Basic redis object cache implementation

Iceshrimp.Backend/Core/Extensions/DistributedCacheExtensions.cs

@@ -0,0 +1,38 @@
+using System.Text.Json;
+using Microsoft.Extensions.Caching.Distributed;
+
+namespace Iceshrimp.Backend.Core.Extensions;
+
+public static class DistributedCacheExtensions {
+	//TODO: named caches, CacheService?
+	//TODO: thread-safe locks to prevent fetching data more than once
+
+	public static async Task<T?> Get<T>(this IDistributedCache cache, string key) {
+		var buffer = await cache.GetAsync(key);
+		if (buffer == null) return default;
+
+		var stream = new MemoryStream(buffer);
+		var data   = await JsonSerializer.DeserializeAsync<T>(stream);
+
+		return data != null ? (T)data : default;
+	}
+
+	public static async Task<T> Fetch<T>(this IDistributedCache cache, string key, TimeSpan ttl,
+	                                     Func<Task<T>> fetcher) {
+		var hit = await cache.Get<T>(key);
+		if (hit != null) return hit;
+
+		var fetched = await fetcher();
+		await cache.Set(key, fetched, ttl);
+		return fetched;
+	}
+
+	public static async Task Set<T>(this IDistributedCache cache, string key, T data, TimeSpan ttl) {
+		using var ms = new MemoryStream();
+		await JsonSerializer.SerializeAsync(ms, data);
+		var buffer = new Memory<byte>();
+		_ = await ms.ReadAsync(buffer);
+		var options = new DistributedCacheEntryOptions { AbsoluteExpirationRelativeToNow = ttl };
+		await cache.SetAsync(key, buffer.ToArray(), options);
+	}
+}

Iceshrimp.Backend/Core/Services/UserService.cs

@@ -9,6 +9,7 @@ using Iceshrimp.Backend.Core.Federation.ActivityPub;
 using Iceshrimp.Backend.Core.Helpers;
 using Iceshrimp.Backend.Core.Middleware;
 using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Caching.Distributed;
 using Microsoft.Extensions.Options;
 
 namespace Iceshrimp.Backend.Core.Services;
@@ -19,7 +20,9 @@ public class UserService(
 	IOptions<Config.InstanceSection> instance,
 	ILogger<UserService> logger,
 	DatabaseContext db,
-	ActivityFetcherService fetchSvc) {
+	ActivityFetcherService fetchSvc,
+	IDistributedCache cache
+) {
 	private (string Username, string? Host) AcctToTuple(string acct) {
 		if (!acct.StartsWith("acct:")) throw new GracefulException(HttpStatusCode.BadRequest, "Invalid query");
 
@@ -161,10 +164,13 @@ public class UserService(
 		return await GetOrCreateSystemUser("relay.actor");
 	}
 
-	//TODO: cache in redis
 	private async Task<User> GetOrCreateSystemUser(string username) {
-		return await db.Users.FirstOrDefaultAsync(p => p.UsernameLower == username && p.Host == null) ??
+		return await cache.Fetch($"systemUser:{username}", TimeSpan.FromHours(24), async () => {
+			logger.LogTrace("GetOrCreateSystemUser delegate method called for user {username}", username);
+			return await db.Users.FirstOrDefaultAsync(p => p.UsernameLower == username &&
+			                                               p.Host == null) ??
 			       await CreateSystemUser(username);
+		});
 	}
 
 	private async Task<User> CreateSystemUser(string username) {