System Design: How to Build a Networking Library Like Retrofit or OkHttp

Networking is one of the most important infrastructure layers in a modern Android application. Most Android developers use libraries such as Retrofit and OkHttp without thinking about everything happening behind a simple API call:

api.getUsers()

Under the hood, a production networking stack may need to handle:

• HTTP and HTTPS
• Request construction
• URL resolution
• DNS
• TCP/TLS connections
• Connection pooling
• Interceptors
• Authentication
• Serialization
• Coroutines
• Threading
• Timeouts
• Cancellation
• Retries
• HTTP caching
• Response parsing
• Error handling
• Certificate validation
• Observability

This article designs a networking library from scratch and explains the architecture behind a Retrofit/OkHttp-style system.

The goal is not to recreate Retrofit or OkHttp internally. The goal is to understand the system-design principles that make a networking library reliable, reusable, testable, and scalable.

1. What Are We Building?

We want an Android networking library that allows developers to write:

interface UserApi {
    @GET("/users")
    suspend fun getUsers(): List<User>
}

Then:

val users = api.getUsers()

Instead of manually writing:

val request = Request.Builder()
    .url("https://example.com/users")
    .get()
    .build()

val response = client.newCall(request).execute()

Our library will convert a high-level API definition into an executable HTTP request.

2. Requirements

Functional Requirements

The library should support:

• GET, POST, PUT, PATCH and DELETE.
• Query parameters.
• Path parameters.
• Headers.
• Request bodies.
• Response bodies.
• JSON serialization/deserialization.
• Interceptors.
• Authentication.
• Logging.
• Coroutines.
• Request cancellation.
• Timeouts.
• Retry policies.
• HTTP caching.
• TLS/HTTPS.
• Connection pooling.
• Error handling.
• Multipart requests.
• Streaming responses.

3. Non-Functional Requirements

The networking library should be:

• Fast
• Thread-safe
• Memory efficient
• Secure
• Testable
• Modular
• Extensible
• Coroutine-friendly
• Lifecycle-aware
• Observable
• Suitable for high request concurrency

4. High-Level Architecture

The architecture can be divided into two major layers.

Android Application
        |
        v
API Interface Layer
        |
        v
Request Builder
        |
        v
Interceptor Chain
        |
        v
HTTP Client Engine
   +-------------+-------------+
   |                           |
   v                           v
Connection Manager         Dispatcher
   |                           |
   v                           v