Lets say you have a complex Lua application, and there is some base function that different parts of your code call repeatedly. It's a stateless function with little to no side effects, and fairly simple.
How does the virtual machine handle this? Does it queue up all the calls, and let them run one by one, waiting for the function to to return before calling it again? Or does it do some trickery to avoid this sort of situation? What if the function had some big side effects, like print()?
Lua is single threaded so every function call must return before the next one is called. If a function is blocked then so is the VM. The only way around that is coroutines or Lua lanes or C threads.
Related
To process HTTP requests, we have to make blocking calls (e.g. JDBC calls) as part of a Mono/Flux-based process. Our current plan looks something like this:
// I renamed getSomething to processJaxrsHttpRequest
CompletionStage<String> processJaxrsHttpRequest(String input) {
return Mono.just(input)
.map(in -> process(in))
.flatMap(str -> Mono.fromCallable(() -> jdbcCall(str)).subscribeOn(fixedScheduler))
.flatMap(str -> asyncHttpCall(str))
.flatMap(str -> Mono.fromCallable(() -> jdbcCall(str)).subscribeOn(fixedScheduler))
.toFuture();
}
where fixedScheduler is used concurrently across HTTP requests.
We were hoping to get some feedback on this strategy for handling block calls within a decent number of fluxes. Of course, we understand that if all our requests were flowing through these blocking calls then we might as well not use reactor (outside of the admittedly nice processing API).
Update: Thanks bsideup for this answer. However, I should have been a little more specific with my questions.
My overall question is how to effectively have a blocking call used across multiple fluxes were these fluxes can be created/subscribed to in large numbers. We tried the suggested approach, but it results in an explosion of threads and quickly OOMs. So we are thinking to use a shared scheduler. So.. here are my questions.
Is using a shared scheduler (fixedScheduler) what you would suggest in the situation I describe? If not, will you point me in any directions?
If using a shared scheduler is good, would this be a good implementation of it: Schedulers.newParallel("blocking-scheduler", maxNumThreads)?
Update 2: Just dug a big on Schedulers#newParallel and realize that won't work since it 'rejects' blocking calls.
Really appreciate any tips!
While subscribeOn is indeed one way of handling blocking calls and your usage is okay, you can as well use publishOn.
It moves processing to the provided Scheduler, unless other publishOn is specified:
CompletionStage<String> getSomething(String input) {
return Mono.just(input)
.map(in -> process(in)) // process must be non-blocking, or go after publishOn
.publishOn(Schedulers.boundedElastic())
.map(::jdbcCall)
.flatMap(str -> asyncHttpCall(str))
.publishOn(Schedulers.boundedElastic())
.map(::jdbcCall)
.toFuture();
}
As you can see, you can continue using async calls too. Just make sure you're not blocking non-blocking schedulers (in that example, I use publishOn again after flatMap because asyncHttpCall may complete from non-blocking scheduler)
For example for gen_tcp vs SSL. Some connections will use SSL and some Http. I can save this into my state as Module=gen_tcp OR I can save it as MyFun=fun gen_tcp:send/2, then I can do
Module:send(Socket,Data)
OR
MyFun(Sock,Data)
which of these methods is more efficient or recommended? I heard apply is slower than funs and doing MFA is just an apply. Are funs referencing static functions any faster than regular funs?
The difference in speed for an operation like this will never be measurable in a real-world program (particularly one dealing with I/O). You should think about what kind of API you want: passing a single callback module that exports send/2 and possibly other functions (maybe adding callback functions in future versions), or passing a specific functional value for each specific user-defined function to be called.
Is there a way to set an entry point in DWScript?
For example, if I start a script execution, I'd like it to execute a procedure Main, rather than the code in the regular entry point (begin ... end.).
I know it's possible to execute functions from Delphi, but I'm not sure this would be quite the same.
Aside from writing your procedure Main(); and then having your regular script entry point consist of nothing but calling Main, which is probably not what you're thinking of, no, there's no way to do that in DWS.
For all its innovations in syntax, DWS is still Pascal, and it still works the way Pascal works. Asking for some sort of named Main routine would be a radical departure from Pascal style.
EDIT: To answer the clarification posted in comments:
If you want your script to spawn a new script thread, you'll have to handle it in external Delphi code. As of this writing, the DWS system doesn't have any concept of multithreading built in. If you wanted to do it, you'd do something like this:
Create an external routine called something like SpawnThread(EntryPoint: string). Its eval method (out in Native-Delphi-land) would spawn a new thread that loads the current script, then finds the routine with the specified name and executes it.
That's about the only way you could get it to work without language-level support. If you'd like a way to spawn threads from within DWS, try adding it as a feature request to the issue tracker.
Calling functions directly is explicited in
https://code.google.com/p/dwscript/wiki/FirstSteps#Functions
If you want to execute a function in a different thread, you'll need some Delphi-side code to create a new thread, a new execution, and then call your functions. The main and threaded-execution will then be sandboxed from each other (so can't share share global vars etc.).
If you need to share data between the threads, you could do that by exposing functions or external variables, which would call into Delphi code with the proper synchronizations and locks in place (what is "proper" will depend on what your code wants to do, like always in multithreading...).
Note that it is possible to pass objects, interfaces and dynamic arrays between script executions (provided they're executions of the same program), but just like with regular code, you'll have to use locks, critical sections or mutexes explicitly.
Despite my best efforts, I am unable to produce the kind of synchronization effects I would like to in ActionScript. The issue is that I need to make several external calls to get various pieces of outside information in response to a user request, and the way items will be laid out on the page is dependent on what each of these external calls returns. So, I don't care that all of these calls return asynchronously. However, is there any way to force some amount of synchronization on ActionScript, so that at least calling the method for doing the final layout and placement of items on the page is dependent on all of my calls finishing?
If I understand the question right, event listeners are probably your best bet. Most loader classes throw an Event.COMPLETE message when they finish doing everything behind the scenes. If you wrote those external calls, it would be easy to dispatch a complete event at the end.
So when you make all these external calls, have a function that listens to when those calls complete. This function would keep track of how many calls have been made, and when there's none left to run, continue building your layout.
Rough Sketch to explain below:
var numProcesses:int = 0;
slowthing.addEventListener(Event.COMPLETE,waitForSlowest);
numProcesses++;
slowthing.load();
quickThing.addEventListener(Event.COMPLETE,waitForSlowest);
numProcesses++;
quickthing.load();
function waitForSlowest(e:Event)
{
numProcesses--;
if(numProcesses == 0)
finalizeLayout();
}
I am creating an action script library.I am calling some APIs which parses some xml and gets me the result. It dispatches an Event.COMPLETE when the parsing is done. I want to monitor whether this event is dispatched in some while loop like "while(eventnotdispatched)"
is it possible? I know the other way would be to addeventlistener. But please let me know if the other thing is possible.
Thanks
NO, it is not possible. Actionscript is single threaded. Thus while you are waiting in your while loop, that is the only thread running, and the process you are waiting for can never complete. This is why everything is done with events, so that's what you should use. If you need to update your display periodically while you are waiting for something to complete...again, use events. Create a Timer object which generates a TIMER event every so often, and use that to make your updates.
EDIT: Davr is right, you would not be able to use the while loop like this. You would need a timer.
Yes, it is possible to poll for it. BUT you will still need to create an event listener. It will work something like this:
private var loadCompleted = false;
private var timer:Timer= new Timer(1);
private function onInitCompleted(event:Event):void
{
timer.addEventListener(TimerEvent.TIMER, timerHandler);
timer.start();
}
private function loadCompleteEventHandler(event:Event):void
{
loadCompleted = true;
...
}
private function timerHandler()
{
if(!loadCompleted)
{
... // stop the timer or something.
timer.stop();
}
}
Please note, this is VERY BAD code. I would NEVER use it in production because Actionscript is a event driven language. There should be absolutely NO REASON for you to need to do this. Whatever you are trying to do could be accomplished using another method much simpler. Tell me what you are trying to accomplish with this and I will present a better solution.
Sorry for yelling, it's late and I am sleepy.
Doing that means forcing a synchronous model of execution on the underlying asynchronous model (that works with callbacks).
What are you trying to achieve exactly, and why not use a callback?
I agree with the statements about it probably being a bad idea and a while loop will certainly not work this way in ActionScript. However, there may be legitimate reasons for doing what you are attempting to do. Only you can prevent bad code. Instead of judging, I'll just get to an answer for your question.
First I'm going to make an assumption, that what you really want to do is monitor a property and for some reason the API for this object does not dispatch an event when this property changes. I'm making this assumption because if you have the event available, I assume you would just use the event.
So... you have an object weirdXmlObj with a property loaded that defaults to false but goes to true when the XML is loaded.
In this case with slight modifications the code posted by CookieOfFortune would in fact work. You wouldn't need the loadCompleteEventHandler function (which was never attached anyway) and in the timer handler you would simply check if( weirdXmlObj.loaded ) and then branch however you wanted to.
Ah but there may be a simpler way, depending on what you are doing.
If you have a display object handy. (i.e. something that makes sense, not just some random object.) You can attach your code to the stage's EnterFrame event instead of using a timer.
myDisplayObject.stage.addEventListner(Event.ENTER_FRAME,frameEnterHandler);
A couple of things to be aware of:
You don't really even need to go to the stage level, all display objects support the EnterFrame event, but it's a nice place to attach the event listener.
You really should keep whatever the function calls to a minimum. In particular the actual frameEnterHandler function should do nothing more than do the if( weirdXmlObj.loaded ) check.
You are attempting to circumvent event-driven programming, which is not a good idea. This is often the case when someone approaches from an older model and does not yet have a good frame of reference to appreciate the elegance of event-driven programming.
Events are your friends. They work very well. Your loadCompleteHandler is all that is required. Want to do something else in response? Add the call in that handler:
private function loadCompletedHandler(event:Event):void
{
waitingObject.fileWasLoadedSoGoDoThatThing();
}
There is no need to make it any more complicated than that. No need for a semaphore or a loop to check the semaphore. Unnecessary environmental semaphores can break the encapsulation that could shield you from unwanted side effects.