AWS Lambda Deep Dive Cheat Sheet
Practical reference for writing, deploying, and configuring AWS Lambda functions including triggers, layers, and CLI usage.
2 PagesIntermediateFeb 12, 2026
Python Handler
Basic Lambda function handler signature.
python
import jsondef lambda_handler(event, context): # event: dict with the trigger payload # context: runtime info (request_id, remaining time, etc.) body = json.loads(event.get('body', '{}')) return { 'statusCode': 200, 'headers': {'Content-Type': 'application/json'}, 'body': json.dumps({'message': f"Hello {body.get('name', 'World')}"}) }
CLI: Create & Update Function
Deploy and update a function via the AWS CLI.
bash
# Zip and createzip function.zip lambda_function.pyaws lambda create-function \ --function-name my-func \ --runtime python3.12 \ --role arn:aws:iam::123456789012:role/lambda-exec-role \ --handler lambda_function.lambda_handler \ --zip-file fileb://function.zip# Update code after changesaws lambda update-function-code \ --function-name my-func --zip-file fileb://function.zip# Invokeaws lambda invoke --function-name my-func out.json
Environment & Config Update
Set memory, timeout, and environment variables.
bash
aws lambda update-function-configuration \ --function-name my-func \ --timeout 30 \ --memory-size 256 \ --environment "Variables={STAGE=prod,LOG_LEVEL=info}"
Common Event Sources
Key common event sources to know.
- API Gateway- HTTP requests invoke the function synchronously (REST or HTTP API)
- S3- Object created/removed events trigger the function asynchronously
- EventBridge- Scheduled (cron) or event-pattern based invocations
- SQS- Lambda polls the queue and processes messages in batches
- DynamoDB Streams- Invoked on table item changes (insert/update/delete)
Key Concepts
Key key concepts to know.
- Cold Start- Latency from initializing a new execution environment before first invoke
- Layer- Shared zip archive of libraries/dependencies attached to multiple functions
- Concurrency- Number of simultaneous executions; can set reserved or provisioned concurrency
- Execution Role- IAM role granting the function permissions to call other AWS services
- Timeout- Max runtime per invocation, configurable up to 900 seconds (15 min)
Provisioned Concurrency (Eliminate Cold Starts)
Keep a fixed number of execution environments pre-initialized for latency-sensitive endpoints.
bash
# Publish a version first — provisioned concurrency targets a version or alias, not $LATESTaws lambda publish-version --function-name my-funcaws lambda put-provisioned-concurrency-config \ --function-name my-func \ --qualifier 3 \ --provisioned-concurrent-executions 5# Auto-scale provisioned concurrency with Application Auto Scalingaws application-autoscaling register-scalable-target \ --service-namespace lambda \ --resource-id function:my-func:prod \ --scalable-dimension lambda:function:ProvisionedConcurrency \ --min-capacity 2 --max-capacity 20
Building & Publishing a Layer
Share dependencies across functions and keep deployment packages small.
bash
mkdir -p python-layer/pythonpip install requests -t python-layer/pythoncd python-layer && zip -r ../layer.zip python && cd ..aws lambda publish-layer-version \ --layer-name shared-deps \ --zip-file fileb://layer.zip \ --compatible-runtimes python3.12aws lambda update-function-configuration \ --function-name my-func \ --layers arn:aws:lambda:us-east-1:123456789012:layer:shared-deps:1
Error Handling: DLQ vs Destinations
Configure asynchronous invocation failure handling — destinations supersede the older dead-letter-queue mechanism.
bash
# Destinations: route based on success/failure, richer payload than DLQaws lambda put-function-event-invoke-config \ --function-name my-func \ --maximum-retry-attempts 2 \ --destination-config '{ "OnSuccess": {"Destination": "arn:aws:sns:us-east-1:123456789012:success-topic"}, "OnFailure": {"Destination": "arn:aws:sqs:us-east-1:123456789012:failure-queue"} }'# Legacy DLQ (only captures the failed event, no context)aws lambda update-function-configuration \ --function-name my-func \ --dead-letter-config TargetArn=arn:aws:sqs:us-east-1:123456789012:dlq
AWS SAM Template
Declarative IaC for a Lambda function with an API Gateway trigger, deployable via 'sam deploy'.
yaml
Transform: AWS::Serverless-2016-10-31Resources: MyFunction: Type: AWS::Serverless::Function Properties: CodeUri: src/ Handler: app.lambda_handler Runtime: python3.12 MemorySize: 256 Timeout: 10 ReservedConcurrentExecutions: 20 Environment: Variables: STAGE: prod Events: Api: Type: Api Properties: Path: /hello Method: get Policies: - DynamoDBReadPolicy: TableName: my-table
Advanced Runtime & Scaling Concepts
Terms that matter once you're tuning throughput and cost, not just shipping a first function.
- Reserved Concurrency- Caps (and guarantees) the max concurrent executions for one function, isolating it from account-wide concurrency exhaustion
- SnapStart- Java-only feature that snapshots an initialized execution environment to cut cold-start latency by up to 90%
- Execution Environment Reuse- Warm invocations reuse /tmp (up to 512MB-10GB ephemeral storage) and module-level state between invocations of the same environment
- Event Source Mapping Batch Window- For SQS/Kinesis/DynamoDB triggers, controls how long Lambda buffers records before invoking, trading latency for batch efficiency
- Function URL- Built-in HTTPS endpoint for a function without needing API Gateway, supports IAM auth or public access
- Lambda Extensions- Companion processes (via Extensions API) for telemetry/monitoring that run alongside the function, independent of the handler
- Graviton (arm64)- ARM-based architecture option offering up to 34% better price-performance versus x86_64 for compatible workloads
Pro Tip
Initialize SDK clients and heavy imports outside the handler function body — code outside the handler runs once per execution environment and is reused across warm invocations, cutting latency significantly.
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