Currently there is MethodBuilder support for HTTP and ThriftMux. We are waiting on user interest before expanding to more protocols.

MethodBuilder is a collection of APIs for client configuration at a higher level than the Finagle 6 APIs while improving upon the deprecated ClientBuilder. MethodBuilder provides:

  • Idempotent classification providing a good default retry policy. It also offers a single knob, max extra load, that can help reduce tail latency through backup requests.
  • Logical success rate metrics.
  • Retries based on application-level requests and responses (e.g. an HTTP 503 response code or a Thrift exception).
  • Configuration of per-attempt and total timeouts.

All of these can be customized per method (or endpoint) while sharing a single underlying Finagle client. Concretely, a single service might offer both GET statuses/show/:id as well as POST statuses/update, whilst each having wildly different characteristics. The GET is idempotent and has a tight latency distribution while the POST is not idempotent and has a wide latency distribution. If users want different configurations, without MethodBuilder they must create separate Finagle clients for each grouping. While long-lived clients in Finagle are not expensive, they are not free. They create duplicate metrics and waste heap, file descriptors, and CPU.

MethodBuilder can also be used without scoping to a methodName, allowing users to wrap an entire client with MethodBuilder, as with totalService in the example below.

Turning the example into code, first in Scala:

import com.twitter.conversions.percent._
import com.twitter.conversions.time._
import com.twitter.finagle.{http, Http, Service}
import com.twitter.finagle.service.{ReqRep, ResponseClass}
import com.twitter.util.Return

// other StackClient configuration done here, e.g. `withSessionQualifier`
val stackClient = Http.client

val builder = stackClient.methodBuilder("inet!localhost:8080")

// a `Service` for "GET statuses/show/:id" with relatively tight timeouts
// and a liberal retry policy.
val statusesShow: Service[http.Request, http.Response] = builder
  // 25 millisecond timeout per attempt. this applies to the initial
  // attempt and each retry, if any.
  // 50 milliseconds timeout in total, including retries
  // retry all HTTP 4xx and 5xx responses
  .withRetryForClassifier {
    case ReqRep(_, Return(rep)) if rep.statusCode >= 400 && rep.statusCode <= 599 =>
  // can reduce tail latency by sending 1% extra load to the backend
  .idempotent(maxExtraLoad = 1.percent)
  // build the service
  .newService(methodName = "get_statuses")

// a `Service` for "POST statuses/update", which is non-idempotent and has
// relatively loose timeouts
val statusesUpdate: Service[http.Request, http.Response] = builder
  // 200 millisecond timeouts per attempt. this applies to the initial
  // attempt and each retry, if any.
  // 500 milliseconds timeouts in total, including retries
  // no retries (except on write exceptions), and no backup requests
  // build the service
  .newService(methodName = "update_status")

// A `Service` that is not scoped to a method
val totalService: Service[http.Request, http.Response] = builder

A similar example, for Java:

import com.twitter.finagle.Http;
import com.twitter.finagle.Service;
import com.twitter.finagle.http.Request;
import com.twitter.finagle.http.Response;
import com.twitter.util.Duration;

Service<Request, Response> exampleService =


MethodBuilder defaults to using the client’s classifier to retry failures that are marked as retryable (com.twitter.finagle.service.ResponseClass.RetryableFailure).

A budget is used to prevent retries from overwhelming the backend service. The budget is shared across clients created from an initial MethodBuilder. As such, even if the retry rules deem the request retryable, it may not be retried if there is insufficient budget.

Finagle automatically retries failures that are known to be safe to retry via RequeueFilter. This includes com.twitter.finagle.WriteException WriteExceptions and retryable nacks. As these should have already been retried, MethodBuilder will avoid retrying them again at this layer.

The classifier set by withRetryForClassifier is used to determine which requests are successful. This is the basis for measuring the logical success metrics of the method and for logging unsuccessful requests.


MethodBuilder provides idempotent and nonIdemptotent methods for a client to signal whether it’s safe to resend requests that have already been sent.

If a client is configured with idempotent, a protocol-dependent ResponseClassifier is combined with any existing classifier to also reissue requests on failure (Thrift exceptions for ThriftMux clients, and 500s for HTTP clients). The parameter to idempotent, maxExtraLoad, is used to configure backup requests and may be useful in reducing tail latency. Backup requests can be disabled by setting maxExtraLoad to 0.0.

If a client is configured with nonIdempotent, any existing configured ResponseClassifier is removed and replaced with the default ResponseClassifier, which only retries on write exceptions (wherein the request was never sent to the server). Any configured backup requests are also disabled, since it’s not safe to reissue requests.

Backup Requests

Backup requests, or hedged requests, are a means of reducing the tail latency of requests that are known to be safe to issue multiple times. This is done by sending an extra copy of a request, or a backup, to the backend after some threshold of time has elapsed before receiving a response for the initial request. This helps tail latency when the backend service has high variability in its response times and the additional backend load is deemed worth the cost of improving tail latency.

MethodBuilder makes backup requests easy through the idempotent method’s maxExtraLoad parameter. This parameter represents the maximum extra load, expressed as a fraction from [0.0 to 1.0), you are willing to send to the backend. If a response for the original request has not been received within some duration, a second, backup, request will be issued so long as the extra load constraints have not been violated. That duration is derived from maxExtraLoad; it is the n-th percentile latency of requests, where n is 100 * (1 - maxExtraLoad). For example, if maxExtraLoad is 0.01, no more than 1% additional requests will be sent and it will be sent at the p99 latency.

To disable backup requests, set maxExtraLoad to 0.0.

Latency is calculated using a windowed history of responses. In order to protect the backend from excessive backup requests should the latency shift suddenly, a RetryBudget based on maxExtraLoad is used. When determining whether or not to send a backup, this local budget is combined with the underlying client’s retry budget; this means that the backend will not receive more extra load than that permitted by the budget, whether through retries due to failures or backup requests.

If you are using TwitterServer along with finagle-stats, a good starting point for determining a value for maxExtraLoad is looking at the details of the PDF histogram for request latency. If you choose a maxExtraLoad of 0.01, for example, you can expect your p999 and p9999 latencies to come in towards the p99 latency. For 0.05, those latencies would shift towards your p95 latency. You should also ensure that your backend can tolerate the increased load.

Here’s an example of how to send backup requests for one endpoint and disable them for another, first in Scala:

import com.twitter.conversions.percent._
import com.twitter.finagle.{http, Http, Service}

val builder = Http.client.methodBuilder("inet!localhost:8080")

val withBackups: Service[http.Request, http.Response] = builder
  .idempotent(maxExtraLoad = 1.percent)
  .newService(methodName = "with_backups")

val noBackups: Service[http.Request, http.Response] = builder
  .idempotent(maxExtraLoad = 0.percent)
  .newService(methodName = "no_backups")

A similar example, for Java:

import com.twitter.finagle.Http;
import com.twitter.finagle.Service;
import com.twitter.finagle.http.Request;
import com.twitter.finagle.http.Response;

Service<Request, Response> withBackups =

Service<Request, Response> noBackups =

While backup requests are integrated nicely with MethodBuilder, this is not a requirement. The functionality is encapsulated in a Filter that can be composed with your other Filters and Services. Take a look at BackupRequestFilter, but note there are subtleties regarding where it is placed and the retry budgets.


Backup requests were popularized by Google in Dean, J. and Barroso, L.A. (2013), The Tail at Scale, Communications of the ACM, Vol. 56 No. 2, Pages 74-80. Non-paywalled slides here.


For per-request timeouts the defaults come from the client’s configuration for TimeoutFilter.Param which is typically set on a client via com.twitter.finagle.$Protocol.withRequestTimeout.

For total total timeouts, the defaults come from the client’s configuration for TimeoutFilter.TotalTimeout.

The total timeout is how long the logical request is given to complete. This includes the time spent on developer configured retries as well as automatic retries issued by Finagle. Per request timeouts apply to each attempt issued, irrespective of if it is the initial request, a Finagle requeue, or a retry based on the developer’s policy.

Take a MethodBuilder configured with 100 ms per-request timeout, 150 ms total timeout, and a policy that will retry all timeouts as an example. If the first request to the backend gets a retryable nack back in 10 ms, Finagle will automatically issue a retry with 100 ms for its timeout. If this retry happens to time out, the application level retry policy on the MethodBuilder applies, and this retry will have 40 ms remaining (150 ms total - 10 ms - 100 ms).


Metrics are scoped to your client’s label and method name.

  • clnt/your_client_label/method_name/logical/requests — A counter of the total number of logical successes and failures. This does not include any retries.
  • clnt/your_client_label/method_name/logical/success — A counter of the total number of logical successes.
  • clnt/your_client_label/method_name/logical/request_latency_ms — A stat of the latency of the logical requests, in milliseconds.
  • clnt/your_client_label/method_name/retries — A stat of the number of times requests are retried.
  • clnt/your_client_label/method_name/backups/send_backup_after_ms - A stat of the time, in milliseconds, after which a request will be re-issued (backup sent) if it has not yet completed. Present only if idempotent is configured.
  • clnt/your_client_label/method_name/backups/backups_sent - A counter of the number of backup requests sent. Present only if idempotent is configured.
  • clnt/your_client_label/method_name/backups/backups_won - A counter of the number of backup requests that completed before the original, regardless of whether they succeeded. Present only if idempotent is configured.
  • clnt/your_client_label/method_name/backups/budget_exhausted - A counter of the number of times the backup request budget (computed using the current value of the maxExtraLoad param) or client retry budget was exhausted, preventing a backup from being sent. Present only if idempotent is configured.

For example:

import com.twitter.conversions.percent._
import com.twitter.finagle.Http

val builder = Http.client
  .idempotent(maxExtraLoad = 1.percent)
val statusesShow = builder.newService(methodName = "get_statuses")

Will produce the following metrics:

  • clnt/example_client/get_statuses/logical/requests
  • clnt/example_client/get_statuses/logical/success
  • clnt/example_client/get_statuses/logical/request_latency_ms
  • clnt/example_client/get_statuses/retries
  • clnt/example_client/get_statuses/backups/send_backup_after_ms
  • clnt/example_client/get_statuses/backups/backups_sent
  • clnt/example_client/get_statuses/backups/backups_won
  • clnt/example_client/get_statuses/backups/budget_exhausted

MethodBuilder adds itself into the process registry which allows for introspection of runtime configuration via TwitterServer’s /admin/registry.json endpoint.


Unsuccessful request, as determined by the classifier set by withRetryForClassifier, are logged at com.twitter.logging.Level.DEBUG level. Further details, including the request and response, are available at TRACE level. There is a Logger per method, named with the format “com.twitter.finagle.client.MethodBuilder.$clientName.$methodName”.


A MethodBuilder is tied to a single logical destination via a Name, though using dtabs allows clients to talk to different physical locations.

Because MethodBuilder is immutable, its methods chain together, and create new instances backed by the original underlying client. This allows for common customizations to be shared across endpoints:

import com.twitter.conversions.time._
import com.twitter.finagle.{http, Http, Service}
import com.twitter.finagle.service.{ReqRep, ResponseClass}
import com.twitter.util.Return

// the `Services` below will use these settings unless they are
// explicitly changed.
val base = Http.client.methodBuilder("inet!localhost:8080")

val longerTimeout: Service[http.Request, http.Response] = base
  // changes the timeout, while leaving retries disabled
  .newService(methodName = "longer_timeout")

val retryOn418s: Service[http.Request, http.Response] = base
  // keeps the 200 ms timeout, while changing the retry policy
  .withRetryForClassifier {
    case ReqRep(_, Return(rep)) if rep.statusCode == 418 =>
  .newService(methodName = "retry_teapots")

As a consequence of the Finagle client being shared, its underlying resources (e.g. connections) are shared as well. Specifically, all Services constructed by a MethodBuilder must be close-ed for the underlying resources to be closed.

One other effect of sharing the Finagle client is that the load balancer and connection pool (when applicable, e.g. HTTP/1.1) are shared resources as well. For most usage patterns this is unlikely to be an issue. In some cases, it may manifest as poor distribution of the different method’s requests across backends. Should it be an issue, we recommend creating and using separate Finagle clients for those methods.

Migrating from ClientBuilder

MethodBuilder is in part intended as a replacement for ClientBuilder and as such there is relatively easy migration path. Users should prefer using the Finagle 6 style StackClients directly for creating a MethodBuilder and work on migrating their code off of ClientBuilder.

Notes and caveats:

  • Metrics will be scoped to the and then the method name.
  • Total timeout defaults to using the ClientBuilder.timeout configuration.
  • Per-request timeout defaults to using the ClientBuilder.requestTimeout configuration.
  • The ClientBuilder metrics scoped to “tries” are not included. These are superseded by the logical MethodBuilder metrics.
  • The ClientBuilder retry policy will not be applied and users must migrate to using withRetryForClassifier.
  • The ClientBuilder must have a destination set via one of hosts, addrs, dest, cluster, or group.
import com.twitter.finagle.client.ClientBuilder
import com.twitter.finagle.{http, Http}

val stackClient = Http.client()
val clientBuilder = ClientBuilder()

val methodBuilder = http.MethodBuilder.from(clientBuilder)

Application-level failure handling

While MethodBuilder encourages developers to consider failure modes in the broadest sense through response classification, this is often insufficient for application developers who need to do more than that. Examples include logging, fallback to a different data source, hiding functionality, and more. As MethodBuilder gives you a standard Service, developers are encouraged to compose them with Filters and/or transform the Services returned Future to handle more granular failures.

Using with ThriftMux

Above we saw an example using HTTP. Next let’s walk through a ThriftMux example, using a hypothetical social graph service with two endpoints, followers and follow, where followers is idempotent and has a tight latency profile and follow is only retryable for a specific error code and has a wide latency distribution. Given the IDL:

#@namespace scala com.twitter.finagle.example.graph

exception NotFoundException { 1: i32 code }

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

We create MethodBuilders which work on Scrooge’s generated Service-per-method, ServiceIface.


Scrooge does not yet generate ServiceIface for Java users, so this is limited to Scala.

import com.twitter.conversions.time._
import com.twitter.finagle.{Service, ThriftMux}
import com.twitter.finagle.example.graph._
import com.twitter.finagle.service.{ReqRep, ResponseClass}
import com.twitter.finagle.thriftmux.service.ThriftMuxResponseClassifier
import com.twitter.util.Throw

val stackClient = ThriftMux.client
val builder = stackClient.methodBuilder("inet!localhost:8989")

// `Service` for "followers" with tight timeouts and liberal retry policy
val followers: Service[GraphService.Followers.Args, Int] =
    .newServiceIface[GraphService.ServiceIface](methodName = "followers")

// `Service` for "follow"
val follow: Service[GraphService.Follow.Args, Int] =
    .withRetryForClassifier {
      case ReqRep(_, Throw(NotFoundException(code))) if code == 5 =>
    .newServiceIface[GraphService.ServiceIface](methodName = "follow")

If you are working with code that prefers Scrooge’s FutureIface you can convert the ServiceIface by wrapping it with a MethodIface.

import com.twitter.conversions.time._
import com.twitter.finagle.{Filter, ThriftMux}
import com.twitter.finagle.example.graph._
import com.twitter.util.Future

val stackClient = ThriftMux.client.withLabel("thriftmux_example")
val serviceIface: GraphService.ServiceIface =
    .newServiceIface[GraphService.ServiceIface](methodName = "followers")

// `FutureIface` is a collection of methods that return `Futures`.
// It will use the configuration from the `ServiceIface` which allows you to decorate
// the endpoints with `Filters`.
val loggingFilter: Filter[GraphService.Follow.Args, GraphService.Follow.SuccessType] = ???
val filtered: GraphService.ServiceIface =
  serviceIface.copy(follow = loggingFilter.andThen(serviceIface.follow))
val futureIface: GraphService.FutureIface =
  new GraphService.MethodIface(filtered)

val result: Future[Int] =
  futureIface.follow(follower = 568825492L, followee = 4196983835L)

Further details on the differences between ServiceIface and FutureIface and how to work with them are in Scrooge’s Finagle docs.

Logical request definition

MethodBuilder‘s logical requests represent the result of the initial request, after any retries have occurred. Concretely, should a request result in a retryable failure on the first attempt, but succeed upon retry, this is considered a single successful logical request while the logical request latency is the sum of both the initial attempt and the retry.