Local Clustering and Failover in Visual Studio .NET

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variables and complexities. We will discuss replication and disaster recovery issues starting in 18, Data Replication. The migration of services from one server to another is called failover. At minimum, the migration of services during a failover should meet the following criteria: Transparent. The failover should be no more intrusive to the clients who access the server s services than a simple reboot. This intrusiveness may not be reflected in the duration of the outage, but rather in what the clients must do to get back to work once services have been restored. In some cases, primarily databases, it may be necessary for the user to log back in to his application. Nonauthenticated web and file services should not require logging back in. Login sessions into the server that failed over do, with today s technology, still require a re-login on the takeover server. Quick. Failover should take no more than five minutes, and ideally less than two minutes. The best way to achieve this goal is for the takeover server to already be booted up and running as many of the underlying system processes as possible. If a full reboot is required in order to failover, failover times will go way up and can, in some cases, take an hour or more. The two- to five-minute goal for failovers is a noble one and can be easily met by most applications. The most glaring exception to this is databases such as Oracle or DB2. Databases can only be restarted after all of the transactions that have been cached are rerun, and the database updated. (Transactions are cached to speed up routine database performance; the trade-off is a slowing of recovery time.) There is no limit to how long it might take a database to run through all of the outstanding transactions, and while those transactions are being rerun, the database is down from a user s perspective. Minimal manual intervention. Ideally, no human intervention at all should be required for a failover to complete; the entire process should be automated. Some sites or applications may require manual initiation for a failover, but that is not generally desirable. As already discussed, the host receiving a failover should never require a reboot. Guaranteed data access. After a failover, the receiving host should see exactly the same data as the original host. Replicating data to another host when disks are not shared adds unnecessary risk and complexity, and is not advised for hosts that are located near to each other. The systems in a failover configuration should also communicate with each other continuously, so that each system knows the state of its partner. This communication is called a heartbeat. Later in this chapter we discuss the implications when the servers lose their heartbeats.
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Local Clustering and Failover
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When a failover occurs, three critical elements must be moved from the failed server to the takeover server in order for users to resume their activities and for the application services to be considered available once again: 1. Network identity. Generally, this means the IP address that the server s clients use. Some network media and applications may require additional information to be transferred, such as a MAC address. If the server is on multiple networks or has multiple public network identities, it may be necessary to move multiple addresses. 2. Access to shared disks. Operating system, and in particular filesystem, technology generally prohibits multiple servers from writing to the same disks at the same time for any reason. In a shared disk configuration, logical access must be restricted to one server at a time. During a failover, the process that prevents the second machine from accessing the disks must reverse itself and lock out the original server, while granting access only to the takeover server. Note that not all operating systems provide this technology. 3. Set of processes. Once the disks have migrated to the takeover server, all the processes associated with the data must be restarted. Data consistency must be ensured from the application s perspective. The collection of these elements is commonly called a service group. A service group is the unit that moves from cluster member to cluster member. Sometimes called a virtual machine, the service group provides the critical services that are being made highly available. A cluster may have multiple service groups, and depending on the software that manages the cluster, there may not be any limit to the number of service groups. Service groups must be totally independent of each other, so that they can live on any eligible server in the cluster, regardless of what the other service groups might be doing. If two or more service groups cannot be independent of each other (that is, they must be together on the same server at all times), then they must be merged into a single service group.
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