Configuring Retries

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In order for Linkerd to do automatic retries of failures, there are two questions that need to be answered:

Which requests should be retried?
How many times should the requests be retried?

Both of these questions can be answered by specifying a bit of extra information in the service profile for the service you’re sending requests to.

Note

Routes configured in service profiles are different from [HTTPRoute] resources. Service profile routes allow you to collect per-route metrics and configure client-side behavior such as retries and timeouts. [HTTPRoute] resources, on the other hand, can be the target of AuthorizationPolicies and allow you to specify per-route authorization.

The reason why these pieces of configuration are required is because retries can potentially be dangerous. Automatically retrying a request that changes state (e.g. a request that submits a financial transaction) could potentially impact your user’s experience negatively. In addition, retries increase the load on your system. A set of services that have requests being constantly retried could potentially get taken down by the retries instead of being allowed time to recover.

Check out the retries section of the books demo for a tutorial of how to configure retries.

Retries

For routes that are idempotent, you can edit the service profile and add isRetryable to the retryable route:

spec:
  routes:
  - name: GET /api/annotations
    condition:
      method: GET
      pathRegex: /api/annotations
    isRetryable: true ### ADD THIS LINE ###

Retries are supported for all idempotent requests, whatever verb they use, and whether or not they have a body. In particular, this mean that gRPC requests can be retried. However, requests will not be retried if the body exceeds 64KiB.

Retry Budgets

A retry budget is a mechanism that limits the number of retries that can be performed against a service as a percentage of original requests. This prevents retries from overwhelming your system. By default, retries may add at most an additional 20% to the request load (plus an additional 10 “free” retries per second). These settings can be adjusted by setting a retryBudget on your service profile.

spec:
  retryBudget:
    retryRatio: 0.2
    minRetriesPerSecond: 10
    ttl: 10s

Monitoring Retries

Retries can be monitored by using the linkerd viz routes command with the --to flag and the -o wide flag. Since retries are performed on the client-side, we need to use the --to flag to see metrics for requests that one resource is sending to another (from the server’s point of view, retries are just regular requests). When both of these flags are specified, the linkerd routes command will differentiate between “effective” and “actual” traffic.

ROUTE                       SERVICE   EFFECTIVE_SUCCESS   EFFECTIVE_RPS   ACTUAL_SUCCESS   ACTUAL_RPS   LATENCY_P50   LATENCY_P95   LATENCY_P99
HEAD /authors/{id}.json     authors             100.00%          2.8rps           58.45%       4.7rps           7ms          25ms          37ms
[DEFAULT]                   authors               0.00%          0.0rps            0.00%       0.0rps           0ms           0ms           0ms

Actual requests represent all requests that the client actually sends, including original requests and retries. Effective requests only count the original requests. Since an original request may trigger one or more retries, the actual request volume is usually higher than the effective request volume when retries are enabled. Since an original request may fail the first time, but a retry of that request might succeed, the effective success rate is usually (but not always) higher than the actual success rate.