Resonate Recursive Fan Out Pattern Java

Resonate Recursive Fan-Out Pattern — Java

Prerelease note. resonate-sdk-java is published on Maven Central — pin io.resonatehq:resonate-sdk-java:0.1.1. The API mirrors the Python SDK and may change before a stable 1.0. Requires Java 21+ (virtual threads, a feature generally available in Java 21). Every code block here is compile-verified against 0.1.1 and drawn from the example-fan-out-fan-in-java / example-recursive-factorial-java repos and develop/java.mdx (docs PR #230).

Overview

Recursive fan-out dispatches multiple child invocations in parallel, awaits them individually, and optionally recurses deeper. The Java expression is a two-loop pattern: a dispatch loop builds a List<ResonateFuture<T>>, then a separate await loop reads each result. Mixing dispatch and await serializes the children — the single most common mistake.

For the language-agnostic mental model (promise deduplication, load-balancing, detached vs. result-gathering), see resonate-recursive-fan-out-pattern-typescript.

When to use

• Batch processing where items are independent and each needs durability
• Map-reduce shaped workflows (fan out N workers, aggregate results)
• Recursive tree / graph traversal with dynamic depth
• Any case where a crash mid-fan-out must resume, not restart from zero

Dispatch-then-await shape

The canonical fan-out from example-fan-out-fan-in-java — dispatch all children first, await all second:

import io.resonatehq.resonate.Context;
import io.resonatehq.resonate.Context.ResonateFuture;
import java.util.ArrayList;
import java.util.List;

public record Delivery(String channel, boolean ok) {}

public static List<Delivery> fanout(Context ctx, List<String> channels, String message) {
    // Fan out: spawn a child per channel, collecting futures without awaiting yet.
    List<ResonateFuture<Delivery>> futures = new ArrayList<>();
    for (String channel : channels) {
        futures.add(ctx.run(FanOutFanIn::send, channel, message));
    }

    // Fan in: await every child and aggregate.
    List<Delivery> delivered = new ArrayList<>();
    for (ResonateFuture<Delivery> future : futures) {
        delivered.add(future.await());
    }
    return delivered;
}

public static Delivery send(Context ctx, String channel, String message) {
    return new Delivery(channel, true);
}

The dispatch loop runs serially in user code, but each ctx.run creates an independent durable promise — all children execute concurrently. The await loop only reads results; it does not gate execution of siblings.

Partial failure. As written, an exception from any child's future.await() propagates and stops the loop (fail-fast). To collect every result instead — recording per-child failures rather than aborting at the first — wrap each future.await() in its own try/catch inside the await loop and append a failure record on the catch. Choose fail-fast when one bad child should abort the batch, collect-all when you want a summary of every delivery.

Recursive fan-out across workers via ctx.rpc

A workflow can call ctx.rpc(NAME, smallerArgs) to recurse through the server. Each recursive call is its own durable promise; with multiple workers registered under the same name, the recursion fans out across them. From example-recursive-factorial-java:

import io.resonatehq.resonate.Context;
import io.resonatehq.resonate.Context.Opts;

public final class Factorial {
    private Factorial() {}

    public static final String NAME = "factorial";
    public static final String WORKER_GROUP = "factorial-workers";

    /**
     * Compute n! by recursively dispatching factorial(n-1) to the worker group via ctx.rpc.
     * Returns long — 13! overflows int. The by-name RPC result is read via Number because Jackson
     * decodes a small JSON integer as Integer and a large one as Long.
     */
    public static long factorial(Context ctx, int n) {
        if (n <= 1) {
            return 1L;
        }
        long sub = ((Number) ctx.options(new Opts().withTarget(WORKER_GROUP))
                .rpc(NAME, n - 1).await()).longValue();
        return (long) n * sub;
    }
}

ctx.rpc(NAME, n - 1) dispatches by name to the worker group, returning a ResonateFuture<Object> — hence the Number read. The base case (if (n <= 1)) must always terminate the recursion.

Worker / client split

The recursion runs across processes that register the function under a named group; a client targets that group without registering anything.

Worker — joins the group and registers the function, then blocks (never stop):

import io.resonatehq.resonate.Resonate;
import java.util.concurrent.CountDownLatch;

public static void main(String[] args) throws InterruptedException {
    String url = System.getenv().getOrDefault("RESONATE_URL", "http://localhost:8001");
    Resonate r = Resonate.builder().url(url).group(Factorial.WORKER_GROUP).build();
    r.register(Factorial::factorial);
    new CountDownLatch(1).await(); // keep the worker alive to receive tasks
}

Client — uses a distinct group so it cannot be assigned worker tasks, and dispatches by name to the worker group:

import io.resonatehq.resonate.Context.Opts;
import io.resonatehq.resonate.Handle.ResonateHandle;
import io.resonatehq.resonate.Resonate;

public static void main(String[] args) {
    int n = args.length > 0 ? Integer.parseInt(args[0]) : 6;
    String url = System.getenv().getOrDefault("RESONATE_URL", "http://localhost:8001");

    Resonate r = Resonate.builder().url(url).group("factorial-client").build();
    try {
        String id = "factorial-" + n; // stable id — a second run returns the cached result
        ResonateHandle<Object> handle =
                r.options(new Opts().withTarget(Factorial.WORKER_GROUP)).rpc(id, Factorial.NAME, n);
        long result = ((Number) handle.result()).longValue();
        System.out.printf("factorial(%d) = %d%n", n, result);
    } finally {
        r.stop();
    }
}

The client never registers factorial — it only dispatches by name to the worker group. Routing through a non-default group keeps clients out of the task pool so only real workers execute the recursion. Start a second worker before invoking and the intermediate steps distribute across both processes.

ctx.run vs ctx.rpc for fan-out

Both methods return a ResonateFuture immediately and both support the two-loop fan-out pattern. ctx.run functions must return promptly — long-running or blocking work belongs in ctx.rpc.

Distinct Java idioms

• List<ResonateFuture<T>> collected in the dispatch loop, drained in the await loop — the two-loop pattern. A single loop that dispatches and awaits in the same iteration serializes children.
• Typed futures from method references, untyped from by-name rpc — ctx.run(Owner::send, ...) gives ResonateFuture<Delivery>; ctx.rpc(NAME, ...) gives ResonateFuture<Object>, so read numerics through Number.
• group(...) on the builder for the worker group, and a distinct group for the client — Java sets the group directly on the builder, not on a separate transport.
• Shared NAME / WORKER_GROUP constants in a class imported by both worker and client — prevents string drift between the worker's group(...) and the client's withTarget(...).
• Stable id for idempotent recursion — "factorial-" + n means a second invocation returns the cached result instead of recomputing.
• new CountDownLatch(1).await() keeps a worker alive; never r.stop() a worker.
• Replay safety is automatic — the body re-runs on resume, the dispatch loop deterministically re-issues the same calls, and settled children short-circuit. No replay-guard code needed.

Avoid

• Awaiting inside the dispatch loop — ctx.run/ctx.rpc immediately followed by await in the same iteration runs children sequentially. Collect all futures first, then await.
• Unbounded recursion without a base case — the server-side promise graph is not free; always guard ctx.rpc(NAME, ...) with a termination condition (if (n <= 1)).
• Sharing the default group between client and worker — a client in the worker group joins the task pool and steals dispatches it cannot fulfill. Give the client its own group.
• Casting a by-name rpc numeric result straight to Long — Jackson may have decoded it as Integer. Read through Number.
• Reading a List<MyRecord> (or any generic) back from a by-name rpc — the by-name form decodes against Object, so Jackson hands back List<LinkedHashMap>, not your record type. Use the method-reference form (ctx.rpc(Owner::fn, ...) / ctx.run(Owner::fn, ...)) for a typed future. See resonate-basic-debugging-java.

Related skills

• resonate-basic-durable-world-usage-java — ctx.run, ctx.rpc, ResonateFuture.await, Opts
• resonate-basic-ephemeral-world-usage-java — the worker/client builder setup, group(...), typed vs untyped handles
• resonate-saga-pattern-java — fan-out alongside compensating transactions
• durable-execution — foundational replay semantics
• resonate-recursive-fan-out-pattern-typescript — canonical mental model, deduplication strategy
• resonate-recursive-fan-out-pattern-python — the closest sibling; the Java API mirrors Python