Contextual logging (corelation id in headers ) with webflux - kotlin

Question

I need some params from the headers inside my coroutine in my controller, and log them as a corelation id for my requests.
Is it possible to use webflux / kotlin coroutines in controller AND to do contextual logging with the params in the header ?
I know Webflux can use WebFilter to intercept headers and log them or modify them, but can it be sent to the coroutine it will trigger ?
#RestController
class ItemController(private val itemRepository: ItemRepository) {
#GetMapping("/")
suspend fun findAllItems(): List<Item> =
// do stuff
logger.log("Corelation id is : " + myCorelationIdHeaderParam) // that's the param i need
return itemService.findAll()
}

Answer 1

Any context you set in the webfilter can be accessed by using subscriberContext down the line in your controllers/services.
Below is an example using Java. You can use similar logic in your Kotlin code:
Your filter: (Here you are setting the header value "someHeaderval" in your "myContext" )
public class MyFilter implements WebFilter {
#Override
public Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain) {
ServerHttpRequest request = exchange.getRequest();
String someHeaderval = request.getHeaders().get("someHeader").get(0);
return chain.filter(exchange).subscriberContext(context -> {
return context.put("myContext",someHeaderval);
});;
}
}
Now you can use this context anywhere:
#GetMapping(value = "/myGetApi")
public Mono<String> sampleGet() {
return Mono.subscriberContext()
.flatMap(context -> {
String myHeaderVal = (String)context.get("myContext");
//do logging with this header value
return someService.doSomething(myHeaderVal);
});
}

Answer 2

It turns out you can access the ReactorContext from the CoroutineContext with
coroutineContext[ReactorContext]
Here is my code :
#Component
class MyWebFilter : WebFilter {
val headerKey = "correlation-token-key"
val contextKey = "correlationId"
override fun filter(exchange: ServerWebExchange, chain: WebFilterChain): Mono<Void> {
val headers: HttpHeaders = exchange.request.headers
return chain.filter(exchange)
.subscriberContext(Context.of(contextKey, headers[headerKey] ?: "unidentified"))
}
}
and the controller part (most important for Kotlin users) :
#RestController
class ItemController(private val itemRepository: ItemRepository) {
#GetMapping("/")
suspend fun findAllItems(): List<Item> =
// do stuff
logger.log("Correlation id of request is : " + coroutineContext[ReactorContext]?.context?.get<List<String>>("correlationId")?.firstOrNull())
return itemService.findAll()
}