Highly Available Serial Devices

The ideal data center will support both past and future hardware devices, and one type of device you are likely to use in your data center from the past is a serial communication device.

Although the need for most serial devices has been eliminated by newer technology, most data centers retain some legacy hardware that still uses this simple method of transferring data. Fortunately, several vendors now build a device that converts serial communication into network packets. These serial-to-IP network devices make it possible to communicate with the serial device from any computer attached to the network.

For example, if you need to send faxes from a business tier application running on the cluster (or anywhere else inside your network), you may want to run a fax server software package such as the open source Hylafax program. The Hylafax server can run on the cluster node manager and provide faxing capabilities to all of the cluster nodes. To prevent the fax modem(s) from becoming a single point of failure, you can purchase double the number of modems you require and connect half of them to the primary cluster node manager and half of them to the backup cluster node manager. Alternatively, you can connect these modems to a serial-to-IP communication device.

Serial-to-IP Communication Device

A serial-to-IP communication device (see Figure 20-5) allows you to failover the application program (Hylafax, in this example) that talks to the serial device from a primary to a backup server. When you use this type of device, you load special driver software from the serial-to-IP device manufacturer that allows you to fool the application programs running on the cluster node manager into thinking they are communicating with a serial device directly connected to the system, when in fact the serial device is really only accessible over the network. Of course, you must purchase two of these devices (and at least two serial devices) to avoid a single point of failure.

Figure 20-5: Serial-to-IP communication devices

High-Availability Modems

The dark lines connecting the modems to the serial-to-IP devices in Figure 20-5 represent serial cables. The network cables connecting the serial-to-IP devices to the Ethernet backbone are not shown.

Note 

As noted in Chapter 8, Heartbeat's cl_respawn program may be required to failover a service that needs to restart automatically when the service fails (typically the case for applications communicating with serial devices).