reisenberger / Polly-Samples

Provides sample implementations of the Polly library. The intent of this project is to help newcomers kick-start their use of Polly within their own projects.

Polly-Samples

Provides sample implementations using the Polly library. The intent of this project is to help newcomers kick-start their use of Polly within their own projects.

About the numbered demos

Background

• The demos run against an example 'faulting server' (also within the solution). To simulate failure, this dummy server rejects more than 3 calls from the same IP in any five-second period.
• Be sure to read the <summary/> at the top of each demo: this explains the intent of that demo, and what it achieves (in relation to the dummy 'faulting server').
• Sometimes the <summary/> also highlights what this demo doesn't achieve - often picked up in the following demo. Explore the demos in sequence, for best understanding.

Sequence

• Demo 00 shows behaviour calling the faulting server every half-second, with no Polly policies protecting the call.
• Demos 01-04 show various flavors of Polly retry.
• Demos 05-06 show retry combined with Circuit-Breaker.
• Demo 07 shows the Polly v5.0 PolicyWrap for combining Retry and CircuitBreaker.
• Demo 08 adds Polly v5.0 Fallback, making the call protected (in a PolicyWrap) by a Fallback, Retry, Circuitbreaker.
• Demo 09 shows the Polly v5.0 Timeout policy for an overall call timeout, in combination with Fallback and WaitAndRetry.

Bulkhead isolation demos

Background

The bulkhead isolation demos place calls against two different imaginary endpoints on a downstream server:

• The good endpoint returns results in a timely manner
• The faulting endpoint simulates a faulting downstream system: it does respond, but only after a long delay.
• (Note: Unlike the other demos, there is no throttling rejection of the caller.)

Sequence

In all bulkhead demos, the upstream system makes a random mixture of calls to the good and faulting endpoints.

• In Demo 00 there is no bulkhead isolation.
  • Sooner or later, the faulting stream of calls saturates all resource in the caller, starving the calls to the good endpoint of resource too.
  • Watch how the the calls to the good endpoint eventually start backing up too (watch the 'pending' count climb), as the faulting stream starves the whole system of resource.
• In demo 01, the calls to faulting and good endpoints are isolated by bulkhead isolation.
  • The faulting stream of calls still backs up.
  • But the calls to the good endpoint are unaffected - they consistently succeed, because they are isolated in a separate bulkhead.

To run the demos

• To start the dummy server, start PollyTestApp.
• Be sure the port number for the dummy server in PollyTestClient\Configuration.cs matches the port on which PollyTestApp has started on your machine (in the previous step).
• To run a demo, uncomment the demo you wish to run in PollyTestClient\program.cs. Then start PollyTestClient.
• Many Polly policies are about handling exceptions. If running the demos in debug mode out of Visual Studio and flow is interrupted by Visual Studio breaking on exceptions, consider unchecking the box "Break when this exception type is user-unhandled" in the dialog shown when Visual Studio breaks on an exception. Deeper hints on debugging with Polly are also available on the Polly wiki

Want further information?

• Any questions about the operation of the demos, ask on this repo; any questions about Polly, ask at Polly.
• For full Polly syntax, see Polly.
• For deeper discussions of transient fault-handling and further Polly patterns, see the Polly wiki

Slide deck from DevIntersections presentation

View the slides presented at DevIntersections and other conferences. You are welcome to use and adapt this presentation for not-for-profit presentations of Polly to co-workers, user groups and similar, subject to the condition that references to the .NET Foundation, App-vNext and Polly team are retained.