ABSTRACT Today's business environment has an increasing need for distributed database and client/server applications as the desire for reliable, scalable and accessible information is steadily rising. Distributed database systems provide an improvement on communication and data processing due to its data distribution throughout different network sites. Not only is data access faster, but a single-point of failure is less likely to occur, and it provides local control of data for users. However, there is some complexity when attempting to manage and control distributed database systems. The DDBMS synchronizes all the data periodically, and in cases where multiple users must access the same data, ensures that updates and deletes performed on the data at one location will be automatically reflected in the data stored elsewhere. A distributed database can also be defined as a collection of multiple, logically interrelated databases distributed over a computer network. A distributed database management system is then defined as the software system that permits the management of the distributed databases and makes this distribution transparent to the users. Distributed database system is to referred as a combination of the distributed databases and the distributed DBMS Current trends in multi-tier client/server networks make DDBS an appropriated solution to provide access to and control over localized databases. Oracle, as a leading Database Management System (DBMS) vendor employs the two-phase commit technique to maintain consistent state for the database. INTRODUCTION DISTRIBUTED DATABASE SYSTEM was first used in mainframe environments in the 1950s and 1960s. But they have flourished best since the development, in the 1980s and 1990s, of minicomputers and powerful desktop and workstation computers, along with fast, capacious telecommunications, has made it (relatively) easy and cheap to distribute computing facilities widely.
Distributed Databases
Dept. of CSE SNGCE, Kolenchery
Users have access to the portion of the database at their location so that they can access the data relevant to their tasks without interfering with the work of others. A centralized distributed database management system (DDBMS) manages the database as if it were all stored on the same computer. The DDBMS synchronizes all the data periodically and, in cases where multiple users must access the same data, ensures that updates and deletes performed on the data at one location will be automatically reflected in the data stored elsewhere. Distributed database is a database that is under the control of, a central database management system in which storage devices are not all attached to a common CPU. Distributed database technology is one of the most important developments of the past decades. The maturation of data base management systems - DBMS - technology has coincided with significant developments in distributed computing and parallel processing technologies and the result is the emergence of distributed DBMSs and parallel DBMSs. These systems have started to become the dominant data management tools for highly intensive applications. The basic motivations for distributing databases are improved performance, increased availability, share ability, expandability, and access flexibility. Although, there have been many research studies in these areas, some commercial systems can provide the whole functionality for distributed transaction processing. Important issues concerned in studies are database placement in the distributed environment, distributed query processing, distributed concurrency control algorithms, reliability and availability protocols and replication strategies. For general purposes a database is a collection of data that is stored and maintained at one central location. A database is controlled by a database management system. The user interacts with the database management system in order to utilize the database and transform data into information. Furthermore, a database offers many advantages compared to a simple file system with regard to speed, accuracy, and accessibility such as: shared access, minimal redundancy, data consistency, data integrity, and controlled access. All of these aspects are enforced by a database management system. Among these things let's review some of the many different types of databases. What is a DISTRIBUTED DATABASE?
Distributed Databases
Dept. of CSE SNGCE, Kolenchery
A database that consists of two or more data files located at different sites on a computer network. Because the database is distributed, different users can access it without interfering with one another. However, the DBMS must periodically synchronize the scattered databases to make sure that they all have consistent data. A distributed database is a database that is under the control of a central database management system in which storage devices are not all attached to a common CPU. It may be stored in multiple computers located in the same physical location, or may be dispersed over a network of interconnected computers. Collections of data can be distributed across multiple physical locations. A distributed database is distributed into separate partitions/fragments. Besides distributed database replication and fragmentation, there are many other distributed database design technologies. For example, local autonomy, synchronous and asynchronous distributed database technologies. These technologies implementation can' and does definitely depend on the needs of the business and the sensitivity/confidentiality of the data to be stored in the database. And hence the price the business is willing to spend on ensuring data security, consistency and integrity.
A database that consists of two or more data files located at different sites on a computer network. Because the database is distributed, different users can access it
Distributed Databases without interfering with one another. However, the DBMS must periodically synchronize the scattered databases to make sure that they all have consistent data.
Databases have firmly moved from the realm of research and experimentation into the commercial world. In this area we will address distributed databases related issues including transaction management, concurrency, recovery, fault-tolerance, security, and mobility. Theory and practice of databases will play a prominent role in these pages.
A distributed database system consists of a collection of sites, connected together via some communication network, in which
a. Each site is a full database system site in its own right.
b. The sites have agreed to work together so that a user at any site can access data anywhere in the network in a transparent manner.
A distributed database is a database that is under the control of a central database management system in which storage devices are not all attached to a common CPU. It may be stored in multiple computers located in the same physical location, or may be dispersed over a network of interconnected computers.
Besides distributed database replication and fragmentation, there are many other distributed database design technologies. For example, local autonomy, synchronous and asynchronous distributed database technologies. These technologies' implementation can and does definitely depend on the needs of the business and the sensitivity/confidentiality of the data to be stored in the database. And hence the price the business is willing to spend on ensuring data security, consistency and integrity
Distributed database system (DDBS) = Databases + Computers + Computer Network + Distributed database management system (DDBMS)
A distributed database system can be simply defined as a collection of multiple logically interrelated databases distributed over a computer network and managed by a distributed database management system.
Distributed Databases
The first generation of data processing is decentralized and unintegrated where data are stored in individual files and specifications are embedded into the programs that manipulate the data. Files are therefore not shared, and any changes in the file structure will affect the data specifications in the programs. The second generation of data processing is centralized and integrated in which data are stored in a centralized database and data specification are stored in a centralized location, normally the same location as the database. The advantages of this model are that changes in database may only affect data specifications but not the programs. The third generation of data processing is distributed and integrated in which data and their local specifications are distributed in a network and there also exists a global view of all the data stored in the network.
What is a Distributed Database Management System?
A DDBMS (distributed database management system) is a centralized application that manages a distributed database as if it were all stored on the same computer. The DDBMS synchronizes all the data periodically in some cases multiple users must access the same data, ensures that updates and deletes performed on the data at one location will be automatically reflected in the data stored elsewhere. A distributed database can also be defined as a collection of multiple, logically interrelated databases distributed over a computer network. A distributed database management system (distributed DBMS) is then defined as the software system that
Distributed Databases permits the management of the distributed databases and makes this distribution transparent to the users. Distributed database system is too referred as a combination of the distributed databases and the distributed DBMS.
The implicit assumptions in a distributed database system are; .
• data is physically stored across several sites
• Each site is typically managed by DBMS that is capable of running independently of the other sites.
Distributed Databases
TYPES OF DISTRIBUTED DATABASE
Homogeneous: Every site runs same type of DBMS. Heterogeneous: Different sites run different DBMSs
In a Homogeneous distributed database
1. All sites use identical software.
2. All sites are aware of each other and agree to cooperate in processing user requests.
3. Each site in the network surrenders part of its autonomy in terms of right to change schemas or software.
4. It appears to user as a single system.
In a Heterogeneous distributed database
1. Different sites may use different schemas and software
Difference in schema is a major problem for query processing • Difference in software is a major problem for transaction processing
2. Sites may not be aware of each other and may provide only limited facilities for cooperation in transaction processing
Why DISTRIBUTED DATABASE?
1. Capacity and incremental growth
The new nodes can be added to the computer network easily without undergoing much complexity
2. Reliability and availability
Using the replicated data at several nodes, the failure of a node still allows access to the replicated copy of the data from another node. It avoids the transportation of data from one site to another
Distributed Databases
3. Reduced communication overhead
4. Data is stored close to the anticipated point of use.
5. Efficiency and Flexibility
6. Data can be dynamically moved or replicated to where it is most needed.
a. It becomes handy when a system crash or system in situation. The efficiency of the system increases due to the availability of data and the speed at which data becomes available for the required client
7. In early days data were stored in file systems, but the file systems have a large number of disadvantages. Databases are used to solve the limitations of file systems and also for easy storing and retrieval of data.
FAILURES IN DISTRIBUTED DBS
Several types of failures may occur in distributed database systems:
Transaction Failures: When a transaction fails, it aborts. Thereby, the database must be restored to the state it was in before the transaction started. Transactions may fail for several reasons. Some failures may be due to deadlock situations or concurrency control algorithms.
Site Failures: Site failures are usually due to software or hardware failures. These failures result in the loss of the main memory contents. In distributed database, site failures are of two types:
1. Total Failure where all the sites of a distributed system fail.
2. Partial Failure where only some of the sites of a distributed system fail.
Media Failures: Such failures refer to the failure of secondary storage devices. In these cases, the media failures result in the inaccessibility of part or the entire database stored on such secondary storage.
Communication Failures: Communication failures, as the name implies, are failures in the communication system between two or more sites. This will lead to network partitioning where each site, or several sites grouped together, operates independently. As such, messages
Distributed Databases from one site won't reach the other sites and will therefore be lost. The reliability protocols then utilize a timeout mechanism in order to detect undelivered messages. A message is undelivered if the sender doesn't receive an acknowledgment. The failure of a communication network to deliver messages is known as performance Failure.
RULES OF DISTRIBUTED DATABASES by C.J.Date
In 1987 one of the founders of relational database theory, C. J. Date, stated 12 goals which, he held, designers should strive to achieve in their DDBs and with the associated DBMSs:
1. Local site independence
2. Central site independence
3. Failure independence
4. Location transparency
5. Fragmentation transparency
6. Replication transparency
7. Distributed query processing
8. Distributed transaction processing
9. Hardware independence
10. Operating system independence
11. Network independence
12. Database independence
Distributed Databases
Local site independence: Each site in the DDB should act independently with respect to vital DBM functions.
• Security e Concurrency Control
• Backup
• Recovery
Central site independence: Each site in the DDB should act independently with respect to
• The central site
• All other remote sites
Note: All sites should have the same capabilities, even though some sites may not necessarily exercise all these capabilities at a given point in time.
Failure independence: The DDBMS should be unaffected by the failure of a node or nodes; the rest of the nodes, and the DDBMS as a whole, should continue to work. Note: In similar fashion, the DDBMS should continue to work if new nodes are added.
Location transparency: Users should not have to know the location of a datum in order to retrieve it.
Fragmentation transparency: The user should be unaffected by, and not even notice, any fragmentation of the DDB. The user can retrieve data without regard to the fragmentation of the DDB.
Replication transparency: The user should be able to use the DDB without being concerned in any way with the replication of the data in the DDB.
Distributed query processing: A query should be capable of being executed at any node in the DDBMS that contains data relevant to the query. Many nodes may participate in the response to the user's query without the user's being aware of such participation.
Distributed Databases
Distributed transaction processing: A transaction may access and modify data at several different sites in the DDB without the user's being aware that multiple sites are participating in the transaction.
Hardware independence: The DDB and its associated DDBMS should be capable of being implemented on any suitable platform, i.e., on any computer with appropriate hardware resources regardless of what company manufactured the computer. Note: Current DBMSs often fail to achieve this goal.
Operating system independence: The DDB and its associated DDBMS should be capable of being implemented on any suitable operating system, i.e., on any operating system capable of handling multiple users.
Note: At present this means Windows NT and 2000, and the various varieties of UNIX including Linux.
Network independence: The DDB and its associated DDBMS should be capable of being implemented on any suitable network platform.
Note: At present, this goal means that the DDBMS should be able to run on Windows NT, on Windows 2000, on any variant of UNIX, and on Novell Networks.
Database independence: The design of the DDB should render it capable of being supported by suitable, i.e., of sufficient power and sophistication, DDBMS from any vendor.
FUNCTIONS OF DDBMS
Distribution of databases across a network leads to increased complexity in the system implementation. To achieve the benefits of a distributed database that we have seen, earlier distributed database management software should be able to perform the following functions in addition to the basic functions performed by a no distributed DBMS: o Distributed query processing - Distributed query-processing means the ability to access remote sites and transmit queries and data among the various sites along the communication network.
Distributed Databases o Data tracking - The distributed DBMS should have the ability to keep track of the data distribution. Fragmentation and replication by expanding the distributed DBMS catalog.. o Distributed transaction management - Distributed transaction management is the ability to devise execution strategies for queries and transactions that access data from more than one site and to synchronize the access to distributed data and maintain the integrity of the overall database, o Replicated data management - This is the ability of the system to decide which copy of the replicated data item to access and to maintain consistency of . copies of a replicated data item. o Distributed data recovery - The distributed DBMS should have the ability to recover from individual site crashes and failures of communication links. o Security - Distributed transactions must be executed with the proper management of the security of the data and the authorization and access privileges of users.
COMPONENTS OF A DISTRIBUTED DATABASE SYSTEM
In this section we will examine the components of a distributed database system. One of the main components in a DDBMS is the Database Manager. "A Database Manager is software responsible for processing a segment of the distributed database. Another main component is the User Request Interface, which is usually a client program that acts as an interface to the Distributed Transaction Manager. A Distributed Transaction Manager is a program that translates requests from the user and converts them into actionable requests for the database manager, which are typically distributed. A Distributed database system is made of both the distributed transaction manager and the database manager.
CLIENT-SERVER DATABASE ARCHITECTURES
1) Client-Server Architecture is an arrangement of components (clients and servers) among computers connected by a network.
2) Client-server architecture supports efficient processing of messages (requests for service) between clients and servers.
1) Two-Tier Architecture
Distributed Databases
Courtesy.www. blueportal. org Author: Michael Mannino,Publication:Tata Mc Graw Hill,2004
To improve performance, the three-tier architecture adds another server layer either by a middleware server or an application server.
- The additional server software can reside on a separate computer.
2) Three-Tier Architecture (Middle Ware Architecture)
Distributed Databases
- Alternatively, the additional server software can be distributed between the database server and PC clients.
3) Multiple-Tier Architecture Client-server architecture with more than three layers: a PC client, a backend database server, an intervening middleware server, and application servers. Provides more flexibility on division of processing
The application servers perform business logic and manage specialized kinds of data such as images.
ADVANTAGES OF CLIENT-SERVER ARCHITECTURES
1. More efficient division of labour
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Distributed Databases
2. Horizontal and vertical scaling of resources
3. Better price/performance on client machines
4. Ability to use familiar tools on client machines
5. Client access to remote data (via standards)
6. Full DBMS functionality provided to client workstations
7. Overall better system price/performance
FUNDAMENTALS OF TRANSACTION MANAGEMENT
Transaction Management deals with the problems of keeping the database in a consistent state even when concurrent accesses and failures occur.
What is a Transaction?
A transaction consists of a series of operations performed on a database. The important issue in transaction management is that if a database was in a consistent state prior to the initiation of a transaction, then the database should return to a consistent state after the transaction is completed. This should be done irrespective of the fact that transactions were successfully executed simultaneously or there were failures during the execution. Thus, a transaction is a unit of consistency and reliability. The properties of transactions will be discussed later in the properties section.
Each transaction has to terminate. The outcome of the termination depends on the success or failure of the transaction. When a transaction starts executing, it may terminate with one of two possibilities:
1. The transaction aborts if a failure occurred during its execution
2. The transaction commits if it was completed successfully.
Distributed Databases
Figure la shows an example of a transaction that aborts during process 2 (P2). On the other hand, Figure lb shows an example of a transaction that commits, since all of its processes are successfully completed.
Commit
Committed transaction Aborted and Committed transaction
Properties of Transactions
A Transaction has four properties that lead to the consistency and reliability of a distributed database.
These are Atomicity, Consistency, Isolation, and Durability. Atomicity: This refers to the fact that a transaction is treated as a unit of operation. Consequently, it dictates that either all the actions related to a transaction are completed or none of them is carried out. For example, in the case of a crash, the system should complete the remainder of the transaction, or it will undo all the actions pertaining to this transaction. The recovery of the transaction is split into two types corresponding to the two types of failures:
Aborted transaction
Distributed Databases the transaction recovery, which is due to the system terminating one of the transactions because of deadlock handling; and the crash recovery, which is done after a system crash or a hardware failure.
Consistency: Referring to its correctness, this property deals with maintaining consistent data in a database system. Consistency falls under the subject of concurrency control. For example, "dirty data" is data that has been modified by a transaction that has not yet committed. Thus, the job of concurrency control is to be able to disallow transactions from reading or updating "dirty data."
Isolation: According to this property, each transaction should see a consistent database at t all times. Consequently, no other transaction can read or modify data that is being modified by another transaction.
If this property is not maintained, one of two things could happen to the data base, as shown in Figure 2:
a. Lost Updates: this occurs when another transaction (T2) updates the same data being modified by the first transaction (Tl) in such a manner that T2 reads the value prior to the writing of Tl thus creating the problem of loosing this update.
b. Cascading Aborts: this problem occurs when the first transaction (Tl) aborts, then the transactions that had read or modified data that has been used by Tl will also abort. time IT
T2
1 iine 1
Read x
Time 1
Read \
Time i
Write \
\ " x- 2