Overview of the high-availability solution To prevent a single point of failure in the Apache httpd, you can use a failover httpd instance that is clustered using one of the many available clustering stacks. Note: When the high-availability solution is used for VxFlex OS Gateway, ESRS is not supported. These operating system solutions are described in this document: Ubuntu, using Keepalived, a lightweight open source clustering stack for Linux. CentOS, using Keepalived and HAProxy A shared (virtual) IP address is used for both Apache httpd servers. Both of the Apache httpd servers have the same configuration, except that one is defined in the Keepalived configuration as the master, while the other is defined as the slave. If the master fails, the slave becomes the master, which means the users do not notice any loss of service. The VxFlex OS Gateway servers are configured the same way. According to the configuration in the Apache httpd, one of the VxFlex OS Gateway servers (machine2) is the master and the other (machine1) is the slave. In the Ubuntu environment, the mod_jk module in the Apache httpd monitors the VxFlex OS Gateway servers and decides to which server to forward the request from the client. In the CentOS environment, the Keepalived service monitors the VxFlex OS Gateway servers and determines to which server to forward the client request. The following figures illustrate the configuration and topology for the Ubuntu solution: Figure 1. High availability Gateway cluster topology - Ubuntu Figure 2. High availability networking example Parent topic IntroductionVxFlex OS is a software-only solution that uses existing servers' local disks and LAN to create a virtual SAN that has all the benefits of external storage—but at a fraction of the cost and complexity.