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To build a networking library like Retrofit or OkHttp, separate the API developers declare from the HTTP transport that performs network I/O. Add request and response conversion as an explicit extension point, and keep application code behind an interface so it can be substituted and tested. Android describes Retrofit as a type-safe JVM HTTP client built on OkHttp; the architecture below uses that documented relationship as a starting point, while marking additional implementation choices as design recommendations rather than claims about Retrofit’s internals.
Start with the API the application should express
A caller should be able to describe an operation in terms of its HTTP method, path, parameters, headers, request body, and typed result. Android’s network connectivity guide shows Retrofit’s declarative style: an annotated interface method represents an endpoint, and its return type describes the result. The guide includes a Kotlin suspend method and a Java method returning Call.
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That is a documented usage pattern, not evidence of how Retrofit parses or executes declarations internally. For a new library, one reasonable design is to read an endpoint declaration once and turn it into an immutable request description. The description can contain the resolved URL, method, headers, query values, body representation, and expected response type. A separate execution layer can then hand that description to a transport.
Declarative interface or imperative builder?
| Approach | What it makes explicit | Main design cost |
|---|---|---|
| Declarative endpoint interface | Endpoint shape and result types appear in method signatures and annotations; Android’s Retrofit example demonstrates this form. | The library must define how declarations are interpreted and how invalid or conflicting declarations are reported. |
| Imperative request builder | Request assembly is expressed as a sequence of operations at the call site. | Callers must assemble each request; the API surface and validation rules are design choices rather than properties established by the Android example. |
The comparison is about ergonomics and implementation complexity, not measured performance. A declarative API can make endpoint shapes easier to inspect, while a builder can make dynamic requests straightforward. Choose based on the consumers and constraints your library intends to support.
#1 Best Overall
Keep HTTP transport separate from the API description
Android describes Retrofit as “a type-safe HTTP client for the JVM from Square, built on top of OkHttp.” The relationship illustrates a useful seam: an API-description layer can provide typed endpoint declarations while a transport handles network I/O. Android also documents the platform’s HttpsURLConnection and lists TLS, streaming, configurable timeouts, IPv6, and connection pooling among its capabilities.
In a new design, define a narrow transport boundary that accepts a prepared request and returns a response or a transport failure. Treat connection setup, TLS, streaming, timeouts, and connection reuse as transport responsibilities. Keep endpoint annotations and application-specific result types out of the transport contract where possible. This separation is an architectural recommendation; the cited guide does not document Retrofit’s internal interfaces or guarantee that different transports behave identically.
Rank #2
Platform client or higher-level client?
| Choice | Grounded facts | What to weigh |
|---|---|---|
Platform HttpsURLConnection |
Android lists TLS, streaming, configurable timeouts, IPv6, and connection pooling as capabilities. | You may build more of the API-description and conversion experience yourself; the guide does not provide a performance comparison. |
| Higher-level API-description layer over a transport | Android documents Retrofit as a type-safe JVM client built on OkHttp and shows typed endpoint declarations. | Decide how much declaration parsing, conversion, and policy belongs in the higher-level layer. Do not assume retry, cancellation, pooling, or timeout behavior without consulting documentation for the specific transport and version. |
Make conversion an explicit extension point
Android says Retrofit supports several serialization libraries. That supports treating conversion as an extension point rather than coupling the endpoint API to one serialization format. A converter can turn a typed request value into a wire representation and decode a response representation into the declared type.
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- Converter selection: define how the library chooses a converter when more than one can handle a type or media type.
- Successful and error responses: decide whether error bodies are exposed raw, converted, or represented separately from successful results.
- Buffering and streaming: buffering can simplify decoding but may retain a whole body in memory; streaming can reduce that requirement but makes consumption and cleanup more demanding. These are general design trade-offs, not claims about a particular library’s defaults.
- Failure reporting: distinguish transport failures from conversion failures and HTTP error responses so callers can handle them deliberately.
Document whether converters may consume a body only once, who closes streams, and what happens when decoding fails. Those contracts matter to callers and to the correctness of the transport boundary.
Keep application code behind a data boundary
Android’s data-layer guidance describes a repository as a class that handles data operations and provides an abstraction over a resource. It recommends hiding the API client behind an interface; Android states, “Relying on interfaces makes API implementations swappable in your app.” It also favors fake data-source implementations for testing.
For an application using your library, a repository or equivalent boundary can keep UI and domain code from depending on transport-specific request objects, response streams, or exceptions. Expose application-relevant operations and values through an interface, then provide the network-backed implementation behind it. A repository is a useful application architecture option, not a requirement for every client or every project.
Test the failures a successful request will not reveal
Square’s OkHttp 5.x MockWebServer socket policy reference documents ways to simulate disconnects at different phases, stalled responses, TLS handshake failures, and partial request or response bodies. These scenarios can exercise the seams between transport, conversion, and application-level error handling.
- Disconnect before a response arrives and verify the failure exposed to the caller.
- Stall a response and check timeout behavior for the selected transport configuration.
- Fail a TLS handshake and ensure the error is not mistaken for a conversion or HTTP-status failure.
- Cut off a response body and verify decoding fails cleanly and resources are released.
- Use a partial request body to test what happens when transmission fails before completion.
- Test retry behavior only where your library defines it, including whether the request body can be replayed safely.
The MockWebServer reference warns that some socket closures can make a subsequent request nondeterministic, especially when the request body is not retryable. Treat replayability and connection reuse as cases to test, not as assumptions. The page is for OkHttp 5.x; check the selected version’s current documentation before relying on exact API details in runnable tests.
Choose the smallest set of policies you can support clearly
A networking library becomes harder to reason about when API declaration, transport mechanics, conversion, and application policy are all entangled. Define each boundary and its ownership before adding convenience features:
- API description: endpoint shape, parameter rules, and response type.
- Transport: the request execution contract and the selected client’s documented behavior.
- Conversion: supported representations, selection rules, and body lifecycle.
- Application boundary: the interface through which app code consumes data, with a repository where it fits the app’s architecture.
The cited documentation establishes the broad Retrofit/OkHttp relationship, Android’s platform-client capabilities, the repository/interface testing rationale, and MockWebServer’s failure simulation mechanisms. It does not establish current Retrofit or OkHttp versions, compatibility matrices, licenses, or library-specific timeout and retry defaults. Check official release and repository documentation for those details before making version-specific decisions.
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