骆驼：文件使用者组件“咬得比它所能咀嚼的更多”，管道因内存不足错误而终止

Question

I have a route defined in Camel that goes something like this: GET request comes in, a file gets created in the file system. File consumer picks it up, fetches data from external web services, and sends the resulting message by POST to other web services.

Simplified code below:

    // Update request goes on queue:
    from("restlet:http://localhost:9191/update?restletMethod=post")
    .routeId("Update via POST")
    [...some magic that defines a directory and file name based on request headers...]
    .to("file://cameldest/queue?allowNullBody=true&fileExist=Ignore")

    // Update gets processed
    from("file://cameldest/queue?delay=500&recursive=true&maxDepth=2&sortBy=file:parent;file:modified&preMove=inprogress&delete=true")
    .routeId("Update main route")
    .streamCaching() //otherwise stuff can't be sent to multiple endpoints
    [...enrich message from some web service using http4 component...]
    .multicast()
        .stopOnException()
        .to("direct:sendUpdate", "direct:dependencyCheck", "direct:saveXML")
    .end();

The three endpoints in the multicast are simply POSTing the resulting message to other web services.

This all works rather well when the queue (ie the file directory cameldest ) is fairly empty. Files are being created in cameldest/<subdir> , picked up by the file consumer and moved into cameldest/<subdir>/inprogress , and stuff is being sent to the three outgoing POSTs no problem.

However, once the incoming requests pile up to about 300,000 files progress slows down and eventually the pipeline fails due to out-of-memory errors (GC overhead limit exceeded).

By increasing logging I can see that the file consumer polling basically never runs, because it appears to take responsibility for all files it sees at each time, waits for them to be done processing, and only then starts another poll round. Besides (I assume) causing the resources bottleneck, this also interferes with my sorting requirements: Once the queue is jammed with thousands of messages waiting to be processed, new messages that would naively be sorted higher up are -if they even still get picked up- still waiting behind those that are already "started".

Now, I've tried the maxMessagesPerPoll and eagerMaxMessagesPerPoll options. They seem to alleviate the problem at first, but after a number of poll rounds I still end up with thousands of files in "started" limbo.

The only thing that sort of worked was making the bottle neck of delay and maxMessages... so narrow that the processing on average would finish faster than the file polling cycle.

Clearly, that is not what I want. I would like my pipeline to process files as fast as possible, but not faster. I was expecting the file consumer to wait when the route is busy.

Am I making an obvious mistake?

(I'm running a somewhat older Camel 2.14.0 on a Redhat 7 machine with XFS, if that is part of the problem.)

Answer 1

Try set maxMessagesPerPoll to a low value on the from file endpoint to only pickup at most X files per poll which also limits the total number of inflight messages you will have in your Camel application.

You can find more information about that option in the Camel documentation for the file component

Answer 2

I would propose an alternative solution unless you really need to save the data as files.

From your restlet consumer, send each request to a message queuing app such as activemq or rabbitmq or something similar. You will quickly end up with lots of messages on that queue but that is ok.

Then replace your file consumer with a queue consumer. It will take some time but the each message should be processed separately and sent to wherever you want. I have tested rabbitmq with about 500 000 messages and that has worked fine. This should reduce the load on the consumer as well.

Answer 3

The short answer is that there is no answer: The sortBy option of Camel's file component is simply too memory-inefficient to accomodate my use-case:

• Uniqueness: I don't want to put a file on queue if it's already there.
• Priority: Files flagged as high priority should be processed first.
• Performance: Having a few hundred thousands of files, or maybe even a few million, should be no problem.
• FIFO: (Bonus) Oldest files (by priority) should be picked up first.

The problem appears to be, if I read the source code and the documentation correctly, that all file details are in memory to perform the sorting, no matter whether the built-in language or a custom pluggable sorter is used. The file component always creates a list of objects containing all details, and that apparently causes an insane amount of garbage collection overhead when polling many files often.

I got my use case to work, mostly, without having to resort to using a database or writing a custom component, using the following steps:

• Move from one file consumer on the parent directory cameldest/queue that sorts recursively the files in the subdirectories ( cameldest/queue/high/ before cameldest/queue/low/ ) to two consumers , one for each directory, with no sorting at all.
• Set up only the consumer from /cameldest/queue/high/ to process files through my actual business logic.
• Set up the consumer from /cameldest/queue/low to simply promote files from "low" to "high" (copying them over, ie .to("file://cameldest/queue/high"); )
• Crucially, in order to only promote from "low" to "high" when high is not busy , attach a route policy to "high" that throttles the other route , ie "low" if there are any messages in-flight in "high"
• Additionally, I added a ThrottlingInflightRoutePolicy to "high" to prevent it from inflighting too many exchanges at once.

Imagine this like at check-in at the airport, where tourist travellers are invited over into the business class lane if that is empty.

This worked like a charm under load, and even while hundreds of thousands of files were on queue in "low", new messages (files) dropped directly into "high" got processed within seconds.

The only requirement that this solution doesn't cover, is the orderedness: There is no guarantee that older files are picked up first, rather they are picked up randomly. One could imagine a situation where a steady stream of incoming files could result in one particular file X just always being unlucky and never being picked up. The chance of that happening, though, is very low.

Possible improvement: Currently the threshold for allowing / suspending the promotion of files from "low" to "high" is set to 0 messages inflight in "high". On the one hand, this guarantees that files dropped into "high" will be processed before another promotion from "low" is performed, on the other hand it leads to a bit of a stop-start-pattern, especially in a multi-threaded scenario. Not a real problem though, the performance as-is was impressive.

---

Source:

My route definitions:

    ThrottlingInflightRoutePolicy trp = new ThrottlingInflightRoutePolicy();
    trp.setMaxInflightExchanges(50);

    SuspendOtherRoutePolicy sorp = new SuspendOtherRoutePolicy("lowPriority");

    from("file://cameldest/queue/low?delay=500&maxMessagesPerPoll=25&preMove=inprogress&delete=true")
    .routeId("lowPriority")
    .log("Copying over to high priority: ${in.headers."+Exchange.FILE_PATH+"}")
    .to("file://cameldest/queue/high");

    from("file://cameldest/queue/high?delay=500&maxMessagesPerPoll=25&preMove=inprogress&delete=true")
    .routeId("highPriority")
    .routePolicy(trp)
    .routePolicy(sorp)
    .threads(20)
    .log("Before: ${in.headers."+Exchange.FILE_PATH+"}")
    .delay(2000) // This is where business logic would happen
    .log("After: ${in.headers."+Exchange.FILE_PATH+"}")
    .stop();

My SuspendOtherRoutePolicy , loosely built like ThrottlingInflightRoutePolicy

public class SuspendOtherRoutePolicy extends RoutePolicySupport implements CamelContextAware {

    private CamelContext camelContext;
    private final Lock lock = new ReentrantLock();
    private String otherRouteId;

    public SuspendOtherRoutePolicy(String otherRouteId) {
        super();
        this.otherRouteId = otherRouteId;
    }

    @Override
    public CamelContext getCamelContext() {
        return camelContext;
    }

    @Override
    public void onStart(Route route) {
        super.onStart(route);
        if (camelContext.getRoute(otherRouteId) == null) {
            throw new IllegalArgumentException("There is no route with the id '" + otherRouteId + "'");
        }
    }

    @Override
    public void setCamelContext(CamelContext context) {
        camelContext = context;
    }

    @Override
    public void onExchangeDone(Route route, Exchange exchange) {
        //log.info("Exchange done on route " + route);
        Route otherRoute = camelContext.getRoute(otherRouteId);
        //log.info("Other route: " + otherRoute);
        throttle(route, otherRoute, exchange);
    }

    protected void throttle(Route route, Route otherRoute, Exchange exchange) {
        // this works the best when this logic is executed when the exchange is done
        Consumer consumer = otherRoute.getConsumer();

        int size = getSize(route, exchange);
        boolean stop = size > 0;
        if (stop) {
            try {
                lock.lock();
                stopConsumer(size, consumer);
            } catch (Exception e) {
                handleException(e);
            } finally {
                lock.unlock();
            }
        }

        // reload size in case a race condition with too many at once being invoked
        // so we need to ensure that we read the most current size and start the consumer if we are already to low
        size = getSize(route, exchange);
        boolean start = size == 0;
        if (start) {
            try {
                lock.lock();
                startConsumer(size, consumer);
            } catch (Exception e) {
                handleException(e);
            } finally {
                lock.unlock();
            }
        }
    }

    private int getSize(Route route, Exchange exchange) {
        return exchange.getContext().getInflightRepository().size(route.getId());
    }

    private void startConsumer(int size, Consumer consumer) throws Exception {
        boolean started = super.startConsumer(consumer);
        if (started) {
            log.info("Resuming the other consumer " + consumer);
        }
    }

    private void stopConsumer(int size, Consumer consumer) throws Exception {
        boolean stopped = super.stopConsumer(consumer);
        if (stopped) {
            log.info("Suspending the other consumer " + consumer);
        }
    }
}