Transient fault handling
and retry strategy
While working with webservices in the middleware
layer (say a subscriber to a service bus queue), it is wise to use a good retry
strategy for transient fault handling for scenarios such as – connectivity
issues, timeout issues etc. Retrying immediately will not help much as the
chances of the resource being unavailable would be high. The code below
explains ExponentialBackoff strategy. Exponentialbackoff
retry strategy means - It ll retry after 2seconds, 4seconds, 8seconds.
Explanation of ExponentialBackoff parameters –
var retryStrategy = new ExponentialBackoff(3,
TimeSpan.FromSeconds(2),
TimeSpan.FromSeconds(20), TimeSpan.FromSeconds(1));
a)
first param - number of retries
b)
second param - minimum backoff time (say
2seconds). Meaning - time it should wait 2 seconds before the first RETRY or
minimum time between any retries.
c)
third param - maximum backoff limit. In this case - the third retry will happen
after about ~ 2 + 4 + 8 = 14 seconds. The third param is there to put a limit
on backoff period (which is 20 in this case).
d)
fourth param - delta to add some randomness,
otherwise all clients would be retrying simultaenously.
in this case random value is 1 second. So 2,4,8
could be 1,5,7 etc.
Please find the code files and detailed steps
below –
1) Add
a nuget package to EnterpriseLibrary.TransientFaultHandling.
2)
Create the following class (to specify
errors/exceptions which should be retried) –
public class TransientErrorDetectionStrategy : ITransientErrorDetectionStrategy
{
// add the list of
exceptions, that you consider transient.
public bool IsTransient(Exception ex)
{
if (ex is WebException)
return true;
if (ex is FakeTimeoutException)
return true;
return false;
}
}
3)
Create a FakeTimeoutException (for demo/testing
purpose) –
public class FakeTimeoutException : Exception
{
public FakeTimeoutException(string msg) : base(msg)
{
}
}
4)
Create the main class and method as shown below
–
class Program
{
static void Main(string[] args)
{
var retryStrategy = new ExponentialBackoff(3, TimeSpan.FromSeconds(2),
TimeSpan.FromSeconds(20), TimeSpan.FromSeconds(1));
TransientErrorDetectionStrategy
errorDetectionStrategy = new TransientErrorDetectionStrategy();
var retryPolicy = new RetryPolicy(errorDetectionStrategy,retryStrategy);
retryPolicy.ExecuteAction(() =>
ExecuteHTTPGET("https://microsoft.sharepoint.com")
);
}
private static void ExecuteHTTPGET(string requestUri)
{
Console.WriteLine(DateTime.Now);
throw new FakeTimeoutException("fake
timeout");
HttpWebRequest request = (HttpWebRequest)WebRequest.Create(requestUri);
request.KeepAlive = false;
request.Method = "GET";
HttpWebResponse webResponse = (HttpWebResponse)request.GetResponse();
int requestStatus = (int)webResponse.StatusCode;
webResponse.Close();
}
