building-restate-services

Restate

Restate is a durable execution runtime. It records every step of a handler in a journal, so if the process crashes, the handler replays from the journal and resumes exactly where it left off. Services are regular applications using the Restate SDK (TypeScript, Python, Java, Go) that run behind a Restate Server (a single Rust binary on ports 8080 for ingress and 9070 for admin UI).

Detect context

Scan the project to determine the SDK and context:

1. Detect SDK:

   • package.json with @restatedev/restate-sdk -> TypeScript
   • requirements.txt or pyproject.toml with restate-sdk -> Python
   • pom.xml or build.gradle with dev.restate -> Java
   • go.mod with github.com/restatedev/sdk-go -> Go
   • If no SDK detected, check for project files to determine language, then ask the user

2. Detect existing Restate code: Grep for @restatedev, restate-sdk, dev.restate.sdk, github.com/restatedev

3. Detect AI frameworks: @ai-sdk/, openai-agents, google-adk, pydantic-ai

4. Detect workflow orchestrators: temporalio, @temporalio, aws-cdk/aws-stepfunctions, etc.

Core reference (always load)

After detecting the SDK, always load the SDK reference:

• references/<sdk>/api-and-pitfalls.md - Setup, API patterns, determinism rules, error handling, and pitfalls for the detected SDK

Context-based references (load when relevant)

Context	Reference
Design a new application, choose service types	references/design-and-architecture.md
Convert from a workflow orchestrator or existing app	references/translate-to-restate.md
Invoking services, interacting with invocations (cancel, attach, idempotency, sends, Kafka)	references/invocation-lifecycle.md
Build AI agent with Vercel AI SDK	references/ts/restate-vercel-ai-agents.md
Build AI agent with OpenAI Agents SDK	references/python/restate-openai-agents-agents.md
Build AI agent with Google ADK	references/python/restate-google-adk-agents.md
Build AI agent with Pydantic AI	references/python/restate-pydantic-ai-agents.md
Debug errors, stuck invocations, journal mismatches	references/debug-applications.md
Testing, deployment, server config, Kafka, advanced topics	Use the bundled restate-docs MCP server
Code examples and templates	github.com/restatedev/examples, github.com/restatedev/ai-examples

Before-you-design checklist

Before designing any Restate service architecture, check:

Question	Consult
Restate service types, stateful entities, keying, concurrency?	references/design-and-architecture.md
Non-Restate orchestrator in the project?	references/translate-to-restate.md
Invoking, cancelling, deduplicating, or attaching to invocations?	references/invocation-lifecycle.md
Error handling, compensation, sagas?	Error handling guide, Sagas guide
AI agent or LLM calls?	The relevant agent integration reference

Always verify before finishing

All checks below are mandatory.

• All side effects, external I/O, and DB calls wrapped in ctx.run()
• No native random, time, sleep, or UUID -- use ctx helpers
• Restate concurrency combinators only (no Promise.all, asyncio.gather/wait, CompletableFuture, goroutines + channels, select)
• No ctx operations inside ctx.run()
• TerminalError raised for non-retryable failures
• Python: no bare except:
• AI agents: set a retry policy for LLM calls
• Virtual Objects: no deadlock cycles
• Service registered and invoked via curl or the UI

Replay test - required on any handler logic change

Any change to handler business logic (new ctx operations, reordered steps, new ctx.run() blocks, new branches) must be covered by a Testcontainers test with always-replay enabled, and that test must pass before declaring the work done. Always-replay forces every journaled step to replay on every invocation, so non-determinism fails the test instead of failing in production on retry.

• TypeScript: alwaysReplay: true in RestateTestEnvironment.start
• Python: always_replay=True in restate.test_harness
• Java / Go: RESTATE_WORKER__INVOKER__INACTIVITY_TIMEOUT=0m on the Restate container

See the Testing section of references/<sdk>/api-and-pitfalls.md for the working scaffold.