AWS Lambda Durable Functions

AWS Lambda durable functions

Build resilient multi-step applications and AI workflows that can execute for up to 1 year while maintaining reliable progress despite interruptions.

Onboarding

Step 1: Validate Prerequisites

Before using AWS Lambda durable functions, verify:

1. AWS CLI is installed (2.33.22 or higher) and configured:

   aws --version
   aws sts get-caller-identity
   

2. Runtime environment is ready:

   • For TypeScript/JavaScript: Node.js 22+ (node --version)
   • For Python: Python 3.11+ (python --version. Note that currently only Lambda runtime environments 3.13+ come with the Durable Execution SDK pre-installed. 3.11 is the min supported Python version by the Durable SDK itself, however, you could use OCI to bring your own container image with your own Python runtime + Durable SDK.)

3. Deployment capability exists (one of):

   • AWS SAM CLI (sam --version) 1.153.1 or higher
   • AWS CDK (cdk --version) v2.237.1 or higher
   • Direct Lambda deployment access

Step 2: Select language and IaC framework

Language Selection

Default: TypeScript

Override syntax:

• "use Python" → Generate Python code
• "use JavaScript" → Generate JavaScript code

When not specified, ALWAYS use TypeScript

IaC framework selection

Default: CDK

Override syntax:

• "use CloudFormation" → Generate YAML templates
• "use SAM" → Generate YAML templates

When not specified, ALWAYS use CDK

Error Scenarios

Unsupported Language

• List detected language
• State: "Durable Execution SDK is not yet available for [framework]"
• Suggest supported languages as alternatives

Unsupported IaC Framework

• List detected framework
• State: "[framework] might not support Lambda durable functions yet"
• Suggest supported frameworks as alternatives

Serverless MCP Server Unavailable

• Inform user: "AWS Serverless MCP not responding"
• Ask: "Proceed without MCP support?"
• DO NOT continue without user confirmation

Step 3: Install SDK

For TypeScript/JavaScript:

npm install @aws/durable-execution-sdk-js
npm install --save-dev @aws/durable-execution-sdk-js-testing

For Python:

pip install aws-durable-execution-sdk-python
pip install aws-durable-execution-sdk-python-testing

When to Load Reference Files

Load the appropriate reference file based on what the user is working on:

• Getting started, basic setup, example, ESLint, or Jest setup -> see getting-started.md
• Understanding replay model, determinism, or non-deterministic errors -> see replay-model-rules.md
• Creating steps, atomic operations, or retry logic -> see step-operations.md
• Waiting, delays, callbacks, external systems, or polling -> see wait-operations.md
• Parallel execution, map operations, batch processing, or concurrency -> see concurrent-operations.md
• Error handling, retry strategies, saga pattern, or compensating transactions -> see error-handling.md
• Advanced error handling, timeout handling, circuit breakers, or conditional retries -> see advanced-error-handling.md
• Testing, local testing, cloud testing, test runner, or flaky tests -> see testing-patterns.md
• Deployment, CloudFormation, CDK, SAM, log groups, deploy, or infrastructure -> see deployment-iac.md
• Advanced patterns, GenAI agents, completion policies, step semantics, or custom serialization -> see advanced-patterns.md
• troubleshooting, stuck execution, failed execution, debug execution ID, or execution history -> see troubleshooting-executions.md

Quick Reference

Basic Handler Pattern

TypeScript:

import { withDurableExecution, DurableContext } from '@aws/durable-execution-sdk-js';

export const handler = withDurableExecution(async (event, context: DurableContext) => {
  const result = await context.step('process', async () => processData(event));
  return result;
});

Python:

from aws_durable_execution_sdk_python import durable_execution, DurableContext

@durable_execution
def handler(event: dict, context: DurableContext) -> dict:
    result = context.step(lambda _: process_data(event), name='process')
    return result

Critical Rules

1. All non-deterministic code MUST be in steps (Date.now, Math.random, API calls)
2. Cannot nest durable operations - use runInChildContext to group operations
3. Closure mutations are lost on replay - return values from steps
4. Side effects outside steps repeat - use context.logger (replay-aware)

Python API Differences

The Python SDK differs from TypeScript in several key areas:

• Steps: Use @durable_step decorator + context.step(my_step(args)), or inline context.step(lambda _: ..., name='...'). Prefer the decorator for automatic step naming.
• Wait: context.wait(duration=Duration.from_seconds(n), name='...')
• Exceptions: ExecutionError (permanent), InvocationError (transient), CallbackError (callback failures)
• Testing: Use DurableFunctionTestRunner class directly - instantiate with handler, use context manager, call run(input=...)

Invocation Requirements

Durable functions require qualified ARNs (version, alias, or $LATEST):

# Valid
aws lambda invoke --function-name my-function:1 output.json
aws lambda invoke --function-name my-function:prod output.json

# Invalid - will fail
aws lambda invoke --function-name my-function output.json

IAM Permissions

Your Lambda execution role MUST have the AWSLambdaBasicDurableExecutionRolePolicy managed policy attached. This includes:

• lambda:CheckpointDurableExecution - Persist execution state
• lambda:GetDurableExecutionState - Retrieve execution state
• CloudWatch Logs permissions

Additional permissions needed for:

• Durable invokes: lambda:InvokeFunction on target function ARNs
• External callbacks: Systems need lambda:SendDurableExecutionCallbackSuccess and lambda:SendDurableExecutionCallbackFailure

Validation Guidelines

When writing or reviewing durable function code, ALWAYS check for these replay model violations:

1. Non-deterministic code outside steps: Date.now(), Math.random(), UUID generation, API calls, database queries must all be inside steps
2. Nested durable operations in step functions: Cannot call context.step(), context.wait(), or context.invoke() inside a step function — use context.runInChildContext() instead
3. Closure mutations that won't persist: Variables mutated inside steps are NOT preserved across replays — return values from steps instead
4. Side effects outside steps that repeat on replay: Use context.logger for logging (it is replay-aware and deduplicates automatically)

When implementing or modifying tests for durable functions, ALWAYS verify:

1. All operations have descriptive names
2. Tests get operations by NAME, never by index
3. Replay behavior is tested with multiple invocations
4. Use LocalDurableTestRunner for local testing

MCP Server Configuration

Write access is enabled by default. The plugin ships with --allow-write in .mcp.json, so the MCP server can create projects, generate IaC, and deploy on behalf of the user.

Access to sensitive data (like Lambda and API Gateway logs) is not enabled by default. To grant it, add --allow-sensitive-data-access to .mcp.json.

Resources

• AWS Lambda durable functions Documentation
• JavaScript SDK Repository
• Python SDK Repository
• IAM Policy Reference