87 lines
2.9 KiB
Markdown
87 lines
2.9 KiB
Markdown
---
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categories:
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- DevOps
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tags: [AWS, aws-lambda, backend]
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---
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# AWS Lambda handler function
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Every Lambda function begins the same way with a handler function. In NodeJS:
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```js
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module.exports.handler = function(event, context, callback){...}
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```
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The handler is the entry point for the Lambda. It is a function that Lambda can
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call to start the execution of your code when an event triggers your Lambda
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function.
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You may put your whole code in the body of the handler, or you may call other
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modules and keep the handler clean and focused on bootstrapping.
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The `module.exports` part is just the standard way of exporting a module in
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Node. `handler` is a property that is being exported.
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You always have access to the parameters `event`, `context` and `callback`.
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- `event`
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- The triggering event of the Lambda function stored as an object. The
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object's contents will be different depending on the trigger. It could be
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any AWS service that serves a trigger (S3, DynamoDB, API Gateway etc).
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- For example, if the triggering function is a request to API Gateway, the
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`event` object will contain information about the HTTP request such as
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headers, path, and the request body.
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- `context`
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- contains information about the runtime environment and the current execution
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of the Lambda function. The context object provides properties and methods
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that you can use to interact with the AWS Lambda service and the current
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invocation.
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- `callback`
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- an optional parameter that represents a callback function provided by AWS
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Lambda. The callback function allows you to signal the completion of your
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Lambda function's execution and return a response or an error. The callback
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function accepts two arguments: error and result. If the first argument is
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not null, AWS Lambda treats it as an error, and the Lambda function
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invocation fails. If the first argument is null, AWS Lambda treats the
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second argument as the result and considers the invocation successful. Using
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the callback function is more common in non-async handler functions. When
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you use async functions, you can return a response or throw an error
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directly, without using the callback function.
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### Full example:
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In JavaScript:
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```js
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module.exports.handler = async (event, context) => {
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// Your logic to handle the incoming event
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// For example, you can call other AWS services, process the event data, etc.
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// Return a response
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return {
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statusCode: 200,
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body: JSON.stringify({
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message: "Your Lambda function has successfully executed!",
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}),
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};
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};
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```
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In Python:
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```py
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import json
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def handler(event, context):
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# Your logic to handle the incoming event
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# For example, you can call other AWS services, process the event data, etc.
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# Return a response
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response = {
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'statusCode': 200,
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'body': json.dumps({
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'message': 'Your Lambda function has successfully executed!'
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})
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}
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return response
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```
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