Polly is a popular transient fault handling library which provides a mechanism to define policies which can be applied when certain failures occur. One of the more commonly used policies is the retry policy. This allows you to wrap some code which, should a failure occur, will be retried; multiple times if necessary. This is very useful in situations where your application needs to communicate with external services. There is the ever-present risk when communicating with services over a transport such as HTTP that a transient fault will occur. A transient fault may prevent your request from being completed but is also likely to be a temporary problem. This makes retrying a sensible option in those cases.
As well as retries, Polly offers a number of other types of policy, many of which you may want to combine with retry to build up sophisticated ways to deal with failures. I will cover a few of the more general examples in this post, but if you want more comprehensive coverage I recommend you check out the Polly wiki.
The ASP.NET team have worked closely with Dylan and Joel, the primary maintainers of Polly, to include an integration pattern to make applying Polly policies to HttpClient instances really straightforward.
Before we can work with the Polly integrations we need to add a package reference to our project. The general IHttpClientFactory functionality lives inside the Microsoft.Extensions.Http package which is included as a dependency in the Microsoft.AspNetCore.App 2.1 meta package. This is a new meta package in ASP.NET Core 2.1 which doesn’t include third-party dependencies. Therefore, in order to use the Polly extensions for IHttpClientFactory we need to add the Microsoft.Extensions.Http.Polly package to our project.
After doing so in a basic project the csproj file will look something like this:
Applying a Policy
The Microsoft.Extensions.Http.Polly package includes an extension method called AddPolicyHandler on the IHttpClientBuilder that we can use to add a handler which will wrap all requests made using an instance of that client in a Polly policy. The IHttpClientBuilder is returned when we define a named or typed client.
We can then use the extensions in our ConfigureServices method…
In this example, we’re defining a client named “github” and we’ve used the AddPolicyHandler method to pass in a timeout policy. The policy you provide here must be an IAsyncPolicy<HttpResponseMessage>. This policy will timeout any requests after 10 seconds.
When using Polly, where possible, it is a good practice to define policies once and share them in cases where the same policy should be applied. This way, to change the rules for a policy, those changes only need to be made in one place. Also, it ensures that the policy is allocated only once. Certainly, policies such as the circuit breaker need to be shared if multiple callers expect to run through the same circuit breaker instance.
For this example, we’ll declare the timeout policy from the last example once and share it with two named clients…
We’ll look at another option for policy reuse a little later in this post when we explore using a PolicyRegistry.
Transient Fault Handling
When dealing with HTTP requests, the most common scenarios we want to handle are transient faults. As this is a common requirement, the Microsoft.Extensions.Http.Polly package includes a specific extension that we can use to quickly setup policies that handle transient faults.
For example, to add a basic retry when a transient fault occurs for requests from a named client we can register the retry policy as follows:
In this case, all requests made through the client will retry when certain failure conditions are met. The AddTransientHttpErrorPolicy method takes a Func<PolicyBuilder<HttpResponseMessage>, IAsyncPolicy<HttpResponseMessage>>. The PolicyBuilder here will be preconfigured to handle HttpRequestExceptions, any responses returning a 5xx status code and also any responses with a 408 (request timeout) status code. This should be suitable for many situations. If you require the policy to apply under other conditions, you will need to use a different overload to pass in a more specific policy.
Be aware; when performing retries we need to consider idempotency. Retrying a HTTP GET is a pretty safe operation. If we’ve made the call and not received any response, we can safely retry the call without any danger. However, consider what might happen if we retry a HTTP POST request. In that case, we have to be more careful since it’s possible that your original request was actually received, but the response we received suggested a failure. In that case, retrying could lead to duplication of data, or corruption of the data stored in the downstream system. Here, you need to have more knowledge of what the downstream service will do if it receives the same request more than once. Is retrying a safe operation? When you own the downstream service, it is easier to control this. You might, for example, use some unique identifier to prevent duplicate POSTs.
When you have less control of the downstream system or you know that a duplicate POST might have negative consequences, you will need to control your policy more carefully. An option that might be suitable is to define different named/typed clients. You could create one for those requests that have no side effects and another for those that do. You can then use the correct client for the action being taken. However, this might become a little difficult to manage. A better option is to use an overload of AddPolicyHandler which gives us access to the HttpRequestMessage so that policies can be applied conditionally. That overload looks like this:
AddPolicyHandler(Func<HttpRequestMessage, IAsyncPolicy<HttpResponseMessage>> policySelector)
You’ll note that the policySelector delegate here has access to the HttpRequestMessage and is expected to return an IAsyncPolicy<HttpResponseMessage>. We don’t have access to a PolicyBuilder setup to handle transient faults as we did in our earlier example. If we want to handle the common transient errors, we’ll need to define the expected conditions for our policy. To make this easier, the Polly project includes a helper extension that we can use that sets up a PolicyBuilder ready to handle the common transient errors. To use the extension method we need to add the Polly.Extensions.Http package from Nuget.
We can then call HttpPolicyExtensions.HandleTranisentHttpError() to get a PolicyBuilder that is configured with the transient fault conditions. We can use that PolicyBuilder to create a suitable retry policy which can then be conditionally applied when the request is a HTTP GET. In this example, any other HTTP methods use the NoOp policy.
Using a PolicyRegistry
The last example I want to cover in this post is a basic demonstration of how policies can be applied from a policy registry. To support policy reuse, Polly provides the concept of a PolicyRegistry which is essentially a container for policies. These can be defined at application startup by adding policies into the registry. The registry can then be passed around and used to access the policies by name.
The extensions available on the IHttpClientBuilder also support adding Polly based handlers to a client using a registry.
First, we must register a PolicyRegistry with DI. The Microsoft.Extensions.Http.Polly package includes some extension methods to make this simple. In the above example, I call the AddPolicyRegistry method which is an extension on the IServiceCollection. This will create a new PolicyRegistry and add register it in DI as the implementation for IPolicyRegistry<string> and IReadOnlyPolicyRegistry<string>. The method returns the policy so that we have access to add policies to it.
In this example, we’ve added two timeout policies and given them names. Now when registering a client we can call the AddPolicyHandlerFromRegistry method available on the IHttpClientBuilder. This takes the name of the policy we want to use. When the factory creates instances of this named client, it will add the appropriate handler, wrapping calls in the “regular” retry policy which will be retrieved from the registry.
As a long time user of Polly, I’m very happy to see the integration being added with IHttpClientFactory. Together these libraries make it really easy to get up and running with HttpClient instances that are able to handle transient faults seamlessly. The examples I’ve shown are quite basic and general, but I hope they give the idea of how policies can be used and registered. For more detailed Polly documentation and examples, I recommend you check out the Polly wiki. It was great being involved in some of the early discussions with both the ASP.NET and Polly teams when this integration was being designed as I was able to suggest the usefulness of the policy registry extensions.
Other Posts in this Series
Part 1 – An introduction to HttpClientFactory
Part 2 – Defining Named and Typed Clients
Part 3 – Outgoing request middleware with handlers
Part 4 – This post
Part 5 – Exploring the default request and response logging