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Middleware factory

Middleware factory provides a decorator factory to create your own middleware to run logic before, and after each Lambda invocation synchronously.

Key features

  • Run logic before, after, and handle exceptions
  • Built-in tracing opt-in capability

Getting started

Tip

All examples shared in this documentation are available within the project repository.

You might need a custom middleware to abstract non-functional code. These are often custom authorization or any reusable logic you might need to run before/after a Lambda function invocation.

Middleware with no params

You can create your own middleware using lambda_handler_decorator. The decorator factory expects 3 arguments in your function signature:

  • handler - Lambda function handler
  • event - Lambda function invocation event
  • context - Lambda function context object

Middleware with before logic

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from dataclasses import dataclass, field
from typing import Callable
from uuid import uuid4

from aws_lambda_powertools.middleware_factory import lambda_handler_decorator
from aws_lambda_powertools.utilities.jmespath_utils import (
    envelopes,
    extract_data_from_envelope,
)
from aws_lambda_powertools.utilities.typing import LambdaContext


@dataclass
class Payment:
    user_id: str
    order_id: str
    amount: float
    status_id: str
    payment_id: str = field(default_factory=lambda: f"{uuid4()}")


class PaymentError(Exception):
    ...


@lambda_handler_decorator
def middleware_before(handler, event, context) -> Callable:
    # extract payload from a EventBridge event
    detail: dict = extract_data_from_envelope(data=event, envelope=envelopes.EVENTBRIDGE)

    # check if status_id exists in payload, otherwise add default state before processing payment
    if "status_id" not in detail:
        event["detail"]["status_id"] = "pending"

    response = handler(event, context)

    return response


@middleware_before
def lambda_handler(event, context: LambdaContext) -> dict:
    try:
        payment_payload: dict = extract_data_from_envelope(data=event, envelope=envelopes.EVENTBRIDGE)
        return {
            "order": Payment(**payment_payload).__dict__,
            "message": "payment created",
            "success": True,
        }
    except Exception as e:
        raise PaymentError("Unable to create payment") from e

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{
    "version": "0",
    "id": "9c95e8e4-96a4-ef3f-b739-b6aa5b193afb",
    "detail-type": "PaymentCreated",
    "source": "app.payment",
    "account": "0123456789012",
    "time": "2022-08-08T20:41:53Z",
    "region": "eu-east-1",
    "detail": {
      "amount": "150.00",
      "order_id": "8f1f1710-1b30-48a5-a6bd-153fd23b866b",
      "user_id": "f80e3c51-5b8c-49d5-af7d-c7804966235f"
    }
  }

Middleware with after logic

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import time
from typing import Callable

import requests
from requests import Response

from aws_lambda_powertools.event_handler import APIGatewayRestResolver
from aws_lambda_powertools.middleware_factory import lambda_handler_decorator
from aws_lambda_powertools.utilities.typing import LambdaContext

app = APIGatewayRestResolver()


@lambda_handler_decorator
def middleware_after(handler, event, context) -> Callable:

    start_time = time.time()
    response = handler(event, context)
    execution_time = time.time() - start_time

    # adding custom headers in response object after lambda executing
    response["headers"]["execution_time"] = execution_time
    response["headers"]["aws_request_id"] = context.aws_request_id

    return response


@app.post("/todos")
def create_todo() -> dict:
    todo_data: dict = app.current_event.json_body  # deserialize json str to dict
    todo: Response = requests.post("https://jsonplaceholder.typicode.com/todos", data=todo_data)
    todo.raise_for_status()

    return {"todo": todo.json()}


@middleware_after
def lambda_handler(event: dict, context: LambdaContext) -> dict:
    return app.resolve(event, context)

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{
    "resource": "/todos",
    "path": "/todos",
    "httpMethod": "POST",
    "body": "{\"title\": \"foo\", \"userId\": 1, \"completed\": false}"
}

Middleware with params

You can also have your own keyword arguments after the mandatory arguments.

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import base64
from dataclasses import dataclass, field
from typing import Any, Callable, List
from uuid import uuid4

from aws_lambda_powertools.middleware_factory import lambda_handler_decorator
from aws_lambda_powertools.utilities.jmespath_utils import (
    envelopes,
    extract_data_from_envelope,
)
from aws_lambda_powertools.utilities.typing import LambdaContext


@dataclass
class Booking:
    days: int
    date_from: str
    date_to: str
    hotel_id: int
    country: str
    city: str
    guest: dict
    booking_id: str = field(default_factory=lambda: f"{uuid4()}")


class BookingError(Exception):
    ...


@lambda_handler_decorator
def obfuscate_sensitive_data(handler, event, context, fields: List) -> Callable:
    # extracting payload from a EventBridge event
    detail: dict = extract_data_from_envelope(data=event, envelope=envelopes.EVENTBRIDGE)
    guest_data: Any = detail.get("guest")

    # Obfuscate fields (email, vat, passport) before calling Lambda handler
    for guest_field in fields:
        if guest_data.get(guest_field):
            event["detail"]["guest"][guest_field] = obfuscate_data(str(guest_data.get(guest_field)))

    response = handler(event, context)

    return response


def obfuscate_data(value: str) -> bytes:
    # base64 is not effective for obfuscation, this is an example
    return base64.b64encode(value.encode("ascii"))


@obfuscate_sensitive_data(fields=["email", "passport", "vat"])
def lambda_handler(event, context: LambdaContext) -> dict:
    try:
        booking_payload: dict = extract_data_from_envelope(data=event, envelope=envelopes.EVENTBRIDGE)
        return {
            "book": Booking(**booking_payload).__dict__,
            "message": "booking created",
            "success": True,
        }
    except Exception as e:
        raise BookingError("Unable to create booking") from e

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{
    "version": "0",
    "id": "9c95e8e4-96a4-ef3f-b739-b6aa5b193afb",
    "detail-type": "BookingCreated",
    "source": "app.booking",
    "account": "0123456789012",
    "time": "2022-08-08T20:41:53Z",
    "region": "eu-east-1",
    "detail": {
      "days": 5,
      "date_from": "2020-08-08",
      "date_to": "2020-08-13",
      "hotel_id": "1",
      "country": "Portugal",
      "city": "Lisbon",
      "guest": {
        "name": "Lambda",
        "email": "lambda@powertool.tools",
        "passport": "AA123456",
        "vat": "123456789"
      }
    }
}

Advanced

For advanced use cases, you can instantiate Tracer inside your middleware, and add annotations as well as metadata for additional operational insights.

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import time
from typing import Callable

import requests
from requests import Response

from aws_lambda_powertools import Tracer
from aws_lambda_powertools.event_handler import APIGatewayRestResolver
from aws_lambda_powertools.middleware_factory import lambda_handler_decorator
from aws_lambda_powertools.utilities.typing import LambdaContext

tracer = Tracer()
app = APIGatewayRestResolver()


@lambda_handler_decorator(trace_execution=True)
def middleware_with_advanced_tracing(handler, event, context) -> Callable:

    tracer.put_metadata(key="resource", value=event.get("resource"))

    start_time = time.time()
    response = handler(event, context)
    execution_time = time.time() - start_time

    tracer.put_annotation(key="TotalExecutionTime", value=str(execution_time))

    # adding custom headers in response object after lambda executing
    response["headers"]["execution_time"] = execution_time
    response["headers"]["aws_request_id"] = context.aws_request_id

    return response


@app.get("/products")
def create_product() -> dict:
    product: Response = requests.get("https://dummyjson.com/products/1")
    product.raise_for_status()

    return {"product": product.json()}


@middleware_with_advanced_tracing
def lambda_handler(event: dict, context: LambdaContext) -> dict:
    return app.resolve(event, context)

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{
    "resource": "/products",
    "path": "/products",
    "httpMethod": "GET"
 }

Middleware advanced Tracer

Tracing middleware execution

If you are making use of Tracer, you can trace the execution of your middleware to ease operations.

This makes use of an existing Tracer instance that you may have initialized anywhere in your code.

Warning

You must enable Active Tracing in your Lambda function when using this feature, otherwise Lambda cannot send traces to XRay.

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import time
from typing import Callable

import requests
from requests import Response

from aws_lambda_powertools.event_handler import APIGatewayRestResolver
from aws_lambda_powertools.middleware_factory import lambda_handler_decorator
from aws_lambda_powertools.utilities.typing import LambdaContext

app = APIGatewayRestResolver()


@lambda_handler_decorator(trace_execution=True)
def middleware_with_tracing(handler, event, context) -> Callable:

    start_time = time.time()
    response = handler(event, context)
    execution_time = time.time() - start_time

    # adding custom headers in response object after lambda executing
    response["headers"]["execution_time"] = execution_time
    response["headers"]["aws_request_id"] = context.aws_request_id

    return response


@app.get("/products")
def create_product() -> dict:
    product: Response = requests.get("https://dummyjson.com/products/1")
    product.raise_for_status()

    return {"product": product.json()}


@middleware_with_tracing
def lambda_handler(event: dict, context: LambdaContext) -> dict:
    return app.resolve(event, context)

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{
    "resource": "/products",
    "path": "/products",
    "httpMethod": "GET"
 }

When executed, your middleware name will appear in AWS X-Ray Trace details as ## middleware_name, in this example the middleware name is ## middleware_with_tracing.

Middleware simple Tracer

Combining Powertools for AWS Lambda (Python) utilities

You can create your own middleware and combine many features of Powertools for AWS Lambda (Python) such as trace, logs, feature flags, validation, jmespath_functions and others to abstract non-functional code.

In the example below, we create a Middleware with the following features:

  • Logs and traces
  • Validate if the payload contains a specific header
  • Extract specific keys from event
  • Automatically add security headers on every execution
  • Validate if a specific feature flag is enabled
  • Save execution history to a DynamoDB table
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import json
from typing import Callable

import boto3
import combining_powertools_utilities_schema as schemas
import requests

from aws_lambda_powertools import Logger, Tracer
from aws_lambda_powertools.event_handler import APIGatewayRestResolver
from aws_lambda_powertools.event_handler.exceptions import InternalServerError
from aws_lambda_powertools.middleware_factory import lambda_handler_decorator
from aws_lambda_powertools.shared.types import JSONType
from aws_lambda_powertools.utilities.feature_flags import AppConfigStore, FeatureFlags
from aws_lambda_powertools.utilities.jmespath_utils import extract_data_from_envelope
from aws_lambda_powertools.utilities.typing import LambdaContext
from aws_lambda_powertools.utilities.validation import SchemaValidationError, validate

app = APIGatewayRestResolver()
tracer = Tracer()
logger = Logger()

table_historic = boto3.resource("dynamodb").Table("HistoricTable")

app_config = AppConfigStore(environment="dev", application="comments", name="features")
feature_flags = FeatureFlags(store=app_config)


@lambda_handler_decorator(trace_execution=True)
def middleware_custom(handler: Callable, event: dict, context: LambdaContext):

    # validating the INPUT with the given schema
    # X-Customer-Id header must be informed in all requests
    try:
        validate(event=event, schema=schemas.INPUT)
    except SchemaValidationError as e:
        return {
            "statusCode": 400,
            "body": json.dumps(str(e)),
        }

    # extracting headers and requestContext from event
    headers = extract_data_from_envelope(data=event, envelope="headers")
    request_context = extract_data_from_envelope(data=event, envelope="requestContext")

    logger.debug(f"X-Customer-Id => {headers.get('X-Customer-Id')}")
    tracer.put_annotation(key="CustomerId", value=headers.get("X-Customer-Id"))

    response = handler(event, context)

    # automatically adding security headers to all responses
    # see: https://securityheaders.com/
    logger.info("Injecting security headers")
    response["headers"]["Referrer-Policy"] = "no-referrer"
    response["headers"]["Strict-Transport-Security"] = "max-age=15552000; includeSubDomains; preload"
    response["headers"]["X-DNS-Prefetch-Control"] = "off"
    response["headers"]["X-Content-Type-Options"] = "nosniff"
    response["headers"]["X-Permitted-Cross-Domain-Policies"] = "none"
    response["headers"]["X-Download-Options"] = "noopen"

    logger.info("Saving api call in history table")
    save_api_execution_history(str(event.get("path")), headers, request_context)

    # return lambda execution
    return response


@tracer.capture_method
def save_api_execution_history(path: str, headers: dict, request_context: dict) -> None:

    try:
        # using the feature flags utility to check if the new feature "save api call to history" is enabled by default
        # see: https://awslabs.github.io/aws-lambda-powertools-python/latest/utilities/feature_flags/#static-flags
        save_history: JSONType = feature_flags.evaluate(name="save_history", default=False)
        if save_history:
            # saving history in dynamodb table
            tracer.put_metadata(key="execution detail", value=request_context)
            table_historic.put_item(
                Item={
                    "customer_id": headers.get("X-Customer-Id"),
                    "request_id": request_context.get("requestId"),
                    "path": path,
                    "request_time": request_context.get("requestTime"),
                    "source_ip": request_context.get("identity", {}).get("sourceIp"),
                    "http_method": request_context.get("httpMethod"),
                }
            )

        return None
    except Exception:
        # you can add more logic here to handle exceptions or even save this to a DLQ
        # but not to make this example too long, we just return None since the Lambda has been successfully executed
        return None


@app.get("/comments")
@tracer.capture_method
def get_comments():
    try:
        comments: requests.Response = requests.get("https://jsonplaceholder.typicode.com/comments")
        comments.raise_for_status()

        return {"comments": comments.json()[:10]}
    except Exception as exc:
        raise InternalServerError(str(exc))


@app.get("/comments/<comment_id>")
@tracer.capture_method
def get_comments_by_id(comment_id: str):
    try:
        comments: requests.Response = requests.get(f"https://jsonplaceholder.typicode.com/comments/{comment_id}")
        comments.raise_for_status()

        return {"comments": comments.json()}
    except Exception as exc:
        raise InternalServerError(str(exc))


@middleware_custom
def lambda_handler(event: dict, context: LambdaContext) -> dict:
    return app.resolve(event, context)

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INPUT = {
    "$schema": "http://json-schema.org/draft-07/schema#",
    "$id": "https://example.com/object1661012141.json",
    "title": "Root",
    "type": "object",
    "required": ["headers"],
    "properties": {
        "headers": {
            "$id": "#root/headers",
            "title": "Headers",
            "type": "object",
            "required": ["X-Customer-Id"],
            "properties": {
                "X-Customer-Id": {
                    "$id": "#root/headers/X-Customer-Id",
                    "title": "X-customer-id",
                    "type": "string",
                    "default": "",
                    "examples": ["1"],
                    "pattern": "^.*$",
                }
            },
        }
    },
}

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{
    "body":"None",
    "headers":{
       "Accept":"*/*",
       "Accept-Encoding":"gzip, deflate, br",
       "Connection":"keep-alive",
       "Host":"127.0.0.1:3001",
       "Postman-Token":"a9d49365-ebe1-4bb0-8627-d5e37cdce86d",
       "User-Agent":"PostmanRuntime/7.29.0",
       "X-Customer-Id":"1",
       "X-Forwarded-Port":"3001",
       "X-Forwarded-Proto":"http"
    },
    "httpMethod":"GET",
    "isBase64Encoded":false,
    "multiValueHeaders":{
       "Accept":[
          "*/*"
       ],
       "Accept-Encoding":[
          "gzip, deflate, br"
       ],
       "Connection":[
          "keep-alive"
       ],
       "Host":[
          "127.0.0.1:3001"
       ],
       "Postman-Token":[
          "a9d49365-ebe1-4bb0-8627-d5e37cdce86d"
       ],
       "User-Agent":[
          "PostmanRuntime/7.29.0"
       ],
       "X-Customer-Id":[
          "1"
       ],
       "X-Forwarded-Port":[
          "3001"
       ],
       "X-Forwarded-Proto":[
          "http"
       ]
    },
    "multiValueQueryStringParameters":"None",
    "path":"/comments",
    "pathParameters":"None",
    "queryStringParameters":"None",
    "requestContext":{
       "accountId":"123456789012",
       "apiId":"1234567890",
       "domainName":"127.0.0.1:3001",
       "extendedRequestId":"None",
       "httpMethod":"GET",
       "identity":{
          "accountId":"None",
          "apiKey":"None",
          "caller":"None",
          "cognitoAuthenticationProvider":"None",
          "cognitoAuthenticationType":"None",
          "cognitoIdentityPoolId":"None",
          "sourceIp":"127.0.0.1",
          "user":"None",
          "userAgent":"Custom User Agent String",
          "userArn":"None"
       },
       "path":"/comments",
       "protocol":"HTTP/1.1",
       "requestId":"56d1a102-6d9d-4f13-b4f7-26751c10a131",
       "requestTime":"20/Aug/2022:18:18:58 +0000",
       "requestTimeEpoch":1661019538,
       "resourceId":"123456",
       "resourcePath":"/comments",
       "stage":"Prod"
    },
    "resource":"/comments",
    "stageVariables":"None",
    "version":"1.0"
 }

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AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31
Description: Middleware-powertools-utilities example

Globals:
  Function:
    Timeout: 5
    Runtime: python3.9
    Tracing: Active
    Architectures:
      - x86_64
    Environment:
      Variables:
        LOG_LEVEL: DEBUG
        POWERTOOLS_LOGGER_SAMPLE_RATE: 0.1
        POWERTOOLS_LOGGER_LOG_EVENT: true
        POWERTOOLS_SERVICE_NAME: middleware

Resources:
  MiddlewareFunction:
    Type: AWS::Serverless::Function # More info about Function Resource: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#awsserverlessfunction
    Properties:
      CodeUri: middleware/
      Handler: app.lambda_handler
      Description: Middleware function
      Policies:
      - AWSLambdaBasicExecutionRole # Managed Policy
      - Version: '2012-10-17' # Policy Document
        Statement:
          - Effect: Allow
            Action:
              - dynamodb:PutItem
            Resource: !GetAtt HistoryTable.Arn
          - Effect: Allow
            Action: # https://docs.aws.amazon.com/appconfig/latest/userguide/getting-started-with-appconfig-permissions.html
              - ssm:GetDocument
              - ssm:ListDocuments
              - appconfig:GetLatestConfiguration
              - appconfig:StartConfigurationSession
              - appconfig:ListApplications
              - appconfig:GetApplication
              - appconfig:ListEnvironments
              - appconfig:GetEnvironment
              - appconfig:ListConfigurationProfiles
              - appconfig:GetConfigurationProfile
              - appconfig:ListDeploymentStrategies
              - appconfig:GetDeploymentStrategy
              - appconfig:GetConfiguration
              - appconfig:ListDeployments
              - appconfig:GetDeployment
            Resource: "*"
      Events:
        GetComments:
          Type: Api
          Properties:
            Path: /comments
            Method: GET
        GetCommentsById:
          Type: Api
          Properties:
            Path: /comments/{comment_id}
            Method: GET

  # DynamoDB table to store historical data
  HistoryTable:
    Type: AWS::DynamoDB::Table
    Properties:
      TableName: "HistoryTable"
      AttributeDefinitions:
        - AttributeName: customer_id
          AttributeType: S
        - AttributeName: request_id
          AttributeType: S
      KeySchema:
        - AttributeName: customer_id
          KeyType: HASH
        - AttributeName: request_id
          KeyType: "RANGE"
      BillingMode: PAY_PER_REQUEST

  # Feature flags using AppConfig
  FeatureCommentApp:
    Type: AWS::AppConfig::Application
    Properties:
      Description: "Comments Application for feature toggles"
      Name: comments

  FeatureCommentDevEnv:
    Type: AWS::AppConfig::Environment
    Properties:
      ApplicationId: !Ref FeatureCommentApp
      Description: "Development Environment for the App Config Comments"
      Name: dev

  FeatureCommentConfigProfile:
    Type: AWS::AppConfig::ConfigurationProfile
    Properties:
      ApplicationId: !Ref FeatureCommentApp
      Name: features
      LocationUri: "hosted"

  HostedConfigVersion:
    Type: AWS::AppConfig::HostedConfigurationVersion
    Properties:
      ApplicationId: !Ref FeatureCommentApp
      ConfigurationProfileId: !Ref FeatureCommentConfigProfile
      Description: 'A sample hosted configuration version'
      Content: |
        {
              "save_history": {
                "default": true
              }
        }
      ContentType: 'application/json'

  # this is just an example
  # change this values according your deployment strategy
  BasicDeploymentStrategy:
    Type: AWS::AppConfig::DeploymentStrategy
    Properties:
      Name: "Deployment"
      Description: "Deployment strategy for comments app."
      DeploymentDurationInMinutes: 1
      FinalBakeTimeInMinutes: 1
      GrowthFactor: 100
      GrowthType: LINEAR
      ReplicateTo: NONE

  ConfigDeployment:
    Type: AWS::AppConfig::Deployment
    Properties:
      ApplicationId: !Ref FeatureCommentApp
      ConfigurationProfileId: !Ref FeatureCommentConfigProfile
      ConfigurationVersion: !Ref HostedConfigVersion
      DeploymentStrategyId: !Ref BasicDeploymentStrategy
      EnvironmentId: !Ref FeatureCommentDevEnv

Tips

  • Use trace_execution to quickly understand the performance impact of your middlewares, and reduce or merge tasks when necessary
  • When nesting multiple middlewares, always return the handler with event and context, or response
  • Keep in mind Python decorators execution order. Lambda handler is actually called once (top-down)
  • Async middlewares are not supported
Last update: 2023-06-08