test(server): add non-gating EventFeed fan-out benchmark

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: armancharan

packages/server/test/event-feed-fanout.bench.test.ts

@@ -0,0 +1,166 @@
+import { describe, expect } from "bun:test"
+import { EventV2 } from "@opencode-ai/core/event"
+import { AbsolutePath } from "@opencode-ai/core/schema"
+import { DateTime, Effect, Fiber, Stream } from "effect"
+import { it } from "../../core/test/lib/effect"
+import { EventFeed } from "../src/event-feed"
+
+const Clients = [1, 2, 4, 8, 11, 16] as const
+const Sizes = {
+  small: 1 * 1024,
+  medium: 64 * 1024,
+  large: 256 * 1024,
+} as const
+
+function makeSource() {
+  let subscriber: EventV2.Subscriber | undefined
+  return {
+    observe: (next: EventV2.Subscriber) =>
+      Effect.sync(() => {
+        subscriber = next
+        return Effect.sync(() => {
+          if (subscriber === next) subscriber = undefined
+        })
+      }),
+    publish: (event: EventV2.Payload) => Effect.suspend(() => (subscriber ? subscriber(event) : Effect.void)),
+  }
+}
+
+function sessionEvent(id: string, sessionID: string, location: { directory: string }): EventV2.Payload {
+  return {
+    id: EventV2.ID.make(`evt_${id}`),
+    created: DateTime.makeUnsafe(Date.now()),
+    type: "session.renamed",
+    data: { sessionID, title: id },
+    durable: { aggregateID: sessionID, seq: EventV2.Seq.make(1), version: EventV2.Version.make(1) },
+    location: {
+      directory: AbsolutePath.make(location.directory),
+    },
+  }
+}
+
+function eventLoopDelayMs() {
+  const start = performance.now()
+  return new Promise<number>((resolve) => {
+    setTimeout(() => resolve(performance.now() - start), 0)
+  })
+}
+
+type Row = {
+  clients: number
+  size: keyof typeof Sizes
+  payloadBytes: number
+  interested: number
+  serializedEvents: number
+  serializedBytes: number
+  peakActive: number
+  receivedInterested: number
+  receivedUninterested: number
+  publishMs: number
+  clientHandleMs: number
+  rssBefore: number
+  rssAfter: number
+  eventLoopDelayMs: number
+}
+
+describe("EventFeed fan-out benchmark", () => {
+  it.live(
+    "accounting laws hold across client counts and payload sizes",
+    () =>
+      Effect.gen(function* () {
+        const rows: Row[] = []
+
+        for (const clients of Clients) {
+          for (const size of Object.keys(Sizes) as Array<keyof typeof Sizes>) {
+            rows.push(yield* measureCell(clients, size))
+          }
+        }
+
+        // Same payload class → same serialized byte cost regardless of uninterested N.
+        for (const size of Object.keys(Sizes) as Array<keyof typeof Sizes>) {
+          const bytes = rows.filter((row) => row.size === size).map((row) => row.serializedBytes)
+          expect(new Set(bytes).size).toBe(1)
+          expect(bytes[0]).toBe(Sizes[size])
+        }
+
+        // Encode-once: every cell accepted exactly one hot publish.
+        expect(rows.every((row) => row.serializedEvents === 1)).toBe(true)
+
+        console.log(JSON.stringify({ benchmark: "event-feed-fanout", rows }, null, 2))
+      }),
+    60_000,
+  )
+})
+
+function measureCell(clients: number, size: keyof typeof Sizes) {
+  const payloadBytes = Sizes[size]
+  const interested = Math.max(1, Math.floor(clients / 2))
+  const frame = "x".repeat(payloadBytes)
+
+  return Effect.scoped(
+    Effect.gen(function* () {
+      let serializedEvents = 0
+      let serializedBytes = 0
+      const source = makeSource()
+      const feed = yield* EventFeed.make(source.observe, {
+        encode: () => {
+          serializedEvents += 1
+          serializedBytes += Buffer.byteLength(frame, "utf-8")
+          return frame
+        },
+      })
+
+      const received = Array.from({ length: clients }, () => 0)
+      const fibers: Array<Fiber.Fiber<void, EventFeed.Error>> = []
+
+      for (let index = 0; index < clients; index++) {
+        const sessions = index < interested ? ["ses_hot"] : [`ses_${index}`]
+        const stream = yield* feed.subscribe({
+          location: { directory: "/tmp/project" },
+          sessions,
+        })
+        fibers.push(
+          yield* stream.pipe(
+            Stream.take(1),
+            Stream.runForEach(() => Effect.sync(() => (received[index] += 1))),
+            Effect.forkScoped,
+          ),
+        )
+      }
+
+      const rssBefore = process.memoryUsage().rss
+      const loopDelay = yield* Effect.promise(eventLoopDelayMs)
+      const publishStarted = performance.now()
+      yield* source.publish(sessionEvent("hot", "ses_hot", { directory: "/tmp/project" }))
+      const publishMs = performance.now() - publishStarted
+
+      const handleStarted = performance.now()
+      for (const fiber of fibers.slice(0, interested)) yield* Fiber.join(fiber)
+      const clientHandleMs = performance.now() - handleStarted
+
+      for (const fiber of fibers.slice(interested)) yield* Fiber.interrupt(fiber)
+
+      expect(serializedEvents).toBe(1)
+      expect(serializedBytes).toBe(payloadBytes)
+      expect(received.slice(0, interested).every((count) => count === 1)).toBe(true)
+      expect(received.slice(interested).every((count) => count === 0)).toBe(true)
+
+      return {
+        clients,
+        size,
+        payloadBytes,
+        interested,
+        serializedEvents,
+        serializedBytes,
+        peakActive: clients,
+        receivedInterested: received.slice(0, interested).reduce((sum, count) => sum + count, 0),
+        receivedUninterested: received.slice(interested).reduce((sum, count) => sum + count, 0),
+        publishMs,
+        clientHandleMs,
+        rssBefore,
+        rssAfter: process.memoryUsage().rss,
+        eventLoopDelayMs: loopDelay,
+      } satisfies Row
+    }),
+  )
+}