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Many commercial databases today support replication, which can take one of several forms With master slave replication, the database allows updates at a primary site, and automatically propagates updates to replicas at other sites Transactions may read the replicas at other sites, but are not permitted to update them An important feature of such replication is that transactions do not obtain locks at remote sites To ensure that transactions running at the replica sites see a consistent
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(but perhaps outdated) view of the database, the replica should re ect a transactionconsistent snapshot of the data at the primary; that is, the replica should re ect all updates of transactions up to some transaction in the serialization order, and should not re ect any updates of later transactions in the serialization order The database may be con gured to propagate updates immediately after they occur at the primary, or to propagate updates only periodically Master slave replication is particularly useful for distributing information, for instance from a central of ce to branch of ces of an organization Another use for this form of replication is in creating a copy of the database to run large queries, so that queries do not interfere with transactions Updates should be propagated periodically every night, for example so that update propagation does not interfere with query processing The Oracle database system supports a create snapshot statement, which can create a transaction-consistent snapshot copy of a relation, or set of relations, at a remote site It also supports snapshot refresh, which can be done either by recomputing the snapshot or by incrementally updating it Oracle supports automatic refresh, either continuously or at periodic intervals With multimaster replication (also called update-anywhere replication) updates are permitted at any replica of a data item, and are automatically propagated to all replicas This model is the basic model used to manage replicas in distributed databases Transactions update the local copy and the system updates other replicas transparently One way of updating replicas is to apply immediate update with two-phase commit, using one of the distributed concurrency-control techniques we have seen Many database systems use the biased protocol, where writes have to lock and update all replicas and reads lock and read any one replica, as their currency-control technique Many database systems provide an alternative form of updating: They update at one site, with lazy propagation of updates to other sites, instead of immediately applying updates to all replicas as part of the transaction performing the update Schemes based on lazy propagation allow transaction processing (including updates) to proceed even if a site is disconnected from the network, thus improving availability, but, unfortunately, do so at the cost of consistency One of two approaches is usually followed when lazy propagation is used: Updates at replicas are translated into updates at a primary site, which are then propagated lazily to all replicas This approach ensures that updates to an item are ordered serially, although serializability problems can occur, since transactions may read an old value of some other data item and use it to perform an update Updates are performed at any replica and propagated to all other replicas This approach can cause even more problems, since the same data item may be updated concurrently at multiple sites Some con icts due to the lack of distributed concurrency control can be detected when updates are propagated to other sites (we shall see how in Section 2354),
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19 Distributed Databases
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19
Distributed Databases
but resolving the con ict involves rolling back committed transactions, and durability of committed transactions is therefore not guaranteed Further, human intervention may be required to deal with con icts The above schemes should therefore be avoided or used with care
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