[christopherread]

Next v7 increment: UseNats() for #84, following the UseRabbitMq() pattern from #112. It also carries message headers, including trace context (#90, as in #116), over NATS. Targets v7.

API

builder.Services.AddObvs(obvs => obvs
    .AddService<IOrderMessage>("orders", service => service
        .UseNats(builder.Configuration.GetSection("Obvs:Orders:Nats"))   // or .UseNats(o => o.Url = "nats://localhost:4222")
        .SerializedWith(new JsonMessageSerializer(), new JsonMessageDeserializerFactory(typeof(JsonMessageDeserializer<>)))));

• Options: NatsOptions has Url (required). Each service's settings are named options keyed by the service name: IOptionsMonitor<NatsOptions>.Get("orders").
• Startup checks: a missing URL or serializer fails at startup, inside the same ObvsConfigurationException as any other problem. The message names the configuration key to set (for example Obvs:Orders:Nats:Url) or the method to call.
• Connections aren't shared. v6's shared NATS connections are process-wide, keyed by URL. With them, stopping one host would close the connection another host in the same process is still using. So each service on each bus gets its own connection.

Headers over NATS

• Sending: the publisher sends ObvsMessageHeaders as NATS headers. A message without headers is sent as before.
• Receiving: the source copies NATS headers back onto the received message before subscribers see it. As a result, the receiver's process span continues the sender's trace, and headers the app sets arrive intact.
• Compatibility: v6 receivers ignore NATS headers, so v6 and v7 services can still exchange messages.

Tests (written first)

• Configuration (no server): 5 tests in Obvs.NATS.Tests/Hosting/TestUseNats.cs. They cover options from configuration and from code, a missing URL (in configuration and in code), and a missing serializer.
• End to end (Explicit, real server): 6 tests in TestNatsHosting.cs. They cover:
  • commands and events;
  • request and response through HandleRequest and RequestAsync;
  • two hosts in one process;
  • disposing the bus from a message callback;
  • trace continuation;
  • headers the app sets.
• Red first: the configuration tests failed before UseNats existed. Once UseNats worked, the two header tests still failed for their real reasons, a missing x-tenant header and a different trace ID, until headers were carried.
• Mutation check: switching back to shared connections makes the two-hosts test fail.
• Results:
  • All 8 NATS broker tests (the 6 new ones plus the 2 existing v6 ones) passed 3 runs in a row against a local nats-server.
  • A clean build of the solution has 0 warnings, and all 210 unit tests pass.

Not in this increment

• Use<Transport>() and header support for Kafka, ActiveMQ and NetMQ.

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