Keep SSL sessions alive between xhr2 requests

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

@@ -0,0 +1,51 @@
+/**
+ * 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 { XMLHttpRequest } from 'xhr2';
+
+/**
+ * Allow HTTP sessions to be kept alive.
+ * Without this configuration, Angular re-connects to REST multiple times per SSR cycle.
+ * 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.
+ *
+ * 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 {
+    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;
+  }
+}

src/modules/app/server-app.module.ts

@@ -34,6 +34,8 @@ import { AuthRequestService } from '../../app/core/auth/auth-request.service';
 import { ServerAuthRequestService } from '../../app/core/auth/server-auth-request.service';
 import { CorrelationIdService } from '../../app/correlation-id/correlation-id.service';
 import { AppConfig, APP_CONFIG_STATE } from '../../config/app-config.interface';
+import { XhrFactory } from '@angular/common';
+import { ServerXhrService } from '../../app/core/services/server-xhr.service';
 
 import { environment } from '../../environments/environment';
 
@@ -121,6 +123,10 @@ export function createTranslateLoader() {
       provide: HardRedirectService,
       useClass: ServerHardRedirectService,
     },
+    {
+      provide: XhrFactory,
+      useClass: ServerXhrService,
+    },
   ]
 })
 export class ServerAppModule {