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repeated-completable-future.md

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RepeatedCompletableFuture

Circumstance

  • There is an idempotent function that fails until suitable conditions are provided.
  • The function must be executed until success.
  • The function must be executed asynchronously.

Using

If it is possible to start dependent microservices concurrently, it can simplify their deployment. But for that we need to make that microservices a little tolerant to short-term unavailability of some resources. Usually, before interacting with a resource, we have to establish a connection with it. Since during the connection, the resource may be unavailable for a while, we perform several connection attempts before reporting the problem.

Imagine your resource has the following client API.

interface SomeResourceClient {
    SomeResourceConnection connect(String uri) throws IOException;
}

interface SomeResourceConnection {
    String loadData();
}

Using the RepeatedCompletableFuture class you can create an asynchronous task that tries to connect to this resource until success.

SomeResourceClient resourceClient;
...

RepeatedCompletableFuture<SomeResourceConnection> resourceConnection = RepeatedCompletableFuture.startAttempts(
        () -> resourceClient.connect("my/resource"),
        error -> log.atWarn().setCause(error).log("The connection attempt failed."),
        Duration.ofSeconds(5)
);

In the last example, the connect() method will be invoked infinitely until it completes without exception. If an attempt fails, this error is logged and a new connection attempt will be performed in 5 seconds.

If infinite attempts are inappropriate, you can interrupt this process by invoking the cancel() method. In the following example, the attempts are interrupted after 10 failures.

SomeResourceClient resourceClient;
AtomicInteger connectionAttempts;
RepeatedCompletableFuture<SomeResourceConnection> resourceConnection;
...

void connect() {
    resourceConnection = RepeatedCompletableFuture.startAttempts(
            () -> resourceClient.connect("my/resource"),
            error -> {
                log.atWarn().setCause(error).log("The connection attempt failed.");

                if (connectionAttempts.incrementAndGet() >= 10) {
                    resourceConnection.cancel(true);
                }
            },
            Duration.ofSeconds(5)
    );
}

To access the connection you can await success or use the current connection state by invoking the lastAttempt() method. In the first case you will not be able to work with the resource until either the connection is not established or this connection process is interrupted. In the second case you may get an error if the last attempt was unsuccessful.

// Await the successful connection
resourceConnection
        .thenAccept(connection -> {
            var data = connection.loadData();
            System.out.println(data);
        });

// Or use the current connection state
resourceConnection.lastAttempt()
        .thenAccept(connection -> {
            var data = connection.loadData();
            System.out.println(data);
        });