Distributed Systems | Characteristics | Advantages | Disadvantages.

In distributed database system, the database is shared on several computers. The computers in a distributed system communicate with one another through various communication media, such as high-speed networks or telephone lines. They do not share main memory or disks.

The computers in distributed system may vary in size and function, ranging from workstations up to mainframe systems.

The computers in distributed system are referred to by a number of different names, such as Sites or Nodes depending on the context in which they are mentioned.

A distributed database system consists of single logical database which is split into different fragments. Each fragment is stored on one or more computers under the control of separate DBMS with computers connected by communication network. Each site is capable of independently processing user requests that require access to local data or file system and is also capable of performing processing on remote machines in the network.

Characteristics of Distributed System:

• Data set can be split in to fragments and can be distributed across different nodes within network.
• Individual data fragments can be replicated and allocated across different nodes.
• Data at each site is under control of a DBMS.
• DBMS at each site can handle local applications autonomously.
• Each DBMS site will participate in at least one global application.

Differences between shared nothing Parallel System and Distributed system are :

1. In distributed system, databases are geographically separated, they are administered separately and have slower interconnection.
2. In distributed systems, we differentiate between local and global transactions. Local transaction is one that accesses data in the single site at that the transaction was initiated. Global transaction is one which either accesses data in a site different from the one at which the transaction was initiated or accesses data in several different sites.

Advantages of Distributed System :

• Sharing Data : There is a provision in the environment where user at one site may be able to access the data residing at other sites.
• Autonomy : Because of sharing data by means of data distribution each site is able to retain a degree of control over data that are stored locally.
• In distributed system there is a global database administrator responsible for the entire system. A part of global data base administrator responsibilities is delegated to local data base administrator for each site. Depending upon the design of distributed database
• each local database administrator may have different degree of local autonomy.
• Availability : If one site fails in a distributed system, the remaining sites may be able to continue operating. Thus a failure of a site doesn't necessarily imply the shutdown of the System.

Disadvantages of Distributed Systems :

The added complexity required to ensure proper co-ordination among the sites, is the major disadvantage. This increased complexity takes various forms :

• Software Development Cost : It is more difficult to implement a distributed database system; thus it is more costly.
• Greater Potential for Bugs : Since the sites that constitute the distributed database system operate parallel, it is harder to ensure the correctness of algorithms, especially operation during failures of part of the system, and recovery from failures. The potential exists for extremely subtle bugs.
• increased Processing Overhead : The exchange of information and additional computation required to achieve intersite co-ordination are a form of overhead that does not arise in centralized system.