Advantages And Disadvantages Of Distributed Database Systems

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This paper discussed the Distributed database systems which are systems that have their data distributed and replicated over several locations; unlike the centralized data base system, where one copy of the data is stored in one location. the Distributed database system has many advantages:
1. Availability of the data in case of failure.
2. Parallelism: queries may be processed by several nodes in parallel.
3. Reduced data transfer.
It also has some disadvantages:
1. A distributed system usually exhibits more complexity and cost more than a centralized one.
2. Handling security across several locations is more complicated.
Types of DDBMS:
1- Homogeneous DDBMS
a. All sites use same DBMS product (eg. Oracle)
b. Fairly easy to design and manage.
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Communication Failures: failures in the communication system between two or more sites.

The 2-phase commit (2PC) protocol:
The major issue in building a distributed database system is the transactions atomicity. When a transaction runs across into two sites, it may happen that one site may commit and other one may fail due to an inconsistent state of transaction. Two-phase commit protocol is widely used to solve this.
The 2-phase commit (2PC) protocol is a distributed algorithm to ensure the consistent termination of a transaction in a distributed environment.
This protocol has two phases, the prepare and the commit phase. The protocol is executed by a coordinator process, while the participating servers are called participants. When the transaction’s initiator issues a request to commit the transaction, the coordinator starts the first phase.
The coordinator initiates the first phase of the protocol by sending PREPARE messages in parallel to all the participants. Each participant that is ready to commit first force writes a prepare log record to its local stable storage and then sends a YES vote to the coordinator. On the other hand, each participant that decides to abort force-writes an abort log record and sends a NO vote to the
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