Application: Intermittently Synchronized Databases

• In some of the applications, a server or a group of servers manages the central database and the clients carry laptops or palmtops with a resident DBMS software to do “local” transaction activity for most of the time.
• The clients connect via a network or a dial-up connection (or possibly even through the Internet) with the server, typically for a short session—say, 30 to 60 minutes.
• They send their updates to the server, and the server must in turn enter them in its central database, which must maintain up-to-date data and prepare appropriate copies for all clients on the system.
• Thus, whenever clients connect—through a process known in the industry as synchronization of a client with a server—they receive a batch of updates to be installed on their local database.
• The primary characteristic of this scenario is that the clients are mostly disconnected; the server is not necessarily able to reach the client. This environment has problems similar to those in distributed and client-server databases, and some from mobile databases, but presents several additional research problems for investigation.
• We refer to this environment as Intermittently Synchronized Database Environment (ISDBE), and the corresponding databases as Intermittently Synchronized Databases (ISDBs).

  

Characteristics of ISDB:

1. A client connects to server when it wants to receive updates from a server or to send its updates to a server or to process transactions that need nonlocal data. This communication may be unicast—one-on-one communication between the server and the client—or multicast—one sender or server may periodically communicate to a set of receivers or update a group of clients.

2. A server cannot connect to a client at will.

3. Issues of wireless versus wired client connections and power conservation are generally immaterial.

4. A client is free to manage its own data and transactions while it is disconnected. It can also perform its own recovery to some extent.

5. A client has multiple ways of connecting to a server and, in case of many servers, may choose a particular server to connect based on proximity, communication nodes available, resources available, etc.

                           Because of such differences, there is a need to address a number of problems related to ISDBs that are different from those typically involving mobile database systems. These include server database design for server databases, consistency and synchronization management among client and server databases transaction and update processing, efficient use of the server bandwidth, and achieving scalability in the ISDB environments.