I have two kinds of Webflux applications, annotation-based and route-based. These applications are called with a set of headers, some of which (Open Tracing) I need to propagate in downstream calls using WebClient.
If these were normal Spring WebMvc applications I would use a Filter to keep the selected headers in a ThreadLocal, access it in a RestTemplate interceptor to send them to subsequent services and clear the ThreadLocal.
What's the proper way to replicate this behaviour in WebFlux applications?
I solved it using Project Reactor's Context to store the headers in a WebFilter. Then they are gotten in the WebClient's ExchangeFilterFunction. Here's the whole solution:
WebFilter
class OpenTracingFilter(private val openTracingHeaders: Set<String>) : WebFilter {
private val logger = LoggerFactory.getLogger(javaClass)
override fun filter(exchange: ServerWebExchange, chain: WebFilterChain): Mono<Void> {
return chain.filter(exchange)
.subscriberContext { ctx ->
var updatedContext = ctx
exchange.request.headers.forEach {
if (openTracingHeaders.contains(it.key.toLowerCase())) {
logger.debug("Found OpenTracing Header - key {} - value {}", it.key, it.value[0])
updatedContext = updatedContext.put(it.key, it.value[0])
}
}
updatedContext
}
}
}
OpenTracingExchangeFilterFunction
class OpenTracingExchangeFilterFunction(private val headers: Set<String>) : ExchangeFilterFunction {
private val logger = LoggerFactory.getLogger(javaClass)
override fun filter(request: ClientRequest, next: ExchangeFunction): Mono<ClientResponse> {
logger.debug("OpenTracingExchangeFilterFunction - filter()")
return OpenTracingClientResponseMono(request, next, headers)
}
}
OpenTracingClientResponseMono
class OpenTracingClientResponseMono(private val request: ClientRequest,
private val next: ExchangeFunction,
private val headersToPropagate: Set<String>) : Mono<ClientResponse>() {
private val logger = LoggerFactory.getLogger(javaClass)
override fun subscribe(subscriber: CoreSubscriber<in ClientResponse>) {
val context = subscriber.currentContext()
val requestBuilder = ClientRequest.from(request)
requestBuilder.headers { httpHeaders ->
headersToPropagate.forEach {
if(context.hasKey(it)) {
logger.debug("Propagating header key {} - value{}", it, context.get<String>(it))
httpHeaders[it] = context.get<String>(it)
}
}
}
val mutatedRequest = requestBuilder.build()
next.exchange(mutatedRequest).subscribe(subscriber)
}
}
OpenTracingConfiguration
@Configuration
class OpenTracingConfiguration(private val openTracingConfigurationProperties: OpenTracingConfigurationProperties) {
@Bean
fun webClient(): WebClient {
return WebClient.builder().filter(openTracingExchangeFilterFunction()).build()
}
@Bean
fun openTracingFilter(): WebFilter {
return OpenTracingFilter(openTracingConfigurationProperties.headers)
}
@Bean
fun openTracingExchangeFilterFunction(): OpenTracingExchangeFilterFunction {
return OpenTracingExchangeFilterFunction(openTracingConfigurationProperties.headers)
}
}
OpenTracingConfigurationProperties
@Configuration
@ConfigurationProperties("opentracing")
class OpenTracingConfigurationProperties {
lateinit var headers: Set<String>
}
application.yml
opentracing:
headers:
- x-request-id
- x-b3-traceid
- x-b3-spanid
- x-b3-parentspanid
- x-b3-sampled
- x-b3-flags
- x-ot-span-context