Selectively Caching Values Inside HttpResponseMessage with Polly – caching series 3/3

Full source code here.

This is the last of three posts on caching with Polly. The first showed how to selectively cache HttpResponseMessages based on the status code of the response.

The second showed how to cache values inside the HttpResponseMessage rather than the whole of the response.

This final post will show how to selectively cache values inside the response based on the status of the response.

If you have read the previous posts the content of this will feel familiar.

Changes to Startup
Like the previous post, add a memory cache, a policy registry and a HttpClientFactory to the service collection.

public void ConfigureServices(IServiceCollection services)
{
    services.AddMemoryCache();
    services.AddSingleton<IAsyncCacheProvider, MemoryCacheProvider>();

    IPolicyRegistry<string> registry = services.AddPolicyRegistry();

    services.AddHttpClient("RemoteServer", client =>
    {
        client.BaseAddress = new Uri("http://localhost:5000/api/");
        client.DefaultRequestHeaders.Add("Accept", "application/json");
    });

    services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
}

What’s new is in the Configure method, fist step is to add the registry to the method parameters, DI will take care of this for us.

public void Configure(IApplicationBuilder app, IHostingEnvironment env,
    IAsyncCacheProvider cacheProvider, IPolicyRegistry<string> registry)
{

Next is the cache filter, this one examines the tuple of int and HttpStatusCode and, if the HttpStatusCode is an OK the tuple will be cached for the duration specified. If the respones is not OK the filter will set the cache time to 0 which is interpreted as do not cache by the cache policy.

Func<Context, (int value, HttpStatusCode StatusCode), Ttl> cacheOnly200OkFilter =
	(context, result) => new Ttl(
		timeSpan: result.StatusCode == HttpStatusCode.OK ? TimeSpan.FromSeconds(5) : TimeSpan.Zero,
		slidingExpiration: true
	);

And this is the cache policy that uses the filter above to store the tuple.

IAsyncPolicy<(int, HttpStatusCode)> cachePolicy =
	Policy.CacheAsync<(int, HttpStatusCode)>(
		cacheProvider.AsyncFor<(int, HttpStatusCode)>(), 
		new ResultTtl<(int, HttpStatusCode)>(cacheOnly200OkFilter),
		onCacheError: null);

Lastly, add the policy to the registry.

registry.Add("cacheOnly200OkFilter", cachePolicy);

Changes to the Controller
Over in the CatalogController there many changes.

At the top I take in the HttpClientFactory and policy registry.

private readonly IHttpClientFactory _httpClientFactory;
private readonly IPolicyRegistry<string> _policyRegistry;

public CatalogController(IHttpClientFactory httpClientFactory, IPolicyRegistry<string> policyRegistry)
{
	_httpClientFactory = httpClientFactory;
	_policyRegistry = policyRegistry;
}

Inside the Get method, I setup grab the HttpClient from the HttpClientFactory, set the endpoint of the remote service, take the cache policy from the registry and setup the context for the cache policy to cache based on the incoming request. All of this is similar to normal usage of the Polly cache.

[HttpGet("{id}")]
public async Task<IActionResult> Get(int id)
{
    string requestEndpoint = $"inventory/{id}";
    var httpClient = _httpClientFactory.CreateClient("RemoteServer");

    var cachePolicy = _policyRegistry.Get<CachePolicy<(int, HttpStatusCode)>>("cacheOnly200Ok");
    Context policyExecutionContext = new Context($"GetInventoryById-{id}");	

As in the previous post I don’t call the HttpClient from inside the cache policy, instead I need to call something that returns the int and the HttpStatusCode. I use a local function that makes the request to the remote service, checks the response, deserializes the value and returns a tuple.

(int ItemsInStock, HttpStatusCode StatusCode) itemsInStockAndStatusCode =
    await cachePolicy.ExecuteAsync(context => MakeRequest(), policyExecutionContext);

// local function
async Task<(int count, HttpStatusCode statusCode)> MakeRequest()
{
    HttpResponseMessage response = await httpClient.GetAsync(requestEndpoint);

    if (response.IsSuccessStatusCode)
    {
        int itemsInStock = await response.Content.ReadAsAsync<int>();
        return (itemsInStock, response.StatusCode);
    }

    return (0, response.StatusCode);
}

When the cachePolicy is executed it first checks if there is value already in the cache, if there is, that will be returned and the local function will not be called. If there is no value in the cache, the local function is called, if the response of the local function is in the 200 range, the value is stored in the cache.

At this point, the action method has not returned anything to its caller.

Below the local function I check the tuple returned from the call to the cache policy and return a response.

if (itemsInStockAndStatusCode.StatusCode != HttpStatusCode.OK)
{
     return StatusCode((int)itemsInStockAndStatusCode.StatusCode);
}
return Ok(itemsInStockAndStatusCode.ItemsInStock);

For clarity, here is the full Get method.

[HttpGet("{id}")]
public async Task<IActionResult> Get(int id)
{
    string requestEndpoint = $"inventory/{id}";
    var httpClient = _httpClientFactory.CreateClient("RemoteServer");

    var cachePolicy = _policyRegistry.Get<CachePolicy<(int, HttpStatusCode)>>("cacheOnly200Ok");
    Context policyExecutionContext = new Context($"GetInventoryById-{id}");

    (int ItemsInStock, HttpStatusCode StatusCode) itemsInStockAndStatusCode =
        await cachePolicy.ExecuteAsync(context => MakeRequest(), policyExecutionContext);

    async Task<(int count, HttpStatusCode statusCode)> MakeRequest()
    {
        HttpResponseMessage response = await httpClient.GetAsync(requestEndpoint);

        if (response.IsSuccessStatusCode)
        {
            int itemsInStock = JsonConvert.DeserializeObject<int>(await response.Content.ReadAsSt
            return (itemsInStock, response.StatusCode);
        }

        // it doesn't matter what int you use here, it won't be cached as the StatusCode is not a
        return (0, response.StatusCode);
    }

    if (itemsInStockAndStatusCode.StatusCode != HttpStatusCode.OK)
    {
         return StatusCode((int)itemsInStockAndStatusCode.StatusCode);
    }
    return Ok(itemsInStockAndStatusCode.ItemsInStock);
}

Here endeth the series on caching.

Full source code here.

Caching Values Inside HttpResponseMessage with Polly – caching series 2/3

Full source code here.

In this, the second of three posts on caching in Polly, I will show how to cache the values returned inside a HttpResponseMessage as opposed to caching the response with all its various elements.

The first post explained how to selectively cache a response, based on the Http StatusCode. The third post will show how to combine these two ideas and selectively cache the values inside the HttpResponseMessage.

Caching the whole response or the value
Caching the whole HttpResponseMessage is useful if your business logic depends on interrogating the response for values outside the main payload or you want a quick and easy caching solution.

If on the other hand, you only need the payload, there is no need to cache the whole response.

Both cases are plausible, and it is up to do decide which is more appropriate for your application.

How to cache the value inside HttpResponseMessage
The first step is to set the cache inside ConfigureServices.

public void ConfigureServices(IServiceCollection services)
{
    services.AddMemoryCache();
    services.AddSingleton<IAsyncCacheProvider, MemoryCacheProvider>();

Then in Configure, setup the policy to store the type you expect to find in the response. In this example it is in a int, but it could easily be a complex layered model representing anything.

public void Configure(IApplicationBuilder app, IHostingEnvironment env,
    IAsyncCacheProvider cacheProvider, IPolicyRegistry<string> registry)
{
    CachePolicy<int> cachePolicy =
        Policy.CacheAsync<int>(cacheProvider, TimeSpan.FromSeconds(10));

    registry.Add("CachingPolicy", cachePolicy);

Note, how I specified an int rather than a HttpResponseMessage.

In the previous post, I used the HttpClientFactory to select the policy from a Polly policy registry. I could do that because the type I was storing was HttpResponseMessage, that is not the case now and HttpClientFactory will not play nice when trying to cache anything other than a HttpResponseMessage. Instead I’m going to pass the policy registry directly to the controller and get the policy from this, I am still using the HttpClientFactory to pass HttpClient, but not for policy selection.

Here is how the controller starts.

public class CatalogController : Controller
{
    private readonly IHttpClientFactory _httpClientFactory;
    private readonly IPolicyRegistry<string> _policyRegistry;

    public CatalogController(IHttpClientFactory httpClientFactory, IPolicyRegistry<string> policyRegistry)
    {
        _httpClientFactory = httpClientFactory;
        _policyRegistry = policyRegistry;
    }

And here is the GET method.

[HttpGet("{id}")]
public async Task<IActionResult> Get(int id)
{
    string requestEndpoint = $"inventory/{id}";
    var httpClient = _httpClientFactory.CreateClient("RemoteServer");

    var cachePolicy = _policyRegistry.Get<CachePolicy<int>>("CachingPolicy");
    Context policyExecutionContext = new Context($"GetInventoryById-{id}");

    int itemsInStock =
        await cachePolicy.ExecuteAsync(context => GetItemsInStockCount(), policyExecutionContext);

    async Task<int> GetItemsInStockCount()
    {
        HttpResponseMessage response = await httpClient.GetAsync(requestEndpoint);

        if (response.IsSuccessStatusCode)
        {
            int itemsInStockCount = await response.Content.ReadAsAsync<int>();
            return itemsInStockCount;
        }

        return 0;
    }

    return Ok(itemsInStock);
}

The first few lines are straight forward, setup the endpoint, get the HttpClient, get the cache policy. Like in the previous post, I use the context when placing items into the cache and retrieving them.

Because I want to cache the value inside the HttpResponse (and not the response itself) I can’t call the HttpClient from inside the policy’s exec method. Instead I need to call something that returns the int from inside the HttpResponse.

The easiest way of doing this is to use a local function which makes the HttpClient request, deserializes the response and returns the int from the response.
In this post I am assuming that all values inside the response should be cached, I am NOT checking the status code of the response.

The policy then caches the int for the period specified in Startup.

That’s all there is to it.

If you are wondering how to selectively cache values inside the HttpResponse, tune in for the next post in this series.

Full source code here.

Selectively Caching a HttpResponseMessage with Polly – caching series 1/3

Full source code here.

When I give talks on Polly I show how to use it in a Web API application that calls another Web Api application. I give a simple example of how to use the cache policy where I store the whole of the HttpResponseMessage. At the end of the talk I often get a couple of questions –

1. is it possible to cache the response only when the status code is in the 200 range.
2. is it possible to cache the values inside the response instead of the whole response.

The answer to both of these is yes, and in this series of three articles I will explain how to do both of these things, and how how to combine them to cache a value inside a response only if the response is in the 200 range.

This article will demonstrate how to selectively cache HttpResponseMessages based on the Http StatusCode of the response. The second will show how to cache values inside the HttpResponseMessage, and the third will show how selectively cache values inside the HttpResponseMessage.

As described in an earlier post, using a cache policy is a little more involved than using other policies. See this post for more info.

Changes in Startup
In Startup inside ConfigureServices I setup the memory cache, the HttpClientFactory and a policy registry (I’m not going to go into a detailed explanation of this code as it is already explained in the earlier article and you have all the source code).

All the action for this post takes place in the Configure method.

Creating the cache filter
The cache timeout filter, it sets the length of time to cache a value. The filter here will cache a response for 10 seconds if the response indicates a success, otherwise it caches the response for 0 seconds, in this case the Polly code does not perform any caching.

public void Configure(IApplicationBuilder app, IHostingEnvironment env,
    IAsyncCacheProvider cacheProvider, IPolicyRegistry<string> registry)
{
    Func<Context, HttpResponseMessage, Ttl> cacheOnly200OkFilter =
        (context, result) => new Ttl(
            timeSpan: result.StatusCode == HttpStatusCode.OK ? TimeSpan.
            slidingExpiration: true
        );

The cache policy says that it caches HttpResponseMessages and uses the filter to decide if a response should be cached.

IAsyncPolicy<HttpResponseMessage> cacheOnlyOkResponsePolicy =
    Policy.CacheAsync<HttpResponseMessage>(
        cacheProvider.AsyncFor<HttpResponseMessage>(), //note the .AsyncFor<HttpResponseMessage>
        new ResultTtl<HttpResponseMessage>(cacheOnly200OkFilter),
        onCacheError: null
    );

Note the AsyncFor, this is very important. Thanks to Dylan Reisenberger for his help here, I spent too many hours trying to figure the overloads with no success.

Try it out in the provided solution, hit F5 and put some breakpoints in the controllers and on the filter in Startup.

That’s it, you now have selective caching of responses.

In the next post, I’ll show you how to cache the value inside the response instead of the whole response.

Full source code here.

How to use HttpClientFactory Inside Program.cs

Full source code here.

Over the past week I have written a few articles about HttpClientFactory and dependency injection in .NET Core 2.1. There is one scenario I didn’t deal with – calling a HttpClient from inside the Main method in Program.cs. If you have read my previous post you will probably know how do it, but in case you landed on this post from a search here is how to do it.

In Startup.cs, add the HttpClientFactory to the service collection.

public void ConfigureServices(IServiceCollection services)
{
    services.AddHttpClient("OpenBreweryDb", client =>
    {
        client.BaseAddress = new Uri("https://api.openbrewerydb.org/breweries/");
        client.DefaultRequestHeaders.Add("Accept", "application/json");
    });
    services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
}

In Progam.cs I split the building the webHost from running it so I can get at the service collection.

public static void Main(string[] args)
{
    IWebHost webHost = CreateWebHostBuilder(args).Build();
    CallSomeRemoteService(webHost.Services);
    webHost.Run();
}

Then I grab a HttpClientFactory from the service collection and a HttpClient from inside the HttpClientFactory.

private static void CallSomeRemoteService(IServiceProvider serviceProvider)
{
    var httpClientFactory = serviceProvider.GetService<IHttpClientFactory>();
    var httpClient = httpClientFactory.CreateClient("OpenBreweryDb");
    var response = httpClient.GetAsync("?by_state=Massachusetts&by_name=night").Result;
    if (response.IsSuccessStatusCode)
    {
        var breweries = response.Content.ReadAsAsync<List<Brewery>>().Result;
    }
}

That’s it, easy when you know how.

Full source code here.

How to Dependency Inject a Service from Startup back in Program

Full source code here.

While writing some recent blog posts on HttpClientFactory I had to work with some of the obscure features of ServiceCollection and dependency injection in .NET Core 2.1. One of the things I wanted to do was get at an instance of a token generator from the service collection inside the Main method in Program.cs.

Some time ago I wrote a post about using the ServiceCollection from Program.cs and while that worked for the example I gave, there are cases where it would not depending on the specifics of what scope you are using, and in the case of a singleton, how you instanced it. When it goes wrong you end up with multiple instances of a singleton, multiple DI containers and plenty of unpredictable results. It’s too painful to explain all the variations, you can read a bit more here where David Folwer says “hosting current[ly] creates 3 service providers and its extremely broken in the general case”.

How to Pass a Service from ServicesCollection into Program
In this approach none there are none of the problems outlined above, only one instance of the singleton is every instantiated.

In Startup.cs add all the services you want inside ConfigureServices(..).

public void ConfigureServices(IServiceCollection services)
{
    services.AddMemoryCache();
    services.AddSingleton<ITokenGenerator, TokenGenerator>();
}

In Program.cs, I have the usual CreateWebHostBuilder(..), no changes.

public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
    WebHost.CreateDefaultBuilder(args)	
        .UseStartup<Startup>();	

But in Main, instead of

CreateWebHostBuilder(args).Build().Run();

I split building the WebHost from running the WebHost.

public static void Main(string[] args)
{		
    IWebHost webHost = CreateWebHostBuilder(args).Build();
    var tokenGenerator = webHost.Services.GetService<ITokenGenerator>();
    string token =  tokenGenerator.GetToken();
    System.Console.WriteLine(token);
    webHost.Run();
}
  • Line 3 creates the webHost, but doesn’t run it
  • this in turn instantiates the startup class and runs ConfigureServices(..)
  • Line 4 reaches into the ServiceCollection for the ITokenGenerator service that was setup in ConfigureServices(..)
  • Lines 5 and 6, use the TokenGenerator service to get a token a print it to screen
  • Line 7 runs the WebHost and the application starts up

Full source code here.

Dynamically Updating the Request Header of a HttpClientFactory Generated HttpClient, Part 2

Full source code here.

This is a alternative to the approach described in a previous post.

On a slack channel there was some discussion around the use of a little known extension method on HttpClientBuilder, ConfigureHttpClient. Using this extension method provides another way to dynamically alter the header of a HttpClient provided by the factory.

In ConfigureServices(..) I setup the two services I need, the MemoryCache and the TokenGenerator.

Then, where I configure the HttpClientFactory I call the ConfigureHttpClient, pass it an Action that takes the ServiceProvider and the HttpClient I’m configuring.

Inside the Action, I take a TokenGenerator from the service collection and then add the token to the client header.

public void ConfigureServices(IServiceCollection services)
{
    services.AddMemoryCache();
    services.AddSingleton<ITokenGenerator, TokenGenerator>();

    services.AddHttpClient("RemoteServer", client =>
    {
        client.BaseAddress = new Uri("http://localhost:5000/api/");
        client.DefaultRequestHeaders.Add("Accept", "application/json");
    }).ConfigureHttpClient((serviceProvider, client) =>
    {
        ITokenGenerator tokenGenerator = serviceProvider.GetService<ITokenGenerator>();
        client.DefaultRequestHeaders.Add("Token", tokenGenerator.GetToken());
    });

    services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
}

That’s it, simpler than the approach in the previous post.

Full source code here.

How to Turn Off Console Logging for Kestrel in .NET Core

This post is mostly a note to myself.

I am often annoyed with the amount of logging to the console that occurs when I start a Kestrel hosted application and I can never remember how to turn it off.

public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
    WebHost.CreateDefaultBuilder(args)
    .ConfigureLogging(loggingBuilder => 
        loggingBuilder.AddFilter<ConsoleLoggerProvider>(level => 
            level == LogLevel.None))
        .UseStartup<Startup>();

Dynamically Updating the Request Header of a HttpClientFactory Generated HttpClient, Part 1

Full source code here.

There are some subtle issues in the way I use DI in this post, see here for an alternative if you don’t want to follow this approach

While using the HttpClientFactory I hit a scenario where I needed to update the value of a token passed in the header of requests, the token changed frequently, so I had to repeatedly update it throughout the lifetime of my application.

You have a couple of options for this, the first is to do it after you have taken a HttpClient from the factory at the point where you make your outbound request, this is straightforward, but now everywhere use a HttpClient you have to be able to get a new token. For some this might be fine, and you can use –

    httpClient.DefaultRequestHeaders.Add("Token", _tokenGenerator.GetToken());

Doing it with HttpClientFactory
The better approach is to put all this logic in the Startup.cs and update the header when the factory returns a new HttpClient, now everywhere you use the HttpClient gets the updated token without any work for you.

In my example case I have a token generator and memory cache. If there is a token in the cache, that one is used, if not the token generator generates and stores the new token in the cache for specified period.

In my Startup.cs all I need is this –

services.AddHttpClient("RemoteServer", client =>
{
    client.BaseAddress = new Uri("http://localhost:5000/api/");
    client.DefaultRequestHeaders.Add("Accept", "application/json");
    client.DefaultRequestHeaders.Add("Token", TokenGenerator.GetToken());
});

Read on to see how to wire everything up.

A little known feature of .NET Core is the ability to DI from Program.cs into Startup.cs, I have written about this before in Using Dependency Injection with Startup.cs in ASP.NET Core and am using it again here.

In Program.cs I add a memory cache and a token generator to the service collection.

Adding to the service collection this way can have some unexpected side effects, check this post for an alternative approach.

public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
    WebHost.CreateDefaultBuilder(args)
    .ConfigureServices(cs => cs.AddMemoryCache())
    .ConfigureServices(cs => cs.AddSingleton<ITokenGenerator, TokenGenerator>())
        .UseStartup<Startup>();

In Startup.cs I pass a ITokenGenerator to the constructor.

public Startup(IConfiguration configuration, ITokenGenerator tokenGenerator)
{
    Configuration = configuration;
    TokenGenerator = tokenGenerator;
    string token = tokenGenerator.GetToken(); // do something with the token
}

private ITokenGenerator TokenGenerator { get; }
// snip

Then a simple call the TokenGenerator.GetToken() updates the header of the client the factory returns to callers.

For completeness, here is the implementation of the TokenGenerator.cs

public class TokenGenerator : ITokenGenerator
{
    private readonly IMemoryCache _memoryCache;
    public TokenGenerator(IMemoryCache memoryCache)
    {
        _memoryCache = memoryCache;
    }

    public string GetToken()
    {
        string token;
        if (_memoryCache.TryGetValue("Token", out token))
        {
            return token;
        }
        else
        {
            // here you would have a more realistic way of generating a new token
            token = Guid.NewGuid().ToString();
            _memoryCache.Set("Token", token, TimeSpan.FromSeconds(10));

            return token;
        }
    }
}

Full source code here.

Caching in Polly and the HttpClientFactory

Full source code here.

Polly allows you to cache a response for reuse by a subsequent request, it supports both local an distributed caches, full information can be found here https://github.com/App-vNext/Polly/wiki/Cache.

The HttpClientFactory lets you define polices in startup.cs and apply them to HttpClient requests anywhere in your application. With most policies this makes it easier for you to apply a policy to a request, but with the cache policy it complicates matters.

With the release of HttpClientFactory there are now two ways to use the cache policy, one taking advantage of HttpClientFactory to apply the policy to requests, and the other bypassing it when executing requests.

Caching with the cache policy inside the HttpClientFactory
First let’s look at how to use the Cache Policy using PolicySelector method on the HttpClientFactory to apply the policy to the request, see this post for more info.

If you are working in Polly V6+ add Polly.Caching.Memory package to your project, this is important, don’t add Polly.Caching.MemoryCache that is for Polly V5.9 and earlier, and many an hour was lost figuring that out.

In Startup.cs add the following to the ConfigureServices method –

public void ConfigureServices(IServiceCollection services)
{
    services.AddMemoryCache();
    services.AddSingleton<IAsyncCacheProvider, MemoryCacheProvider>();

    IPolicyRegistry<string> registry = services.AddPolicyRegistry();

    services.AddHttpClient("RemoteServer", client =>
    {
        client.BaseAddress = new Uri("http://localhost:5000/api/");
        client.DefaultRequestHeaders.Add("Accept", "application/json");
    }).AddPolicyHandlerFromRegistry(PolicySelector);

This sets up the in memory cache and policy registry, adding both to ServicesCollection. The cache policy is not added here, instead it is added in the Configure(..) method (thanks Andrew Lock for reminding me about this).

Next, add the HttpClientFactory and a method to pick the appropriate policy from the registry.

Inside the Configure(..) I setup the cache policy. The registry and cache provider are passed via dependency injection.

public void Configure(IApplicationBuilder app, IHostingEnvironment env, 
    IAsyncCacheProvider cacheProvider, IPolicyRegistry<string> registry)
{
    CachePolicy<HttpResponseMessage> cachePolicy = 
        Policy.CacheAsync<HttpResponseMessage>(cacheProvider, TimeSpan.FromSeconds(30));
    registry.Add("CachingPolicy", cachePolicy);

In the controller I have a constructor that takes one argument, the HttpClientFactory.

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

In the action method I get the HttpClient from the HttpClientFactory and specify the remote endpoint I want to call, but now I can’t use usual httpClient.GetAsync(..) because I can’t pass it a cache context key (i.e. the name of where I look for previously cached responses and store new responses).
Instead I build a HttpRequestMessage, specifying the HttpMethod, Uri and using a Polly extension method I set the cache context.

HttpRequestMessage httpRequestMessage = new HttpRequestMessage(HttpMethod.Get, 
    new System.Uri(httpClient.BaseAddress + requestEndpoint));

httpRequestMessage.SetPolicyExecutionContext(new Context($"GetInventoryById-{id}"));

HttpResponseMessage response = await httpClient.SendAsync(httpRequestMessage);

This approach feels a bit awkward and not many people use the HttpRequestMessage to send requests to remote services.

Caching with the cache policy outside the HttpClientFactory
An alternative to the above is to add the cache to the registry, but not add the registry to the HttpClientFactory.

In this case the adding the HttpClientFactory to the services collection changes slightly –

services.AddHttpClient("RemoteServer", client =>  
{
    client.BaseAddress = new Uri("http://localhost:5000/api/");
    client.DefaultRequestHeaders.Add("Accept", "application/json");
}); // no policy selector

In the controller we now pass in the registry as HttpClientFactory as constructor arguments.

public StockMgtController(IHttpClientFactory httpClientFactory, IPolicyRegistry<string> policyRegistry) 
{
    _httpClientFactory = httpClientFactory;
    _policyRegistry = policyRegistry;
}

There are changes inside the action method also –

[HttpGet("{id}")]
public async Task<IActionResult> Get(int id)
{
    string requestEndpoint = $"inventory/{id}";
    var httpClient = _httpClientFactory.CreateClient("RemoteServer");

    var cachePolicy = _policyRegistry.Get<CachePolicy<HttpResponseMessage>>("CachingPolicy");
    Context policyExecutionContext = new Context($"GetInventoryById-{id}");

    HttpResponseMessage response = await cachePolicy.ExecuteAsync(
        context => httpClient.GetAsync(requestEndpoint), policyExecutionContext);
	//snip..	

The httpClient is retrieved from the HttpClientFactory as before, but now the cachePolicy is taken from the policy registry, the Context is defined and then we use the cachePolicy.ExecuteAsync(..) method to make call the httpClient.GetAsync(..) method.

The other way
You might have noticed that there is another way of using the cache policy. You could use the HttpClientFactory and a PolicySelector method to apply most policies to your HttpClient requests, but not for the cache policy. I wouldn’t do this, it will confuse everyone.

Conclusion
The outcome of both is the same, I think the first approach is better, even though the call to the httpClient is a little more convoluted, it will be more consistent with how you call other policies in your application, there is also a chance the Polly team will add an extension method to make the use of HttpRequestMessage transparent to us developers.

Full source code here.

Alter response header in Web API to return machine name

Full source code available here.

I recently hit a problem where I was getting incorrect responses from a server behind a load balancer. Looking at the logs didn’t help because there was no error.

It was likely due to a misconfigured environmental variable but I couldn’t even identify which server was having the problem because of the load balancer.

Luckily the application was a .NET Core Web Api and I could use a little bit of middleware to add the server name to the response header.

It’s a trivial change in the Configure(..) method of Startup.cs

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    app.Use((context, next) =>
    {
        context.Response.Headers.Add("MachineName", $"{Environment.MachineName}");
        return next();
    });
    app.UseMvc();
}

That’s all I had to do.

Here’s how the response looks now.

.

Full source code available here.