feat(metrics): expose Redis GitHub response cache counters

src/selfhost/redis-response-cache.ts

@@ -6,25 +6,40 @@
 // NOT rate-limit headers (a cache hit consumed no quota) or content-encoding (the body is decoded).
 import type { Redis } from "ioredis";
 import type { CachedGitHubResponse, GitHubResponseCache } from "../github/client";
+import { incr } from "./metrics";
 
+const REDIS_GITHUB_RESPONSE_CACHE_METRIC = "gittensory_redis_gh_response_cache_total";
 const keyFor = (key: string): string => `gh:resp:${key}`;
 
 function isReplayableCachedStatus(status: unknown): status is number {
   return status === 200 || status === 403 || status === 404;
 }
 
+function recordRedisResponseCacheMetric(result: "hit" | "miss" | "set" | "error"): void {
+  incr(REDIS_GITHUB_RESPONSE_CACHE_METRIC, { result });
+}
+
 export function createRedisResponseCache(
   redis: Redis,
   ttlSeconds: number,
 ): GitHubResponseCache {
   return {
     async get(key: string) {
-      const raw = await redis.get(keyFor(key));
-      if (!raw) return null;
+      let raw: string | null;
+      try {
+        raw = await redis.get(keyFor(key));
+      } catch (error) {
+        recordRedisResponseCacheMetric("error");
+        throw error;
+      }
+      if (!raw) {
+        recordRedisResponseCacheMetric("miss");
+        return null;
+      }
       try {
         const value = JSON.parse(raw) as Partial<CachedGitHubResponse>;
         const status = value.status;
-        return isReplayableCachedStatus(status) &&
+        const cached = isReplayableCachedStatus(status) &&
           typeof value.body === "string" &&
           typeof value.contentType === "string"
           ? {
@@ -36,17 +51,26 @@ export function createRedisResponseCache(
               ...(typeof value.lastModified === "string" ? { lastModified: value.lastModified } : {}),
             }
           : null;
+        recordRedisResponseCacheMetric(cached ? "hit" : "miss");
+        return cached;
       } catch {
+        recordRedisResponseCacheMetric("miss");
         return null;
       }
     },
     async set(key: string, value: CachedGitHubResponse, ttlOverrideSeconds?: number) {
-      await redis.set(
-        keyFor(key),
-        JSON.stringify(value),
-        "EX",
-        Math.max(1, ttlOverrideSeconds ?? ttlSeconds),
-      );
+      try {
+        await redis.set(
+          keyFor(key),
+          JSON.stringify(value),
+          "EX",
+          Math.max(1, ttlOverrideSeconds ?? ttlSeconds),
+        );
+      } catch (error) {
+        recordRedisResponseCacheMetric("error");
+        throw error;
+      }
+      recordRedisResponseCacheMetric("set");
     },
   };
 }

test/unit/selfhost-redis-response-cache.test.ts

@@ -1,5 +1,6 @@
 import type { Redis } from "ioredis";
-import { describe, expect, it } from "vitest";
+import { afterEach, describe, expect, it } from "vitest";
+import { renderMetrics, resetMetrics } from "../../src/selfhost/metrics";
 import { createRedisResponseCache } from "../../src/selfhost/redis-response-cache";
 
 function fakeRedis(): {
@@ -24,11 +25,16 @@ function fakeRedis(): {
 
 const URL_A = "https://github.com/@api/repos/o/r/pulls/1";
 
+afterEach(() => resetMetrics());
+
 describe("createRedisResponseCache (#perf GitHub GET cache)", () => {
   it("get returns null for a missing url", async () => {
     expect(
       await createRedisResponseCache(fakeRedis().redis, 20).get(URL_A),
     ).toBeNull();
+    expect(await renderMetrics()).toContain(
+      'gittensory_redis_gh_response_cache_total{result="miss"} 1',
+    );
   });
 
   it("set then get round-trips status/body/content-type with the configured TTL", async () => {
@@ -51,6 +57,13 @@ describe("createRedisResponseCache (#perf GitHub GET cache)", () => {
       etag: '"abc123"',
       lastModified: "Mon, 29 Jun 2026 20:00:00 GMT",
     });
+    const metrics = await renderMetrics();
+    expect(metrics).toContain(
+      'gittensory_redis_gh_response_cache_total{result="set"} 1',
+    );
+    expect(metrics).toContain(
+      'gittensory_redis_gh_response_cache_total{result="hit"} 1',
+    );
   });
 
   it("replays cached branch-protection permission denials and missing resources", async () => {
@@ -105,12 +118,18 @@ describe("createRedisResponseCache (#perf GitHub GET cache)", () => {
     const f = fakeRedis();
     f.store.set("gh:resp:" + URL_A, "{nope");
     expect(await createRedisResponseCache(f.redis, 20).get(URL_A)).toBeNull();
+    expect(await renderMetrics()).toContain(
+      'gittensory_redis_gh_response_cache_total{result="miss"} 1',
+    );
   });
 
   it("get returns null when the stored shape is wrong", async () => {
     const f = fakeRedis();
     f.store.set("gh:resp:" + URL_A, JSON.stringify({ status: "200", body: 1 }));
     expect(await createRedisResponseCache(f.redis, 20).get(URL_A)).toBeNull();
+    expect(await renderMetrics()).toContain(
+      'gittensory_redis_gh_response_cache_total{result="miss"} 1',
+    );
   });
 
   it("get returns null for non-replayable cached responses", async () => {
@@ -178,5 +197,42 @@ describe("createRedisResponseCache (#perf GitHub GET cache)", () => {
       body: "{}",
       contentType: "application/json",
     });
+    expect(await renderMetrics()).toContain(
+      'gittensory_redis_gh_response_cache_total{result="hit"} 1',
+    );
+  });
+
+  it("records and rethrows Redis read errors", async () => {
+    const redis = {
+      async get() {
+        throw new Error("redis read failed");
+      },
+    } as unknown as Redis;
+
+    await expect(createRedisResponseCache(redis, 20).get(URL_A)).rejects.toThrow(
+      "redis read failed",
+    );
+    expect(await renderMetrics()).toContain(
+      'gittensory_redis_gh_response_cache_total{result="error"} 1',
+    );
+  });
+
+  it("records and rethrows Redis write errors", async () => {
+    const redis = {
+      async set() {
+        throw new Error("redis write failed");
+      },
+    } as unknown as Redis;
+
+    await expect(
+      createRedisResponseCache(redis, 20).set(URL_A, {
+        status: 200,
+        body: "{}",
+        contentType: "application/json",
+      }),
+    ).rejects.toThrow("redis write failed");
+    expect(await renderMetrics()).toContain(
+      'gittensory_redis_gh_response_cache_total{result="error"} 1',
+    );
   });
 });