Read in a node.js related web document that it is a single threaded server. So it confuses me whether all data structures by default be thread-safe in a node server!
I have multiple call-backs accessing a global object like this :
callback1{
global_var['key'] = val;
}
callback2{
globalv_var['key'] = val;
}
'key' may be same at times and may be different as well. Will the global_var be thread-safe ?
callbacks, as intended gets called back as and when something is done, in no particular order.
Node.JS contains a "dispatcher." It accepts web requests and hands them off for asynchronous processing. That dispatcher is single threaded. But the dispatcher spins up a new thread for each task, and quickly hands off the task to the new thread, freeing the dispatcher's thread for servicing a new request.
To the extent that those task threads are kept separate (i.e. they don't modify each other's state), yes, they are threadsafe.
All of the javascript you write for your node.js applocation executes as if it were running in a single thread.
Any multithreading occurs behind the scenes, in the I/O code and in other native modules. So there's no need to worry about the thread safety of any application code, regardless.
Related
I want to understand internal working of node.js, I am intentionally including computation task ( for loop). But I see it is still blocking main thread.
Here is my script
console.log("start");
for (let i = 0; i < 10; i++) {
console.log(i)
}
console.log("end")
And the o/p is :
start
1
2
3
....
10
end
But according to node.js architecture shouldn't high computation tasks be executed by different thread picked from thread pool and event loop continue executing non-blocking task?
I am referencing node.js internal architecture using this link enter link description here
Can someone please explain the architecture and behavior of the script?
By default, nodejs uses only ONE thread to run your Javascript with. That means that (unless you engage WorkerThreads which are essentially an entirely separate VM), only one piece of Javascript is ever running at once. Nodejs does not "detect" some long running piece of Javascript and move it to another thread. It has no features like that at all. If you have some long running piece of synchronous Javascript, it will block the event loop and block all other Javascript and all other event processing.
Internal to its implementation, nodejs has a thread pool that it uses for certain types of native code (internal implementations of file I/O and crypto operations). That only supports the implementation of asynchronous implementations for file I/O and crypto operations - it does not parallelize the running of Javascript.
So, your script you show:
console.log("start");
for (let i = 0; i < 10; i++) {
console.log(i)
}
console.log("end")
Is entirely synchronous and runs sequentially and blocks all other Javascript from running while it is running because it is using the one thread for running Javascript while it is running.
Nodejs gets its excellent scalability from its asynchronous I/O model that does not have to use a separate thread in order to have lots of asynchronous operations in flight at the same time. But, keep in mind that these asynchronous I/O operations all have native code behind them (some of which may use threads in their native code implementations).
But, if you have long running synchronous Javascript operations (like say something like image analysis written in Javascript), then those typically need to be moved out of the main event loop thread either by shunting them off to WorkerThreads or to other processes or to a native code implementation that may use OS threads.
But according to node.js architecture shouldn't high computation tasks be executed by different thread picked from thread pool and event loop continue executing non-blocking task?
No, that is not how nodejs works and is not a correct interpretation of the diagram you show. The thread pool is NOT used for running your Javascript. It is used for internal implementation of some APIs such as file I/O and some crypto operations. It is not used for running your Javascript. There is just one main thread for running your Javascript (unless you specifically run your code in a WorkerThread).
I want to understand internal working of node.js, I am intentionally including computation task ( for loop). But I see it is still blocking main thread.
Yes, a for loop (that does not contain an await statement that is awaiting a promise) will completely occupy the single Javascript thread and will block the event loop from processing other events while the for loop is running.
JS executes its code Synchronouse. there are few things that gets "Asynchronouse" like setInterval or setTimout for exmple. But thats actually not fully true. Asynchronouse means things get done in parallel witch is not true. Take a look at setTimeout. By executing it you add the function into the task que, later the event loop grabs it from the que and put it onto the stack and executes it, syncrhonouse. If you want to execute something really parallel then you should consider using an worker thread
There are absolutely no threads in JS (unless you explicitly use worker threads). Javascript uses cooperative multi-tasking which means that a function will always complete before the next one will start. The only other way to yield control back to the scheduler is to separate a task out into another function that is called asynchronously. So in your example, e.g., you could do:
console.log("start");
setTimeout(() => {
for (let i = 0; i < 10; i++) {
console.log(i)
}}, 0);
console.log("end")
and you would get:
start
end
1
2
..
9
This also answers your question about heavy computations: unless you use the relatively new worker threads, you cannot run heavy computations in node.js "in the background" without the use of native code.
So if you really have heavy loads you have three options:
worker threads,
native code that is multi-threaded, e.g., written in C/C++, or
breaking your computation down into small pieces, each one yielding control back to the scheduler when done (e.g., using map/reduce).
I would like to create a worker thread in a node.js app and pass the current context to the new thread, so I would be able to access my variables and functions within the new thread, Is there is a library to support that? And if not can I a least pass an anonymous function between them?
There is no way to share a context with a worker thread. This isn't "an ideology of the Node.js team", instead it's a limitation of the JavaScript language, which doesn't allow concurrency (such as concurrent access to objects from a worker thread).
The one exception is that you can share numerical data between multiple threads by using a SharedArrayBuffer.
Aside from that, the way to send data to or receive it from a worker thread is to use postMessage. See also Node's full worker threads documentation.
For completeness: there is an early-stage proposal to add a new kind of cross-thread-shareable object to JavaScript. As with all early-stage proposals, there's no guarantee that it'll be finalized at all or how long that might take, but it does indicate that there's some interest in this space.
If I use CacheStorage.open() to get the same Cache in the UI thread as in a Worker thread, and begin reading/writing from/to it on both sides, is it safe? Or can there be race conditions? I'm hoping the Cache API is thread safe (I would assume that it is, like all else in JS).
is it safe?
Yes. All methods return Promises, as the Cache might be managed by another thread, the Promise then resolves if the other thread performed the operation. There cannot be concurrent modifications or other such weird things.
Or can there be race conditions?
Whenever there are multiple threads there can be race conditions, thats in the nature of things. That means that if you add() a cache entry while trying to retrieve it woth get() from another WebWorker in parallel might or might not get you a result.
I have some tasks I want to do in JS that are resource intensive. For this question, lets assume they are some heavy calculations, rather then system access. Now I want to run tasks A, B and C at the same time, and executing some function D when this is done.
The async library provides a nice scaffolding for this:
async.parallel([A, B, C], D);
If what I am doing is just calculations, then this will still run synchronously (unless the library is putting the tasks on different threads itself, which I expect is not the case). How do I make this be actually parallel? What is the thing done typically by async code to not block the caller (when working with NodeJS)? Is it starting a child process?
2022 notice: this answer predates the introduction of worker threads in Node.js
How do I make this be actually parallel?
First, you won't really be running in parallel while in a single node application. A node application runs on a single thread and only one event at a time is processed by node's event loop. Even when running on a multi-core box you won't get parallelism of processing within a node application.
That said, you can get processing parallelism on multicore machine via forking the code into separate node processes or by spawning child process. This, in effect, allows you to create multiple instances of node itself and to communicate with those processes in different ways (e.g. stdout, process fork IPC mechanism). Additionally, you could choose to separate the functions (by responsibility) into their own node app/server and call it via RPC.
What is the thing done typically by async code to not block the caller (when working with NodeJS)? Is it starting a child process?
It is not starting a new process. Underneath, when async.parallel is used in node.js, it is using process.nextTick(). And nextTick() allows you to avoid blocking the caller by deferring work onto a new stack so you can interleave cpu intensive tasks, etc.
Long story short
Node doesn't make it easy "out of the box" to achieve multiprocessor concurrency. Node instead gives you a non-blocking design and an event loop that leverages a thread without sharing memory. Multiple threads cannot share data/memory, therefore locks aren't needed. Node is lock free. One node process leverages one thread, and this makes node both safe and powerful.
When you need to split work up among multiple processes then use some sort of message passing to communicate with the other processes / servers. e.g. IPC/RPC.
For more see:
Awesome answer from SO on What is Node.js... with tons of goodness.
Understanding process.nextTick()
Asynchronous and parallel are not the same thing. Asynchronous means that you don't have to wait for synchronization. Parallel means that you can be doing multiple things at the same time. Node.js is only asynchronous, but its only ever 1 thread. It can only work on 1 thing at once. If you have a long running computation, you should start another process and then just have your node.js process asynchronously wait for results.
To do this you could use child_process.spawn and then read data from stdin.
http://nodejs.org/api/child_process.html#child_process_child_process_spawn_command_args_options
var spawn = require('child_process').spawn;
var process2 = spawn('sh', ['./computationProgram', 'parameter'] );
process2.stderr.on('data', function (data) {
//handle error input
});
process2.stdout.on('data', function (data) {
//handle data results
});
Keep in mind I/O is parallelized by Node.js; only your JavaScript callbacks are single threaded.
Assuming you are writing a server, an alternative to adding the complexity of spawning processes or forking is to simply build stateless node servers and run an instance per core, or better yet run many instances each in their own virtualized micro server. Coordinate incoming requests using a reverse proxy or load balancer.
You could also offload computation to another server, maybe MongoDB (using MapReduce) or Hadoop.
To be truly hardcore, you could write a Node plugin in C++ and have fine-grained control of parallelizing the computation code. The speed up from C++ might negate the need of parallelization anyway.
You can always write code to perform computationally intensive tasks in another language best suited for numeric computation, and e.g. expose them through a REST API.
Finally, you could perhaps run the code on the GPU using node-cuda or something similar depending on the type of computation (not all can be optimized for GPU).
Yes, you can fork and spawn other processes, but it seems to me one of the major advantages of node is to not much have to worry about parallelization and threading, and therefor bypass a great amount of complexity altogether.
Depending on your use case you can use something like
task.js Simplified interface for getting CPU intensive code to run on all cores (node.js, and web)
A example would be
function blocking (exampleArgument) {
// block thread
}
// turn blocking pure function into a worker task
const blockingAsync = task.wrap(blocking);
// run task on a autoscaling worker pool
blockingAsync('exampleArgumentValue').then(result => {
// do something with result
});
Just recently came across parallel.js but it seems to be actually using multi-core and also has map reduce type features.
http://adambom.github.io/parallel.js/
So I have this seriously recursive function that I would like to use with my code. The issue is it doesn't really take advantage of dual core machines because js is single threaded. I have tried using webworkers but don't really know much about multicore programming. Would someone point me to some material that could explain how it is done. I googled to find this sample link but its not really much help without documentation! =/
I would be glad if someone could show me how this could be done without webworkers though! That would be just awesome! =)
I came across this link on whatwg. This is really weird because it explains how to use multicore programming in webworkers etc, but on executing on my chrome browser it throws errors. Same goes with other browsers.
Error: 9Uncaught ReferenceError: Worker is not defined in worker.js
UPDATE (2018-06-21): For people coming here in search of multi-core programming in JavaScript, not necessarily browser JavaScript (for that, the answer still applies as-is): Node.js now supports multi-threading behind a feature flag (--experimental-workers): release info, relevant issue.
Writing this off the top of my head, no guarantees for source code. Please go easy on me.
As far as I know, you cannot really program in threads with JavaScript. Webworkers are a form of multi-programming; yet JavaScript is by its nature single-threaded (based on an event loop).
A webworker is seperate thread of execution in the sense that it doesn't share anything with the script that started it; there is no reference to the script's global object (typically called "window" in the browser), and no reference to any of your main script's variables other than data you send to the thread.
Think as the web worker as a little "server" that gets asked a question and provides an answer. You can only send strings to that server, and it can only parse the string and send back what it has computed.
// in the main script, one starts a worker by passing the file name of the
// script containing the worker to the constructor.
var w = new Worker("myworker.js");
// you want to react to the "message" event, if your worker wants to inform
// you of a result. The function typically gets the event as an argument.
w.addEventListener("message",
function (evt) {
// process evt.data, which is the message from the
// worker thread
alert("The answer from the worker is " + evt.data);
});
You can then send a message (a String) to this thread using its postMessage()-Method:
w.postMessage("Hello, this is my message!");
A sample worker script (an "echo" server) can be:
// this is another script file, like "myworker.js"
self.addEventListener("message",
function (evt) {
var data = JSON.parse(evt.data);
/* as an echo server, we send this right back */
self.postMessage(JSON.stringify(data))
})
whatever you post to that thread will be decoded, re-encoded, and sent back. of course you can do whatever processing you would want to do in between. That worker will stay active; you can call terminate() on it (in your main script; that'd be w.terminate()) to end it or calling self.close() in your worker.
To summarize: what you can do is you zip up your function parameters into a JSON string which gets sent using postMessage, decoded, and processed "on the other side" (in the worker). The computation result gets sent back to your "main" script.
To explain why this is not easier: More interaction is not really possible, and that limitation is intentional. Because shared resources (an object visible to both the worker and the main script) would be subject to two threads interfering with them at the same time, you would need to manage access (i.e., locking) to that resource in order to prevent race conditions.
The message-passing, shared-nothing approach is not that well-known mainly because most other programming languages (C and Java for example) use threads that operate on the same address space (while others, like Erlang, for instance, don't). Consider this:
It is really hard to code a larger project with mutexes (a mutual exclusion mechanism) because of the associated deadlock/race condition complexities. This is stuff that can make grown men cry!
It is really easy in comparison to do message-passing, shared-nothing semantics. The code is isolated; you know exactly what goes into your worker and what comes out of your worker. Deadlocks and race conditions are impossible to achieve!
Just try it out; it is capable of doing interesting things, probably all you want. Bear in mind that it is still implementation defined whether it takes advantage of multicore as far as I know.
NB. I just got informed that at least some implementations will handle JSON encoding of messages for you.
So, to give an answer to your question (it's all above; tl;dr version): No, you cannot do this without web workers. But there is nothing really wrong about web workers aside from browser support, as is the case with HTML5 in general.
As far as I remember this is only possible with the new HTML5 standard. The keyword is "Web-Worker"
See also:
HTML5: JavaScript Web Workers
JavaScript Threading With HTML5 Web Workers
Web workers are the answer to the client side. For NodeJS there are many approaches. Most popular - spawn several processes with pm2 or similar tool. Run single process and spawn/fork child processes. You can google around these and will find a lot of samples and tactics.
Web workers are already well supported by all browsers. https://caniuse.com/#feat=webworkers
API & samples: https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Using_web_workers