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content/posts/2023-04-11-dismantling-a-monolithic-golang-application.md

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+---
+type: "blog-post"
+title: "Dismantling a Monolithic Golang Application"
+description: "The follow-up article showcases a practical example of how to apply the strangler pattern to a monolithic system, using a Go application as an example. The process involves incrementally extracting small pieces of functionality into separate APIs, testing them, and gradually replacing the old monolithic code. This approach allows for a smoother and less risky transition to a microservices architecture, with minimal disruption to the existing system."
+draft: false
+date: "2023-04-13"
+updates:
+- time: "2023-04-13"
+  description: "first iteration"
+tags:
+- '#blog'
+- '#microservices'
+- '#monolith'
+- '#softwarearchitecture'
+- '#golang'
+- '#stranglerpattern'
+- '#migration'
+- '#api'
+- '#deployment'
+---
+
+In this follow-up article to
+[Strategies for Dismantling Monolithic Systems](https://blog.kasperhermansen.com/posts/breaking-down-the-monolith/),
+we will explore a practical example of dismantling a monolithic Golang
+application using the strategies discussed in the previous article. We will walk
+through the process step by step, demonstrating the application of the
+Strangler, Decorator, and Sprig strategies, and provide a simple diagram to
+illustrate the architectural changes.
+
+## Initial Monolithic Application
+
+Consider a simple monolithic Golang application that handles user registration
+and authentication:
+
+```go
+package main
+
+import (
+	"fmt"
+	"net/http"
+)
+
+func main() {
+	http.HandleFunc("/register", registerHandler)
+	http.HandleFunc("/login", loginHandler)
+	http.ListenAndServe(":8080", nil)
+}
+
+func registerHandler(w http.ResponseWriter, r *http.Request) {
+	// Register user logic
+	fmt.Fprint(w, "User registered")
+}
+
+func loginHandler(w http.ResponseWriter, r *http.Request) {
+	// Authenticate user logic
+	fmt.Fprint(w, "User logged in")
+}
+```
+
+This application has two main functionalities: registering a new user and
+logging in an existing user.
+
+## Breaking Down the Monolith
+
+### Step 1: Identify the functionalities to be extracted
+
+We will start by identifying the functionalities that can be extracted into
+separate microservices. In our example, we will extract the user registration
+and authentication functionalities into two separate services:
+
+1. User Registration Service
+2. Authentication Service
+
+### Step 2: Apply the Strangler Pattern
+
+Next, we will apply the Strangler Pattern to gradually replace the monolithic
+application with the new microservices.
+
+First, create the new User Registration and Authentication services:
+
+```go
+// User Registration Service
+func newUserRegistrationHandler(w http.ResponseWriter, r *http.Request) {
+	// New user registration logic
+	fmt.Fprint(w, "New user registered")
+}
+
+// Authentication Service
+func newAuthenticationHandler(w http.ResponseWriter, r *http.Request) {
+	// New authentication logic
+	fmt.Fprint(w, "New user authenticated")
+}
+```
+
+Now, we will modify the main function of the application to use these new
+services:
+
+```go
+func main() {
+	http.HandleFunc("/register", newUserRegistrationHandler)
+	http.HandleFunc("/login", newAuthenticationHandler)
+	http.ListenAndServe(":8080", nil)
+}
+```
+
+During this transition, we can use feature flags or canary deployments to
+control the traffic between the old and new services.
+
+### Step 3: Apply the Decorator and Sprig Patterns
+
+As we develop new features, we can leverage the Decorator and Sprig Patterns to
+add functionality to the new microservices without further complicating the
+monolithic application.
+
+For example, if we want to implement a password reset functionality, we can
+create a new endpoint in the Authentication Service:
+
+```go
+// Password Reset
+func passwordResetHandler(w http.ResponseWriter, r *http.Request) {
+	// Password reset logic
+	fmt.Fprint(w, "Password reset")
+}
+
+// Updated main function
+func main() {
+	http.HandleFunc("/register", newUserRegistrationHandler)
+	http.HandleFunc("/login", newAuthenticationHandler)
+	http.HandleFunc("/reset-password", passwordResetHandler)
+	http.ListenAndServe(":8080", nil)
+}
+```
+
+By following these strategies, we can gradually dismantle the monolithic
+application while maintaining a functional system throughout the process.
+
+## System Diagram
+
+### Step 1: Identify and Isolate a Component
+
+The first step is to identify and isolate a component that can be extracted from
+the monolith. This should be a well-defined, self-contained unit that can be
+broken off and turned into a separate service without affecting the rest of the
+application. Once you have identified the component, you should create a new API
+that can handle its responsibilities.
+
+```mermaid
+graph TD
+    A[User] --> B[Monolith]
+    B --> C[Monolithic Application]
+    subgraph C["Monolithic Application"]
+        E[UserRegistrationService]
+        F[AuthenticationService]
+    end
+```
+
+### Step 2: Create a New API
+
+Next, you need to create a new API that can handle the responsibilities of the
+isolated component. This API should be designed to work independently of the
+monolith, so it can be easily swapped in or out as needed. The API should be
+thoroughly tested to ensure it works as expected.
+
+### Step 3: Test and Roll Out the New API
+
+Once you have created the new API, you need to test it to ensure it works as
+expected. You can use a canary rollout or feature flags to gradually roll out
+the new API while still keeping the old one in place. This will allow you to
+catch any issues or bugs before fully switching over to the new API.
+
+### Step 4: Switch Over to the New API
+
+Once you have thoroughly tested the new API, it's time to switch over to it. You
+can do this by updating the monolith to use the new API instead of the old one.
+You should monitor the application closely to ensure there are no issues or
+bugs, and be prepared to roll back if necessary.
+
+```mermaid
+graph TD
+    A[User] --> B[Monolith]
+    B --> C[Monolithic Application]
+    B --> D
+    subgraph C[Monolithic Application]
+        E[UserRegistrationService]
+        F[AuthenticationService]
+    end
+    subgraph D[Microservices]
+        NewUserRegistrationService
+        NewAuthenticationService
+    end
+```
+
+### Step 5: Remove monolithic application
+
+Once you're satisfied with the performance of the new API, delete the old parts
+of the monolith. This process can take a long time, as the old code will exist
+as a form of backup for a while.
+
+```mermaid
+graph TD
+    A[User] --> B[Microservices]
+    subgraph B["Microservices"]
+        C[UserRegistrationService]
+        D[AuthenticationService]
+        E[PasswordResetService]
+    end
+```
+
+### Step 6: Repeat
+
+Finally, you should repeat the process by identifying and isolating another
+component that can be extracted from the monolith. This process can be repeated
+until the monolith has been completely broken down into a set of smaller,
+independent services.
+
+## Conclusion
+
+By following these strategies, we can gradually dismantle a monolithic Golang
+application while maintaining a functional system throughout the process. The
+practical example and the PlantUML diagram demonstrate how the Strangler,
+Decorator, and Sprig Patterns can be applied to effectively break down a
+monolithic application into smaller, more manageable microservices.