Retry

Retry

Syntax

RetryPolicy retry = Policy
  .Handle<HttpRequestException>()
  .Retry(3);

The above example will create a retry policy which will retry up to three times if an action fails with an exception handled by the Policy.

For full retry syntax and overloads (including retry-forever, wait-and-retry, and related variants), see https://github.com/App-vNext/Polly#retry.

Syntax examples given are sync; comparable async overloads exist for asynchronous operation: see readme and wiki.

How Polly Retry works

When an action is executed through the policy:

• The retry policy attempts the action passed in the .Execute(…) (or similar) delegate.
  • If the action executes successfully, the return value (if relevant) is returned and the policy exits.
  • If the action throws an unhandled exception, it is rethrown and the policy exits: no further tries are made.
• If the action throws a handled exception, the policy:
  • Counts the exception
  • Checks whether another retry is permitted.
    • If not, the exception is rethrown and the policy terminates.
    • If another try is permitted, the policy:
      • for wait-and-retry policies, calculates the duration to wait from the supplied sleep duration configuration
      • raises the onRetry delegate (if configured)
      • for wait-and-retry policies, waits for the calculated duration.
      • Returns to the beginning of the cycle, to retry executing the action again.

Overall number of attempts

The overall number of attempts that may be made to execute the action is one plus the number of retries configured. For example, if the policy is configured .Retry(3), up to four attempts are made: the initial attempt, plus up to three retries.

Exponential backoff

A common retry strategy is exponential backoff: this allows for retries to be made initially quickly, but then at progressively longer intervals, to avoid hitting a subsystem with repeated frequent calls if the subsystem may be struggling.

Exponential backoff can be achieved by configuring waits-between-retries manually (useful if you desire some custom or an easily-readable scheme):

Policy
  .Handle<SomeExceptionType>()
  .WaitAndRetry(new[]
  {
    TimeSpan.FromSeconds(1),
    TimeSpan.FromSeconds(2),
    TimeSpan.FromSeconds(4),
    TimeSpan.FromSeconds(8),
    TimeSpan.FromSeconds(15),
    TimeSpan.FromSeconds(30)
  });

or by calculation:

Policy
  .Handle<SomeExceptionType>()
  .WaitAndRetry(3, retryAttempt => 
    TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)) 
  );

Jitter

In very high throughput scenarios it can be beneficial to add jitter to wait-and-retry strategies, to prevent retries bunching into further spikes of load.

RetryAfter: When the response specifies how long to wait

Some systems specify how long to wait before retrying as part of the fault response returned. This is typically expressed as a Retry-After header with a 429 response code.

This can be handled by using WaitAndRetry/Forever/Async(...) overloads where the sleepDurationProvider takes the handled fault/exception as an input parameter (example overload; discussion and sample code).

Some SDKs wrap RetryAfter in custom responses or exceptions: for example, the underlying Azure CosmosDB architecture sends a 429 response code (too many requests) with a x-ms-retry-after-ms header, but the Azure client SDK expresses this back to calling code by throwing a DocumentClientException with RetryAfter property. The same overloads can be used to handle these.

Retry to refresh authorization

A retry policy can be used to maintain authorisation against a third-party system, where that authorisation periodically lapses. For example:

var authorisationEnsuringPolicy = Policy
    .HandleResult<HttpResponseMessage>(r => r.StatusCode == HttpStatusCode.Unauthorized) 
    .RetryAsync(
       retryCount: 1, // Consider how many retries. If auth lapses and you have valid credentials, one should be enough; too many tries can cause some auth systems to blacklist. 
       onRetry: (e, i) => FooRefreshAuthorization(), 
      /* more configuration */); 

var response = authorisationEnsuringPolicy.ExecuteAsync(ct => DoSomethingThatRequiresAuthorization(...), cancellationToken);

If you wish to combine retry-for-reauthentication (as above) with retry-for-transient-faults, each retry policy can be expressed separately, and then combined with PolicyWrap.

Other uses

While the original premise of retry policies is to handle transient faults, other uses are possible. We have heard of retry policies used to make BDD tests more robust (retrying actions that might initially fail only because some page element was yet to load).

Thread-safety

Each call to .Execute(…) (or similar) through a retry policy maintains its own private state. A retry policy can therefore be re-used safely in a multi-threaded environment.

The internal operation of the retry policy is thread-safe, but this does not magically make delegates you execute through the policy thread-safe: if the delegates you execute through the policy are not thread-safe, they remain not thread-safe.