Using Polly with Any Type of Request
Full source code here.
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I recently presented a talk on Polly at the DevUp conference in St. Louis. In the presentation I showed examples using Polly with HttpClient requests only because this is my most common use case. I showed how to use retries, circuit breakers, fallbacks, caching, timeouts and bulkheads isolation.
At the end of the presentation a few people asked me if Polly could be used with other types of requests, of course it can and I should have said so during the talk. I’ve added a new slide for the next time I present on Polly - (as of October 2018 that will be Granite State Code Camp, Boston Code Camp, and NDC London).
Polly can be used with any type of request you make, whether it returns a value or not. You can use Polly on code that throws exceptions from time to time (though I strongly recommend fixing your code), for database calls, any method call for that matter. Below are some examples of using the retry policy in a variety of scenarios.
Exceptions
This policy retries if an exception is thrown, you can be more specific about exception types.
1ErrorProneCode errorProneCode = new ErrorProneCode();
2
3RetryPolicy retryIfException = Policy.Handle<Exception>()
4 .Retry(4, (exception, retryCount) =>
5 {
6 Console.WriteLine($"Got a response of {exception.Message} (expected 0), retrying {retryCount}");
7 });
8
9retryIfException.Execute(errorProneCode.DoSomethingThatMightThrowException);Int
Here I check if the int returned is anything other than 0, or if there has been a DivideByZeroException. If so, I retry up to four times and also print some text to the console.
1RetryPolicy<int> retryPolicyNeedsAResponseOfOne = Policy.HandleResult<int>(i => i != 0)
2 .Or<DivideByZeroException>()
3 .Retry(4, (response, retryCount) =>
4 {
5 Console.WriteLine($"Got a response of {response.Result} (expected 0), retrying {retryCount}");
6 });
7
8int number = retryPolicyNeedsAResponseOfOne.Execute(() => errorProneCode.GetSomeNumber());
9Console.WriteLine($"Got expected response = {number}\n\n");IEnumerable
In this one I check that the IEnumerable has three items in it, if not, I retry and print some info to the console.
1RetryPolicy<IEnumerable<int>> retryPolicyNeedsResponseWithTwoNumbers = Policy.HandleResult<IEnumerable<int>>(l => l.Count() != 3)
2 .Retry(4, (response, retryCount) =>
3 {
4 Console.WriteLine($"Got a response with {response.Result.Count()} entries (expected 3), retrying {retryCount}");
5 });
6
7var numbers = retryPolicyNeedsResponseWithTwoNumbers.Execute(() => errorProneCode.GetListOfNumbers());
8Console.WriteLine($"Got expected response of {numbers.Count()} entries\n\n");Bool
In this policy I check the bool returned, if it is false, I retry.
1RetryPolicy<bool> retryPolicyNeedsTrueResponse = Policy.HandleResult<bool>(b => b != true)
2 .Retry(4, (response, retryCount) =>
3 {
4 Console.WriteLine($"Got a response of {response.Result} entries (expected true), retrying {retryCount}");
5 });
6
7bool result = retryPolicyNeedsTrueResponse.Execute(() => errorProneCode.GetBool());
8Console.WriteLine($"Got expected response of {result}\n\n");Polly can check the value of any return type or exception making it possible to use Polly for any call you can think of.
Full source code here.