1. Learning the Basics of the Polly Framework Flashcards
What two categories can Polly resilience strategies be broken into?
- Reactive
2. Proactive
What is a reactive resilience strategy?
A resilience strategy that is triggered in response to a current and perhaps transient issue, like a server fault.
What is a proactive resilience strategy?
A resilience strategy that monitors for ongoing problems and attempts to stabilise the system when facing degraded quality or longer outages in remote services.
What reactive resilience strategies does Polly offer us?
- The
RetryPolicy - The
Wait and RetryPolicy - The
Circuit BreakerPolicy - The
FallbackPolicy
What does the Retry policy do?
Immediately retries the request some specified number of times without any delay.
What does the Wait and Retry policy do?
Works like the Retry but introduces a delay before each retry.
What does the Circuit Breaker policy do?
Stops all requests to an endpoint if some failure threshold is reached.
What does the Fallback policy do?
Returns some specified default value if the request fails
ps. Often used in conjunction with other policies
What can the use of delegates with the policies offered by Polly allow us to do?
Run custom code as part of the policy. For example, we can check if the cause of a failure was a specific error and look to resolve it before retrying the original request.
What proactive resilience strategies does Polly offer us?
- The
TimeoutPolicy - The
CachingPolicy - The
Bulkhead IsolationPolicy
What does the Timeout policy do?
Set the timeout for a request rather than waiting for default as implemented by the HttpClient.
What does the Caching policy do?
Cache responses for a period of time and then channel future requests to the cache rather than the remote service.
What does the Bulkhead Isolation policy do?
Manage the load on your own application by limiting the number of concurrently executing requests and limiting the size of the queue of requests that are waiting to execute.
What is a resilience framework?
There is no agreed-upon definition, but you can think of it as a set of libraries that help an application to recover from transient or longer failures in services or infrastructure upon which it depends when recovery is possible, and that facilitates graceful degradation of your application when it is not.
When does a Retry policy retries the request?
Immediately after failure.
What does the Wait and Retry policy allow us to define?
Backoff between entries, allowing us to increase the backoff of each subsequent request.
How does the Wait and Retry policy work for handling a request that failed?
It will decide whether to retry.
- If it decides to retry, it waits a specified amount of time before tries to send the request again.
- If no retries should be performed, it returns the response.
What parameters do the WaitAndRetry method take?
It takes the number of times to retry; and a lambda function that takes in an int parameter which tells us the retry count and returns a TimeSpan that determines how long to wait before retrying.
...
.WaitAndRetryAsync(3,
retryAttempt => TimeSpan.FromSeconds(
Math.Pow(2, retryAttempt) / 2));
What benefit does the Wait and Retry policy offer?
It gives the remote service time to recover, and in some scenarios, will make the retry more likely to succeed.
What is idempotence in the context of web requests and how does it relate to Polly policies?
A request is idempotent if it can be called any number of times and the result is the same each time.
GET, PUT, and DELETE are usually designed to be idempotent, but you should not expect POSTs to be. So, be careful when retrying POSTs.
What does the use of delegates allow us to do when used with policies?
It allows us to execute arbitrary code before the request is retried — log a failure, call some other method, etc.
/...
.RetryAsync(3,
onRetry: (httpResponseMessage, _) =>
{
if (httpResponseMessage.Result.StatusCode == HttpStatusCode.Unauthorized)
{
PerformReauthoripzation();
}
});
What does the Fallback policy do?
It allows us to return some meaningful default if all else has failed.
How does the Fallback policy work when used in conjunction with a HTTP request?
If the request succeeds, the response is returned as normal. If the request fails, the Fallback returns some default response.
What’s a common policy to use in conjunction with the Fallback policy?
The Retry policy — if the retries fail, use the Fallback to return a default.
