General Finagle FAQ

What are CancelledRequestException, CancelledConnectionException, and ClientDiscardedRequestException?

When a client connected to a Finagle server disconnects, the server raises a cancellation interrupt on the pending Future. This is done to conserve resources and avoid unnecessary work: the upstream client may have timed the request out, for example. Interrupts on futures propagate, and so if that server is in turn waiting for a response from a downstream server it will cancel this pending request, and so on.

The topology, visually:

Upstream ---> (Finagle Server -> Finagle Client) ---> Downstream

Interrupts propagate between the Finagle Server and Client only if the Future returned from the Server is chained 1 to the Client.

A simplified code snippet that exemplifies the intra-process structure:

import com.twitter.finagle.Mysql
import com.twitter.finagle.mysql
import com.twitter.finagle.Service
import com.twitter.finagle.Http
import com.twitter.finagle.http
import com.twitter.util.Future

val client: Service[mysql.Request, mysql.Result] =

val service: Service[http.Request, http.Response] =
  new Service[http.Request, http.Response] {
    def apply(req: http.Request): Future[http.Response] = {
      client(...).map(req: mysql.Result => http.Response())

val server = Http.server.serve(..., service)

“Chained” in this context means that calling Future#raise will reach the interrupt handler on the Future that represents the RPC call to the client. This is clearly the case in the above example where the call to the client is indeed the returned Future. However, this will still hold if the client call was in the context of a Future combinator (e.g. Future#select, Future#join, etc.)

This is the source of the CancelledRequestException – when a Finagle client receives the cancellation interrupt while a request is pending, it fails that request with this exception. A special case of this is when a request is in the process of establishing a session and is instead interrupted with a CancelledConnectionException

Note, in mux, when work is interrupted/cancelled on behalf of a request we encode this failure with a ClientDiscardedRequestException. The concept is the same as above though.

Server operators may prefer to measure their server side success rates excluding these classes of exceptions. The rationale is that these failures don’t necessarily represent a server side issue and are dependent on client side configuration (e.g. timeouts). The recommended way to achieve this is to configure a custom com.twitter.finagle.service.ResponseClassifier via $Protocol.server.withResponseClassifier(…).

An important note is that these failures may sometimes show up in client stats! The interrupt will propagate through the call graph until it finds an unsatisfied future. In some cases, that outstanding future will have originated from a client backend within a server (i.e. Finagle Client in the topology above). Thus, when the interrupt reaches the client future, all references to the future will see the interrupt with the cancellation exception.

You can disable this behavior by using the MaskCancelFilter:

import com.twitter.finagle.filter.MaskCancelFilter
import com.twitter.finagle.Http
import com.twitter.finagle.http.

val service: Service[http.Request, http.Response] =
val masked = new MaskCancelFilter[http.Request, http.Response]

val maskedService = masked.andThen(service)


Most protocols do not natively support request cancellations (though modern RPC protocols like Mux do). In practice, this means that for these protocols, we need to disconnect the client to signal cancellation, which in turn can cause undue connection churn.

How do I configure clients and servers with Finagle 6 APIs?

As of Finagle 6.x, we introduced a new, preferred API for constructing Finagle Clients and Servers. Where the old API used ServerBuilder/ClientBuilder, the new APIs use $Protocol.client.newClient and $Protocol.server.serve 2.

Old ClientBuilder APIs:

import com.twitter.finagle.Http
import com.twitter.finagle.builder.ClientBuilder
import com.twitter.finagle.stats.StatsReceiver
import com.twitter.finagle.tracing.Tracer
import com.twitter.util.Duration

val statsReceiver: StatsReceiver = ???
val tracer: Tracer = ???
val requestTimeout: Duration = ???
val connectTimeout: Duration = ???

val client = ClientBuilder()

New Stack APIs:

import com.twitter.finagle.Http
import com.twitter.finagle.stats.StatsReceiver
import com.twitter.finagle.tracing.Tracer
import com.twitter.util.Duration

val statsReceiver: StatsReceiver = ???
val tracer: Tracer = ???
val requestTimeout: Duration = ???
val connectTimeout: Duration = ???

val client = Http.client
  // if `withStatsReceiver` is not specified, it will use the
  // `c.t.f.stats.DefaultStatsReceiver` scoped to the value of `newClient` or
  // `newService`'s label. If that is not provided, it will be scoped to the
  // value of `withLabel`.

More configuration options and the details about them are available for clients and servers. Additionally, the Scaladocs for most methods on ServerBuilder and ClientBuilder include the Stack-based API’s alternative. A few methods do not yet have one-to-one equivalents, such as ClientBuilder.retries and for these you should migrate to using MethodBuilder.


Protocol implementors are encouraged to provide sensible defaults and leave room for application specific behavior to be built on top of the base layer via Filters or synchronization mechanisms.

Why do clients see com.twitter.finagle.FailedFastException’s?

While the FailFast service factory generally shields clients from downed hosts, sometimes clients will see FailedFastExceptions. A common cause is when all endpoints in the load balancer’s pool are marked down as fail fast, then the load balancer will pass requests through, resulting in a com.twitter.finagle.FailedFastException.

See this example on how to disable Fail Fast for a given client.

Refer to the fail fast section for further context.

What is a com.twitter.finagle.service.ResponseClassificationSyntheticException?

While typically, a StatsFilter counts Exceptions as failures, a user may supply a ResponseClassifier that treats non-Exceptions as failures. In that case, while no exceptions have occurred, a ResponseClassificationSyntheticException is used as a “synthetic” exception for bookkeeping purposes.

One specific example can be seen when using the ThriftResponseClassifier.ThriftExceptionsAsFailures. Successful ThriftResponses which deserialize into Thrift Exceptions use this exception to be counted as failures in StatsFilter.

How long should my Clients live?

One client should be made per set of fungible services. You should not be reinstantiating your client on every request, and you should not have a different client per instance–finagle can handle load-balancing for you.

There are a few use cases, like link shortening, or web crawling, where a service must communicate with many other non-fungible services, in which it makes sense to proliferate clients that are created, used, and thrown away, but in the vast majority of cases, clients should be persistent, not ephemeral.

When can I use a null?

None of Finagle’s APIs admits nulls unless noted otherwise. Finagle is written in Scala, and by convention, we use Scala Options when a parameter or a result is optional.

Where is time spent in the client stack?

Finagle’s clients and servers have many modules that are tasked with a wide assortment of jobs. When there is unexpected latency, it can be useful to have visibility into where time is spent. Finagle’s RequestLogger can help with this. It can be enabled by setting the com.twitter.finagle.request.Logger level to TRACE and enabling the stack param:

// scala
import com.twitter.finagle.filter.RequestLogger

// java
import com.twitter.finagle.filter.RequestLogger;


The logs include synchronous and asynchronous time for each stack module’s Filter. Synchronous here means the time spent from the beginning of the Filter.apply call to when the Future is returned from the Filter. Asynchronous here is how long it takes from the beginning of the Filter.apply call to when the returned Future is satisfied.

As an example, given this stack module with the name “slow-down-module”:

import com.twitter.conversions.DurationOps._
import com.twitter.finagle.Filter
import com.twitter.finagle.util.DefaultTimer

class SlowFilterDoNotUse extends Filter[Int, Int, Int, Int] {
  def apply(request: Int, service: Service[Int, Int]): Future[Int] = {
    // this delays the synchronous path

    // the call to `Future.delayed` delays the asynchronous path

The output of RequestLogger would look something like:

traceId=b07d63561ed1a9b9.b07d63561ed1a9b9<:b07d63561ed1a9b9 server-name slow-down-module begin
traceId=b07d63561ed1a9b9.b07d63561ed1a9b9<:b07d63561ed1a9b9 server-name slow-down-module end cumulative sync elapsed 1000025 us
traceId=b07d63561ed1a9b9.b07d63561ed1a9b9<:b07d63561ed1a9b9 server-name slow-down-module end cumulative async elapsed 1500045 us

There will be these lines for every stack module and the log format is: traceId=$traceId $client-or-server-label $module-name.

Mux-specific FAQ

What service behavior will change when upgrading to Mux?

Connecting Pooling Metrics

With Mux, Finagle multiplexes several requests onto a single connection. As a consequence, traditional forms of connection-pooling are no longer required. Thus Mux employs SingletonPool, which exposes new stats:

  • connects, connections, and closes stats should drop, since there will be less channel opening and closing.

  • connection_duration, connection_received_bytes, and connection_sent_bytes stats should increase, since connections become more long-lived.

  • connect_latency_ms and failed_connect_latency_ms stats may become erratic because their sampling will become more sparse.

  • pool_cached, pool_waiters, pool_num_waited, pool_size stats all pertain to connection pool implementations not used by Mux, so they disappear from stats output.

ClientBuilder configuration

Certain ClientBuilder settings related to connection pooling become obsolete: hostConnectionCoresize, hostConnectionLimit, hostConnectionIdleTime, hostConnectionMaxWaiters, and expHostConnectionBufferSize

Server Connection Stats

The server-side connection model changes as well. Expect the following stats to be impacted:

  • connects, connections, and closes stats should drop.

  • connection_duration, connection_received_bytes, and connection_sent_bytes should increase.

  • Obsolete stats: idle/idle, idle/refused, and idle/closed

ServerBuilder configuration Certain ServerBuilder connection management settings become obsolete: openConnectionsThresholds.

What is ThriftMux?

ThriftMux is an implementation of the Thrift protocol built on top of Mux.