Hi I'm making a business application in which user can write his custom code and execute it.
How can i handle situations when the custom code generates infinite loops ?
A way is to handle each request in separate child process and kill that process on request completion but if there are thousands of users this doesn't seem to be appropriate. What else can be done in such cases ?
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I'm learning how to emit events using the NodeJS Event module but I'm struggling on a workflow interrogation.
I'd like to listen for new post created on a specific website using web scraping as it doesn't provide any other way.
For now, all I could find was by using a loop every X minutes and emit it once it notices a difference. But it involves a lot of inconvenience :
How to do it in a loop without blocking the whole script
That involves that you need to wait before continuing in the loop
So, how should you listen to events such as a new post? Is looking for differences at regular intervals a good practice?
The scraping is not blocking the whole script : the loading itself is pretty asynchronous, only the parsing part is synchronous.
If the scraping action is done with a promise, you can run promisses in parallel with promises.all, you could also run scraping with setInterval that way it will be started at regular interval even if some of the scrapings are slow or even failing
If you really want to not block the eventloop at all, you can run a worker for each scraping process.
I wonder how Camunda manage multiple instances of a sub-process.
For example this BPMN:
Let's say multi-instances process would iterate on a big collection, 500 instances.
I have a function in a web app that call the endpoint to complete the user common task, and perform another call to camunda engine to get all tasks (on first API call callback). I am supposed to get a list of 500 sub-process user tasks (the ones generated by the multi-instances process).
What if the get tasks call is performed before Camunda Engine successfully instantiated all sub-processes?
Do i get a partial list of task ?
How to detect that main and sub processes are ready?
I don't really know if Camunda is able to manage this problematic by itself so I though of the following solution, knowing I only can use Modeler environment with Groovy to add code (Javascript as well, but the entire code parts already added are groovy):
Use of a sub process throw event to catch in main process, then count and compare tasks ready with awaited tasks number for each signal emitted.
Thanks
I would maybe likely spawn the tasks as parallel process (or 500 of them) and then got to a next step in which I signal or otherwise set a state that indicates the spawning is completed. I would further join the parallel processes all together again and have here a task signaling or otherwise setting a state that indicates all the parallel processes are done. See https://docs.camunda.org/manual/7.12/reference/bpmn20/gateways/parallel-gateway/. This way you can know exactly at what point (after spawning is done and before the join) you have a chance of getting your 500 spawned sub processes
I have a nodejs app that has a finite specific sequence of actions.
One of the actions is getting an array of images, sending it to a client, and displaying it for a manual human filtering.
After filtering was done, (say a button was pressed), I need the nodejs app to keep executing the sequence until it's done.
I've been wondering over and over how to perform such a thing (and if possible, without the use of sockets.)
I tried creating a boolean representing if filtering was done, and using
while (!boolean), but server seems to be busy running it so it can't event handle the response which should update that same boolean.
Is there a better way?
I'm pretty new to node and am trying to setup an express server. The server will handle various requests and if any of them fail call a common failure function that will send e-mail. If I was doing this in something like Java I'd likely use something like a synchronized block and a boolean to allow the first entrance into the code to send the mail.
Is there anything like a synchronized block in Node? I believe node is single threaded and has a few helper threads to handle asyncronous/callback code. Is it at all possible that the same line of code could run at exactly the same time in Node?
Thanks!
Can the same line of Node.js code run at the same time? Is it at all possible that the same line of code could run at exactly the same time in Node?
No, it is not. Your Javascript in node.js is entirely single threaded. An event is pulled from the event queue. That calls a callback associated with that event. That callback runs until it returns. No other events can be processed until that first one returns. When it returns, the interpreter pulls the next event from the event queue and then calls the callback associated with it.
This does not mean that there are not concurrency issues in node.js. There can be, but it is caused not by code running at the same physical time and creating conflicting access to shared variables (like can happen in threaded languages like Java). Concurrency issues can be caused by the asynchronous nature of I/O in node.js. In the asynchronous case, you call an asynchronous function, pass it a callback (or expect a promise in return). Your code then continues on and returns to the interpreter. Some time later an event will occur inside of node.js native code that will add something to the event queue. When the interpreter is free from running other Javascript, it will process that event and then call your callback which will cause more of your code to run.
While all this is "in process", other events are free to run and other parts of your Javascript can run. So, the exposure to concurrency issues comes, not from simultaneous running of two pieces of code, but from one piece of your code running while another piece of your code is waiting for a callback to occur. Both pieces of code are "in process". They are not "running" at the same time, but both operations are waiting from something else to occur in order to complete. If these two operations access variables in ways that can conflict with each other, then you can have a concurrency issue in Javascript. Because none of this is pre-emptive (like threads in Java), it's all very predictable and is much easier to design for.
Is there anything like a synchronized block in Node?
No, there is not. It is simply not needed in your Javascript code. If you wanted to protect something from some other asynchronous operation modifying it while your own asynchronous operation was waiting to complete, you can use simple flags or variables in your code. Because there's no preemption, a simple flag will work just fine.
I believe node is single threaded and has a few helper threads to handle asyncronous/callback code.
Node.js runs your Javascript as single threaded. Internally in its own native code, it does use threads in order to do its work. For example, the asynchronous file system access code internal to node.js uses threads for disk I/O. But these threads are only internal and the result of these threads is not to call Javascript directly, but to insert events in the event queue and all your Javascript is serialized through the event queue. Pull event from event queue, run callback associated with the event. Wait for that callback to return. Pull next event from the event queue, repeat...
The server will handle various requests and if any of them fail call a common failure function that will send e-mail. If I was doing this in something like Java I'd likely use something like a synchronized block and a boolean to allow the first entrance into the code to send the mail.
We'd really have to see what your code looks like to understand what exact problem you're trying to solve. I'd guess that you can just use a simple boolean (regular variable) in node.js, but we'd have to see your code to really understand what you're doing.
I made function which use do while(true) - infinite loop (which controls other apps)
I am searching solution how to make form accessable - when I start app, access just loading and my form cannot be changed. At least I tried to make some "listening function" for key "S" to stop while and it doesnt work too (maybe I am doing it bad, is it possible?)
I just used
Do While (run) <code> Loop
I guess I will need at least 2 thread (1 thread handle infinite while, 2 thread for form, but I read access cannot handle Multithreading.)
I can ask, is there some possibility of make this work?
Jan.
MS Access or MS office are single threaded platforms and cannot perform parallel executions. Generally, having a blind loop or uncontrolled loop is not a good practice at all. If you have a blind loop function without a controller, you are wasting process cycles of your vba application.
what you can do:
use timer and set intervals how often your code must execute
Add "Do events" within the loop to use the process cycles to execute some other code when a code within loop is waiting for an external signal.
Add any sort of controller to break the loop.. simple if else condition
Migrate your code to an external .DLL and execute/control the function outside VBA
In short, VBA cannot perform threading. Using uncontrolled will freeze your application which will force you to use [CTRL] + F12 to stop the execution manually.
I used function "DoEvents" (it at least react for input - keyboard / mouse)