AWS Lambda Durable Functions

Summary

Lambda durable functions solve the problem of building reliable long-running workflows on serverless infrastructure. Traditional Lambda functions have execution limits, but durable functions use checkpointing to track progress and replay from interruptions—skipping completed work while maintaining consistency.

Key Concepts

• Durable Execution: A function execution that persists across interruptions. When resumed, it replays from the beginning but skips checkpointed work using stored results.
• Steps: Execute business logic with built-in retries and progress tracking. Wrap service calls in steps for automatic checkpointing.
• Waits: Suspend execution without compute charges. Ideal for human-in-the-loop processes or external events.

How Checkpoint/Replay Works

flowchart LR
    Start[Function Start] --> Run[Execute Code]
    Run --> Check{Checkpoint
Reached?}
    Check -->|Yes| Store[Store Result]
    Store --> Continue[Continue Execution]
    Continue --> Check
    Check -->|No| Continue

    Interrupt[Interruption] -.-> Replay[Replay from Start]
    Replay --> Skip{Already
Checkpointed?}
    Skip -->|Yes| UseStored[Use Stored Result]
    Skip -->|No| Execute[Execute Normally]
    UseStored --> Continue
    Execute --> Store

The SDK wraps your Lambda handler and provides a DurableContext with access to steps and waits. You write sequential code; the SDK handles state management transparently.

Use Cases

1. AI Workflows: Chain model calls, incorporate human feedback, handle long-running inference tasks
2. Payment Processing: Multi-step authorization with transaction state maintenance and automatic rollback
3. Order Fulfillment: Coordinate inventory, payment, shipping, and notifications across services
4. Business Processes: Employee onboarding, loan approvals, compliance workflows spanning days or weeks

Trade-offs

Connections

• building-effective-agents - Discusses workflow patterns for AI agents that durable functions can reliably execute at scale
• build-autonomous-agents-using-prompt-chaining-with-ai-primitives - Prompt chaining workflows are a natural fit for durable function orchestration