to think about it: When you call a function within a single thread, the state for the current method is pushed onto a stack (i.e. local variables). The subroutine is invoked and eventually returns, at which time the original state is popped off the stack.
With an asynchronous callback, the same thing happens! The difference is that the subroutine is invoked by the system, not by the current code invoking a subroutine.
A couple notes about JavaScript in particular:
XMLHttpRequests are non-blocking by default. The send() method returns immediately after the request has been relayed to the underlying network stack. A response from the server will schedule an invocation of your callback on the event loop as discussed by the other excellent answers.
This does not require a new thread. The underlying socket API is selectable, similar to java.nio.channels in Java.
It's possible to construct synchronous XMLHttpRequest objects by passing false as the third parameter to open(). This will cause the send() method to block until a response has been received from the server, thus placing the event loop at the mercy of network latency and potentially hanging the browser until network timeout. This is a Bad Thing™.
Firefox 3.5 will introduce honest-to-god multithreaded JavaScript with the Worker class. The background code runs in a completely separate environment and communicates with the browser window by scheduling callbacks on the event loop.
In many GUI applications, an async call (like Java's invokeLater) merely adds the Runnable object to its GUI thread queue. The GUI thread is already created, and it doesn't create a new thread. But threads aren't even strictly required for an asynchronous system. Take, for example, libevent, which uses select/poll/kqueue, etc. to make non-blocking calls to sockets, which then fires callbacks to your code, completely without threads.
No, but more than one thread will be involved.
An asynchronous call might launch another thread to do the work, or it might post a message into a queue on another, already running thread. The caller continues and the callee calls back once it processes the message.
If you wanted to do a synchronous call in this context, you'd need to post a message and actively wait for the callback to happen.
So in summary: More than one thread will be involved, but it doesn't necessarily create a new thread.
I don't know about javascript, but for instance in the Windows Forms world, asynchronous invocations can be made without multiple threads. This has to do with the way the Windows Message Pump operates. Basically a Windows Forms application sets up a message queue through which Windows places messages notifying it about events. For instance, if you move the mouse, messages will be placed on that queue. The Windows Forms application will be in an endless loop consuming all the messages that are thrown at it. According to what each message contains it will move windows around, repaint them or even invoke user-defined methods, amongst other things. Calls to methods are identified by delegates. When the application finds a delegate instance in the queue, it happily invokes the method referred by the delegate.
So, if you are in a method doing something and want to spawn some asynchronous work without creating a new thread, all you have to do is place a delegate instance into the queue, using the Control.BeginInvoke method. Now, this isn't actually multithreaded, but if you throw very small pieces of work to the queue, it will look like multithreaded. If, on the other hand you give it a time consuming method to execute, the application will freeze until the method is done, which will look like a jammed application, even though it is doing something.