L2 Cache retry intial commit

src/Microsoft.Identity.Web/TokenCacheProviders/Distributed/MsalDistributedTokenCacheAdapter.cs

@@ -82,6 +82,12 @@ protected override async Task RemoveKeyAsync(string cacheKey)
             catch (Exception ex)
             {
                 _logger.LogError($"[IdWebCache] Connection issue encountered with Distributed cache. Currently using In Memory cache only. Error message: {ex.Message} ");
+
+                if (_distributedCacheOptions.OnL2CacheFailure != null && _distributedCacheOptions.OnL2CacheFailure(ex))
+                {
+                    _logger.LogDebug($"[IdWebCache] DistributedCache: Retry to remove cacheKey {cacheKey}. ");
+                    await _distributedCache.RemoveAsync(cacheKey).ConfigureAwait(false);
+                }
             }
         }
 

src/Microsoft.Identity.Web/TokenCacheProviders/Distributed/MsalDistributedTokenCacheAdapterOptions.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT License.
 
+using System;
 using Microsoft.Extensions.Caching.Distributed;
 
 namespace Microsoft.Identity.Web.TokenCacheProviders.Distributed
@@ -18,6 +19,17 @@ public class MsalDistributedTokenCacheAdapterOptions : DistributedCacheEntryOpti
         /// </summary>
         public long L1CacheSizeLimit { get; set; } = 500;
 
+        /// <summary>
+        /// Callback offered to the app to be notified when the L2 cache fails.
+        /// This way the app is given the possibility to act on the L2 cache,
+        /// for instance, in the case of Redis, to reconnect. This is left to the application as it's
+        /// the only one that knows about the real implementation of the L2 cache.
+        /// The handler should return <c>true</c> if the cache should try again the operation, and
+        /// <c>false</c> otherwise. When <c>true</c> is passed and the retry fails, an exception
+        /// will be thrown.
+        /// </summary>
+        public Func<Exception, bool>? OnL2CacheFailure { get; set; }
+
         /// <summary>
         /// Value more than 0, less than 1, to set the In Memory (L1) cache
         /// expiration time values relative to the Distributed (L2) cache.

tests/WebAppCallsWebApiCallsGraph/Client/Startup.cs

@@ -14,6 +14,7 @@
 using Microsoft.Extensions.Hosting;
 using Microsoft.AspNetCore.Authentication.OpenIdConnect;
 using Microsoft.Identity.Web.UI;
+using Microsoft.Identity.Web.TokenCacheProviders.Distributed;
 
 namespace WebApp_OpenIDConnect_DotNet
 {
@@ -48,6 +49,18 @@ public void ConfigureServices(IServiceCollection services)
                 options.Configuration = Configuration.GetConnectionString("Redis");
                 options.InstanceName = "RedisDemos_"; //should be unique to the app
             });
+            services.Configure<MsalDistributedTokenCacheAdapterOptions>(options => 
+            {
+                options.OnL2CacheFailure = (ex) =>
+                {
+                    if (ex is StackExchange.Redis.RedisConnectionException)
+                    {
+                        // action: try to reconnect or something
+                        return true; //try to do the cache operation again
+                    }
+                    return false;
+                };
+            });
 #endif
 
             services.AddAuthentication(OpenIdConnectDefaults.AuthenticationScheme)