Merge pull request #2042 from atmire/poc-keepalive-during-ssr

Keep SSL sessions alive during SSR

src/app/core/services/server-xhr.service.ts

@@ -1,16 +1,58 @@
+/**
+ * The contents of this file are subject to the license and copyright
+ * detailed in the LICENSE and NOTICE files at the root of the source
+ * tree and available online at
+ *
+ * http://www.dspace.org/license/
+ */
+
 import { XhrFactory } from '@angular/common';
 import { Injectable } from '@angular/core';
+import { Agent as HttpAgent, AgentOptions as HttpAgentOptions } from 'http';
+import { Agent as HttpsAgent } from 'https';
 import { prototype, XMLHttpRequest } from 'xhr2';
 
 /**
- * Overrides the default XhrFactory server side, to allow us to set cookies in requests to the
+ * Allow HTTP sessions to be kept alive.
- * backend. This was added to be able to perform a working XSRF request from the node server, as it
+ * Without this configuration, Angular re-connects to REST multiple times per SSR cycle.
- * needs to set a cookie for the XSRF token
+ * https://nodejs.org/api/http.html#new-agentoptions
+ */
+const agentOptions: HttpAgentOptions = {
+  keepAlive: true,
+  keepAliveMsecs: 60 * 1000,
+};
+
+// Agents need to be reused between requests, otherwise keep-alive doesn't help.
+const httpAgent = new HttpAgent(agentOptions);
+const httpsAgent = new HttpsAgent(agentOptions);
+
+/**
+ * Contructs the XMLHttpRequest instances used for all HttpClient requests.
+ * Emulated by https://github.com/pwnall/node-xhr2 on the server.
+ * This class overrides the built-in Angular implementation to set additional configuration.
+ *
+ * Changes:
+ * - Turn off restriction for cookie headers to allow us to set cookies in requests to the backend.
+ *   This was added to be able to perform a working XSRF request from the node server, as it needs to set a cookie for the XSRF token.
+ * - Override NodeJS HTTP(S) agents to keep sessions alive between requests.
+ *   This improves SSR performance by reducing REST request overhead on the server.
+ *
+ * Note that this must be provided in ServerAppModule;
+ * it doesn't work when added as a Universal engine provider.
  */
 @Injectable()
 export class ServerXhrService implements XhrFactory {
   build(): XMLHttpRequest {
     prototype._restrictedHeaders.cookie = false;
-    return new XMLHttpRequest();
+    const xhr = new XMLHttpRequest();
+
+    // This call is specific to xhr2 and will probably break if we use another library.
+    // https://github.com/pwnall/node-xhr2#features
+    (xhr as any).nodejsSet({
+      httpAgent,
+      httpsAgent,
+    });
+
+    return xhr;
   }
 }