Implementing a Flexible Middleware Pipeline

The middleware pattern is a powerful architectural concept that allows you to build flexible and extensible request processing pipelines. It enables you to chain together multiple functions (middleware) that can inspect, modify, or even terminate a request before it reaches its final destination. This challenge will guide you through implementing a basic middleware pipeline.

Problem Description

You need to implement a system that can execute a series of functions (middleware) in order for a given request. Each middleware function will receive the current request context and a reference to the next middleware in the pipeline. A middleware can choose to:

1. Perform an action and pass the request to the next middleware: This is the most common scenario, where middleware adds functionality like logging, authentication, or data transformation.
2. Perform an action and terminate the pipeline: If a middleware determines that the request should not proceed further (e.g., due to an authentication failure or invalid input), it can choose to stop the pipeline execution.

You should create a mechanism to register middleware functions and a way to execute them for a given request.

Key Requirements:

• Middleware Registration: A way to add middleware functions to a pipeline.
• Pipeline Execution: A function to trigger the execution of the registered middleware in the order they were added.
• next() Functionality: Each middleware must receive a next function. Calling next() will execute the subsequent middleware in the pipeline.
• Pipeline Termination: Middleware should be able to prevent further execution by not calling next().
• Request Context: A mechanism to pass data through the pipeline. This could be a simple object or structure.

Expected Behavior:

When the pipeline is executed, middleware functions should be called sequentially. If a middleware calls next(), the subsequent middleware is invoked. If a middleware does not call next(), the pipeline stops at that point.

Edge Cases:

• An empty pipeline: What happens when there are no middleware registered?
• A pipeline with only one middleware: It should execute and decide whether to terminate or proceed (though there's nothing to proceed to).
• Middleware that throws an error: How should the system handle unexpected errors within a middleware? (For this challenge, we'll assume graceful termination without explicit error handling beyond what the language provides).

Examples

Example 1: Simple Logging and Data Modification

Input:
request = { body: "hello" }
middleware1 (logs request body, calls next):
  print("Received request with body:", request.body)
  next()
middleware2 (modifies body, calls next):
  request.body = request.body.toUpperCase()
  print("Modified request body to:", request.body)
  next()
middleware3 (logs final body, terminates):
  print("Final request body:", request.body)

Output:
Received request with body: hello
Modified request body to: HELLO
Final request body: HELLO

Explanation: middleware1 logs the initial body and calls next(). middleware2 converts the body to uppercase and calls next(). middleware3 logs the uppercase body. Since middleware3 doesn't call next(), the pipeline terminates.

Example 2: Early Termination due to Validation

Input:
request = { user: "guest" }
middleware1 (checks for authentication, calls next if authenticated):
  if request.user == "admin":
    print("Authenticated as admin.")
    next()
  else:
    print("Authentication failed. User is not admin.")
    // Does NOT call next()
middleware2 (only runs if middleware1 calls next):
  print("This middleware will not be executed.")

Output:
Authentication failed. User is not admin.

Explanation: middleware1 checks the user property. Since it's "guest" and not "admin", it prints a failure message and does not call next(). This terminates the pipeline, and middleware2 is never executed.

Example 3: Empty Pipeline

Input:
request = { data: "some data" }
pipeline = [] // No middleware registered

Output:
(No output, pipeline executes without error)

Explanation: When the pipeline is executed with no registered middleware, it should simply complete without any actions or errors.

Constraints

• The number of middleware functions in a pipeline can range from 0 to 1000.
• The request context will be a generic object or dictionary.
• Middleware functions should be designed to execute within a reasonable time frame (e.g., milliseconds per middleware) to avoid performance bottlenecks in a real-world scenario.

Notes

• Consider how you will manage the state of the request object as it passes through different middleware.
• The next function is the key to enabling the chaining of middleware. Ensure it's correctly passed and invoked.
• Think about how you would represent the pipeline itself. A list or array is a common choice.
• The core of this challenge is managing the flow of control between functions.