Transactions in Page-Server Systems - Transaction Processing - page 331

Assume that a client c performs the call fix-and-read-latch .p/ (Algorithm 14.1 )
for a page p that currently is not cached at c.Thenc sends to the server the request
get-page .p; m/,wherep is the page-id of the requested page and m D “read” is
the requested caching privilege. The request is processed at the server by the call
service-get-page .p;m;c/ (Algorithm 14.5 ). The server fixes and write-latches the
server version of page p and examines the cached-page table to see if the page is
cached for writing at some other client c 0 . If so, the server calls the page back from
c 0 . Then the server ships a copy of the now current server version of p to client c,
records p as cached at c for reading, and unlatches and unfixes p. Client c installs
the page in its cache and continues with the fix-and-read-latch .p/ call.
Algorithm 14.5 Procedure service-get-page .p;m;c/
fix-and-write-latch .p/
if page p is cached for writing at a client c 0 6D c then
callback-page .p;m;c 0 /
else {page p is not cached for writing}
if m D “write” then
for all clients c 0 6D c that cache p do
callback-page .p;m;c 0 /
end for
end if
end if
if page p is cached at c and m D “write” then
send write privilege on p to client c
else
send a copy of p with m privilege to client c
end if
record p as cached with m privilege at client c
unlatch-and-unfix .p/
Algorithm 14.6 Procedure service-get-write-privilege .p; c/
fix-and-write-latch .p/
for all clients c 0 6D c that cache p do
callback-page .p;m;c 0 /
end for
send write privilege on p to client c
record p as cached for writing at client c
unlatch-and-unfix .p/
Assume then that a client c performs the call fix-and-write-latch .p/ (Algo-
rithm 14.2 )forapagep that currently is not cached at c.Thenc sends to the server
the request get-page .p; m/,wherem D “write.” When processing the request at the
server by the call service-get-page .p;m;c/ (Algorithm 14.5 ), if page p is cached
for updating at some other client c 0 , the page is called back, and its current version