Transactions ensure that data changes are processed reliably in databases. They guarantee that all parts of an operation succeed (atomicity) or none succeed to keep the database consistent. Transaction logs record actions to allow restoring the database state if needed. Common transaction types are explicit, defined by code, and implicit, occurring with data changes. Transactions must meet ACID properties for reliability.

1. Transactions
CIS-182

2. Transactions
• Transaction is an action that occurs
– Student enrolls in class
– Often associated to movement
• When enroll, student ID is “moved” to requested enrollment
list
• In a database, data moving from one place to
another, and ensuring proper and complete
movement, is a transaction

3. Database Transactions
• Provides method for canceling an
operation
• Can restore rows, columns to original
state in event of error or business logic
failure
• Use when changes will either be
committed or discarded in entirety

4. Transaction Types
• Explicit: Define where the transaction begins,
ends
• Implicit: When data movement occurs without a
specific beginning, a transaction is implied
• Goal is to ensure that database is always correct
– Movement (storage) is successful; or
– Movement is cancelled
– No partial movement is allowed

5. ACID
• ACID describes conditions that a valid
transaction must meet
• Atomic: All steps in an operation are treated as
one piece of work
– All steps succeed or no steps succeed
• Consistent: Outcome is always predictable
– An operation will always work or never work

6. ACID – 2
• Isolated: If multiple transactions occur, data
is never available in an intermediate state
(partial change)
• Durable: Once finished, all changes are complete
and changes can only be done by another
transaction/unit of work

7. Transaction Log
• Tracks each action in the database
– Provides a record of what happened in sequential
order
• Used by SQL Server to ensure that ACID
requirements are met
• Also used in event of failure to recreate database
since last backup
– Backups create record of database contents at specific
points in time

8. CRUD
• Four operations affecting data
• Create (or INSERT) an entry
• Retrieve (or SELECT) an entry
• Update an entry
• Delete an entry

9. CIS-182 Transactions
• Using implied transactions
• When executing INSERT, UPDATE, DELETE, if
any part of the statement fails, the change to the
row is not saved to disk
– In INSERT, if one value doesn’t satisfy a CHECK
constraint, no values are added to the table
– When UPDATE, if referential integrity is violated the
change won’t be saved