The drbd is a block device which is designed to build high availability clusters. You can think it as a networked RAID device. The Corosync Cluster Engine is a Group Communication System with additional features for implementing high availability within applications. Pacemaker is an open source high availability resource manager software. You can combine all of three to build a cluster on CentOS Linux version 7.x.
From the blog post:
When configuring a cluster, you want tot keep managing the server as simple as possible. Theoretically, the results given by any node in the cluster should be equal as you want the cluster to be transparent to the end-user. Part of doing this, is having the same data available on every node of the cluster when it’s active. One way to do this, is using a central file-share, for example over NFS but this also has disadvantages. Another way is to have a distributed file system that stays on the nodes itself. DRBD is one of them. This post explains how to integrate DRBD in a cluster with Corosync and Pacemaker.