Data Replication – Mobile Computing

Data Replication – Mobile Computing

Data Replication in Mobile Computing

Data replication is the process of making copies of data stored on various sites in order to improve –

• reliability,
• efficiency,
• robustness,
• simpler transaction,
• fault tolerance and
• reduce network load

Goals of data replication

Data replication is performed with the purpose to

• Increase the availability of data.
• Speed up the query evaluation.

Types of data replication

 

 

 

 

 

 

There are two types of data replication

1. Synchronous Replication:
   In synchronous replication, the replica of the database is modified immediately after changes are made in the relation table.

So there is no difference between original data and replicated data table.

2. Asynchronous replication:
In asynchronous replication, the replica will be modified after commit action is fired on to the database.

 

Replication Schemes

The three replication schemes are as follows:

1. Full Replication scheme

In full replication scheme, the database is available at all the locations to ease the user in communication network

 

 

 

 

 

 

 

 

Advantages of full replication

• It gives high availability of data. In this scheme database is available to at each location.
• It support faster execution of queries.

Disadvantages of full replication

• In full replication scheme concurrency control is difficult to achieve in full replication.
• During updating each and every site need to be updated therefore update operation is slower.

2. No Replication

No replication means, each fragment is stored exactly at one location only.

Advantages of no replication

• Concurrency can be easily minimized.
• Easy recovery of data becomes easy.

Disadvantages of no replication

• Poor availability of data.
• Slows down the query execution process, because multiple clients are accessing the same data at the same server.

 

 

3. Partial replication

Partial replication scheme means only part of the or data fragments are replicated.

File System

The general goal of a file system is to support efficient, transparent, and consistent access to files, no matter where the client requesting files or the server(s) offering files are located.

 

In Distributed File System clients or users access files and folders that are provided by the file servers.

The client gives the request and the server provides a service to the client through a file service interface. Service fulfills the request of the client by providing the requested file view, part of the file view or folders.

The important challenges of distributed systems apply to DFS are as below:

Transparency of Dfs

Dfs must be completely transparent to ensure the following to its users

• Location

A Dfs user or client cannot informed be about the exact location of file the client is accessing

 

 

 

• Migration

In DFS a file can be transparently moved from one server to another

• Replication:

In Dfs multiple copies of the same file may present

• Concurrency:

At the same time, multiple clients can access the same file in DFS.

• Consistency:

Due to replicated copies of a file and permitted concurrent access to files inconsistency may occur in the data

• Security:

In DFS each and every client must authenticate themselves before using a file and servers must decide whether the clients are authorized to do the requested operation.

Also, communication between clients and the file server must be done with complete security.

 

 

• Fault tolerance:

In DFS all the clients should be able to work continuously if a file server crashes. Similarly, data must not be lost. Also, a restarted file server must be recovered to a valid state.