Sequential composition for arbitrary number of calls in Vertx with Futures

Question

We use Futures in vertx in examples like:

Future fetchVehicle = getUserBookedVehicle(routingContext, client);

        fetchVehicle.compose         


        

Answer 1

If you want to feed the response from the previous request to the next request, and suppose you have different handlers for each response. You can add a helper method

private <T> Future<T> chain(Future<T> init, List<Function<T, Future<T>>> handlers) {
    Future<T> result = init;
    for (Function<T, Future<T>> handler : handlers) {
        result = result.compose(handler);
    }
    return result;
}

And then change your code like this

    Future<JsonObject> fetchVehicle = getUserBookedVehicle(routingContext, client);

    Function<JsonObject, Future<JsonObject>> vehicleResponseHandler = vehicleJson ->
        vehicleDoor(routingContext, client, vehicleJson, lock);

    Function<JsonObject, Future<JsonObject>> anotherTrivialHandler = someJsonObj -> {
        // add here new request by using information from someJsonObj
        LOG.info("Hello from trivial handler {} ", someJsonObj);
        return Future.succeededFuture(someJsonObj);
    };

    List<Function<JsonObject, Future<JsonObject>>> handlers = new ArrayList<>();

    handlers.add(vehicleResponseHandler);
    handlers.add(anotherTrivialHandler);

    chain(fetchVehicle, handlers).setHandler( asyncResult -> {
        if (asyncResult.succeeded()) {
            handler.handle(Future.succeededFuture(new AsyncReply(200, "OK")));
        } else {
            handler.handle(Future.failedFuture(asyncResult.cause()));
        }
    });

But there is a limitation for this implementation which requires each chained Future must have the same type parameter T.

Answer 2

Here is a solution using map & reduce that executes a method in an orderly fashion and returns the accumulated result in the form of a Future<String>

 public static <T> Future<String> chainCall(List<T> list, Function<T, Future<String>> method){
        return list.stream().reduce(Future.succeededFuture(),// the initial "future"
                (acc, item) -> acc.compose(v -> method.apply(item)), // we return the compose of the previous "future" with "future" returned by next item processing
                (a,b) -> Future.future()); // not used! only useful for parallel stream.
    }

can be used as in the example below:

 chainCall(conversation.getRequestList(), this::sendApiRequestViaBus);

where sendApiRequestViaBus is:

/**
     * @param request The request to process
     * @return The result of the request processing. 
     */
    Future<String> sendApiRequestViaBus(ApiRequest request) {
        Future<String> future = Future.future();
        String address = CommandUtilsFactory.getInstance(request.getImplementation()).getApiClientAddress();
        log.debug("Chain call start msgId {}", request.getId());

        vertx.eventBus().send(address, JsonObject.mapFrom(request), deliveryOptions, res -> {
            log.debug("Chain call returns {}", request.getId());
            if (res.succeeded()) {
                future.complete("OK");
            } else {
                future.fail("KO");
            }
        });
        return future;
    }

I hope it helps.

Answer 3

Here's something handy. Hope it helps.

public static <R> Future<List<R>> allOfFutures(List<Future<R>> futures) {
    return CompositeFutureImpl.all(futures.toArray(new Future[futures.size()]))
            .map(v -> futures.stream()
                    .map(Future::result)
                    .collect(Collectors.toList())
            );
}