Information Technology Reference
In-Depth Information
client
client
database
server
·
data-
base
·
·
·
·
client
Fig. 1.1
Typical two-tier transaction server
execute database actions (queries and updates) on the database stored at the server.
The server processes the requests coming from the clients and returns the results
to the clients. The requests are specified by
SQL
queries and update statements
embedded in an application program or by a special database programming interface
consisting of a collection of procedures and functions for communicating between
an application program and the database server. Java Database Connectivity (
JDBC
)
and Open Database Connectivity (
ODBC
) are examples of such interfaces.
The mode of operation of a transaction server is called
transaction shipping
or
query shipping
(or
function shipping
): a transaction (or a query or a function)
is “shipped” from a client to the server, to be executed on the data stored at the
server. Figure
1.1
shows a typical two-tier organization of many client machines
connected to a database machine. The two “tiers” are the client tier and the database
tier. For scalability, a large system with thousands of clients may be organized with
additional tiers between the client and database tiers, such that a machine at a lower
tier services only requests from a subset of machines at the next higher tier.
There are one or more
server processes
running at the server. Such a process
is usually
multi-threaded
, that is, has several subprocesses or
threads
running
concurrently in the same virtual address space. A typical server process services
a number of clients, so that for each client application process there is a dedicated
server-process thread.
A connection between a client application process and a server-process thread
is created by the embedded
SQL
statement
connect to
s,wheres is an identifier
(network address) of the server. The statement creates a new server-process thread
to service the application process or assigns an existing idle thread for the purpose.
1.2
Logical Database
A database application programmer sees the database as a
logical database
,
a collection of data items defined and manipulated by the data definition and
manipulation language offered by the database management system. In most cases
