fixes #178: implement SSE retry without fetch-event-source

package.json

@@ -31,7 +31,6 @@
     "release": "np"
   },
   "dependencies": {
-    "@microsoft/fetch-event-source": "^2.0.1",
     "deepmerge": "^4.2.2",
     "linkify-html": "^3.0.5",
     "linkifyjs": "^3.0.5",

src/network/TockEventSource.ts

@@ -1,20 +1,46 @@
 import { BotConnectorResponse } from '../model/responses';
-import {
-  EventSourceMessage,
-  EventStreamContentType,
-  fetchEventSource,
-} from '@microsoft/fetch-event-source';
-
-class RetriableError extends Error {}
-class FatalError extends Error {}
 
 const INITIAL_RETRY_DELAY = 0;
 const RETRY_DELAY_INCREMENT = 1000;
 const MAX_RETRY_DELAY = 15000;
 
+enum SseStatus {
+  /**
+   * The server is not answering, or answering with a 1XX, 3XX, 429, or 5XX HTTP status code
+   */
+  SERVER_UNAVAILABLE = -1,
+  /**
+   * The server is answering with a 4XX HTTP status code, except 429 (rate limit)
+   */
+  UNSUPPORTED = 0,
+  /**
+   * The server is answering with a 2XX HTTP status code
+   */
+  SUPPORTED = 1,
+}
+
+async function getSseStatus(url: string) {
+  try {
+    const response = await fetch(url);
+    if (response.ok) {
+      return SseStatus.SUPPORTED;
+    } else if (
+      response.status >= 400 &&
+      response.status < 500 &&
+      response.status !== 429
+    ) {
+      return SseStatus.UNSUPPORTED;
+    } else {
+      return SseStatus.SERVER_UNAVAILABLE;
+    }
+  } catch (_) {
+    return SseStatus.SERVER_UNAVAILABLE;
+  }
+}
+
 export class TockEventSource {
   private initialized: boolean;
-  private abortController: AbortController;
+  private eventSource: EventSource | null;
   private retryDelay: number;
   onResponse: (botResponse: BotConnectorResponse) => void;
   onStateChange: (state: number) => void;
@@ -38,60 +64,55 @@ export class TockEventSource {
    */
   open(endpoint: string, userId: string): Promise<void> {
     this.onStateChange(EventSource.CONNECTING);
-    this.abortController = new AbortController();
+    const url = `${endpoint}/sse?userid=${userId}`;
     return new Promise<void>((resolve, reject): void => {
-      fetchEventSource(`${endpoint}/sse?userid=${userId}`, {
-        signal: this.abortController.signal,
-        onopen: async (response) => {
-          if (
-            response.ok &&
-            response.headers
-              .get('content-type')
-              ?.includes(EventStreamContentType)
-          ) {
-            this.onStateChange(EventSource.OPEN);
-            this.initialized = true;
-            resolve();
-            return;
-          } else if (
-            response.status >= 400 &&
-            response.status < 500 &&
-            response.status !== 429
-          ) {
-            throw new FatalError();
-          } else {
-            throw new RetriableError();
-          }
-        },
-        onmessage: (e: EventSourceMessage) => {
-          if (e.event === 'message') {
-            this.onResponse(JSON.parse(e.data));
-          }
-        },
-        onerror: (err) => {
-          if (err instanceof FatalError) {
-            throw err; // rethrow to stop the operation
-          } else {
-            const retryDelay = this.retryDelay;
-            this.retryDelay = Math.min(
-              MAX_RETRY_DELAY,
-              retryDelay + RETRY_DELAY_INCREMENT,
-            );
-            return retryDelay;
-          }
-        },
-      })
-        .catch((e) => console.error(e))
-        .finally(() => {
-          reject();
-          this.onStateChange(EventSource.CLOSED);
-          this.initialized = false;
-        });
+      this.tryOpen(url, resolve, reject);
+    });
+  }
+
+  private tryOpen(url: string, resolve: () => void, reject: () => void) {
+    this.eventSource = new EventSource(url);
+    this.eventSource.addEventListener('open', () => {
+      this.onStateChange(EventSource.OPEN);
+      this.initialized = true;
+      this.retryDelay = INITIAL_RETRY_DELAY;
+      resolve();
     });
+    this.eventSource.addEventListener('error', () => {
+      this.eventSource?.close();
+      this.retry(url, reject, resolve);
+    });
+    this.eventSource.addEventListener('message', (e) => {
+      this.onResponse(JSON.parse(e.data));
+    });
+  }
+
+  private retry(url: string, reject: () => void, resolve: () => void) {
+    const retryDelay = this.retryDelay;
+    this.retryDelay = Math.min(
+      MAX_RETRY_DELAY,
+      retryDelay + RETRY_DELAY_INCREMENT,
+    );
+    setTimeout(async () => {
+      switch (await getSseStatus(url)) {
+        case SseStatus.UNSUPPORTED:
+          reject();
+          this.close();
+          break;
+        case SseStatus.SUPPORTED:
+          this.tryOpen(url, resolve, reject);
+          break;
+        case SseStatus.SERVER_UNAVAILABLE:
+          this.retry(url, reject, resolve);
+          break;
+      }
+    }, retryDelay);
   }
 
   close() {
-    this.abortController?.abort();
+    this.eventSource?.close();
+    this.eventSource = null;
     this.initialized = false;
+    this.onStateChange(EventSource.CLOSED);
   }
 }

yarn.lock

@@ -1381,11 +1381,6 @@
     "@types/mdx" "^2.0.0"
     "@types/react" ">=16"
 
-"@microsoft/fetch-event-source@^2.0.1":
-  version "2.0.1"
-  resolved "https://registry.yarnpkg.com/@microsoft/fetch-event-source/-/fetch-event-source-2.0.1.tgz#9ceecc94b49fbaa15666e38ae8587f64acce007d"
-  integrity sha512-W6CLUJ2eBMw3Rec70qrsEW0jOm/3twwJv21mrmj2yORiaVmVYGS4sSS5yUwvQc1ZlDLYGPnClVWmUUMagKNsfA==
-
 "@ndelangen/get-tarball@^3.0.7":
   version "3.0.9"
   resolved "https://registry.yarnpkg.com/@ndelangen/get-tarball/-/get-tarball-3.0.9.tgz#727ff4454e65f34707e742a59e5e6b1f525d8964"