- Transaction processing in
distributed databases:
- All processing sites
must commit the
updates/changes to the
database
or
- All processing sites must abort
- All processing sites
must commit the
updates/changes to the
database
or
- Problem in
distributed database systems:
- A failure in a
distributed database system results in
partial system failure:
which can result in partial updates
- A failure in a
distributed database system results in
partial system failure:
- Goal of the
2-phase commit
protocol:
- Ensure that
updates at
multiple processing sites
are
all incorporated
or
- None of the updates are incorporated
- Ensure that
updates at
multiple processing sites
are
all incorporated
or
- Coordinator site:
- One of the
processing site is special:
- The coordinator (site)
All other sites are participants (sites)
- One of the
processing site is special:
- 2 Phase commit protocol
overview:
- The coordinator sends
control messages to
participants to
direct them to
complete
neccessary
preparations
- In Phase 1:
- The participant sites must
vote (agree) to
commit/abort the
transaction
- If a participant votes to commit the transaction, the participant must also prapare to commit by its flushing logs
- The participant sites must
vote (agree) to
commit/abort the
transaction
- In Phase 2:
- The coordinator makes the
final dicision to
commit or abort
the transaction
- The final decision is recorded at the coordinator site for reference (in case some site crashes)
- The coordinator makes the
final dicision to
commit or abort
the transaction
- The coordinator sends
control messages to
participants to
direct them to
complete
neccessary
preparations
- Assumptions:
- Each site
logs the
actions
at that site
(using the
log protocol(s)
discussed previously)
- Each message that a site sends is logged at the sending site (to aid in recovery if necessary)
- Each site
logs the
actions
at that site
(using the
log protocol(s)
discussed previously)
- Phase 1:
Protocol Explanation:
- The coordinator
writes the
log record
"Prepare T" into the
log at its
site.
- The coordinator sends the
message:
"Prepare T"to all participant sites
- A participant that
receives the
Prepare T message must
report its
commit decision for its
local operations:
- If the local operations
results in a serializable schedule,
the participant will decide to
commit the
local transaction
- Otherwise, the participant will decide to abort the local transaction
- If the local operations
results in a serializable schedule,
the participant will decide to
commit the
local transaction
- If
the participant decides to
commit the
local transaction,
the participant must:
- Flush the
local logs to disk
(so the participant can
ensure the
persistency of the
local updates)
- Write a
Ready T log record to its
local log
- Flush the Ready T log record to disk
- Flush the
local logs to disk
(so the participant can
ensure the
persistency of the
local updates)
- If
the participant decides to
abort the
local transaction,
the participant must:
- Write a
Don't Commit T (= abort)
log record to its
local log
- Flush the Don't Commit T log record to disk
- Write a
Don't Commit T (= abort)
log record to its
local log
- The coordinator
writes the
log record
"Prepare T" into the
log at its
site.
- Phase 2:
Protocol Explanation:
- If the
coordination received
"Commit T" from
all participants, then:
- The coordinator
writes a
"Commit T" (decision record)
to its local log file
- The coordinator
sends a
"Commit T" (final decision)
message
to all participants
- When a participant receives the "Commit T" (final decision) message, the participant writes a "Commit T" log record into its local log and make all updates of the transaction permanent at its site.
- The coordinator
writes a
"Commit T" (decision record)
to its local log file
- Otherwise (the
coordination received
at least one
"Don't commit T" message), then:
- The coordinator
writes an
"Abort T" (decision record)
to its local log file
- The coordinator
sends an
"Abort T" (final decision)
message to all participants
- When a participant receives the "Abort T" (final decision) message, the participant writes a "Abort T" log record into its local log and make all updates of the transaction void (e.g., undo) at its site.
- The coordinator
writes an
"Abort T" (decision record)
to its local log file
- If the
coordination received
"Commit T" from
all participants, then: