Run NodeJS event loop / wait for child process to finish

Question

I first tried a general description of the problem, then some more detail why the usual approaches don't work. If you would like to read these abstracted explanations go on. In the end I explain the greater problem and the specific application, so if you would rather read that, jump to "Actual application".

I am using a node.js child-process to do some computationally intensive work. The parent process does it's work but at some point in the execution it reaches a point where it must have the information from the child process before continuing. Therefore, I am looking for a way to wait for the child-process to finish.

My current setup looks somewhat like this:

importantDataCalculator = fork("./runtime");
importantDataCalculator.on("message", function (msg) {
    if (msg.type === "result") {
        importantData = msg.data;
    } else if (msg.type === "error") {
        importantData = null;
    } else {
        throw new Error("Unknown message from dataGenerator!");
    }
});

and somewhere else

function getImportantData() {
    while (importantData === undefined) {
        // wait for the importantDataGenerator to finish
    }

    if (importantData === null) {
        throw new Error("Data could not be generated.");
    } else {
        // we should have a proper data now
        return importantData;
    }
}

So when the parent process starts, it executes the first bit of code, spawning a child process to calculate the data and goes on doing it's own bit of work. When the time comes that it needs the result from the child process to continue it calls getImportantData(). So the idea is that getImportantData() blocks until the data is calculated.

However, the way I used doesn't work. I think this is due to me preventing the event loop from executing by using the while-loop. And since the Event-Loop does not execute no message from the child-process can be received and thus the condition of the while-loop can not change, making it an infinite loop.

Of course, I don't really want to use this kind of while-loop. What I would rather do is tell node.js "execute one iteration of the event loop, then get back to me". I would do this repeatedly, until the data I need was received and then continue the execution where I left of by returning from the getter.

I realize that his poses the danger of reentering the same function several times, but the module I want to use this in does almost nothing on the event loop except for waiting for this message from the child process and sending out other messages reporting it's progress, so that shouldn't be a problem.

Is there way to execute just one iteration of the event loop in Node.js? Or is there another way to achieve something similar? Or is there a completely different approach to achieve what I'm trying to do here?

The only solution I could think of so far is to change the calculation in such a way that I introduce yet another process. In this scenario, there would be the process calculating the important data, a process calculating the bits of data for which the important data is not needed and a parent process for these two, which just waits for data from the two child-processes and combines the pieces when they arrive. Since it does not have to do any computationally intensive work itself, it can just wait for events from the event loop (=messages) and react to them, forwarding the combined data as necessary and storing pieces of data that cannot be combined yet. However this introduces yet another process and even more inter-process communication, which introduces more overhead, which I would like to avoid.

Edit

I see that more detail is needed.

The parent process (let's call it process 1) is itself a process spawned by another process (process 0) to do some computationally intensive work. Actually, it just executes some code over which I don't have control, so I cannot make it work asynchronously. What I can do (and have done) is make the code that is executed regularly call a function to report it's progress and provided partial results. This progress report is then send back to the original process via IPC.

But in rare cases the partial results are not correct, so they have to be modified. To do so I need some data I can calculate independently from the normal calculation. However, this calculation could take several seconds; thus, I start another process (process 2) to do this calculation and provide the result to process 1, via an IPC message. Now process 1 and 2 are happily calculating there stuff, and hopefully the corrective data calculated by process 2 is finished before process 1 needs it. But sometimes one of the early results of process 1 needs to be corrected and in that case I have to wait for process 2 to finish its calculation. Blocking the event loop of process 1 is theoretically not a problem, since the main process (process 0) would not be be affected by it. The only problem is, that by preventing the further execution of code in process 1 I am also blocking the event loop, which prevents it from ever receiving the result from process 2.

So I need to somehow pause the further execution of code in process 1 without blocking the event loop. I was hoping that there was a call like process.runEventLoopIteration that executes an iteration of the event loop and then returns.

I would then change the code like this:

function getImportantData() {
    while (importantData === undefined) {
        process.runEventLoopIteration();
    }

    if (importantData === null) {
        throw new Error("Data could not be generated.");
    } else {
        // we should have a proper data now
        return importantData;
    }
}

thus executing the event loop until I have received the necessary data but NOT continuing the execution of the code that called getImportantData().

Basically what I'm doing in process 1 is this:

function callback(partialDataMessage) {
    if (partialDataMessage.needsCorrection) {
        getImportantData();
        // use data to correct message
        process.send(correctedMessage); // send corrected result to main process
    } else {
        process.send(partialDataMessage); // send unmodified result to main process
    }
}

function executeCode(code) {
    run(code, callback); // the callback will be called from time to time when the code produces new data
    // this call is synchronous, run is blocking until the calculation is finished
    // so if we reach this point we are done
    // the only way to pause the execution of the code is to NOT return from the callback 
}

Actual application/implementation/problem

I need this behaviour for the following application. If you have a better approach to achieve this feel free to propose it.

I want to execute arbitrary code and be notified about what variables it changes, what functions are called, what exceptions occur etc. I also need the location of these events in the code to be able to display the gathered information in the UI next to the original code.

To achieve this, I instrument the code and insert callbacks into it. I then execute the code, wrapping the execution in a try-catch block. Whenever the callback is called with some data about the execution (e.g. a variable change) I send a message to the main process telling it about the change. This way, the user is notified about the execution of the code, while it is running. The location information for the events generated by these callbacks is added to the callback call during the instrumentation, so that is not a problem.

The problem appears, when an exception occurs. I also want to notify the user about exceptions in the tested code. Therefore, I wrapped the execution of the code in a try-catch and any exceptions that get out of the execution are caught and send to the user interface. But the location of the errors is not correct. An Error object created by node.js has a complete call stack so it knows where it occurred. But this location if relative to the instrumented code, so I cannot use this location information as is, to display the error next to the original code. I need to transform this location in the instrumented code into a location in the original code. To do so, after instrumenting the code, I calculate a source map to map locations in the instrumented code to locations in the original code. However, this calculation might take several seconds. So, I figured, I would start a child process to calculate the source map, while the execution of the instrumented code is already started. Then, when an exception occurs, I check whether the source map has already been calculated, and if it hasn't I wait for the calculation to finish to be able to correct the location.

Since the code to be executed and watched can be completely arbitrary I cannot trivially rewrite it to be asynchronous. I only know that it calls the provided callback, because I instrumented the code to do so. I also cannot just store the message and return to continue the execution of the code, checking back during the next call whether the source map has been finished, because continuing the execution of the code would also block the event-loop, preventing the calculated source map from ever being received in the execution process. Or if it is received, then only after the code to execute has completely finished, which could be quite late or never (if the code to execute contains an infinite loop). But before I receive the sourceMap I cannot send further updates about the execution state. Combined, this means I would only be able to send the corrected progress messages after the code to execute has finished (which might be never) which completely defeats the purpose of the program (to enable the programmer to watch what the code does, while it executes).

Temporarily surrendering control to the event loop would solve this problem. However, that does not seem to be possible. The other idea I have is to introduce a third process which controls both the execution process and the sourceMapGeneration process. It receives progress messages from the execution process and if any of the messages needs correction it waits for the sourceMapGeneration process. Since the processes are independent, the controlling process can store the received messages and wait for the sourceMapGeneration process while the execution process continues executing, and as soon as it receives the source map, it corrects the messages and sends all of them off.

However, this would not only require yet another process (overhead) it also means I have to transfer the code once more between processes and since the code can have thousands of line that in itself can take some time, so I would like to move it around as little as possible.

I hope this explains, why I cannot and didn't use the usual "asynchronous callback" approach.

Answer 1

Adding a third ( :) ) solution to your problem after you clarified what behavior you seek I suggest using Fibers.

Fibers let you do co-routines in nodejs. Coroutines are functions that allow multiple entry/exit points. This means you will be able to yield control and resume it as you please.

Here is a sleep function from the official documentation that does exactly that, sleep for a given amount of time and perform actions.

function sleep(ms) {
    var fiber = Fiber.current;
    setTimeout(function() {
        fiber.run();
    }, ms);
    Fiber.yield();
}

Fiber(function() {
    console.log('wait... ' + new Date);
    sleep(1000);
    console.log('ok... ' + new Date);
}).run();
console.log('back in main');

You can place the code that does the waiting for the resource in a function, causing it to yield and then run again when the task is done.

For example, adapting your example from the question:

var pausedExecution, importantData;
function getImportantData() {
    while (importantData === undefined) {
        pausedExecution = Fiber.current;
        Fiber.yield();
        pausedExecution = undefined;
    }

    if (importantData === null) {
        throw new Error("Data could not be generated.");
    } else {
        // we should have proper data now
        return importantData;
    }
}

function callback(partialDataMessage) {
    if (partialDataMessage.needsCorrection) {
        var theData = getImportantData();
        // use data to correct message
        process.send(correctedMessage); // send corrected result to main process
    } else {
        process.send(partialDataMessage); // send unmodified result to main process
    }
}

function executeCode(code) {
    // setup child process to calculate the data
    importantDataCalculator = fork("./runtime");
    importantDataCalculator.on("message", function (msg) {
        if (msg.type === "result") {
            importantData = msg.data;
        } else if (msg.type === "error") {
            importantData = null;
        } else {
            throw new Error("Unknown message from dataGenerator!");
        }

        if (pausedExecution) {
            // execution is waiting for the data
            pausedExecution.run();
        }
    });


    // wrap the execution of the code in a Fiber, so it can be paused
    Fiber(function () {
        runCodeWithCallback(code, callback); // the callback will be called from time to time when the code produces new data
        // this callback is synchronous and blocking,
        // but it will yield control to the event loop if it has to wait for the child-process to finish
    }).run();
}

Good luck! I always say it is better to solve one problem in 3 ways than solving 3 problems the same way. I'm glad we were able to work out something that worked for you. Admittingly, this was a pretty interesting question.