feat(metrics): expose Redis token cache counters

src/selfhost/redis-token-cache.ts

@@ -6,25 +6,42 @@
 // Also makes the cache shared across instances if the stack is ever scaled horizontally.
 import type { Redis } from "ioredis";
 import type { InstallationTokenStore } from "../github/app";
+import { incr } from "./metrics";
+
+const REDIS_TOKEN_CACHE_METRIC = "gittensory_redis_token_cache_total";
 
 const keyFor = (installationId: number): string =>
   `gh:insttoken:${installationId}`;
 
+function recordTokenCacheMetric(result: "hit" | "miss"): void {
+  incr(REDIS_TOKEN_CACHE_METRIC, { result });
+}
+
 export function createRedisTokenCache(redis: Redis): InstallationTokenStore {
   return {
     async get(installationId: number) {
       const raw = await redis.get(keyFor(installationId));
-      if (!raw) return null;
+      if (!raw) {
+        recordTokenCacheMetric("miss");
+        return null;
+      }
       try {
         const value = JSON.parse(raw) as {
           token?: unknown;
           expiresAtMs?: unknown;
         };
-        return typeof value.token === "string" &&
-          typeof value.expiresAtMs === "number"
-          ? { token: value.token, expiresAtMs: value.expiresAtMs }
-          : null;
+        if (typeof value.token !== "string") {
+          recordTokenCacheMetric("miss");
+          return null;
+        }
+        if (typeof value.expiresAtMs !== "number") {
+          recordTokenCacheMetric("miss");
+          return null;
+        }
+        recordTokenCacheMetric("hit");
+        return { token: value.token, expiresAtMs: value.expiresAtMs };
       } catch {
+        recordTokenCacheMetric("miss");
         return null;
       }
     },

test/unit/selfhost-redis-token-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 { createRedisTokenCache } from "../../src/selfhost/redis-token-cache";
 
 /** Minimal ioredis stand-in that records the TTL passed to set(). */
@@ -23,20 +24,32 @@ function fakeRedis(): {
   return { redis, store, ttl: () => lastTtl };
 }
 
+afterEach(() => resetMetrics());
+
 describe("createRedisTokenCache (#perf installation-token persistence)", () => {
   it("get returns null for a missing installation", async () => {
     const { redis } = fakeRedis();
     expect(await createRedisTokenCache(redis).get(42)).toBeNull();
+
+    expect(await renderMetrics()).toContain(
+      'gittensory_redis_token_cache_total{result="miss"} 1',
+    );
   });
 
   it("set then get round-trips the token + expiry, with TTL ~ the token lifetime", async () => {
     const f = fakeRedis();
     const cache = createRedisTokenCache(f.redis);
     const expiresAtMs = Date.now() + 3_600_000;
-    await cache.set(7, { token: "tok", expiresAtMs });
+    await cache.set(7, { token: "sensitive-value", expiresAtMs });
     expect(f.ttl()).toBeGreaterThan(3500); // ~3600s
     expect(f.ttl()).toBeLessThanOrEqual(3600);
-    expect(await cache.get(7)).toEqual({ token: "tok", expiresAtMs });
+    expect(await cache.get(7)).toEqual({ token: "sensitive-value", expiresAtMs });
+
+    const metrics = await renderMetrics();
+    expect(metrics).toContain(
+      'gittensory_redis_token_cache_total{result="hit"} 1',
+    );
+    expect(metrics).not.toContain("sensitive-value");
   });
 
   it("floors the TTL at 1s for an already-near-expiry token", async () => {
@@ -52,14 +65,35 @@ describe("createRedisTokenCache (#perf installation-token persistence)", () => {
     const f = fakeRedis();
     f.store.set("gh:insttoken:9", "{not json");
     expect(await createRedisTokenCache(f.redis).get(9)).toBeNull();
+
+    expect(await renderMetrics()).toContain(
+      'gittensory_redis_token_cache_total{result="miss"} 1',
+    );
+  });
+
+  it("get returns null when the cached token is not a string", async () => {
+    const f = fakeRedis();
+    f.store.set(
+      "gh:insttoken:9",
+      JSON.stringify({ token: 123, expiresAtMs: Date.now() + 60_000 }),
+    );
+    expect(await createRedisTokenCache(f.redis).get(9)).toBeNull();
+
+    expect(await renderMetrics()).toContain(
+      'gittensory_redis_token_cache_total{result="miss"} 1',
+    );
   });
 
-  it("get returns null when the stored shape is wrong", async () => {
+  it("get returns null when the cached expiry is not a number", async () => {
     const f = fakeRedis();
     f.store.set(
       "gh:insttoken:9",
-      JSON.stringify({ token: 123, expiresAtMs: "soon" }),
+      JSON.stringify({ token: "sensitive-value", expiresAtMs: "soon" }),
     );
     expect(await createRedisTokenCache(f.redis).get(9)).toBeNull();
+
+    expect(await renderMetrics()).toContain(
+      'gittensory_redis_token_cache_total{result="miss"} 1',
+    );
   });
 });