Concurrency & Fibers

When to use

• Parallelizing independent work safely with limits
• Coordinating background tasks and lifecycle
• Racing operations for latency control

Parallel Patterns

const results = yield* Effect.all(tasks, { concurrency: 10 })

const processed = yield* Effect.forEach(items, processItem, { concurrency: 5 })

Fiber Lifecycle

const fiber = yield* Effect.fork(work)
const value = yield* Fiber.join(fiber)
yield* Fiber.interrupt(fiber)

Racing / Timeouts

const fastest = yield* Effect.race(slow, fast)
const withTimeout = yield* Effect.timeout(operation, "5 seconds")

Guidance

• Limit concurrency to protect resources
• Use

  fork

  for background loops; always manage interruption
• Prefer

  Effect.all

  for independent operations
• Use

  Effect.forEach

  with

  concurrency

  for pools
• Combine with retries and timeouts for resilient parallelism

Pitfalls

• Unbounded concurrency can exhaust CPU/IO or hit rate limits
• Always interrupt background fibers on shutdown
• Don’t block inside fibers; keep work asynchronous/effectful

Cross-links

• Errors & Retries: backoff + jitter for transient failures
• Streams & Pipelines: concurrent map over streams
• EffectPatterns inspiration: https://github.com/PaulJPhilp/EffectPatterns

Local Source Reference

CRITICAL: Search local Effect source before implementing

The full Effect source code is available at

docs/effect-source/

. Always search the actual implementation before writing Effect code.

Key Source Files

• Effect:

  docs/effect-source/effect/src/Effect.ts

• Fiber:

  docs/effect-source/effect/src/Fiber.ts

• Duration:

  docs/effect-source/effect/src/Duration.ts

Example Searches

# Find Effect.all and concurrency patterns
grep -F "Effect.all" docs/effect-source/effect/src/Effect.ts

# Find forEach with concurrency
grep -rF "forEach" docs/effect-source/effect/src/ | grep -F "concurrency"

# Study Fiber lifecycle operations
grep -F "export" docs/effect-source/effect/src/Fiber.ts | grep -E "fork|join|interrupt"

# Find race and timeout implementations
grep -F "race" docs/effect-source/effect/src/Effect.ts
grep -F "timeout" docs/effect-source/effect/src/Effect.ts

Workflow

1. Identify the concurrency API you need (e.g., Effect.all, fork)
2. Search

   docs/effect-source/effect/src/Effect.ts

   for the implementation
3. Study the types and concurrency options
4. Look at test files for usage examples
5. Write your code based on real implementations

Real source code > documentation > assumptions

References

• Agent Skills overview: https://www.anthropic.com/news/skills
• Skills guide: https://docs.claude.com/en/docs/claude-code/skills