Information Technology Reference
In-Depth Information
Fig. 14.2
Processes in a page
server
We assume that the following processes appear in a page-server system:
1. The server s runs one or more multi-threaded
database-server processes
S that
process the requests coming from the client machines.
2. Each client c
i
runs one or more multi-threaded
client database processes
C
i
that
process the requests coming from the application processes running at c
i
.
3. Each client c
i
runs one or more
client application processes
A
ij
, each of which
executes a database application program and sends embedded database requests
to the local client database process using the query-shipping (transaction-
shipping) paradigm.
From the point of view of an application process A
ij
, the page-server system is a
local service that functions using the query-shipping paradigm (Fig.
14.2
).
As said previously, every transaction runs from beginning to end at a single client.
For simplicity, we might assume that the server runs no transactions. If there is need
to run transactions at the server, we can always run an instance of the client database
system at the server machine, besides the instance of the database server system.
The client database process C
i
parses and optimizes an
SQL
query or update
statement received from its client application processes A
ij
and executes the
corresponding database-action sequence on the pages buffered at client c
i
in the
name of the application's transaction. From the clients, the log records and the
updated pages are eventually shipped to the server.
14.3
Client Caching and Page-Server State
To keep track of pages cached at the clients, the server maintains in its main memory
a
cached-page table
, that contains, for each page p cached at some client c, the page-
id of page p, the identifier of c, and the privilege under which p is cached at c.A
page is cached under a
caching privilege
that indicates the modes of access allowed
on the cached page. A
read privilege
(or
read token
) on a page entitles the client to
