Kevin Oliver


Posted February 9, 2016

Make application errors great again

make application errors great again

Finagle’s new response classifiers improve client’s avoidance of faulty nodes thus increasing your success rate. To get this benefit, you must wire up the application’s rules into your clients and how to do so is explained below.

First, a pop quiz — does Finagle treat an HTTP 500 response as a success or failure? How about a Thrift Exception?

If you answered failures, sadly, you are in for a surprise. Finagle lacks application level domain knowledge of what kinds of responses are failures. Without this, Finagle uses a conservative policy and treats all Returns as successful and all Throws as failures. Unfortunately, both HTTP 500s and Thrift Exceptions are Returns, and thus, successful responses.

By having you, the developers, give Finagle this application level knowledge, it can then accurately track failures in its failure accrual module which directly helps your client’s success rate. Finagle’s built-in success rate metrics (e.g. clnt/tweetsvc/success) also become accurate and this in turn means you may be able remove additional success rate metrics you may be wrapping on top of a Finagle client.

In the future, we are considering wiring this into load balancing which enables us to penalize servers which are returning failures or partial results.

Cool story. How do I use it?

As of release 6.33 you wire up a ResponseClassifier to your client. For HTTP clients, using HttpResponseClassifier.ServerErrorsAsFailures often works great as it classifies any HTTP 5xx response code as a failure. For Thrift/ThriftMux clients you may want to use ThriftResponseClassifier.ThriftExceptionsAsFailures which classifies any deserialized Thrift Exception as a failure. For a large set of use cases these should suffice. Classifiers get wired up to your client in a straightforward manner, for example:

// Scala
import com.twitter.finagle.ThriftMux
import com.twitter.finagle.builder.ClientBuilder
import com.twitter.finagle.thrift.service.ThriftResponseClassifier

// Discoverable Parameters API

// ClientBuilder API
// Java
import com.twitter.finagle.Http;
import com.twitter.finagle.builder.ClientBuilder;
import com.twitter.finagle.http.service.HttpResponseClassifier;

// Discoverable Parameters API

// ClientBuilder API

If a classifier is not specified on a client or if a user’s classifier isn’t defined for a given request/response pair then ResponseClassifier.Default is used. This gives us the same behavior Finagle had prior to classification — responses that are Returns are successful and Throws are failures.

Rad. How do I write a custom classifier?

To do this we should understand the few classes used. A ResponseClassifier is a PartialFunction from ReqRep to ResponseClass.

Let’s work our way backwards through those, beginning with ResponseClass. This can be either Successful or Failed and those values are self-explanatory. There are three constants which will cover the vast majority of usage: Success, NonRetryableFailure and RetryableFailure. While as of today there is no distinction made between retryable and non-retryable failures, it was a good opportunity to lay the groundwork for use in the future.

A ReqRep is a request/response struct with a request of type Any and a response of type Try[Any]. While the lack of typing may initially disturb you, our hope is that it is not an issue in practice. While all of this functionality is called response classification, you’ll note that classifiers make judgements on both a request and response.

Writing a custom PartialFunction is easy in Scala given its syntactic sugar. As with many things it is a bit more work from Java but is doable. Here is an example that counts HTTP 503s as failures (for Java examples, take a look at HttpResponseClassifierCompilationTest and ResponseClassifierCompilationTest):

// Scala
import com.twitter.finagle.http
import com.twitter.finagle.service.{ReqRep, ResponseClass, ResponseClassifier}
import com.twitter.util.Return
val classifier: ResponseClassifier = {
  case ReqRep(_, Return(r: http.Response)) if r.statusCode == 503 =>

Note that this PartialFunction isn’t total which is ok due to Finagle always using user defined classifiers in combination with ResponseClassifier.Default which will cover all cases.

Thrift and ThriftMux classifiers require a bit more care as the request and response types are not as obvious. This is because there is only a single Service from Array[Byte] to Array[Byte] for all the methods of an IDL’s service. To make this workable, there is support in Scrooge and Thrift/ThriftMux.newService and Thrift/ThriftMux.newClient code to deserialize the responses into the expected application types so that classifiers can be written in terms of the Scrooge generated request type, $Service.$Method.Args, and the method’s response type. Given an IDL:

exception NotFoundException { 1: string reason }

service SocialGraph {
  i32 follow(1: i64 follower, 2: i64 followee) throws (1: NotFoundException ex)

One possible classifier would be:

val classifier: ResponseClassifier = {
  // #1
  case ReqRep(_, Throw(_: NotFoundException)) =>
  // #2
  case ReqRep(_, Return(x: Int)) if x == 0 =>
  // #3
  case ReqRep(SocialGraph.Follow.Args(a, b), _) if a <= 0 =>

If you examine that classifier you’ll note a few things. First (#1), the deserialized NotFoundException can be treated as a failure. Next (#2), a “successful” response can be examined to enable services using status codes to classify errors. Lastly (#3), the request can be introspected to make the decision.

But what’s it really gonna do?

It’s important to understand what the impact will be if you customize response classification for your client. Perhaps most importantly, when responses are classified as failures, this affects how failure accrual sees responses. In the past, you may have had a Thrift service returning nothing but exceptions, but this node would continue getting traffic due to failure accrual’s lack of visibility. While this changes lets you fix this visibility, you should consider what causes those responses. For example, if the service is simply proxying a failure from its downstream service, you may not want to count that as a failure.

There isn’t a strict rule on what is the right thing to do with classification. However, with some minimal thought, many services can improve their success rate both in terms of how it’s reported as well as through avoidance of bad nodes.


We’re really hopeful that this makes a significant difference in how well Finagle works for you but it needs you, the application developers, to make these choices.

If you have any questions on how to use this or feedback on how it’s working, please get in touch through @finagle or the Finaglers mailing list.