Doesn't code take an efficiency hit by being synchronous? Why is coding synchronously a win?
I found these two links in doing some research: http://bjouhier.wordpress.com/2012/03/11/fibers-and-threads-in-node-js-what-for/, https://github.com/Sage/streamlinejs/
If the goal is to prevent spaghetti code, then clearly you can have asynchronous code, with streamline.js for example, that isn't a callback pyramid, right?
You have to distinguish two things here:
Synchronous functions like node's fs.readFileSync, fs.statSync, etc. All these functions have a Sync in their names (*). These functions are truly synchronous and blocking. If you call them, you block the event loop and you kill node's performance. You should only use these functions in your server's initialization script (or in command-line scripts).
Libraries and tools like fibers or streamline.js. These solutions allow you to write your code in sync-style but the code that you write with them will still execute asynchronously. They do not block the event loop.
(*) require is also blocking.
Meteor uses fibers. Its code is written in sync-style but it is non-blocking.
The win is not on the performance side (these solutions have their own overhead so they may be marginally slower but they can also do better than raw callbacks on specific code patterns like caching). The win, and the reason why these solutions have been developed, is on the usability side: they let you write your code in sync-style, even if you are calling asynchronous functions.
Jan 25 2017 edit: I created 3 gists to illustrate non-blocking fibers:
fibers-does-not-block.js, fibers-sleep-sequential.js, fibers-sleep-parallel.js
The code is not "synchronous" when using something like streamlinejs. The actual code will still run asynchronously. It's not very pretty to write lots of anonymous callback functions, thats where these things helps.
Related
I have a NodeJS/Express web app, where TypeOrm is used for many database functions. To avoid callback hell I usually use async/await to call and wait for database actions from my endpoint methods.
I've heard, however, that methods like fs.readFileSync should always be avoided, since they are blocking and will force all other requests to wait. Does this also apply to async/await? Do I have to use promises and callbacks to get decent multi-user performance?
Sync functions really BLOCK the event loop, until the io is complete. But in other hand async-await is just syntactic sugar to the Promise chaining. It is not actually synchronous, it just looks like it. That is the biggest difference you need to understand.
But, Some problems are introduces with async-await making the promises too sequential.
For example, two independent promises should be executed parallelly with Promise.all But when you use async-await you tend to do
await func1();
await func2();
So, logically you may make bottleneck yourself. But syntactically it has no problems like Sync ones whatsoever.
You can check out ES5 transpilation using Babel REPL, you may understand a little bit better.
The reason you shouldn't use *Sync functions is that they block the event loop. That causes your application to become irresponsive while synchronous functions are being executed.
Promises help you avoid the callback hell problem when you are dealing with async code. Additionally, async/await syntax allows you to write asynchronous code that LOOKS like synchronous code. So it's perfectly fine to use async and await.
I've heard, however, that methods like fs.readFileSync should always be avoided, since they are blocking and will force all other requests to wait.
This is true, mostly.
Remember that your server doesn't run on a single pipeline. Rather, you use the cluster API to run multiple worker processes side-by-side (where the number of cores of the server's CPU limits the number of worker processes that should be run).
In reality then, even if a single event loop is blocked by a synchronous IO and other requests that are assigned to the very same loop are forced to wait, other worker processes are still able to process incoming requests.
On the other hand, if you make a request await an IO operation, the loop is able to pick up another request and process it. But, there's a trade off.
Suppose first request comes, you await a fs.readFile. The second request comes and it's processed. But the second request doesn't wait for any IO, rather, it's a CPU-bound operation (a heavy calculation maybe?). The IO operation triggered by the first request completes but it has to wait until the second request completes and only then the continuation can be picked up by the event loop and the response can be sent back.
Do I have to use promises and callbacks to get decent multi-user performance?
A simple answer would be yes, however, be careful and monitor your app to not to fall in a pitfall (e.g. mixing IO requests with CPU intensive tasks where the performance of async IO could be worse from the client's perspective).
Using async/await in Node.js syntax is preferable to the alternatives, which are stock promises or especially callbacks. It allows for much cleaner code which is easier to understand and maintain. We used to have to use babel to transpile to access these in older times but they've been in Node for ages now so I'd recommend for people to use them.
Kind of a weird question, Imagine you have a situation where you need to run 10 SYNCRONOUS functions, it doesn't matter when they complete, you just want to know when all 10 are done: I.E.
f1()
f2()
f3()
...
f10()
doStuffWithResult();
Now, If you use promises like so, assuming you have rewrote each as promoises:
Promise.All([f1,f2,f3,f4,f5,f6,f7,f8,f9,f10])
.then(() => {
doStuffWithResult();
})
Would you see a performance increase? Theoretically, I want to say no because these functions are still synchronous, and everything is still running on one thread.
Thanks!
Would you see a performance increase?
No, what you are proposing would not be faster.
Promises do not create threads. All they do is provide a cooperative system for keeping track of when asynchronous operations are complete and then notifying interested parties of success or failure. They also provide services for propagating errors when asynchronous operations are nested.
And, your proposed Promise.all() code would not even work. You must pass an array of promises to Promise.all(), not an array of function references. In your example, your functions would not even be called.
And, if you changed your code to something that would actually execute, then it would likely be slower than just calling the synchronous functions directly because you'd be executing promise code and all .then() handlers execute on a future tick (not synchronously).
In node.js, the only way to execute synchronous things in parallel is to launch child processes (that can execute in parallel) or pass the operations to some native code that can use actual OS threads.
I'm trying to connect to a 3rd party library, that has a function that can block. I would like to use it, but without blocking. Is it possible to wrap a blocking call that I don't have the control of, to make it async?
// calling this function will block the nodejs thread
blockingCall();
What I would like would be something like this.
// wrapper for the blocking call
var wrapper = wrapBlockingCall(blockingCall);
wrapper.on('complete', function() {});
Is this possible? Does this make sense?
There is no way to make a blocking JavaScript code non-blocking in Node.js - the mechanism which Node uses for its non-blocking behaviour is implemented in the C/C++ layer, which in turn is used only when doing I/O operations (reading from disk, networking etc.).
In reality, every line of JavaScript your program uses will be executed one-by-one, because it is always executed on the same thread, no matter what you do.
The only option I see is to execute the offending code in a separate Node process using the built-in Child Process module. However, this will have significant performance impact, even bigger one if the code needs to be executed frequently.
Note:
After reading the comments under your question, it seems that you are actually the author of the blocking function, which in turn calls a C API which performs blocking I/O. There are ways of calling C functions which would normally block in a manner which does not block the upper JavaScript layer.
While I am not a C expert, I think this is accomplished using the libuv library included in Node - have a look at the addons documentation for more info.
I found many talks saying that Node.js is bad because of callback hell and Go is good because of its synchronous model.
What I feel is Go can also do callback as same as Node.js but in a synchronous way. As we can pass anonymous function and do closure things
So, why are they comparing Go and Node.js in callback perspective as if Go cannot become callback hell.
Or I misunderstand the meaning of callback and anonymous function in Go?
A lot of things take time, e.g. waiting on a network socket, a file system read, a system call, etc. Therefore, a lot of languages, or more precisely their standard library, include asynchronous version of their functions (often in addition to the synchronous version), so that your program is able to do something else in the mean-time.
In node.js things are even more extreme. They use a single-threaded event loop and therefore need to ensure that your program never blocks. They have a very well written standard library that is built around the concept of being asynchronous and they use callbacks in order to notify you when something is ready. The code basically looks like this:
doSomething1(arg1, arg2, function() {
doSomething2(arg1, arg2, function() {
doSomething3(function() {
// done
});
});
});
somethingElse();
doSomething1 might take a long time to execute (because it needs to read from the network for example), but your program can still execute somethingElse in the mean time. After doSomething1 has been executed, you want to call doSomething2 and doSomething3.
Go on the other hand is based around the concept of goroutines and channels (google for "Communicating Sequential Processes", if you want to learn more about the abstract concept). Goroutines are very cheap (you can have several thousands of them running at the same time) and therefore you can use them everywhere. The same code might look like this in Go:
go func() {
doSomething1(arg1, arg2)
doSomething2(arg1, arg2)
doSomething3()
// done
}()
somethingElse()
Whereas node.js focus on providing only asynchronous APIs, Go usually encourages you to write only synchronous APIs (without callbacks or channels). The call to doSomething1 will block the current goroutine and doSomething2 will only be executed after doSomething1 has finished. But that's not a problem in Go, since there are usually other goroutines available that can be scheduled to run on the system thread. In this case, somethingElse is part of another goroutine and can be executed in the meantime, just like in the node.js example.
I personally prefer the Go code, since it's easier to read and reason about. Another advantage of Go is that it also works well with computation heavy tasks. If you start a heavy computation in node.js that doesn't need to wait for network of filesystem calls, this computation basically blocks your event loop. Go's scheduler on the other hand will do its best to dispatch the goroutines on a few number of system threads and the OS might run those threads in parallel if your CPU supports it.
What I feel is Golang can also do callback as same as Node.js but in a synchronous way. As we can pass anonymous function and do closure things
So, why are they comparing Golang and Node.js in callback perspective as if Golang cannot become callback hell.
Yes, of course it is possible to mess things up in Go as well. The reason why you don't see as much callbacks as in node.js is that Go has channels for communication, which allow for a way of structuring your code without using callbacks.
So, since there are channels, callbacks are not used as often therefore it is unlikely to stumble over callback infested code. Of course this doesn't mean that you cannot write scary code with channels as well...
Redis is very fast. For most part on my machine it is as fast as say native Javascript statements or function calls in node.js. It is easy/painless to write regular Javascript code in node.js because no callbacks are needed. I don't see why it should not be that easy to get/set key/value data in Redis using node.js.
Assuming node.js and Redis are on the same machine, are there any npm libraries out there that allow interacting with Redis on node.js using blocking calls? I know this has to be a C/C++ library interfacing with V8.
I suppose you want to ensure all your redis insert operations have been performed. To achieve that, you can use the MULTI commands to insert keys or perform other operations.
The https://github.com/mranney/node_redis module queues up the commands pushed in multi object, and executes them accordingly.
That way you only require one callback, at the end of exec call.
This seems like a common bear-trap for developers who are trying to get used to Node's evented programming model.
What happens is this: you run into a situation where the async/callback pattern isn't a good fit, you figure what you need is some way of doing blocking code, you ask Google/StackExchange about blocking in Node, and all you get is admonishment on how bad blocking is.
They're right - blocking, ("wait for the result of this before doing anything else"), isn't something you should try to do in Node. But what I think is more helpful is to realize that 99.9% of the time, you're not really looking for a way to do blocking, you're just looking for a way to make your app, "wait for the result of this before going on to do that," which is not exactly the same thing.
Try looking into the idea of "flow control" in Node rather than "blocking" for some design patterns that could be a clearer fit for what you're trying to do. Here's a list of libraries to check out:
https://github.com/joyent/node/wiki/modules#wiki-async-flow
I'm new to Node too, but I'm really digging Async: https://github.com/caolan/async
Blocking code creates a MASSIVE bottleneck.
If you use blocking code your server will become INCREDIBLY slow.
Remember, node is single threaded. So any blocking code, will block node for every connected client.
Your own benchmarking shows it's fast enough for one client. Have you benchmarked it with a 1000 clients? If you try this you will see why blocking code is bad
Whilst Redis is quick it is not instantaneous ... this is why you must use a callback if you want to continue execution ensuring your values are there.
The only way I think you could (and am not suggesting you do) achieve this use a callback with a variable that is the predicate for leaving a timer.