Push scalability (#3080)

* Update status through increment
* adds support for incrementing nested keys
* fix issue when having spaces in keys for ordering
* Refactors PushController to use worker
* Adds tests for custom push queue config
* Makes PushController adapter independant
* Better logging of _PushStatus in VERBOSE

src/Push/PushQueue.js

@@ -0,0 +1,60 @@
+import { ParseMessageQueue }  from '../ParseMessageQueue';
+import rest                   from '../rest';
+import { isPushIncrementing } from './utils';
+
+const PUSH_CHANNEL = 'parse-server-push';
+const DEFAULT_BATCH_SIZE = 100;
+
+export class PushQueue {
+  parsePublisher: Object;
+  channel: String;
+  batchSize: Number;
+
+  // config object of the publisher, right now it only contains the redisURL,
+  // but we may extend it later.
+  constructor(config: any = {}) {
+    this.channel = config.channel || PUSH_CHANNEL;
+    this.batchSize = config.batchSize || DEFAULT_BATCH_SIZE;
+    this.parsePublisher = ParseMessageQueue.createPublisher(config);
+  }
+
+  static defaultPushChannel() {
+    return PUSH_CHANNEL;
+  }
+
+  enqueue(body, where, config, auth, pushStatus) {
+    const limit = this.batchSize;
+    // Order by badge (because the payload is badge dependant)
+    // and createdAt to fix the order
+    const order = isPushIncrementing(body) ? 'badge,createdAt' : 'createdAt';
+
+    return Promise.resolve().then(() => {
+      return rest.find(config,
+                       auth,
+                       '_Installation',
+                       where,
+                       {limit: 0, count: true});
+    }).then(({results, count}) => {
+      if (!results) {
+        return Promise.reject({error: 'PushController: no results in query'})
+      }
+      pushStatus.setRunning(count);
+      let skip = 0;
+      while (skip < count) {
+        const query = { where,
+          limit,
+          skip,
+          order };
+
+        const pushWorkItem = {
+          body,
+          query,
+          pushStatus: { objectId: pushStatus.objectId },
+          applicationId: config.applicationId
+        }
+        this.parsePublisher.publish(this.channel, JSON.stringify(pushWorkItem));
+        skip += limit;
+      }
+    });
+  }
+}