improved streaming

aws/aws-lambda-nodejs-runtime-interface-client#94 (comment)

examples/app-router/app/ssr/layout.tsx

@@ -1,13 +1,11 @@
 import { PropsWithChildren } from "react";
 
-import Filler from "@example/shared/components/Filler";
-
 export default function Layout({ children }: PropsWithChildren) {
   return (
     <div>
       <h1>SSR</h1>
       {/* 16 kb seems necessary here to prevent any buffering*/}
-      <Filler size={16} />
+      {/* <Filler size={16} /> */}
       {children}
     </div>
   );

packages/open-next/src/http/openNextResponse.ts

@@ -14,6 +14,7 @@ export interface StreamCreator {
   // Just to fix an issue with aws lambda streaming with empty body
   onWrite?: () => void;
   onFinish: () => void;
+  waitForFirstWrite?: boolean;
 }
 
 // We only need to implement the methods that are used by next.js
@@ -23,6 +24,7 @@ export class OpenNextNodeResponse extends Transform {
   headers: OutgoingHttpHeaders = {};
   private _cookies: string[] = [];
   private responseStream?: Writable;
+  private hasDoneFirstWrite: boolean = false;
   headersSent: boolean = false;
   _chunks: Buffer[] = [];
 
@@ -144,7 +146,18 @@ export class OpenNextNodeResponse extends Transform {
     if (!this.headersSent) {
       this.flushHeaders();
     }
-    this._internalWrite(chunk, encoding);
-    callback();
+    if (this.streamCreator?.waitForFirstWrite && !this.hasDoneFirstWrite) {
+      const waitTime = parseInt(
+        process.env.STREAMING_INITIAL_WRITE_WAIT_TIME ?? "25",
+      );
+      new Promise((resolve) => setTimeout(resolve, waitTime)).then(() => {
+        this._internalWrite(chunk, encoding);
+        this.hasDoneFirstWrite = true;
+        callback();
+      });
+    } else {
+      this._internalWrite(chunk, encoding);
+      callback();
+    }
   }
 }

packages/open-next/src/wrappers/aws-lambda-streaming.ts

@@ -119,6 +119,7 @@ const handler: WrapperHandler = async (handler, converter) =>
             compressedStream?.end(new Uint8Array(8));
           }
         },
+        waitForFirstWrite: true,
       };
 
       const response = await handler(internalEvent, streamCreator);