Distributed Processing

Snapshots are read-only copies of a master table located on a remote node which is periodically refreshed to reflect changes made to the master table.

Prepare phase - The global coordinator (initiating node) ask a participants to prepare (to promise to commit or rollback the transaction, even if there is a failure) Commit - Phase - If all participants respond to the coordinator that they are prepared, the coordinator asks all nodes to commit the transaction, if all participants cannot prepare, the coordinator asks all nodes to roll back the transaction.

SQL *NET is ORACLEs mechanism for interfacing with the communication protocols used by the networks that facilitate distributed processing and distributed databases. It is used in Clint-Server and Server-Server communications.

► A simple snapshot is based on a query that does not contains GROUP BY clauses, CONNECT BY clauses, JOINs, sub-query or snashot of operations.

► A complex snapshots contain atleast any one of the above.

Time the references to occur when master tables are not in use.

Peform the reference the manually immdiately locking the master tables. We can join tables in snopshots by creating a complex snapshots that will based on the master tables.

It is a table that maintains a record of modifications to the master table in a snapshot. It is stored in the same database as master table and is only available for simple snapshots. It should be created before creating snapshots.

Two-phase commit is mechanism that guarantees a distributed transaction either commits on all involved nodes or rolls back on all involved nodes to maintain data consistency across the global distributed database. It has two phase, a Prepare Phase and a Commit Phase.

► Replication of data in distributed environment.

► Using snapshots to replicate data.

► Using remote procedure calls.

Simple and Complex Snapshots

COMPLETE - Tables are completely regenerated using the snapshots query and the master tables every time the snapshot referenced.

FAST - If simple snapshot used then a snapshot log can be used to send the changes to the snapshot tables.

FORCE - Default value. If possible it performs a FAST refresh; Otherwise it will perform a complete refresh.

A distributed database is a network of databases managed by multiple database servers that appears to a user as single logical database. The data of all databases in the distributed database can be simultaneously accessed and modified.

We must be able to create a after row trigger on table (i.e., it should be not be already available) After giving table privileges.

We cannot specify snapshot log name because oracle uses the name of the master table in the name of the database objects that support its snapshot log. The master table name should be less than or equal to 23 characters. (The table name created will be MLOGS_tablename, and trigger name will be TLOGS name).