middleware_factory.md

title	description
Middleware factory	Utility

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{target="_blank"}.

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

=== "getting_started_middleware_before_logic_function.py" python hl_lines="5 23 24 29 30 32 37 38" --8<-- "examples/middleware_factory/src/getting_started_middleware_before_logic_function.py"

=== "getting_started_middleware_before_logic_payload.json"

```json hl_lines="9-13"
--8<-- "examples/middleware_factory/src/getting_started_middleware_before_logic_payload.json"
```

Middleware with after logic

=== "getting_started_middleware_after_logic_function.py" python hl_lines="7 14 15 21-23 37" --8<-- "examples/middleware_factory/src/getting_started_middleware_after_logic_function.py"

=== "getting_started_middleware_after_logic_payload.json"

```json
--8<-- "examples/middleware_factory/src/getting_started_middleware_after_logic_payload.json"
```

Middleware with params

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

=== "getting_started_middleware_with_params_function.py" python hl_lines="6 27 28 29 33 49" --8<-- "examples/middleware_factory/src/getting_started_middleware_with_params_function.py"

=== "getting_started_middleware_with_params_payload.json"

```json hl_lines="18 19 20"
--8<-- "examples/middleware_factory/src/getting_started_middleware_with_params_payload.json"
```

Advanced

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

=== "advanced_middleware_tracer_function.py" python hl_lines="7 9 12 16 17 19 25 42" --8<-- "examples/middleware_factory/src/advanced_middleware_tracer_function.py"

=== "advanced_middleware_tracer_payload.json"

```json
--8<-- "examples/middleware_factory/src/advanced_middleware_tracer_payload.json"
```

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.

=== "getting_started_middleware_tracer_function.py" python hl_lines="8 14 15 36" --8<-- "examples/middleware_factory/src/getting_started_middleware_tracer_function.py"

=== "getting_started_middleware_tracer_payload.json"

```json hl_lines="18 19 20"
--8<-- "examples/middleware_factory/src/getting_started_middleware_tracer_payload.json"
```

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.

Combining Powertools utilities

You can create your own middleware and combine many features of Lambda Powertools 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

=== "combining_powertools_utilities_function.py" python hl_lines="11 28 29 119 52 61 73" --8<-- "examples/middleware_factory/src/combining_powertools_utilities_function.py"

=== "combining_powertools_utilities_schema.py" python hl_lines="12 14" --8<-- "examples/middleware_factory/src/combining_powertools_utilities_schema.py"

=== "combining_powertools_utilities_event.json" python hl_lines="10" --8<-- "examples/middleware_factory/src/combining_powertools_utilities_event.json"

=== "SAM TEMPLATE" python hl_lines="66 83 89 96 103 108-113 119 130" --8<-- "examples/middleware_factory/sam/combining_powertools_utilities_template.yaml"

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{target="_blank"}. Lambda handler is actually called once (top-down)
• Async middlewares are not supported