Resonate Recursive Fan Out Pattern Go

Resonate Recursive Fan-Out Pattern — Go

Version note. The Go SDK's first tagged release is 0.1.0. The tag is 0.1.0, not v0.1.0, so go get …@latest does not resolve to it — pin the tag explicitly: go get github.com/resonatehq/resonate-sdk-go@0.1.0. APIs may still change before a 1.0. Every code block here is verified against the 0.1.0 tag source and the example-fan-out-fan-in-go / example-recursive-factorial-go repos.

Overview

Recursive fan-out dispatches multiple child invocations in parallel, awaits them individually, and optionally recurses deeper. The Go expression is a two-loop pattern: a dispatch loop builds a []*resonate.Future slice, then a separate await loop reads each result. Mixing dispatch and await serializes the children — that is 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-go — dispatch all children first, await all second:

type FanoutArgs struct {
    Channels []string `json:"channels"`
    Message  string   `json:"message"`
}

type FanoutResult struct {
    Delivered []Delivery `json:"delivered"`
}

type SendArgs struct {
    Channel string `json:"channel"`
    Message string `json:"message"`
}

type Delivery struct {
    Channel string `json:"channel"`
    OK      bool   `json:"ok"`
    Reason  string `json:"reason,omitempty"`
}

func fanout(ctx *resonate.Context, args FanoutArgs) (FanoutResult, error) {
    // Dispatch loop — create all durable promises before awaiting any.
    futures := make([]*resonate.Future, 0, len(args.Channels))
    for _, ch := range args.Channels {
        f, err := ctx.RPC("send", SendArgs{Channel: ch, Message: args.Message})
        if err != nil {
            return FanoutResult{}, err
        }
        futures = append(futures, f)
    }

    // Await loop — read results in order; record per-child failures.
    out := FanoutResult{Delivered: make([]Delivery, 0, len(futures))}
    for i, f := range futures {
        var d Delivery
        if err := f.Await(&d); err != nil {
            out.Delivered = append(out.Delivered, Delivery{
                Channel: args.Channels[i],
                OK:      false,
                Reason:  err.Error(),
            })
            continue
        }
        out.Delivered = append(out.Delivered, d)
    }
    return out, nil
}

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

Recursive fan-out

A workflow can call ctx.RPC(SameName, smallerArgs) to recurse. Each recursive call is its own durable promise; with multiple workers running the same registered name, the promise tree fans out across them automatically. From example-recursive-factorial-go:

// factorial/factorial.go

const Name = "Factorial"
const WorkerGroup = "factorial-workers"

type Args struct {
    N int `json:"n"`
}

func Workflow(ctx *resonate.Context, args Args) (int, error) {
    // Base case — must always be present to terminate the recursion.
    if args.N <= 1 {
        return 1, nil
    }
    // Recurse via ctx.RPC so the sub-call is dispatched as a separate
    // durable promise; with multiple workers it may run on a different process.
    f, err := ctx.RPC(Name, Args{N: args.N - 1})
    if err != nil {
        return 0, err
    }
    var sub int
    if err := f.Await(&sub); err != nil {
        return 0, err
    }
    return args.N * sub, nil
}

Worker binary — joins the named group so only registered workers receive dispatches:

// cmd/worker/main.go

import "github.com/resonatehq/resonate-sdk-go/httpnet"

r, err := resonate.New(resonate.Config{
    Network: httpnet.NewHTTP(url, httpnet.HTTPOptions{
        PID:   pid,
        Group: factorial.WorkerGroup, // "factorial-workers"
    }),
})
if _, err := resonate.Register(r, factorial.Name, factorial.Workflow); err != nil {
    log.Fatalf("Register: %v", err)
}
// block on SIGINT / SIGTERM; do NOT call r.Stop() on long-running workers

Client binary — uses a distinct group so it cannot be mistakenly assigned worker tasks:

// cmd/client/main.go

r, err := resonate.New(resonate.Config{
    Network: httpnet.NewHTTP(url, httpnet.HTTPOptions{
        PID:   pid,
        Group: "factorial-client", // distinct — not in WorkerGroup
    }),
})
target := fmt.Sprintf("poll://any@%s", factorial.WorkerGroup)
h, err := r.RPC(ctx, "factorial-10", factorial.Name, factorial.Args{N: 10},
    resonate.RPCOptions{Target: target})
var result int
if err := h.Result(ctx, &result); err != nil { ... }

Collecting results and partial-failure handling

The example-fan-out-fan-in-go pattern records per-child failures rather than aborting the parent at the first error. Use this when you want a summary of all deliveries rather than a short-circuit:

for i, f := range futures {
    var d Delivery
    if err := f.Await(&d); err != nil {
        // Record the failure and continue to the next child.
        out.Delivered = append(out.Delivered, Delivery{
            Channel: channels[i],
            OK:      false,
            Reason:  err.Error(),
        })
        continue
    }
    out.Delivered = append(out.Delivered, d)
}

To fail-fast on the first child error, replace the continue block with return ..., err.

ctx.Run vs ctx.RPC for fan-out

Both methods return *resonate.Future immediately. Both support the two-loop fan-out pattern. ctx.Run functions must return promptly — long-running or blocking work belongs in ctx.RPC.

Distinct Go idioms

• []*resonate.Future slice with make(..., 0, len(items)) — pre-allocate to the known capacity; the two-loop pattern iterates this slice twice (dispatch, then await).
• Two-loop pattern is mandatory — a single loop that dispatches and awaits in the same iteration serializes children, defeating the purpose of fan-out.
• Struct args with json tags — args are JSON-encoded into the durable promise payload; use exported fields with json tags on every type that crosses ctx.RPC or ctx.Run.
• WorkerGroup constant — define group names as shared constants in a package imported by both worker and client binaries (see factorial.WorkerGroup). This prevents string drift between the httpnet.HTTPOptions{Group: ...} in the worker and the poll://any@... target in the client.
• Distinct client group — processes that only create promises (CLI tools, HTTP gateways, test harnesses) must use a different group name from the worker group. Without this, the server dispatches tasks to the client process, which hasn't registered the function, and the task is dropped as "function not registered".
• r.Stop() only on short-lived processes — call it on demo binaries and one-shot clients. Do not call it on long-running workers; let SIGINT/SIGTERM end the lifecycle.
• Replay safety — the whole workflow body re-runs on resume. The dispatch loop deterministically re-issues the same ctx.RPC / ctx.Run calls, and already-settled children short-circuit. No replay guard code is needed.

Avoid

• Awaiting inside the dispatch loop — ctx.RPC followed immediately by f.Await in the same loop body 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(SameName, ...) with a termination condition (if args.N <= 1, if depth == 0, etc.).
• Sharing the default group between client and worker — Config.URL shorthand builds a transport in the "default" group. If both the worker and the client use Config.URL, the client joins the worker pool and steals dispatches it cannot fulfill.

Related skills

• resonate-basic-durable-world-usage-go — ctx.Run, ctx.RPC, Future.Await, options structs
• resonate-saga-pattern-go — 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-rust — .spawn() / Vec<DurableFuture> sibling