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Reintegration of a repaired site or link into the system requires care When a failed site recovers, it must initiate a procedure to update its system tables to re ect changes made while it was down If the site had replicas of any data items, it must obtain the current values of these data items and ensure that it receives all future updates Reintegration of a site is more complicated than it may seem to be at rst glance, since there may be updates to the data items processed during the time that the site is recovering An easy solution is to halt the entire system temporarily while the failed site rejoins it In most applications, however, such a temporary halt is unacceptably disruptive Techniques have been developed to allow failed sites to reintegrate while concurrent updates to data items proceed concurrently Before a read or write lock is granted on any data item, the site must ensure that it has caught up on all updates to the data item If a failed link recovers, two or more partitions can be rejoined Since a partitioning of the network limits the allowable operations by some or all sites, all sites should be informed promptly of the recovery of the link See the bibliographical notes for more information on recovery in distributed systems
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1964 Comparison with Remote Backup
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Remote backup systems, which we studied in Section 1710, and replication in distributed databases are two alternative approaches to providing high availability The main difference between the two schemes is that with remote backup systems, actions such as concurrency control and recovery are performed at a single site, and only data and log records are replicated at the other site In particular, remote backup systems help avoid two-phase commit, and its resultant overheads Also, transactions need to contact only one site (the primary site), and thus avoid the overhead of running transaction code at multiple sites Thus remote backup systems offer a lower-cost approach to high availability than replication On the other hand, replication can provide greater availability by having multiple replicas available, and using the majority protocol
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1965 Coordinator Selection
Several of the algorithms that we have presented require the use of a coordinator If the coordinator fails because of a failure of the site at which it resides, the system can continue execution only by restarting a new coordinator on another site One way to continue execution is by maintaining a backup to the coordinator, which is ready to assume responsibility if the coordinator fails A backup coordinator is a site that, in addition to other tasks, maintains enough information locally to allow it to assume the role of coordinator with minimal disruption to the distributed system All messages directed to the coordinator are received by both the coordinator and its backup The backup coordinator executes the same algorithms and maintains the same internal state information (such as, for a concurrency coordinator, the lock table) as does the actual coordinator The only difference in function between the coordinator and its backup is that the backup does not take any action that affects other sites Such actions are left to the actual coordinator In the event that the backup coordinator detects the failure of the actual coordinator, it assumes the role of coordinator Since the backup has all the information available to it that the failed coordinator had, processing can continue without interruption The prime advantage to the backup approach is the ability to continue processing immediately If a backup were not ready to assume the coordinator s responsibility, a newly appointed coordinator would have to seek information from all sites in the system so that it could execute the coordination tasks Frequently, the only source of some of the requisite information is the failed coordinator In this case, it may be necessary to abort several (or all) active transactions, and to restart them under the control of the new coordinator Thus, the backup-coordinator approach avoids a substantial amount of delay while the distributed system recovers from a coordinator failure The disadvantage is the overhead of duplicate execution of the coordinator s tasks Furthermore, a coordinator and its backup need to communicate regularly to ensure that their activities are synchronized In short, the backup-coordinator approach incurs overhead during normal processing to allow fast recovery from a coordinator failure In the absence of a designated backup coordinator, or in order to handle multiple failures, a new coordinator may be chosen dynamically by sites that are live Election algorithms enable the sites to choose the site for the new coordinator in a decentralized manner Election algorithms require that a unique identi cation number be associated with each active site in the system The bully algorithm for election works as follows To keep the notation and the discussion simple, assume that the identi cation number of site Si is i and that the chosen coordinator will always be the active site with the largest identi cation number Hence, when a coordinator fails, the algorithm must elect the active site that has the largest identi cation number The algorithm must send this number to each active site in the system In addition, the algorithm must provide a mechanism by which a site recovering from a crash can identify the current coordinator Suppose that site Si sends a request that is not answered by the coordinator within a prespeci ed time
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