Ktor: ZipkinIds feature

A Ktor installable plugin (formerly called feature) for particiapting in Zipkin distributed tracing.

Why might you want this?

You are building microservices in Ktor and want to instrument them for tracing using Zipkin.

What does it do?

Incoming headers

The feature reads incoming HTTP tracing headers, either multiple headers:

• X-B3-TraceId
• X-B3-SpanId
• X-B3-ParentSpanId
• X-B3-Sampled or X-B3-Flags

or a single b3 header. See zipkin-b3-propagation for details.

Initiating tracing

The feature initiates tracing if configured, optionally only for specified paths.

Response headers

The feature returns tracing headers in the response that match those received or initiated.

Client requests

The feature propagates tracing into downstream client requests where installed into Ktor clients.

Logging MDC items

Optionally you can install the current tracing information into Slf4j mapped diagnostic context (MDC).

How do you use it?

There are two parts: a server feature and a client feature.

Server feature

An example is:

fun Application.module() {

    install(ZipkinIds) {
        initiateTracePathPrefixes = arrayOf("/api")
        b3Header = true
    }

    // Other feature installations

    routing {
        get("/health") {
            call.respond(mapOf("status" to "UP"))
        }

        post("/api/v1/service") {
            try {
                val message = call.receive<Message>()
                val result = call.processMessage(message)
                call.respond(HttpStatusCode.OK, result)
            } catch (e: Exception) {
                call.respond(HttpStatusCode.BadRequest, e.message ?: "There was an error processing your request")
            }
        }
    }
}

In this example, the feature is installed so it will initiate tracing only for requests starting with "/api".

• Requests to /health without any tracing headers do not initiate tracing.
• Requests to /api/v1/service without tracing headers initiate tracing.
• Tracing initiated by the service respond with b3 headers and use them in client calls.
• If requests contain tracing headers, those headers and their type (either b3 or X-B3-*) will be maintained.
• Tracing information is stored in the ApplicationCall instance.

Client feature

To propagate tracing information into client calls, define extension functions on ApplicationCall, for example:

suspend fun ApplicationCall.processMessage(message: Message) {

    val url = "https://provider.com/api/v1/other-service"
    try {
        val client = HttpClient(CIO) {
            tracingParts?.let { it ->
                install(ClientIds) {
                    tracingParts = it
                }
            }
        }
        val response = client.post<String>(url) {
            body = TextContent(
                json.writeValueAsString(OutgoingMessage(message)),
                contentType = ContentType.Application.Json
            )
        }
    } catch (e: Exception) {
        logger.error("Client request failure", e)
    }
}

In this example:

• When the client is being constructed, if the ApplicationCall contains a tracingParts attribute, install the ClientIds feature with that attribute.

Logging MDC

Install the keys into logging MDC in the application as part of call logging:

    install(CallLogging) {
        level = Level.INFO
        zipkinMdc()
        filter { call -> call.request.path().startsWith("/") }
    }

Keys available are:

• b3Id
• traceId
• spanId
• parentSpanId

They can be set in logback.xml like so:

    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <pattern>%d{YYYY-MM-dd HH:mm:ss.SSS} [%thread] [%X{b3Id}] %-5level %logger{36} - %msg%n</pattern>
        </encoder>
    </appender>

Gradle dependency

repositories {
    // ...
    maven { url 'https://kotlin.bintray.com/ktor' }
    mavenCentral()
}

dependencies {
    // ...
    implementation 'com.michaelstrasser:ktor-features-zipkin:0.2.12'
}