Ktor for HTTP

Use Ktor for Both Client and Server

• HTTP client work: io.ktor:ktor-client-core + ktor-client-cio (CIO is the coroutine-native engine; pick it unless you have a specific reason to use Apache or OkHttp)
• HTTP server work: io.ktor:ktor-server-core + ktor-server-cio for embedded servers; ktor-server-netty when you need Netty's tuning surface
• Both run on coroutines. client.get(...) / client.post(...) are suspend functions. They compose with Flow and withContext(Dispatchers.IO) naturally.

Versions

• Use Ktor 3.4.x (current). Stick to a single Ktor version across client + server in the same module — mixed 2.x / 3.x will not link cleanly.
• Ktor 3.4 depends on kotlinx-coroutines 1.10.2; align if you're pinning coroutines.

Idioms

• Configure the client once at the application top level; pass it where needed. Per-call HttpClient(CIO) allocation leaks under load.
• Use parameter("key", value) instead of string-concatenating the URL — type-safe and URL-encoded for free.
• Close the client on shutdown: Runtime.getRuntime().addShutdownHook(Thread { client.close() }).
• For JSON, install ContentNegotiation + kotlinx-serialization-json rather than hand-parsing response bodies.

Anti-patterns

• ❌ java.net.HttpURLConnection or URL("…").openStream() in new code — pre-coroutine, no async, painful headers
• ❌ OkHttp + Retrofit when there's no Android constraint — adds an Android-shaped abstraction layer on top of a JVM project
• ❌ requests-style synchronous Apache HttpClient in a coroutine context — blocks the dispatcher thread
• ❌ Two HTTP clients in the same app for no reason (legacy RestTemplate + new Ktor side-by-side)