Polly, HttpClientFactory and the Policy Registry in a console application

Full source code available here.

How to use the HttpClientFactory with a console application is not immediately obvious. I thought it would be a simple matter, but it’s not because it relies on the dependency injection infrastructure you get with a web application. I’ve written about using HttpClientFactory with Polly in a Web Api here.

The easiest way to use HttpClientFactory within a console application is inside a HostBuilder. This gives you access to the services collection, now everything is easy.

Start with a standard console application, if you’re wondering about the async Task on my Main method, this was introduced in C# 7.1.

static async Task Main(string[] args)
{
    var builder = new HostBuilder()
        .ConfigureServices((hostContext, services) =>
        {

Inside the ConfigureServices, we configure the HttpClientFactory in the same way I showed in my previous post. You can also configure other things like logging, configuration sources, more DI, etc.

But first off, I’m going to add a Polly registry –

IPolicyRegistry<string> registry = services.AddPolicyRegistry();
	
IAsyncPolicy<HttpResponseMessage> httWaitAndpRetryPolicy =
    Policy.HandleResult<HttpResponseMessage>(r => !r.IsSuccessStatusCode)
        .WaitAndRetryAsync(3, retryAttempt => TimeSpan.FromSeconds(retryAttempt));

registry.Add("SimpleWaitAndRetryPolicy", httWaitAndpRetryPolicy);

IAsyncPolicy<HttpResponseMessage> noOpPolicy = Policy.NoOpAsync()
    .AsAsyncPolicy<HttpResponseMessage>();

registry.Add("NoOpPolicy", noOpPolicy);

Then add the HttpClientFactory, passing in the lambda to pick the right policy based on the HTTP verb.

services.AddHttpClient("JsonplaceholderClient", client =>
{
    client.BaseAddress = new Uri("https://jsonplaceholder.typicode.com");
    client.DefaultRequestHeaders.Add("Accept", "application/json");
}).AddPolicyHandlerFromRegistry((policyRegistry, httpRequestMessage) =>
{
    if (httpRequestMessage.Method == HttpMethod.Get || httpRequestMessage.Method == HttpMethod.Delete)
    {
        return policyRegistry.Get<IAsyncPolicy<HttpResponseMessage>>("SimpleWaitAndRetryPolicy");
    }
    return policyRegistry.Get<IAsyncPolicy<HttpResponseMessage>>("NoOpPolicy");
});

Next, add the hosted service we want to start.

services.AddSingleton<IHostedService, BusinessService>();

A hosted service is a class that implements IHostedService, more on this below.

Finally at the end of the the Main method, start the hosted service.

await builder.RunConsoleAsync();

For clarity, here is the full listing of the main method –

static async Task Main(string[] args)
{
    var builder = new HostBuilder()
        .ConfigureServices((hostContext, services) =>
        {
            IPolicyRegistry<string> registry = services.AddPolicyRegistry();

            IAsyncPolicy<HttpResponseMessage> httWaitAndpRetryPolicy =
                Policy.HandleResult<HttpResponseMessage>(r => !r.IsSuccessStatusCode)
                    .WaitAndRetryAsync(3, retryAttempt => TimeSpan.FromSeconds(retryAttempt));

            registry.Add("SimpleWaitAndRetryPolicy", httWaitAndpRetryPolicy);

            IAsyncPolicy<HttpResponseMessage> noOpPolicy = Policy.NoOpAsync()
                .AsAsyncPolicy<HttpResponseMessage>();

            registry.Add("NoOpPolicy", noOpPolicy);

            services.AddHttpClient("JsonplaceholderClient", client =>
            {
                client.BaseAddress = new Uri("https://jsonplaceholder.typicode.com");
                client.DefaultRequestHeaders.Add("Accept", "application/json");
            }).AddPolicyHandlerFromRegistry((policyRegistry, httpRequestMessage) =>
            {
                if (httpRequestMessage.Method == HttpMethod.Get || httpRequestMessage.Method == HttpMethod.Delete)
                {
                    return policyRegistry.Get<IAsyncPolicy<HttpResponseMessage>>("SimpleWaitAndRetryPolicy");
                }
                return policyRegistry.Get<IAsyncPolicy<HttpResponseMessage>>("NoOpPolicy");
            });
                
            services.AddSingleton<IHostedService, BusinessService>();
        });

    await builder.RunConsoleAsync();
}

The hosted service
The hosted service is where you put your business logic, it is a simple c# class that implements IHostedService giving it two methods, StartAsync and StopAsync.

Its constructor takes an IHttpClientFactory as a parameter, which is satisfied by the dependency injection infrastructure.

public BusinessService(IHttpClientFactory httpClientFactory)
{
    _httpClientFactory = httpClientFactory;
}

From StartAsync, you can do anything you need.

In this example I call another method which in turn uses the HttpClientFactory to get an instance of a HttpClient to make requests to the the remote server. The requests are executed inside the appropriate Polly policy.

public async Task StartAsync(CancellationToken cancellationToken)
{
    await MakeRequestsToRemoteService();
}

public async Task MakeRequestsToRemoteService()
{
    HttpClient httpClient = _httpClientFactory.CreateClient("JsonplaceholderClient");
    var response = await httpClient.GetAsync("/photos/1");
    Photo photo = await response.Content.ReadAsAsync<Photo>();
    Console.WriteLine(photo);
}

Full source code available here.

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