Lambda Functions are now "Durable"
State Machines Without the Pain
AWS just launched Lambda Durable Functions at re:Invent 2025, and they might quietly be one of the most important additions to the serverless toolbox in years.
What are Durable Functions?
Durable functions let you write multi‑step, long‑running workflows directly inside Lambda. You write sequential code in Python or TypeScript, wrap calls in steps (with automatic retries and checkpointing), and use waits to suspend execution for minutes, hours, or even up to a year — without paying for idle compute. When the function resumes, it replays from the start but skips completed checkpoints, guaranteeing consistency.
Think of it as Lambda with a built‑in state machine.
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How do they compare to Step Functions?
Step Functions have always been the “official” way to orchestrate workflows in AWS. But let’s be honest: they’re verbose, hard to debug, and painful to maintain. Durable functions flip the model: instead of drawing JSON diagrams, you just write code. The SDK handles state, retries, and recovery behind the scenes. It’s simpler, more intuitive, and closer to how developers actually think.
Why not just spin up a long‑running service?
Sure, you could run a workflow engine on EC2, ECS, or Fargate. But then you’re back to managing infrastructure, scaling, and idle costs. Durable functions inherit Lambda’s scale‑to‑zero model: no servers, no idle billing, no ops overhead. For workflows that wait on human approvals, external APIs, or multi‑day processes, this is a huge win.
Does it make sense pricing‑wise?
Yes. The killer feature is that waits don’t cost you compute. You only pay for actual execution time. That makes durable functions far cheaper than keeping a container alive for hours or days. For businesses, this means you can finally build resilient, long‑running workflows without worrying about runaway costs.
The bigger picture
AWS is marketing this heavily for AI agent orchestration — chaining model calls, human feedback loops, etc. That’s fine, but don’t get distracted. The real opportunity is much broader: everything you used Step Functions for, but easier. Payments, order processing, onboarding flows, compliance checks — all now possible with clean, sequential Lambda code.
Durable functions are not just an AI play. They’re a simpler state machine baked into Lambda. And that opens the door to a whole new class of serverless applications.