Service poling in microservice

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I know that below code is to avoid error throw when the microservice is not available and also configure to do retries to reach the service.

I am confused how "_" is recognized as an integer which is used as a parameter for
WaitAndRetryAsync Method.

FYI : this function accepts a delegate

public static AsyncRetryPolicy<TResult> WaitAndRetryAsync<TResult>(this PolicyBuilder<TResult> policyBuilder, int retryCount, Func<int, TimeSpan> sleepDurationProvider);

services.AddHttpClient("RydoProductsService", rydoconfig =>
            {
                rydoconfig.BaseAddress = new Uri(Configuration["Services:Products"]);
            }).AddTransientHttpErrorPolicy(p=>p.WaitAndRetryAsync(5,_=> TimeSpan.FromMilliseconds(500))) ;

What I have tried:

Googled........................................................................

Posted 19-Jun-21 2:10am

vaibhav1800

Updated 21-Jun-21 4:08am

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2 solutions

Solution 1

It represents the result of the lambda expression on the right of the => operator. See => operator - C# reference | Microsoft Docs[^].

Posted 19-Jun-21 2:50am

Richard MacCutchan

Updated 19-Jun-21 2:51am

v2

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vaibhav1800 19-Jun-21 9:06am

But then as part of delegate definition we are expecting two parameters right ? One is integer and other is timespan. I see the timespan parameter defined. What about the integer parameter. Or is it optional ? If yes then where it is defined as optional

Richard MacCutchan 19-Jun-21 9:22am

The integer is there in the first parameter position: 5.

vaibhav1800 19-Jun-21 12:56pm

That 5 is retrycount. Its not the one mentioned in the delegate.

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Richard Deeming · Accepted Answer · 2021-06-21T04:08:00

The WaitAndRetryAsync method accepts two parameters:

An integer representing the maximum number of times to retry. You have passed 5.
A Func<int, TimeSpan> delegate, which takes a single parameter - the attempt number - and returns the length of time to wait before trying again. You have passed the lambda method: _ => TimeSpan.FromMilliseconds(500)

The _ here is a discard, which simply means that you don't care about the parameter passed to the delegate. Your policy always waits 500ms between attempts.

Implement HTTP call retries with exponential backoff with Polly | Microsoft Docs[^]
Discards - unassigned discardable variables | Microsoft Docs[^]