Estimated reading time: 2 minutes

The CallId feature allows to identify a request/call and can work along the CallLogging feature.

It has been introduced in Ktor 0.9.5.

Generating Call IDs

install(CallId) {
    // Tries to retrieve a callId from an ApplicationCall. You can add several retrievers and all will be executed coalescing until one of them is not null.  
    retrieve { // call: ApplicationCall ->
    // If can't retrieve a callId from the ApplicationCall, it will try the generate blocks coalescing until one of them is not null.
    val counter = atomic(0)
    generate { "generated-call-id-${counter.getAndIncrement()}" }
    // Once a callId is generated, this optional function is called to verify if the retrieved or generated callId String is valid. 
    verify { callId: String ->
    // Allows to process the call to modify headers or generate a request from the callId
    reply { call: ApplicationCall, callId: String ->

    // Retrieve the callId from a headerName
    retrieveFromHeader(headerName: String)
    // Automatically updates the response with the callId in the specified headerName
    replyToHeader(headerName: String)
    // Combines both: retrieveFromHeader and replyToHeader in one single call
    header(headerName: String)

Extending CallLogging

The CallId feature includes a callIdMdc extension method to be used when configuring the CallLogging. It allows to associate the callId to the specified key to be put in the MDC context.

install(CallLogging) {