Retry logic with CompletableFuture

Question

I need to submit a task in an async framework I'm working on, but I need to catch for exceptions, and retry the same task multiple times before "aborting".

The code I'm working with is:

int retries = 0;
public CompletableFuture<Result> executeActionAsync() {

    // Execute the action async and get the future
    CompletableFuture<Result> f = executeMycustomActionHere();

    // If the future completes with exception:
    f.exceptionally(ex -> {
        retries++; // Increment the retry count
        if (retries < MAX_RETRIES)
            return executeActionAsync();  // <--- Submit one more time

        // Abort with a null value
        return null;
    });

    // Return the future    
    return f;
}

This currently doesn't compile because the return type of the lambda is wrong: it expects a Result, but the executeActionAsync returns a CompletableFuture<Result>.

How can I implement this fully async retry logic?

Answer 1

I think I was successfully. Here's an example class I created and the test code:

---

RetriableTask.java

public class RetriableTask
{
    protected static final int MAX_RETRIES = 10;
    protected int retries = 0;
    protected int n = 0;
    protected CompletableFuture<Integer> future = new CompletableFuture<Integer>();

    public RetriableTask(int number) {
        n = number;
    }

    public CompletableFuture<Integer> executeAsync() {
        // Create a failure within variable timeout
        Duration timeoutInMilliseconds = Duration.ofMillis(1*(int)Math.pow(2, retries));
        CompletableFuture<Integer> timeoutFuture = Utils.failAfter(timeoutInMilliseconds);

        // Create a dummy future and complete only if (n > 5 && retries > 5) so we can test for both completion and timeouts. 
        // In real application this should be a real future
        final CompletableFuture<Integer> taskFuture = new CompletableFuture<>();
        if (n > 5 && retries > 5)
            taskFuture.complete(retries * n);

        // Attach the failure future to the task future, and perform a check on completion
        taskFuture.applyToEither(timeoutFuture, Function.identity())
            .whenCompleteAsync((result, exception) -> {
                if (exception == null) {
                    future.complete(result);
                } else {
                    retries++;
                    if (retries >= MAX_RETRIES) {
                        future.completeExceptionally(exception);
                    } else {
                        executeAsync();
                    }
                }
            });

        // Return the future    
        return future;
    }
}

---

Usage

int size = 10;
System.out.println("generating...");
List<RetriableTask> tasks = new ArrayList<>();
for (int i = 0; i < size; i++) {
    tasks.add(new RetriableTask(i));
}

System.out.println("issuing...");
List<CompletableFuture<Integer>> futures = new ArrayList<>();
for (int i = 0; i < size; i++) {
    futures.add(tasks.get(i).executeAsync());
}

System.out.println("Waiting...");
for (int i = 0; i < size; i++) {
    try {
        CompletableFuture<Integer> future = futures.get(i);
        int result = future.get();
        System.out.println(i + " result is " + result);
    } catch (Exception ex) {
        System.out.println(i + " I got exception!");
    }
}
System.out.println("Done waiting...");

---

Output

generating...
issuing...
Waiting...
0 I got exception!
1 I got exception!
2 I got exception!
3 I got exception!
4 I got exception!
5 I got exception!
6 result is 36
7 result is 42
8 result is 48
9 result is 54
Done waiting...

---

Main idea and some glue code (failAfter function) come from here.

Any other suggestions or improvement are welcome.