Distributed Oracle Databases

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Distributed Oracle Databases

• By
• Yemi Ogunrombi
• IS8030 - Integrated Computing Systems
• Professor - Dr Hoganson

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Distributed Oracle Databases

• A distributed database is a set of databases
  stored on multiple computers that typically
  appears to applications as a single database.
• Consequently, an application can simultaneously
  access and modify the data in several databases
  in a network.

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Distributed Oracle Databases

• Each Oracle database in the system is controlled
  by its local Oracle Server but cooperates to
  maintain the consistency of the global
  distributed database.

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Distributed Oracle Databases

• A database server is the Oracle software managing
  a database, and a client is an application that
  requests information from a server.
• Each computer in a system is a node.
• A node in a distributed database system act as a
  client, a server, or both, depending on the
  situation.

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Net8

• All Oracle databases in a distributed database
  system use Oracle's networking software, Net8, to
  facilitate inter-database communication across a
  network.
• Net8 allows database servers to communicate
  across networks to support remote and distributed
  transactions in a distributed database.

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Database Names

• Each database in a distributed database is
  distinct from all other databases in the system
  and has its own distinct global database name.
• Oracle forms a database's global database name by
  prefixing the database's network domain with the
  individual database's name.

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Database Links

• To facilitate application requests in a
  distributed database system, Oracle uses database
  links.
• A database link defines a one-way communication
  path from an Oracle database to another database.

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Distributed Processing

• Distributed processing occurs when an application
  system distributes its tasks among different
  computers in a network.
• Distributed database applications typically use
  distributed transactions to access both local and
  remote data and modify the global database in
  real-time.

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Database Replication

• Replication is the process of copying and
  maintaining database objects in multiple
  databases that make up a distributed database
  system.
• Most commonly, replication is useful to improve
  the performance and protect the availability of
  applications because alternate data access
  options exist.

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Heterogeneous Distributed Databases

• In a heterogeneous distributed database system at
  least one of the databases is a non-Oracle
  system.
• Oracle Open Gateways provide access to the
  non-Oracle system from an Oracle Server.
• The Oracle Server, together with the gateway,
  provides full heterogeneity transparency to the
  application.

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Distributed Database Applications

• A distributed query retrieves information from
  two or more nodes.
• A distributed transaction is a transaction that
  includes one or more statements that,
  individually or as a group, update data on two or
  more distinct nodes of a distributed database.

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Two-Phase Commit Mechanism

• Oracle's two-phase commit mechanism guarantees
  that all database servers participating in a
  distributed transaction either all commit or all
  roll back the statements in the transaction.
• The two phases are
• Prepare Phase
• Commit Phase

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Two-Phase Commit-Prepare Phase

• The first phase in committing a distributed
  transaction is the prepare phase in which the
  commit of the transaction is not actually carried
  out. Instead, all nodes referenced in a
  distributed transaction (except one, known as the
  commit point site, ) are told to prepare (to
  commit).
• By preparing, a node records enough information
  so that it can subsequently either commit or
  abort the transaction (in which case, a rollback
  will be performed), regardless of intervening
  failures.

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Two-Phase Commit-Prepare Phase

• When a node responds to its requestor that it has
  prepared, the prepared node has made a promise to
  be able to either commit or roll back the
  transaction later and not to make a unilateral
  decision on whether to commit or roll back the
  transaction.

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Two-Phase Commit-Prepare Phase

• When a node is told to prepare, it can respond
  with one of three responses
• Prepared Data on the node has been modified by a
  statement in the distributed transaction, and the
  node has successfully prepared.
• Read-onlyNo data on the node has been, or can
  be, modified (only queried), so no prepare is
  necessary.
• AbortThe node cannot successfully prepare.

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Two-Phase Commit-Prepare Phase

• To complete the prepare phase, each node performs
  the following actions
• The node requests its descendants (nodes
  subsequently referenced) to prepare.
• The node checks to see if the transaction changes
  data on that node or any of its descendants. If
  there is no change, the node skips the next steps
  and replies with a read-only message

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Two-Phase Commit-Prepare Phase

• The node allocates all resources it needs to
  commit the transaction if data is changed.
• The node flushes any entries corresponding to
  changes made by that transaction to its local
  redo log.
• The node guarantees that locks held for that
  transaction are able to survive a failure.
• The node responds to the node that referenced it
  in the distributed transaction with a prepared
  message or, if its prepare or the prepare of one
  of its descendents was unsuccessful, with an
  abort message

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Two-Phase Commit-Commit Phase

• The second phase in committing a distributed
  transaction is the commit phase.
• Before this phase occurs, all nodes referenced in
  the distributed transaction have guaranteed that
  they have the necessary resources to commit the
  transaction. That is, they are all prepared.

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Two-Phase Commit-Commit Phase

• The commit phase consists of the following steps
  
• The global coordinator send a message to all
  nodes telling them to commit the transaction.
• At each node, Oracle8 commits the local portion
  of the distributed transaction (releasing locks)
  and records an additional redo entry in the local
  redo log, indicating that the transaction has
  committed.
• When the commit phase is complete, the data on
  all nodes of the distributed system are
  consistent with one another.

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Conclusion

• Distributed Oracle Databases allow applications
  to transact across various systems and network
  topologies, and ensures that consistency is
  maintained amongst these databases using the
  two-phase commit principle when updating.

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Bibliography

• www.csee.umbc.edu/help/oracle8/server803/A54653_01
  /toc.htm
• technet.oracle.com/products/rdb7/htdocs/fs_dist.ht
  m