[core] Add configurable stream flush interval per world

.changeset/configurable-stream-flush-interval.md

@@ -0,0 +1,8 @@
+---
+"@workflow/world": patch
+"@workflow/core": patch
+"@workflow/world-local": patch
+"@workflow/world-postgres": patch
+---
+
+Add `streamFlushIntervalMs` option to `Streamer` interface, allowing non-Vercel world implementations to configure the stream write buffer flush interval. Defaults to 10ms (unchanged behavior). Set to 0 for immediate flushing on backends with sub-millisecond writes.

packages/core/src/serialization.ts

@@ -522,7 +522,7 @@ export class WorkflowServerWritableStream extends WritableStream<Uint8Array> {
             for (const w of currentWaiters) w.reject(err);
           }
         );
-      }, STREAM_FLUSH_INTERVAL_MS);
+      }, world.streamFlushIntervalMs ?? STREAM_FLUSH_INTERVAL_MS);
     };
 
     super({
@@ -570,8 +570,7 @@ export class WorkflowServerWritableStream extends WritableStream<Uint8Array> {
         // unsettled promise because the cleared timer will never fire.
         const waiters = flushWaiters;
         flushWaiters = [];
-        const abortError =
-          reason ?? new Error("Stream aborted");
+        const abortError = reason ?? new Error('Stream aborted');
         for (const w of waiters) w.reject(abortError);
       },
     });

packages/core/src/writable-stream.test.ts

@@ -11,6 +11,7 @@ describe('WorkflowServerWritableStream', () => {
     writeToStream: ReturnType<typeof vi.fn>;
     writeToStreamMulti: ReturnType<typeof vi.fn>;
     closeStream: ReturnType<typeof vi.fn>;
+    streamFlushIntervalMs?: number;
   };
 
   beforeEach(async () => {
@@ -248,4 +249,52 @@ describe('WorkflowServerWritableStream', () => {
       );
     });
   });
+
+  describe('streamFlushIntervalMs', () => {
+    it('should use world.streamFlushIntervalMs when set to 0 (immediate flush)', async () => {
+      mockWorld.streamFlushIntervalMs = 0;
+
+      const stream = new WorkflowServerWritableStream('s', 'run-1');
+      const writer = stream.getWriter();
+
+      // With interval=0, the flush fires on the next microtask tick via setTimeout(fn, 0)
+      await writer.write(new Uint8Array([1]));
+      expect(mockWorld.writeToStream).toHaveBeenCalledTimes(1);
+
+      await writer.close();
+    });
+
+    it('should fall back to default interval when streamFlushIntervalMs is undefined', async () => {
+      // mockWorld has no streamFlushIntervalMs set — uses default 10ms
+      delete mockWorld.streamFlushIntervalMs;
+
+      const stream = new WorkflowServerWritableStream('s', 'run-1');
+      const writer = stream.getWriter();
+
+      await writer.write(new Uint8Array([1]));
+      expect(mockWorld.writeToStream).toHaveBeenCalledTimes(1);
+
+      await writer.close();
+    });
+
+    it('should respect a custom non-zero flush interval', async () => {
+      mockWorld.streamFlushIntervalMs = 50;
+
+      const stream = new WorkflowServerWritableStream('s', 'run-1');
+      const writer = stream.getWriter();
+
+      // Start a write — the flush is scheduled 50ms from now
+      const writePromise = writer.write(new Uint8Array([1]));
+
+      // After 10ms (the old default), data should NOT have flushed yet
+      await new Promise((r) => setTimeout(r, 10));
+      expect(mockWorld.writeToStream).not.toHaveBeenCalled();
+
+      // Wait for the write to complete (will resolve after the 50ms timer fires)
+      await writePromise;
+      expect(mockWorld.writeToStream).toHaveBeenCalledTimes(1);
+
+      await writer.close();
+    });
+  });
 });

packages/world-local/src/config.ts

@@ -22,6 +22,11 @@ export type Config = {
    * `.workflow-data` directory.
    */
   tag?: string;
+  /**
+   * Override the flush interval (in ms) for buffered stream writes.
+   * Default is 10ms. Set to 0 for immediate flushing.
+   */
+  streamFlushIntervalMs?: number;
 };
 
 export const config = once<Config>(() => {

packages/world-local/src/index.ts

@@ -64,10 +64,12 @@ export function createLocalWorld(args?: Partial<Config>): LocalWorld {
   return {
     ...queue,
     ...createStorage(mergedConfig.dataDir, tag),
-    ...instrumentObject(
-      'world.streams',
-      createStreamer(mergedConfig.dataDir, tag)
-    ),
+    ...instrumentObject('world.streams', {
+      ...createStreamer(mergedConfig.dataDir, tag),
+      ...(mergedConfig.streamFlushIntervalMs !== undefined && {
+        streamFlushIntervalMs: mergedConfig.streamFlushIntervalMs,
+      }),
+    }),
     async start() {
       await initDataDir(mergedConfig.dataDir);
     },

packages/world-postgres/src/config.ts

@@ -7,4 +7,9 @@ type PgConnectionConfig =
 export type PostgresWorldConfig = PgConnectionConfig & {
   jobPrefix?: string;
   queueConcurrency?: number;
+  /**
+   * Override the flush interval (in ms) for buffered stream writes.
+   * Default is 10ms. Set to 0 for immediate flushing.
+   */
+  streamFlushIntervalMs?: number;
 };

packages/world-postgres/src/index.ts

@@ -59,6 +59,9 @@ export function createWorld(
     ...storage,
     ...streamer,
     ...queue,
+    ...(config.streamFlushIntervalMs !== undefined && {
+      streamFlushIntervalMs: config.streamFlushIntervalMs,
+    }),
     async start() {
       await queue.start();
     },

packages/world/src/interfaces.ts

@@ -30,6 +30,19 @@ import type {
 } from './steps.js';
 
 export interface Streamer {
+  /**
+   * Override the default flush interval (in milliseconds) for buffered stream writes.
+   * Chunks are accumulated in a buffer and flushed together on this interval.
+   *
+   * The default is 10ms, which is appropriate for HTTP-based backends like world-vercel
+   * where each flush is a network round-trip. For backends with sub-millisecond writes
+   * (e.g., Redis, local filesystem), a lower value (or 0 for immediate flushing) reduces
+   * end-to-end stream latency.
+   *
+   * Not supported by world-vercel.
+   */
+  streamFlushIntervalMs?: number;
+
   writeToStream(
     name: string,
     runId: string,