How to Create an Oak Cluster With MongoMK Failover

By Rama Kiran Kadambari 3 min read

Are you looking for scalability (MongoDB) and high availability (oak clustering with replica set)? If so, read on. This post is a first step towards achieving a minimal set-up of an AEM 6.0 cluster with MongoDB in real-life scenarios.

To get the greatest benefit from this blog post, here’s what you’ll need :


With the release of AEM 6.0, clustering has been rethought—resulting in the introduction of “Oak” and the concept of microkernels. Here I’ll provide a working example of a minimal AEM 6 (Oak) cluster, similar to the image below. This example implies multiple Oak instances accessing one MongoDB instance. A secondary MongoDB is provided for failover within the same data center.

Fig.1. AEM 6 Oak cluster

How Does Oak Clustering Work?

An Oak cluster is formed by two AEM 6.0 instances and a MongoDB for persistence. All these instances are linked together to ensure that if one node fails, the other nodes are active and accessible for your application, with no system interruption. This enables you to easily recover and re-start failed nodes.

Why Do You Need It?

MongoDB offers high availability with replicaSets and scalability with sharding, to support deployments with very large data sets and high throughput operations.

Advantages and Disadvantages

The advantages:

  • The ability to scale AEM instances
  • Acceptable complexity

The disadvantages:

  • Lesser performance than TarMK

Three-Step Setup

Step 1:
First, set up three instances of MongoDB with replicaSet. For help creating a replicaSet, please refer to this MongoDB tutorial.

Fig.2. After successfully creating the three-node replica set, check the status either using the rs.status () command on the command line or using your native browser. You should see something like this.

As you can see in this example, a three-node replica set has been configured. This same process can be applied to configuring any number of nodes in a replica set.

Step 2:
Set up the AEM 6.0 servers.

  • Take two new AEM 6.0 instances (since we are creating an Oak cluster with two AEM 6.0 instances and one MongoDB).
  • Run each of the AEM instances with the following commands.

Note: Make sure that all of the MongoDB instances are up and running. Also check to make sure the replica set status (replicaSet) is active and running with PRIMARY, SECONDARY and arbiter.

java -jar <aem instance jar name> -r crx3,crx3mongo -Doak.mongo.uri=mongodb://<primaryhost>:<port>,<secondaryhost>:<port>/?replicaSet=<replicaSet name>

The actual command would look like this:

java -jar aem-author-4512.jar -r crx3,crx3mongo -Doak.mongo.uri=mongodb://localhost:27017,localhost:27018/?replicaSet=oak-cluster

java -jar aem-author-4513.jar -r crx3,crx3mongo -Doak.mongo.uri=mongodb://localhost:27017,localhost:27018/?replicaSet=oak-cluster

In the above command, two hosts are given in the mongo URI—the first being the PRIMARY and the second being the SECONDARY MongoDB instance for automatic failover.

Note: Initial startup takes some time, so please be patient and monitor the log files of both AEM and MongoDB.

  • Once the AEM servers are up, log onto the servers and open the Geometrixx outdoors website for testing.
  • Open any of the two AEM servers and open the Geometrixx outdoors website. Try adding/modifying/deleting some content on the pages. These changes will get reflected in the other AEM server instantaneously.

Step 3:
Test the automatic failover

  • Shut down the primary MongoDB server (ctl+c).
  • Open the Mongo shell of the secondary Mongo instance.
  • Now check the replSet status using the rs.status() command or in your native browser.
  • The PRIMARY server will shut down and the SECONDARY server will get elected as PRIMARY.
Fig.3. When you test the automatic failover, note how the primary will shut down and the secondary server will get elected as primary.
  • Open the AEM instance and Geometrixx outdoors website, which should still be working. You should be able to add/modify/delete content.

Explore Additional Resources

Learn more on how to select the right CMS for your business and improve your customer’s Digital Experience.

Our Adobe Experience Cloud Consulting team specializes in helping you deliver a seamless, intelligent, and personalized customer experience.


We have one of the largest and deepest multi-solutions digital consulting teams in the world. Our proprietary processes and years of Digital Experience expertise have earned us a 97% customer satisfaction rating with our clients ranging from Global Fortune 1000 to Mid-Market Enterprises, leading educational institutions, and Non-Profits.

Contact us today if you would like more help on AEM 6.0 cluster with MongoDB.

About TA Digital

TA Digital is the only global boutique agency that delivers the “best of both worlds” to clients seeking to achieve organizational success through digital transformation. Unlike smaller, regional agencies that lack the ability to scale or large organizations that succumb to a quantity-over-quality approach, we offer resource diversity while also providing meticulous attention to the details that enable strategic success.

Over the past 20 years, TA Digital has positioned clients to achieve digital maturity by focusing on data, customer-centricity and exponential return on investment; by melding exceptional user experience and data-driven methodologies with artificial intelligence and machine learning, we enable digital transformations that intelligently build upon the strategies we set into motion. We are known as a global leader that assists marketing and technology executives in understanding the digital ecosystem while identifying cultural and operational gaps within their business – ultimately ushering organizations toward a more mature model and profitable digital landscape.

Recognized in 2013, 2014, 2015, 2019, and 2020 Inc. 5000 list as one of the most successful technology companies in the United States, TA Digital is pleased also to share high-level strategic partnerships with world class digital experience platform companies like Adobe, SAP and Salesforce and possess global partnerships with industry leaders such as Sitecore, Episerver, Elastic Path, BigCommerce, AWS, Azure and Coveo.

Rama Kiran Kadambari

Written By

Rama Kiran Kadambari