▌Introduction
Ocelot is the API Gateway framework for ASP.NET Core, it supports memory caching and the cache key contains a hash value by default. We’ll implement the Redis caching and use our own cache key generating strategy.
The full sample code is at KarateJB/AspNetCore.Profiler.Sample.
▌Install Package
<PackageReference Include="StackExchange.Redis" Version="2.8.24" />
<PackageReference Include="Ocelot" Version="23.2.2" />
<PackageReference Include="Ocelot.Provider.Polly" Version="23.2.2" />
▌Implement
First we have to enable Ocelot in Program.cs.
using Ocelot.Cache;
using Ocelot.Configuration.File;
using Ocelot.DependencyInjection;
using Ocelot.Middleware;
using Ocelot.Provider.Polly;
var builder = WebApplication.CreateBuilder(args);
builder.Logging.ClearProviders();
// Add Ocelot configuration file
builder.Configuration.AddJsonFile("ocelot.json", optional: false, reloadOnChange: true);
builder.Services.AddOcelot(builder.Configuration).AddPolly();
builder.Services.AddControllers();
var app = builder.Build();
app.UseHttpsRedirection();
app.UseRouting();
app.UseAuthentication();
app.UseAuthorization();
await app.UseOcelot();
app.MapControllers();
app.Run();
Then complete the ocelot.json configuration, see Ocelot:Configuration.
▋Ocelot’s caching implementation and interfaces
We can find the default caching implementation DefaultMemoryCache<T> in Ocelot source code; it implements the interface IOcelotCache<T>.
using Microsoft.Extensions.Caching.Memory;
namespace Ocelot.Cache;
public class DefaultMemoryCache<T> : IOcelotCache<T>
{
}
For the cache key generator, the implementation class is DefaultCacheKeyGenerator., which implements the interface ICacheKeyGenerator.
namespace Ocelot.Cache;
public class DefaultCacheKeyGenerator : ICacheKeyGenerator
{
}
Ocelot registers them by the following code in Ocelot source code “src\Ocelot\DependencyInjection\Features.cs”.
public static IServiceCollection AddOcelotCache(this IServiceCollection services) => services
.AddSingleton<IOcelotCache<Regex>, DefaultMemoryCache<Regex>>()
.AddSingleton<IOcelotCache<FileConfiguration>, DefaultMemoryCache<FileConfiguration>>()
.AddSingleton<IOcelotCache<CachedResponse>, DefaultMemoryCache<CachedResponse>>()
.AddSingleton<ICacheKeyGenerator, DefaultCacheKeyGenerator>()
.AddSingleton<ICacheOptionsCreator, CacheOptionsCreator>();
In ASP.NET Core Dependency Injection, we can add multiple implementation classes for a single interface into the service containers. If we don’t specify which one to use, the last injected implementation class will be the default one. However, Ocelot checks if the service container already has the custom implementation of the two interfaces, it won’t add its own implementations to the DI container to make sure that the our custom implementations are used.
Thus, all we have to do is create our custom Redis caching and Redis key generator classes(services) and implement the interfaces, then register our services into the DI container.
▋Implement IOcelotCache<CachedResponse>
Let’s create a new class RedisCacheStore that implements IOcelotCache<CachedResponse>.
namespace AspNetCore.Profiler.Gateway.Services
{
public class RedisCacheStore : IOcelotCache<CachedResponse>
{
private readonly ILogger _logger;
private readonly RedisSetting _redisSetting;
private readonly IDatabase? _redisDb;
private Func<string, string, string> RedisKey = (string region, string key) => $"{region}:{key}";
public RedisCacheStore(ILogger<RedisCacheStore> logger, IOptions<AppSettings> options)
{
_redisSetting = options?.Value?.Redis ?? throw new ArgumentNullException(nameof(RedisSetting));
// Initialize Redis connection
var redis = ConnectionMultiplexer.Connect(_redisSetting.ConnectionString);
_redisDb = redis.GetDatabase();
}
public void Add(string key, CachedResponse value, TimeSpan ttl, string region)
{
string redisKey = RedisKey(region, key);
if (value is not null && _redisDb != null)
{
var cacheValue = JsonConvert.SerializeObject(value);
_redisDb.StringSet(redisKey, cacheValue, expiry: ttl);
}
}
public void AddAndDelete(string key, CachedResponse value, TimeSpan ttl, string region)
{
Add(key, value, ttl, region);
}
public CachedResponse Get(string key, string region)
{
string redisKey = RedisKey(region, key);
RedisValue cachedData = _redisDb != null ? _redisDb.StringGet(redisKey) : RedisValue.Null;
if (!cachedData.IsNullOrEmpty)
{
return JsonConvert.DeserializeObject<CachedResponse>(cachedData);
}
return null;
}
public bool TryGetValue(string key, string region, out CachedResponse value)
{
value = Get(key, region);
return value != null;
}
public void ClearRegion(string region)
{
// Skip
}
}
}
The code has nothing special, but we have to know what “region” and “key” are, because they are related to the cache key management in Ocelot’s design.
To enable caching for an upstreaming routine, we add the following configuration in Ocelot.json (see Ocelot document).
"FileCacheOptions": {
"TtlSeconds": 3600,
"Region": "payment",
"EnableContentHashing": false
}
The Region’s value is for the parameter “region”. A region is a concept of grouping of cache keys. For example, we can put cache keys “user:aaa” and “vip-bbb” in the same group: “users”, and “users” is the region. When we want to clear all the caches in the same group, the “region” is specified to clear all the cache keys that were put into this region. I suggest reading Ocelot’s memory cache implementation DefaultMemoryCache<T> and you will find out how Ocelot manages the cache keys with “region”.
In Redis, we often set the Redis key in this format, e.g. “users:aaa” or “users:bbb” for grouping keys; that’s the same concept of region. The “region” here is “users”, and we can find all the Redis keys of the same region with pattern “users:*”.
The “key” is the MD5 hash of the request’s “HTTP method + URL” that is generated by the implementation (DefaultCacheKeyGenerator by default) of ICacheKeyGenerator. If we don’t implement our own cache key generator, our Redis key will be like this:
payment:5D828182FD57BC692F0055A01C27374F
The hash “5D828182FD57BC692F0055A01C27374F” is the “key”. The caching (read/write) won’t have any problem with the Redis key; however, if we want to delete the cache before it expires (manually or by another application), the hash will make it hard to delete the cache unless we know how to get the same hash value. Let’s implement our cache key generator at the next step.
▋Implement ICacheKeyGenerator
Before implementing our custom cache key generator, we have to think about the key pattern. Your Redis key may contain the information from the request’s URL parameter or from a HTTP header, etc. The way to generate a Redis key might be different by each upstreaming request or HTTP method.
The following sample code generates the “key” with the HTTP header “X-Redis-Key”.
public class RedisKeyGenerator : ICacheKeyGenerator
{
private const string RedisKeyHeaderName = "X-Redis-Key";
public async ValueTask<string> GenerateRequestCacheKey(DownstreamRequest downstreamRequest, DownstreamRoute downstreamRoute)
{
StringBuilder customRedisKey = await this.TryGenRedisKeyByHttpHeader(downstreamRequest);
if (customRedisKey.Length > 0)
{
string cacheKey = customRedisKey.ToString();
return cacheKey;
}
else
{
#region Official implementation
// You can copy the original Ocelot implementation here if the client doesn't give the HTTP header.
#endregion
}
}
private static Task<string> ReadContentAsync(DownstreamRequest downstream) => downstream.HasContent && downstream.Request?.Content != null
? downstream.Request.Content.ReadAsStringAsync() ?? Task.FromResult(string.Empty)
: Task.FromResult(string.Empty);
private Task<StringBuilder> TryGenRedisKeyByHttpHeader(DownstreamRequest downstreamRequest)
{
StringBuilder sbRedisKey = new();
var httpHeaderValues = downstreamRequest.Headers.FirstOrDefault(x => x.Key.Equals(RedisKeyHeaderName));
if (httpHeaderValues.Value != null && httpHeaderValues.Value.Any())
{
string headerValue = httpHeaderValues.Value.FirstOrDefault();
if (!string.IsNullOrEmpty(headerValue))
{
sbRedisKey.Append(headerValue);
}
}
return Task.FromResult(sbRedisKey);
}
}
Don’t forget that we also put the “region” into our Redis key pattern “region:key” in RedisCacheStore. While we set the “Region” with value: “payment” and the request’s HTTP header “X-Redis-Key” is a payment transaction ID: “dedcf2fb-c8e0-4965-a590-d674e2094304”, then the Redis key will be
payment:dedcf2fb-c8e0-4965-a590-d674e2094304
The cache key seems more readable, because the client application knows the payment transaction ID and it can delete the cache anytime by itself.
▋Register the new services
Now we can register our new services RedisKeyGenerator and RedisCacheStore into DI containers to let Ocelot use them instead of its default memory caching and cache key generator.
builder.Services.AddSingleton<ICacheKeyGenerator, RedisKeyGenerator>();
builder.Services.AddSingleton<IOcelotCache<CachedResponse>, RedisCacheStore>();
▋The Cache Value
The cache value by Ocelot contains:
localhost:6379> mget payment:dedcf2fb-c8e0-4965-a590-d674e2094304
1) {"StatusCode":200,"Headers":{"Date":["Sat, 22 Mar 2025 16:45:22 GMT"],"Server":["Kestrel"],"Transfer-Encoding":["chunked"]},"ContentHeaders":{"Content-Type":["application/json; charset=utf-8"]},"Body":"eyJpZCI6IjkyMDQxNThiLTJhZjEtNDUxNS1iMTk4LTJlNjk5MmRmMjA4NiIsIml0ZW0iOiJEREQiLCJhbW91bnQiOjEwMCwiY3JlYXRlT24iOiIyMDI1LTAzLTE5VDAxOjQxOjQzLjQ5MjM1MzIrMDA6MDAifQ==","ReasonPhrase":"OK"}
By storing the cache in Redis and using the custom Redis key, we can easily enhance and manage the caching, or trouble-shooting.
▌Reference
▋Ocelot: Caching
