C# Polly WaitandRetry or delay and then retry-CodePudding

I have created a policy which will wait and retry for: My step is depending on output of a SELECT query. Sometimes database takes a long time around 35-45 seconds to generate the value in a table. So, I have to wait till that time to check if value populated in database table or not using Polly retry.

Below is my logic for it:

var parameters = new { PackageID = packageId };
var query = $"Select ID From Staging..Log Where StagePkg=@PackageID";

var _retryPolicy = Policy
    .Handle<Exception>()
    .WaitAndRetryAsync(5, retryAttempt => {
            var timeToWait = TimeSpan.FromSeconds(Math.Pow(10, retryAttempt));
            Console.WriteLine($"Waiting {timeToWait.TotalSeconds} seconds");
            return timeToWait;
        }
    );

return await _retryPolicy.ExecuteAsync<BatchMailLog>(async () => await SybaseConnection.WithConnectionAsync(c => c.QueryFirstOrDefaultAsync<StageLog>(query, parameters)));

It's not getting executed the way I was thinking like: execute the query, check if query returning a value. If returned value is 0 or null then retry the query after 15 seconds and repeat till we get the value from select query.

I am getting error below:

System.NullReferenceException : Object reference not set to an instance of an object.

This error is happening because it didn't get the value from a query so it has to wait for few seconds and retry.

CodePudding user response：

The declared policy catch exception to retry the action, but the action don't throw exception, it just return null.

You can use HandleResult to retry on a specific value like null :

var _retryPolicy = Policy
    .HandleResult<BatchMailLog>(b => b != null)
    ...

If you want also retry on exception :

var _retryPolicy = Policy
    .Handle<Exception>()
    .OrResult<BatchMailLog>(b => b != null)
    ...

CodePudding user response：

if returned value is 0 or null then retry the query after 15 seconds and repeat till we get the value from select query

This means you need to setup your policy in the following way:

Trigger: if returned value is 0 or null
Sleep duration: retry the query after 15 seconds
Times: till we get the value from select query

With Polly you can describe this desired behaviour in the following way:

var retry = Policy
    .HandleResult<BatchMailLog>(b => (b?.XYZ ?? 0) == 0) //Trigger
    .WaitAndRetryForeverAsync( //Times
      _ => TimeSpan.FromSeconds(15)); //Sleep duration