Chapter 17: Clustering identically configured in hardware and
Chapter 17: Clustering identically configured in hardware and OS, and they both host the same software. Interconnection exists between the machines (for example, over a LAN). One machine is designated as the master server and processes incoming requests. The rest of the machines are backup servers. The health of the master server is monitored constantly, either by the backup servers or by an independent hardware/software component. Whenever the master server crashes, one of the backup servers is made the master and request processing continues as if no crash has occurred. This action of the backup server taking over operation after a crash of the master is an instance of the fail-over behavioral pattern, covered in the next section. Master-backup combined with fail-over form the basis of most HA implementations. BACKUP Server1 BACKUP Server2 MASTER Server Request processing on MASTER only Local area network Figure 17-2: The master-backup topological pattern Fail-Over Behavioral Pattern Figure 17-3 illustrates the fail-over behavioral pattern. Fail-over occurs when a server crashes in a master- backup system. It refers to the way and means by which the master server is taken over by the former backup server. While in a hardware implementation, this may involve sophisticated switching and communications link isolation; in a software scenario, a state transfer and synchronization mechanism must be in place. New MASTER Server (formerly BACKUP Server1) BACKUP Server2 MASTER Server Request processing fail-over from MASTER to BACKUP Local area network CRASHED! Figure 17-3: The fail-over behavioral pattern In other words, simply having identically configured hardware, OS, and software applications (as in the master-backup pattern) is not enough to guarantee a logically transparent fail-over. The backup server taking over the duties of the master server must know what the master server is up to, and continue from where it left off. This requires the synchronization and sharing of dynamic state information, as shown in Figure 17-3 .
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