Parallel Workers with Effect

Golem runs invocations to one durable agent instance sequentially. For actual parallel work, send the work to distinct agent instances and compose their typed remote-call Effects with Effect v4 structured-concurrency operators. Do not translate Promise.all, floating promises, or async handlers from the Promise-based TypeScript SDK.

Use the mechanisms according to their semantics:

Mechanism	Purpose
Effect.all	Run remote-call Effects concurrently and collect ordered results
Effect.forkChild / Fiber.join	Manage individual child fibers within the current invocation
remote.method.trigger(input)	Submit durable background work when no result is needed
Agents.fork	Clone the current Golem agent and its state into a phantom instance

Fan Out to Durable Child Agents

Define each worker as a normal durable agent. Constructor parameters are its durable identity, so different id values address different worker instances.

import { Effect, Schema } from "effect";
import { defineAgent, method } from "@golemcloud/effect-golem";

const Numbers = Schema.Array(Schema.Number);

export const Worker = defineAgent({
  name: "Worker",
  mode: "durable",
  id: {
    id: Schema.Number,
  },
  methods: {
    compute: method({
      input: { value: Schema.Number },
      success: Schema.Number,
    }),
  },
}).implement({
  init: () => Effect.void,
  methods: () => ({
    compute: ({ value }) => Effect.succeed(value * 2),
  }),
});

export const Coordinator = defineAgent({
  name: "Coordinator",
  mode: "durable",
  id: {
    name: Schema.String,
  },
  methods: {
    fanOut: method({
      input: { items: Numbers },
      success: Numbers,
    }),
  },
}).implement({
  init: () => Effect.void,
  methods: () => ({
    fanOut: ({ items }) =>
      Effect.all(
        items.map((value, id) =>
          Effect.gen(function* () {
            const worker = yield* Worker.client.get({ id });
            return yield* worker.compute({ value });
          }),
        ),
        { concurrency: "unbounded" },
      ).pipe(Effect.orDie),
  }),
});

Worker.client.get({ id }) and worker.compute({ value }) both return lazy Effects. Effect.all executes those Effects concurrently, retains input order in its output, and returns an empty array for empty input. In a stateful existing application, add the method contract and handler to the existing coordinator instead of registering a duplicate agent type.

Import every implementation module from src/main.ts using its emitted .js suffix:

import "./worker.js";
import "./coordinator.js";

Bound Concurrency

Use a numeric concurrency value when the input can be large or a downstream system has a capacity limit:

const calls = items.map((value, id) =>
  Effect.gen(function* () {
    const worker = yield* Worker.client.get({ id });
    return yield* worker.compute({ value });
  }),
);

const results = yield * Effect.all(calls, { concurrency: 5 });

This runs at most five lookup-and-call Effects at once. It is preferable to manually slicing the input into batches because Effect owns scheduling, interruption, and result ordering. Pick stable worker identities when calls across invocations should reach the same worker; include a job identity when each fan-out must use a separate durable worker set.

Fail-Fast and Partial Results

By default, Effect.all fails on the first typed failure. Effects that have not started are skipped, and already-started siblings may be interrupted. Remote cancellation is best-effort: a worker invocation that already began can still finish and apply side effects even when its caller fiber is interrupted. Make remote work idempotent when retries or interruption can overlap.

When every worker outcome must be retained, turn each typed failure into data before collecting:

const outcomes =
  yield *
  Effect.all(
    calls.map((call) =>
      Effect.match(call, {
        onFailure: (error) => ({
          _tag: "Failure" as const,
          error: String(error),
        }),
        onSuccess: (value) => ({
          _tag: "Success" as const,
          value,
        }),
      }),
    ),
    { concurrency: 8 },
  );

Effect.match handles typed failures, not defects or interruption. If those outcomes must also be returned as data, use Effect.exit for each call and declare a schema-safe public result. Do not silently apply Effect.orDie when the caller is expected to recover; either handle the remote error or map it to a domain error declared in the coordinator method.

Child Fibers Are Structured, Not Background Agents

Use child fibers when individual calls need separate handles, cancellation, or joins:

import { Effect, Fiber } from "effect";

const left = yield * Effect.forkChild(firstWorker.compute({ value: 1 }));
const right = yield * Effect.forkChild(secondWorker.compute({ value: 2 }));

const results =
  yield *
  Effect.all([Fiber.join(left), Fiber.join(right)], {
    concurrency: "unbounded",
  });

Effect.forkChild creates a fiber in the current agent invocation; it does not create a Golem agent, clone state, or make work outlive the parent. Prefer direct Effect.all unless individual fiber control is needed.

Fire-and-Forget Fan-Out

If results are intentionally not needed, submit each remote method through its real .trigger form. Yield the trigger Effects—the calls do nothing if their Effects are discarded:

yield *
  Effect.all(
    regions.map((region) =>
      Effect.gen(function* () {
        const worker = yield* RegionWorker.client.get({ region });
        yield* worker.runReport.trigger({ reportId });
      }),
    ),
    { concurrency: "unbounded" },
  );

Trigger success means the host accepted the invocation, not that the worker completed. There is no result or remote domain error to collect afterward. Use awaited calls when fan-in needs the result. Host-backed Agents.Promises can implement an explicit later rendezvous, but it is unrelated to a JavaScript Promise; this SDK version does not export a ready-made Effect Schema for passing its PromiseId through an agent method.

Clone the Current Agent with Agents.fork

Agents.fork is an Effect value that asks Golem to clone the current agent at the current execution point. Both agents resume from it: the source sees tag: "original", and the clone sees tag: "forked". This is different from Effect.forkChild.

Use a host-backed Golem promise created before the fork to return data from the cloned agent:

import { Effect } from "effect";
import { Agents } from "@golemcloud/effect-golem";

const parallelCompute = Effect.gen(function* () {
  const promiseId = yield* Agents.Promises.create;
  const branch = yield* Agents.fork;

  if (branch.tag === "forked") {
    const payload = new TextEncoder().encode("forked-result");
    yield* Agents.Promises.complete(promiseId, payload);
    return "forked done";
  }

  const payload = yield* Agents.Promises.await(promiseId);
  const forkedResult = new TextDecoder().decode(payload);
  return `Combined: original + ${forkedResult}`;
}).pipe(Effect.orDie);

The forked branch must finish its handler after completing its slice; otherwise it can continue through coordinator-only logic. Agents.Promises.await is durably suspendable. Interrupting the JavaScript wait does not cancel or complete the host promise, so every created promise still needs a peer that completes it.

Choosing a Pattern

Requirement	Use
Independent work with persistent worker identities	Distinct durable agents + Effect.all
Limit pressure on workers or dependencies	Numeric Effect.all concurrency
Need every typed success and failure	Per-call Effect.match + Effect.all
Work should continue without returning a value	Remote .trigger(...)
Need individual in-invocation cancellation or joins	Effect.forkChild + Fiber
Need a copy of the current agent state	Agents.fork + Agents.Promises

Key Constraints

• Calls to the same agent identity are still processed sequentially; use distinct identities for real worker parallelism.
• Compose SDK-returned Effects. Do not use Promise.all, plain async handlers, or floating Effects.
• Use named records for client.get and remote method inputs, including {} for no parameters.
• Avoid synchronous RPC cycles in which two sequential agents await each other; use .trigger only when the callback result is not required.
• Durable child agents remain addressable after fan-in. The pinned Effect SDK client has no remote agent deletion method, so do not invent delete, remove, or destroy on a remote handle.
• Put arbitrary external or nondeterministic side effects behind host-backed SDK APIs or an appropriate durable wrapper; Effect concurrency alone does not make a raw JavaScript side effect replay-safe.