AWS Lambda Execution Model: Event Sources, Triggers, and Handlers
AWS Lambda is a cornerstone of serverless computing, allowing you to run code without provisioning or managing servers. Understanding its execution model is crucial for building efficient and scalable serverless applications. This module delves into how Lambda functions are invoked, the role of event sources and triggers, and how your code (the handler) processes these events.
The Core Components: Event Sources, Triggers, and Handlers
At its heart, the Lambda execution model revolves around three key concepts:
- Event Sources: These are AWS services or custom applications that generate events. An event is a change in state or an occurrence that Lambda can respond to.
- Triggers: A trigger is a configuration that connects an event source to a Lambda function. When an event occurs in the source, the trigger invokes your Lambda function.
- Handler: This is the specific method or function within your Lambda deployment package that Lambda executes when it's invoked. It receives event data and context information.
Lambda functions are event-driven and stateless.
Lambda functions don't run continuously. They are invoked in response to specific events and execute only when needed. Each invocation is independent, meaning they don't retain state from previous invocations.
When an event source generates an event, a trigger activates your Lambda function. Lambda then provisions an execution environment, loads your code, and executes your handler function with the event data. After the handler completes, the execution environment may be kept warm for a short period for subsequent invocations, but it's not guaranteed. This stateless nature is a fundamental aspect of serverless design, promoting scalability and resilience.
Common Event Sources and Triggers
Lambda integrates with a vast array of AWS services, acting as a powerful glue for your cloud architecture. Here are some common examples:
- Amazon S3: Triggers can be configured for object creation, deletion, or modification events in an S3 bucket. This is useful for image resizing, data processing, or file validation.
- Amazon API Gateway: Provides HTTP endpoints that trigger Lambda functions, enabling you to build RESTful APIs and web applications.
- Amazon DynamoDB Streams: Captures changes to items in a DynamoDB table, allowing you to react to data modifications in real-time.
- Amazon SQS (Simple Queue Service): Lambda can poll an SQS queue and process messages in batches.
- Amazon SNS (Simple Notification Service): Lambda can be subscribed to SNS topics to process messages published to those topics.
- AWS CodeCommit, AWS CodePipeline: Triggers can be set up for code repository events or pipeline stage changes.
| Event Source | Typical Event | Lambda Trigger Action |
|---|---|---|
| Amazon S3 | Object Created | Invoke Lambda with object details |
| API Gateway | HTTP Request | Invoke Lambda with request payload |
| DynamoDB Streams | Item Modified | Invoke Lambda with stream record |
| SQS | Message Available | Invoke Lambda with batch of messages |
The Lambda Handler Function
Your handler function is the entry point for your Lambda code. It typically receives two arguments:
- Event Object: A JSON object containing the data passed from the event source. The structure of this object varies depending on the event source.
- Context Object: Provides runtime information about the Lambda invocation, such as the request ID, function name, remaining execution time, and logging capabilities.
Your handler's responsibility is to process the event data and perform the desired actions. It can return a value, which might be used by the invoking service or simply discarded.
Imagine a visitor arriving at a library. The visitor (event source) wants a book (event). The librarian at the desk (trigger) receives the request and directs the visitor to the correct aisle and shelf (handler). The librarian then retrieves the book (processing the event) and hands it over. The execution environment is like the library itself, ready to serve visitors. The handler is the specific action the librarian takes.
Text-based content
Library pages focus on text content
Event Sources, Triggers, and Handlers.
Lambda functions are stateless.
Understanding the structure of the event object for each specific AWS service you integrate with is critical for writing effective Lambda handlers.
Learning Resources
The official AWS documentation detailing the various event sources that can trigger Lambda functions and how they work.
Provides a foundational understanding of AWS Lambda, including its execution model and core concepts.
Explains how to write and configure the handler function for Python Lambda deployments.
Details the specifics of creating and managing handler functions for Node.js Lambda functions.
Describes the structure of event payloads for various AWS services when they invoke Lambda functions.
Understand how Lambda is priced, which is tied to execution time and the number of requests, reinforcing the importance of efficient execution.
A deep dive into the Lambda execution environment, including cold starts and warm starts, which impacts function performance.
A comprehensive whitepaper on building serverless architectures with AWS services, including Lambda's role.
An introductory video from AWS explaining the basics of Lambda, including its event-driven nature.
Details the information available in the Lambda context object, which your handler function can use.