RELIABILITY
Reading
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Textbook: Ch. 12.4
CSCE 824 - Spring 2011
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Design Issues
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Placing of data and programs
(DBMS and application)
Network issues
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Data Distribution
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Fully redundant
Partially redundant
Partitioned: each data item is
stored at exactly one site
How can we guaranty atomicity of transactions?
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CSCE 824 - Spring 2011
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Distributed Processing
Site A
Site B
T2
Site C
T3
T1
Distributed DBMS
T:
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T4
Site D
must be atomic
CSCE 824 - Spring 2011
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Distributed Reliability
Protocols
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Ensure that distributed transactions
are atomic
Protocol evaluation:
– Max. number of messages
– Max. number of log forces
Checkpoint: for recovery, indicates the
latest consistent point (undo has to go
back to this point, recovery can start
from this point)
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Reliability Protocols
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2-Phase Commit
Presumed Commit
Early-Prepare Commit
Presumed Abort
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Two-Phase Commit 1.
Coordinator
P
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P
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P
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Participant
User initiates transaction T at the site that
acts as a coordinator C. C sends the
appropriate work request to the
participants. (N number of messages)
Executes the
work request
and sends reply
to C. (N number
of messages)
User issues a commit command. C sends a
prepare-to-commit message to each
participant. (N number of messages)
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CSCE 824 - Spring 2011
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Two-Phase Commit 2.
Coordinator
Window of
Uncertainty (If the
participant voted
yes, it cannot
decode whether to
commit or abort
until hears from C)
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Participant
If the participant is able to
commit, it forces a prepare
log record and sends a yes
vote to C. (N forces, N
messages)
If the participant cannot
commit, it forces an abort
log record and sends a no
vote to C. (N forces, N
messages)
CSCE 824 - Spring 2011
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Two-Phase Commit 3.
D
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Coordinator
Participant
If all votes were yes, C forces a commit log
record and sends a commit message to all
participants (1 force, N messages)
At this point the user can be told that the
transaction has been committed.
W
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N
D
O
W
If even one node voted no, C forces an abort log
record that contains transaction membership
and sends an abort message to each participant
that voted yes or didn’t send a vote. At this
point, the user can be told that the transaction
has been aborted.
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CSCE 824 - Spring 2011
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Two-Phase Commit 4.
Coordinator
Participant
When the participant receives a
commit message from C, it forces a
commit log record, sends an ACK to C,
and forgets about the transaction. (N
forces, N messages)
If it receives an abort message, the
participant forces an abort log record,
sends an ACK to C, and forgets about
the transaction.
After receiving ACKs from all
participants, C writes an
end-of-transaction record to
the log and forgets about the
transaction.
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CSCE 824 - Spring 2011
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Efficiency of 2-Phase
Commit
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Total number of messages: 6N
Log forces; 2N +1
Can be combined with 2-Phase
locking
CSCE 824 - Spring 2011
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Next Lectures
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Replica Update Protocols
CSCE 824 - Spring 2011
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