This content originally appeared on DEV Community and was authored by Michael Uanikehi
Serverless is amazing for scale, but cold starts are the silent tax we pay. In this post, we’ll unpack AWS Lambda SnapStart, how it works, what limitations matter, and how to enable it across Terraform, SAM, and CDK to slash your cold starts.
What SnapStart actually does:
When you publish a version of your Lambda, SnapStart:
1. Initializes your function once (imports, SDK clients, DB pools, frameworks, etc.).
2. Takes a snapshot of memory + runtime state after init.
3. Caches that snapshot.
4. On new execution environments, restores from that snapshot instead of doing INIT again.
Result: far less startup time, often ~10× faster for heavy-initialization workloads. You’ll see a new “Restore Duration” in logs and X-Ray that reflects snapshot restore time.  
Supported runtimes & key limitations
Runtimes (managed):
• Java 11+, Python 3.12+, .NET 8+. Not supported for Node.js, Ruby, OS-only runtimes, or container images. 
Limits & incompatibilities:
• Not compatible with Provisioned Concurrency (choose one). 
• No EFS, and /tmp must be ≤ 512 MB. 
• Works on published versions (and aliases pointing to them) — not $LATEST. 
• Pricing: charged for cache time (while version is active) and per restore; cost scales with memory size. 
Lifecycle nuance:
• Snapshots for Python/.NET stay active as long as the version is active.
• For Java, a snapshot may expire after 14 days of inactivity. 
When SnapStart shines
• Heavy init: big frameworks (Spring, .NET DI), large dependency graphs, JDBC/SDK client setup.
• Spiky or unpredictable traffic: where paying for Provisioned Concurrency would be wasteful.
• Latency-sensitive paths that still tolerate low-double-digit ms on restore.
When SnapStart is not the right tool:
• You must use EFS, >512MB ephemeral storage, or Provisioned Concurrency.
• Your function depends on unique state during INIT (see “uniqueness” below). 
Enabling SnapStart with IaC
Terraform
resource "aws_lambda_function" "fn" {
function_name = "snapstart-demo"
runtime = "python3.12" # or java11/java17/.NET 8+
handler = "app.handler"
role = aws_iam_role.lambda.arn
filename = "build.zip"
# SnapStart only works on published versions — publish must be true
publish = true
snap_start {
apply_on = "PublishedVersions"
}
}
resource "aws_lambda_alias" "live" {
name = "live"
function_name = aws_lambda_function.fn.function_name
function_version = aws_lambda_function.fn.version
}
Terraform requires publish = true and snap_start.apply_on = “PublishedVersions”. Use an alias for safe releases. 
AWS SAM (template.yaml)
Resources:
SnapStartFn:
Type: AWS::Serverless::Function
Properties:
Runtime: python3.12
Handler: app.handler
CodeUri: .
AutoPublishAlias: live
SnapStart:
ApplyOn: PublishedVersions
SnapStart appears under Edit > Basic settings in console, but with SAM you set it declaratively and always publish a version + alias. 
AWS CDK (TypeScript)
const fn = new lambda.Function(this, 'Fn', {
runtime: lambda.Runtime.PYTHON_3_12,
code: lambda.Code.fromAsset('dist'),
handler: 'app.handler',
snapStart: { applyOn: lambda.SnapStartApplyOn.PUBLISHED_VERSIONS },
currentVersionOptions: { removalPolicy: cdk.RemovalPolicy.DESTROY }
});
new lambda.Alias(this, 'Live', { aliasName: 'live', version: fn.currentVersion });
Measuring the impact
• CloudWatch Logs: Each cold start shows Restore Duration and Billed Restore Duration in the REPORT line. Track these to quantify improvements. 
• AWS X-Ray: Enable tracing to see a Restore subsegment alongside invocation. Great for comparing before/after.  
Best practices & “uniqueness” gotchas
Because multiple execution environments start from the same snapshot, don’t create per-invoke unique state during init. Move anything that must be unique into the handler (or use after-restore hooks if your runtime supports them). Watch for:
• Random/UUID seeded at init → generate inside the handler.
• Ephemeral tokens/leases → fetch per request.
• Time-based logic in init → recompute on first invoke (or use after-restore).
See AWS docs on uniqueness and runtime hooks if you rely on special init behavior. 
What’s safe in INIT?
• SDK clients / DB pools (connection creation may still occur lazily).
• Static configuration (env vars, constants).
• Framework boot (Spring/.NET DI, serializers).
Cost awareness
You pay for:
• Snapshot cache time while the version is active (per-ms, minimum 3 hours).
• Snapshot restore GB per resume.
Use smaller memory sizes where possible and clean up old versions to avoid lingering cache cost.   
Real-world tuning checklist
• Publish versions + use aliases (live, beta) so each release creates a fresh snapshot. 
• Warm critical paths: A canary/health check can invoke the alias after deploy to “prime” the version state.
• Log & trace: Compare cold start vs. restore using Restore Duration and X-Ray. 
• Right-size memory: Faster CPU → faster restore, but balance against cache/restore costs.
• CI/CD: Account for snapshot creation time when publishing (deploy steps may take longer). 
Quick examples by runtime
Python 3.12
app.py
import boto3
# OK to create clients in init – captured in snapshot
s3 = boto3.client("s3")
def handler(event, context):
# Generate per-invoke UUIDs/timestamps inside the handler, not at import time
# ...your logic...
return {"ok": True}
Java (11+/17+/21)
• Favor GraalVM native image (where possible) or tune your framework bootstrap (Spring AOT, Micronaut, Quarkus).
• Avoid init-time randomness; move uniqueness to the handler.
• Enable X-Ray to visualize Restore vs Invocation.
.NET 8
• Heavy DI? Great SnapStart candidate.
• Ensure libraries don’t assume “fresh process” uniqueness during initialization.
SnapStart vs Provisioned Concurrency (PC)
• SnapStart: Pay per snapshot cache/restore; great for many bursty workloads; no EFS/PC.
• PC: Always-warm environments; extra charges constantly; works with EFS; deterministic lowest-latency.
You can’t enable both on the same version—pick the one that fits your constraints. 
Final thoughts
If you’re running Java, Python 3.12+, or .NET 8+, SnapStart is a no-brainer to reduce latency without the always-on bill of Provisioned Concurrency. Enable it once, measure your gains, and enjoy faster cold starts at scale.
This content originally appeared on DEV Community and was authored by Michael Uanikehi