Transaction Processing
Lecture 2005 09 22
ACID
2 phase commit
What is a transaction?
• It is a program, executing within a database
application on data in the DB.
• The DB instance reflects the current state of
the world. A transaction is used to:
– Update the state of the DB to reflect the world’s
new state.
– Effect ( trigger ) a real-world event, depending on
new state or a state transition in the DB.
– Return DB’s current state information.
• Unlike a program, a transaction must
maintain a consistent DB instance.
– Even if system crashes or there are
multiple interleaved accesses to the same
data element.
– ACID properties help to maintain data
consistency.
•
•
•
•
Atomic
Consistent
Isolated
Durable
ACID
– Consistent, essentially, means that all the
data reflect the current real world state.
– Further, no action can be taken on the DB
that creates an instance that violates
business rules.
ACID
• Integrity constraints: Examples
– Domain constraints -- set of values allowed for an
attribute (e.g., can’t be NULL, or must be INT).
– Referential integrity -- a value for a foreign key
must appear in the related primary key field (in a
different table).
– Business (or enterprise) rules -- e.g., no one can
receive a higher salary than the CEO.
ACID
• Transaction integrity (Lewis, et al)
The transaction designer can assume the DB
satisfies all integrity constraints before the
transaction begins. The transaction, when
completed, MUST leave the DB in a state
where
- (1) all integrity constraints are satisfied, and
- (2) the new state reflects the transformation
as specified.
ACID
Ways to check integrity:
- Some via SQL’s DDL (PRIMARY KEY, NOT
NULL)
- CHECK constraint (very time consuming
because lots of unnecessary checking)
- ASSERTION ( ditto )
- error checking within the transaction’s code.
- increased vulnerability to programmer
error
- difficult to maintain as system spec
changes
ACID
A transaction is a unit of work. That is, a
transaction does ALL the work required
to ensure the DB is left in a consistent
state.
ACID
• Atomicity (Lewis, et al) -- the system
must ensure that either
– the transaction completes, or
– (if it doesn’t complete) it has no effect at all.
• Notice the <huge> difference from a
task.
ACID
• Committed -- A transaction completes and the
system guarantees to maintain its effect.
• Aborted -- A transaction does not complete,
the system must undo any changes.
• Rolled back -- the operations the system
takes to undo the effects of an aborted
transaction.
Why might a transaction
abort?
• System crash during execution.
• Transaction attempts a change that violates
integrity.
• Transaction effects violate the ISOLATION
requirement of ACID.
• Transaction involved in deadlock
• Transaction code has abort statement (error
checking or debugging)
• Transaction responds to user initiated abort.
ACID
• Durability (Lewis, et al) -- Once a
transaction commits, its effects remain
in the DB, even if the computer or
storage subsequently fails.
ACID
Causes of failure
- CPU crash
- Disk failure
- Multiple disk failures
- External accidents (fire, EMP)
- Malice
ACID
• Isolation is the property that each
transaction’s effect on the data is
correct, even when other executing
transactions may be accessing the
same data.
• Note the related meaning to interleaving
tasks (critical section, re-entrancy,
independence, mutexes, …)
ACID
• Serial execution -- one transaction
commits before another transaction
begins.
• Concurrent execution -- more than one
transaction is active simultaneously
(many active, partially completed (all
uncommitted) transactions)
ACID
• A typical transaction will read a data element
(“global”) into local variable, then do some
processing on the local, then write a data
element.
• The important thing to look at is the
operations (the reads, writes and updates) on
the data base.
• The operations of a serial transaction
schedule will leave the DB in a consistent and
correct state. Interleaving those same
transactions may not.
ACID
• Isolation (Lewis, et al) -- when multiple
transactions are executed concurrently,
the final effect is as though the
transactions had executed serially.
• Concurrent schedules that satisfy the
isolation property are serializable.
Transaction Transistion State
Diagram
Two phase commit protocol
2PC
• There is a coordinator ( C ) that knows
the set of current transactions (by
keeping track of current transaction
control blocks) -- I.e., it is a bookkeeper.
• When a transaction requests commit,
the 2PC begins (next slide).
• Any transaction can unilaterally abort.
All must agree to commit.
2PC -- 2 rounds of messages: vote,
then termination.
• C initiates vote phase; Sends prepare message to
each active transaction.
• When an active receives a prepare, it decides
whether to abort or commit. It sends no (abort) or yes
(commit) message to C.
• C initiates termination phase; If C receives all yes
messages, it sends commit message to all actives. If
it receives one or more no (or no response), it sends
abort to all actives.
• When transaction receives abort, it aborts. When it
receives a commit, it commits.