C# Retry two queries with Polly Retry

Question

So I have 2 queries:

var a = await _aQuery.Execute(aId);
var b = await _bQuery.Execute(bId);

How can I retry execution of these queries with Polly Retry so that if any of them succeeds to get a non-null value, the code can proceed?

For example: 5 retries every 200ms and if query succeeds on a or on b, break the retries and proceed.

There are lots of examples available on the link below. https://github.com/App-vNext/Polly — maxspan, Sep 24 '21 at 12:52
*retries every 200ms* << Did you mean 200 milliseconds delay between each retry attempts, right? — Peter Csala, Sep 24 '21 at 13:28
yes thats correct, but the timing isn't a problem, the problem is how to wrap these two queries to be retried instead of retrying them one by one — tad, Sep 24 '21 at 14:09

score 3 · Answer 1 · answered Sep 24 '21 at 14:26

Let me alter your two operations a bit for the sake of simplicity.

static Random rnd = new();
static async Task<int?> OperationA()
{
    await Task.Delay(100); //Simulate I/O call's latency
    var number = rnd.Next();
    return number % 5 == 0 ? number : null;
}

static async Task<int?> OperationB()
{
    await Task.Delay(100);  //Simulate I/O call's latency
    var number = rnd.Next();
    return number % 4 == 0 ? number + 1 : null;
}

We have two operations (OperationA and OperationB) and they will either return null or an int
Their implementation are not important, they just serve demonstration purposes

Now If you need to define a retry policy which

Triggers if both operations failed
Triggers at most 5 times
Delays 200 milliseconds between each attempt

then you can do that like this:

var retryUntilGetResult = Policy<(int?, int?)>
    .HandleResult(((int? OpA, int? OpB) results) => !results.OpA.HasValue && !results.OpB.HasValue)
    .WaitAndRetryAsync(retryCount: 5, _ => TimeSpan.FromMilliseconds(200));

The Policy<(int?, int?)> part says that your return type is a tuple with two nullable integers
The HandleResult part says that if the returned tuple (results) does not contain any integer (!...HasValue) then the policy should trigger
The WaitAndRetryAsync part says you want to decorate an async method with this policy
The policy should fire at most 5 times, but wait 200 milliseconds between each attempt

The usage of this policy looks like this:

var result = await retryUntilGetResult.ExecuteAsync(async () =>
{
    Task<int?> opA = OperationA();
    Task<int?> opB = OperationB();
    await Task.WhenAll(opA, opB);
    return (await opA, await opB);
});

We fire of two Tasks and we are waiting for both to complete
Then we return a new tuple where we pass the results of the async operations

For the sake of completeness here the full source code:

static async Task Main()
{
    var retryUntilGetResult = Policy<(int?, int?)>
        .HandleResult(((int? OpA, int? OpB) results) => !results.OpA.HasValue && !results.OpB.HasValue)
        .WaitAndRetryAsync(retryCount: 5, _ => TimeSpan.FromMilliseconds(200));

    var result = await retryUntilGetResult.ExecuteAsync(async () =>
    {
        Task<int?> opA = OperationA();
        Task<int?> opB = OperationB();
        await Task.WhenAll(opA, opB);
        return (await opA, await opB);
    });
    Console.WriteLine(result);
}

static Random rnd = new();
static async Task<int?> OperationA()
{
    await Task.Delay(100); //Simulate I/O call's latency
    var number = rnd.Next();
    return number % 5 == 0 ? number : null;
}

static async Task<int?> OperationB()
{
    await Task.Delay(100);  //Simulate I/O call's latency
    var number = rnd.Next();
    return number % 4 == 0 ? number + 1 : null;
}

C# Retry two queries with Polly Retry

1 Answers1