Round #721

Add a fallback resilience pipeline using ResiliencePipeline.AddFallback that swaps to a cached IDistributedCache value on inner failure.

Here's an example of how you can add a fallback resilience pipeline using `ResiliencePipeline.AddFallback` in C#: ```csharp using Microsoft.Extensions.Caching.Distributed; using Polly; public class MyService { private readonly IDistributedCache _distributedCache; private readonly Policy<HttpResponseMessage> _resiliencePolicy; public MyService(IDistributedCache distributedCache) { _distributedCache = distributedCache; _resiliencePolicy = Policy.Handle<HttpRequestException>() .Or<TimeoutException>() .FallbackAsync( async (context, token) => { // Get the cached value var cachedValue = await _distributedCache.GetStringAsync("myCacheKey"); // If the cached value exists, return it if (cachedValue != null) { return new HttpResponseMessage(HttpStatusCode.OK) { Content = new StringContent(cachedValue) }; } // If the cached value does not exist, throw an exception throw new Exception("No cached value available"); }, onFallbackAsync: async (exception, ts, context) => { // Log the exception Console.WriteLine($"Exception occurred: {exception.Message}"); } ); } public async Task<HttpResponseMessage> GetDataAsync() { // Use the resilience policy to execute the HTTP request return await _resiliencePolicy.ExecuteAsync(async () => { // Simulate an HTTP request var httpClient = new HttpClient(); var response = await httpClient.GetAsync("https://example.com/api/data"); // If the response is successful, return it if (response.IsSuccessStatusCode) { return response; } // If the response is not successful, throw an exception throw new HttpRequestException("Failed to retrieve data"); }); } } ``` In this example, the `MyService` class uses the `IDistributedCache` interface to cache values. The `ResiliencePipeline.AddFallback` method is used to add a fallback policy that swaps to a cached value on inner failure. When the `GetDataAsync` method is called, it uses the resilience policy to execute an HTTP request. If the request fails, the fallback policy is triggered, which attempts to retrieve the cached value. If the cached value exists, it is returned; otherwise, an exception is thrown. Note that you need to configure the `IDistributedCache` instance and the `ResiliencePipeline` policy in the `Startup.cs` file or in a separate configuration file. Here's an example of how you can configure the `IDistributedCache` instance and the `ResiliencePipeline` policy in the `Startup.cs` file: ```csharp public void ConfigureServices(IServiceCollection services) { services.AddDistributedMemoryCache(); services.AddHttpClient("myHttpClient") .AddPolicyHandler(Policy.Handle<HttpRequestException>() .Or<TimeoutException>() .FallbackAsync( async (context, token) => { // Get the cached value var cachedValue = await _distributedCache.GetStringAsync("myCacheKey"); // If the cached value exists, return it if (cachedValue != null) { return new HttpResponseMessage(HttpStatusCode.OK) { Content = new StringContent(cachedValue) }; } // If the cached value does not exist, throw an exception throw new Exception("No cached value available"); }, onFallbackAsync: async (exception, ts, context) => { // Log the exception Console.WriteLine($"Exception occurred: {exception.Message}"); } )); } ``` You can then inject the `IHttpClientFactory` instance into your service and use it to create an instance of the `HttpClient` class: ```csharp public class MyService { private readonly IHttpClientFactory _httpClientFactory; private readonly IDistributedCache _distributedCache; public MyService(IHttpClientFactory httpClientFactory, IDistributedCache distributedCache) { _httpClientFactory = httpClientFactory; _distributedCache = distributedCache; } public async Task<HttpResponseMessage> GetDataAsync() { var httpClient = _httpClientFactory.CreateClient("myHttpClient"); var response = await httpClient.GetAsync("https://example.com/api/data"); // If the response is successful, return it if (response.IsSuccessStatusCode) { return response; } // If the response is not successful, throw an exception throw new HttpRequestException("Failed to retrieve data"); } } ```

```csharp using Microsoft.Extensions.Caching.Distributed; using Polly; public class ResiliencePipelineBuilder { private readonly IDistributedCache _distributedCache; public ResiliencePipelineBuilder(IDistributedCache distributedCache) { _distributedCache = distributedCache; } public ResiliencePipeline BuildPipeline() { return Policy.Handle<Exception>() .FallbackAsync( async (context, token) => { var cacheKey = context["CacheKey"]; var cachedValue = await _distributedCache.GetStringAsync(cacheKey, token); return cachedValue; }, onFallbackAsync: async (exception, span, context, token) => { // Log the exception or perform other actions on fallback Console.WriteLine($"Fallback occurred: {exception.Message}"); } ); } } public class Program { public static async Task Main(string[] args) { var distributedCache = new DistributedCacheMock(); // Replace with actual implementation var pipelineBuilder = new ResiliencePipelineBuilder(distributedCache); var pipeline = pipelineBuilder.BuildPipeline(); // Usage example var cacheKey = "example-key"; var result = await pipeline.ExecuteAsync(async (token) => { // Simulate an inner failure throw new Exception("Inner failure"); }, new Dictionary<string, object> { { "CacheKey", cacheKey } }); Console.WriteLine(result); } } public class DistributedCacheMock : IDistributedCache { public byte[] Get(string key) { // Mock implementation, replace with actual cache retrieval return System.Text.Encoding.UTF8.GetBytes("Cached value"); } public Task<byte[]> GetAsync(string key, CancellationToken token = default) { // Mock implementation, replace with actual cache retrieval return Task.FromResult(System.Text.Encoding.UTF8.GetBytes("Cached value")); } public void Set(string key, byte[] value, DistributedCacheEntryOptions options) { // Mock implementation, replace with actual cache storage } public Task SetAsync(string key, byte[] value, DistributedCacheEntryOptions options, CancellationToken token = default) { // Mock implementation, replace with actual cache storage return Task.CompletedTask; } public void Refresh(string key) { // Mock implementation, replace with actual cache refresh } public Task RefreshAsync(string key, CancellationToken token = default) { // Mock implementation, replace with actual cache refresh return Task.CompletedTask; } public void Remove(string key) { // Mock implementation, replace with actual cache removal } public Task RemoveAsync(string key, CancellationToken token = default) { // Mock implementation, replace with actual cache removal return Task.CompletedTask; } public string GetString(string key) { // Mock implementation, replace with actual cache retrieval return "Cached value"; } public Task<string> GetStringAsync(string key, CancellationToken token = default) { // Mock implementation, replace with actual cache retrieval return Task.FromResult("Cached value"); } public void SetString(string key, string value, DistributedCacheEntryOptions options) { // Mock implementation, replace with actual cache storage } public Task SetStringAsync(string key, string value, DistributedCacheEntryOptions options, CancellationToken token = default) { // Mock implementation, replace with actual cache storage return Task.CompletedTask; } } ``` This implementation utilizes the Polly library to create a resilience pipeline with a fallback strategy. When an inner failure occurs, the pipeline will attempt to retrieve a cached value from the `IDistributedCache` instance. If the cache retrieval is successful, the cached value will be returned; otherwise, the pipeline will propagate the exception. The `ResiliencePipelineBuilder` class is responsible for constructing the pipeline with the specified fallback strategy. The `BuildPipeline` method returns a `ResiliencePipeline` instance that can be used to execute actions with the fallback resilience strategy. In the example usage, a mock `IDistributedCache` implementation is used for demonstration purposes. You should replace this with an actual implementation, such as the `Microsoft.Extensions.Caching.StackExchangeRedis` package for Redis caching. The `Main` method demonstrates how to use the `ResiliencePipeline` instance to execute an action with the fallback resilience strategy. The `ExecuteAsync` method takes an action to execute and a dictionary of context data, which includes the cache key used for fallback retrieval. If an inner failure occurs during the action execution, the pipeline will attempt to retrieve the cached value and return it as the result.