Introduction

Middleware is a software pattern that allows you to intercept and modify the processing of messages in a chain. It is a powerful tool for customizing the behavior of a system and adding additional functionality, such as logging, retries, deadlettering, backoffs, and tracing.

In the KP library, every feature is implemented as a middleware. This means that you can add or remove features by adding or removing middlewares from the middleware chain.

Adding Middleware to the KP Library

The KP library provides a way to add middlewares to the middleware chain through the AddMiddleware

This method takes a middleware as an argument and returns a new MessageProcessor with the middleware added to the chain. Here is an example of how to use the AddMiddleware method:

Example

package main

import (
    "context"
    "fmt"
    "time"

    v2 "github.com/honestbank/kp/v2"
    "github.com/honestbank/kp/v2/middlewares/measurement"
    "github.com/honestbank/kp/v2/middlewares/tracing"
    "go.opentelemetry.io/otel"
)

type UserLoggedInEvent struct {
    UserID string
}

func main() {
    retryCount := 10
    kp := v2.New[UserLoggedInEvent]("user-logged-in", getConfig()).
        WithRetryOrPanic("send-login-notification-retries", retryCount).
        WithDeadletterOrPanic("send-login-notification-failures")
    kp.AddMiddleware(measurement.NewMeasurementMiddleware("/path/to/push-gateway", "application-name"))
    tp := otel.GetTracerProvider() // you'll need to set the tracer provider as well
    kp.AddMiddleware(tracing.NewTracingMiddleware(tp))
    err := kp.Process(processUserLoggedInEvent)
    if err != nil {
        panic(err) // do better error handling
    }
}

func processUserLoggedInEvent(ctx context.Context, message UserLoggedInEvent) error {
    // here, you can focus on your business logic.
    fmt.Printf("processing %v\n", message)
    time.Sleep(time.Millisecond * 200) // simulate long running process
    return nil                         // or error
}

func getConfig() any {
    return nil // return your config
}

Notes

• Middlewares are executed in the order they are added to the chain. It is important to consider the order in which middlewares are added.
• The KP library provides several built-in middlewares for common features, such as consuming messages, retries, deadletters, backoffs, and tracing. You can also write custom middlewares to meet your specific