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/[email protected]. 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
}