AWS Lambda SnapStart
AWS Lambda SnapStart is a performance feature that dramatically reduces cold-start latency by taking a cached, encrypted snapshot of an initialized execution environment (memory and disk state) and resuming new function invocations from…
Definition
AWS Lambda SnapStart is a performance feature that dramatically reduces cold-start latency by taking a cached, encrypted snapshot of an initialized execution environment (memory and disk state) and resuming new function invocations from that snapshot instead of starting from scratch.
Overview
Cold starts — the delay incurred when AWS Lambda must initialize a new execution environment before running a function for the first time or after scaling out — have long been a pain point for latency-sensitive serverless workloads, particularly for runtimes with heavier startup costs like Java. SnapStart addresses this by performing the expensive initialization (loading the runtime, executing static initializers, warming JIT-compiled code paths) once, then snapshotting the resulting memory and disk state to encrypted, low-latency storage. When a new invocation needs an execution environment, instead of re-running the full initialization sequence, Lambda resumes the sandboxed environment directly from the cached snapshot, which can cut cold-start latency by up to 90 percent or more for supported workloads. AWS applies this optimization automatically once SnapStart is enabled on a function version — no code changes are strictly required, though developers are encouraged to use runtime hooks (`beforeCheckpoint` and `afterRestore`) to handle state that should not be reused across invocations, such as database connections, random seeds, or unique identifiers generated during initialization. SnapStart originally launched for Java (Corretto) functions and has since expanded to other managed runtimes including Python and .NET, with the caching mechanism designed around each runtime's initialization characteristics. It applies to published function versions, not the unpublished `$LATEST` version, and works best for functions with expensive, deterministic startup work and relatively infrequent cold starts, since the snapshot itself needs to be created and refreshed as code changes. A key operational consideration is uniqueness: because a resumed environment restores prior in-memory state, values generated before the snapshot (like cryptographic material or connection tokens) can be inadvertently reused across invocations unless explicitly regenerated in the `afterRestore` hook, which is a common source of subtle bugs if overlooked.
Key Features
- Snapshots a fully initialized Lambda execution environment for fast resume
- Reduces cold-start latency, often by up to 90% for supported runtimes
- Requires no code changes to enable, though runtime hooks improve correctness
- Supports `beforeCheckpoint` and `afterRestore` lifecycle hooks
- Applies to published function versions, not `$LATEST`
- Originally targeted Java, later extended to Python and .NET runtimes
- Snapshots are encrypted and cached by AWS, not user-managed
- Best suited for functions with expensive, deterministic initialization