Request Hedging

Overview

Hedging is one of two configurable retry policies supported by gRPC. With hedging, a gRPC client sends multiple copies of the same request to different backends and uses the first response it receives. Subsequently, the client cancels any outstanding requests and forwards the response to the application.

Use cases

Hedging is a technique to reduce tail latency in large scale distributed systems. While naive implementations could add significant load to the backend servers, it is possible to get most of the latency reduction effects while increasing load only modestly.

For an in-depth discussion on tail latencies, see the seminal article, The Tail At Scale, by Jeff Dean and Luiz André Barroso.

Configuring hedging in gRPC

Hedging is configurable via gRPC Service Config, at a per-method granularity. The configuration contains the following knobs:

"hedgingPolicy": {
  "maxAttempts": INTEGER,
  "hedgingDelay": JSON proto3 Duration type,
  "nonFatalStatusCodes": JSON array of grpc status codes (int or string)
}

• maxAttempts: maximum number of in-flight requests while waiting for a successful response. This is a mandatory field, and must be specified. If the specified value is greater than 5, gRPC uses a value of 5.
• hedgingDelay: amount of time that needs to elapse before the client sends out the next request while waiting for a successful response. This field is optional, and if left unspecified, results in maxAttempts number of requests all sent out at the same time.
• nonFatalStatusCodes: an optional list of grpc status codes. If any of hedged requests fails with a status code that is not present in this list, all outstanding requests are canceled and the response is returned to the application.

Hedging policy

When the application makes an RPC call that contains a hedgingPolicy configuration in the Service Config, the original RPC is sent immediately, as with a standard non-hedged call. After hedgingDelay has elapsed without a successful response, the second RPC will be issued. If neither RPC has received a response after hedgingDelay has elapsed again, a third RPC is sent, and so on, up to maxAttempts. gRPC call deadlines apply to the entire chain of hedged requests. Once the deadline has passed, the operation fails regardless of in-flight RPCS, and regardless of the hedging configuration.

When a successful response is received (in response to any of the hedged requests), all outstanding hedged requests are canceled and the response is returned to the client application layer.

If an error response with a non-fatal status code (controlled by the nonFatalStatusCodes field) is received from a hedged request, then the next hedged request in line is sent immediately, shortcutting its hedging delay. If any other status code is received, all outstanding RPCs are canceled and the error is returned to the client application layer.

If all instances of a hedged RPC fail, there are no additional retry attempts. Essentially, hedging can be seen as retrying the original RPC before a failure is even received.

If server pushback that specifies not to retry is received in response to a hedged request, no further hedged requests should be issued for the call.

Throttling Hedged RPCs

gRPC provides a way to throttle hedged RPCs to prevent server overload. Throttling can be configured via the Service Config as well using the RetryThrottlingPolicy message. The throttling configuration contains the following:

"retryThrottling": {
  "maxTokens": 10,
  "tokenRatio": 0.1
}

For each server name, the gRPC client maintains a token_count which is initially set to max_tokens. Every outgoing RPC (regardless of service or method invoked) changes token_count as follows:

• Every failed RPC will decrement the token_count by 1.
• Every successful RPC will increment the token_count by token_ratio.

With hedging, the first request is always sent out, but subsequent hedged requests are sent only if token_count is greater than the threshold (defined as max_tokens / 2). If token_count is less than or equal to the threshold, hedged requests do not block. Instead they are canceled, and if there are no other already-sent hedged RPCs the failure is returned to the client application.

The only requests that are counted as failures for the throttling policy are the ones that fail with a status code that qualifies as a non-fatal status code, or that receive a pushback response indicating not to retry. This avoids conflating server failure with responses to malformed requests (such as the INVALID_ARGUMENT status code).

Server Pushback

Servers may explicitly pushback by setting metadata in their response to the client. If the pushback says not to retry, no further hedged requests will be sent. If the pushback says to retry after a given delay, the next hedged request (if any) will be issued after the given delay has elapsed.

Server pushback is specified using the metadata key, grpc-retry-pushback-ms. The value is an ASCII encoded signed 32-bit integer with no unnecessary leading zeros that represents how many milliseconds to wait before sending the next hedged request. If the value for pushback is negative or unparseble, then it will be seen as the server asking the client not to retry at all.

Resources

• The Tail At Scale
• gRPC Service Config
• gRPC Retry Design

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