DISTRIBUTED DATABASE SYSTEMS - Database Design and Development: An Essential Guide for IT Professionals - page 575

B

D

E

C

D

A E

E

D

UPDATE EMPLOYEE
SET Salary = Salary * 1.15
WHERE EMPLOYEE.DeptNo = DEPARTMENT.DeptNo
AND DeptName = 'InfoTech';

LEGEND

D - Order File E - Employee File

Vertical fragment
of File

Horizontal fragment
of File A

Full File

Figure 18-17 Transaction processing example.

• Completes database transactions of reads, writes, and so on.
• When ready to commit, sends “prepare to commit” message to all cooperating
DBMSs where the database objects reside.
• Also, sends “prepare to commit” message to all other sites where replicated
copies of the updated database objects are held.
• All sites where database updates are being made and those sites with replicated
copies send back response messages to transaction initiating site (each response
is either “ready to commit” or “not prepared to commit, aborted”).
• The execution protocol may allow a transaction to commit only if all partici-
pating sites can commit or may permit a transaction to commit if a suffi-
cient number of sites can commit. The coordinating transaction examines the
responses and determines the course of action.
• If the coordinating transaction decides to commit, it issues a “commit” message
to participating sites.
• Coordinator waits for “commit” responses from participating sites. When all
responses are received, coordinator completes processing of the transaction.
• Sends “commit complete” message to participating sites.

If the coordinating transaction decides to abort, it issues messages to abort to all
participating sites. The sequence to abort and roll back at every participating site
and to complete the abort process is similar to the sequence following the decision
to commit.