This document provides an overview and whirlwind tour of resource managers and transaction processing. It discusses: - The key actors in transaction processing including resource managers, transaction managers, log managers, and communication managers. - The application programming interface verbs for beginning, committing, rolling back, and managing transactions. - How transactions are executed normally and when failures occur. - The transaction manager's role in coordinating transaction commit, recovery, and restart. - Idempotent and testable operations and the challenges of logical versus physical logging.

1. 1Gray & Reuter: Resource Manager
Resource ManagersResource Managers
9:00
11:00
1:30
3:30
7:00
Overview
Faults
Tolerance
T Models
Party
TP mons
Lock Theory
Lock Techniq
Queues
Workflow
Log
ResMgr
CICS & Inet
Adv TM
Cyberbrick
Files &Buffers
COM+
Corba
Replication
Party
B-tree
Access Paths
Groupware
Benchmark
Mon Tue Wed Thur Fri
Jim GrayJim Gray
Microsoft, Gray @ Microsoft.comMicrosoft, Gray @ Microsoft.com
Andreas ReuterAndreas Reuter
International University, Andreas.Reuter@i-u.deInternational University, Andreas.Reuter@i-u.de

2. 2Gray & Reuter: Resource Manager
Whirlwind Tour: The Actors
Resource managers
– provide ACID objects (transactional objects)provide ACID objects (transactional objects)
– Use log manager to record changesUse log manager to record changes
– Use transaction manager to coordinate multi-RM changesUse transaction manager to coordinate multi-RM changes
– Use communication manager to make transactional RPCsUse communication manager to make transactional RPCs
Transaction
Manager
Log
Manager
Log
Objects
Resource
Managers
Objects
Resource
Managers
Volatile Storage
Durable Storage
Volatile Storage
Durable Storage
Communication
Manager
Transaction
Manager
Log
Manager
Communication
Manager
Log

3. 3Gray & Reuter: Resource Manager
Whirlwind Tour: the Application Verbs
TRID Begin_Work(context *); /* begin a transaction */
Boolean Commit_Work(context *); /* commit the transaction */
void Abort_Work(void); /* rollback to savepoint zero */
savepoint Save_Work(context *); /* establish a savepoint */
savepoint Rollback_Work(savepoint); /*return to savept (savept 0 = abort)*/
Boolean Prepare_Work(context *); /* put transaction in prepared state */
context Read_Context(void); /* return current savepoint context */
TRID Chain_Work(context *); /* end current and start next trans */
TRID My_Trid(void); /* return current transaction identifier*/
TRID Leave_Transaction(void); /*set process trid null, return current
id*/
Boolean Resume_Transaction(TRID); /* set process trid to desired trid */
enum tran_status { ACTIVE , PREPARED , ABORTING , COMMITTING , ABORTED , COMMITTED};
tran_status Status_Transaction(TRID); /* transaction identifier status */

4. 4Gray & Reuter: Resource Manager
Whirlwind Tour
Types Of Transaction Executions
Shaded stuff is “undone”Shaded stuff is “undone”
Save Persistent
Begin
Action
Action
Save
Action
Save
Action
Action
Action
Save
Action
Action
Commit
Commit
A Simple
Commit
A Simple
Abort
Begin
Action
Action
Save
Action
Save
Action
Action
Action
Save
Action
Rollback
Action
Action
Action
Save
Action
A Partial
Rollback
Begin
Action
Action
Save
Action
Save
Action
Action
Action
Save
Action
Rollback
A Persistent Transaction
Surviving A System Restart
Begin
Action
Action
Action
Save
Action
Restart
Action
Save
Action
Commit

5. 5Gray & Reuter: Resource Manager
Whirlwind Tour: the TRID Flow
Call graph: who calls whom.
TRIDs flow on all such calls.
Application is typically root.
RM can be an application (use a transactional RM to store state)
Application
Application
Servers
Resource
Managers
Resource
Managers
Transaction Application
Servers

6. 6Gray & Reuter: Resource Manager
Whirlwind tour Normal (no failure) Transaction
Execution
TM generates the TRID at Begin_Work().
Coordinates Commit,
RM joins work, generates log records, allows commit
T r a n s a c t i o n
M a n a g e r
W r i t e C o m m i t
L o g R e c o r d &
F o r c e L o g
C o m m i t P h a s e 1 ?
Y e s / N o
C o m m i t P h a s e 2
a c k
T r a n s a c t i o n
C a l l b a c k s
F u n c t i o n s
W o r k R e q u e s t s R e s o u r c e
M a n a g e r
N o r m a l
F u n c i t o n s
L o c k R e q u e s t s
L o g R e c o r d s
W o r k R e q u e s t s
L o c k
M a n a g e r
t r a n s i d
L o g
M a n a g e r
A p p lic a t io n
B e g i n _ W o r k ( )
C o m m i t _ W o r k ( )
J o i n _ W o r k

7. 7Gray & Reuter: Resource Manager
WW tour: The Resource Manger view
Resource
Manager
resource manager's own service interface
rmCall(...)
transaction
management
other
resource
managers
rmCall(...)
TP monitor
administrative functions
and callbacks to install, start, and
schedule a resource manager
response
invocation
callbacks
(depends on application)
Save
Prepare
Commit
UNDO
REDO
Checkpoint
Transaction
Manager
functions
callbacks
Identify
SaveWork
RollbackWork
Join
StatusTransaction
Leave
Resume

8. 8Gray & Reuter: Resource Manager
WW tour: The Resource manager view
BooleanSavepoint(LSN *); /* invoked at tran Save_Work(). Returns RM vote */
BooleanPrepare(LSN *); /* invoked at phase_1. Return vote on commit */
void Commit(); /* called at commit ¯2 */
void Abort(); /* called at failed commit ¯2 or abort */
void UNDO(LSN); /* Undo the log record with this LSN */
void REDO(LSN); /* Redo the log record with this LSN */
BooleanUNDO_Savepoint(LSN);/* Vote TRUE if can return to savepoint */
void REDO_Savepoint(LSN);/* Redo a savepoint. */
void TM_Startup(LSN); /* TM restarting. Passes RM ckpt LSN */
LSN Checkpoint(LSN * low_water); /* TM checkpointing, Return RM ckpt LSN,
set low water LSN */
Boolean Join_Work(RMID, TRID); /* Become part of a transaction */

9. 9Gray & Reuter: Resource Manager
WW Tour: The Transaction Manager
Transaction rollback.
coordinates transaction rollback to a savepoint or abort
rollbacks can be initiated by any participant.
Resource manager restart.
If an RM fails and restarts, TM presents checkpoint anchor & RM undo/redo log
System restart.
TM drives local RM recovery (like RM restart)
TM resolves any in-doubt distributed transactions
Media recovery.
TM helps RM reconstruct damaged objects by providing
archive copies of object + the log of object since archived.
Node restart.
Transaction commit among independent TMs when a TM fails.

10. 10Gray & Reuter: Resource Manager
WW Tour: When a Transaction Aborts
At transaction rollback
TM drives undo of each RM joined to the transaction
Can be to savepoint 0 (abort) or partial rollback.
T ra n s a c tio n
M a n a g e r
R e a d T ra n s a c tio n 's
L o g R e c o rd s &
C a ll U n d o
W rite A b o rt R e c o rd
in L o g
T ra n s a c tio n
C a llb a c k s
W o rk R e q u e s ts
N o rm a l
F u n c ito n s
L o c k R e q u e s ts
L o g R e c o rd s
W o rk R e q u e s ts
L o c k
M a n a g e r
tra n s id
L o g
M a n a g e r
A p p l i c a t i o n
B e g in _ W o rk ()
R o llb a c k _ W o rk ()
U n d o (lo g re c o rd )
A b o rte d (tra n s id )
J o in _ W o rk
R e s o u rc e
M a n a g e r

11. 11Gray & Reuter: Resource Manager
WW tour: the Transaction Manager
at Restart/Recovery
At restart, TM reading the log drives RM recovery.
Single log scan.
Single resolver of transactions.
Multiple logs possible, but more complex/more work.
Transaction
Manager
Find Checkpoint
Read log forward
Redo each op
At end,
Undo Soft
Savepoints &
Transactions
Undo (log record)
Log RecordsLog
Manager
Undo (log record)
Undo(log record)
Resource
Manager
Redo (log record)
Redo (log record)
Redo (log record)
Redo (log record)
Redo (log record)
Redo(log record)
Log Records

12. 12Gray & Reuter: Resource Manager
End of Whirl-Wind TourEnd of Whirl-Wind Tour

13. 13Gray & Reuter: Resource Manager
Resource Manager Concepts:
Undo Redo Protocol
DO
Old State New State
DO-UNDO- REDO Protocol
log record
New State
Old State
UNDO
log record
Old State
log record
New State
REDO

14. 14Gray & Reuter: Resource Manager
Resource Manager Concepts:
Transaction UNDO Protocol
declare cursor for transaction_log
select rmid, lsn /* a cursor on the transaction's log */
from log /* it returns the resource manager name */
where trid = :trid /* and record id (log sequence number) */
descending lsn; /* and returns records in LIFO order */
void transaction_undo(TRID trid) /* Undo the specified transaction. */
{ int sqlcode; /* event variables set by sql */
open cursor transaction_log; /* open an sql cursor on the trans log */
while (TRUE) /* scan trans log backwards & undo each*/
{ /* fetch the next most recent log rec */
fetch transaction_log into :rmid, :lsn; /* */
if (sqlcode != 0) break; /* if no more, trans is undone, end loop*/
rmid.undo(lsn); /* tell RM to undo that record */
} /* tell RM to undo that record */
close cursor transaction_log; /* Undo scan is complete, close cursor */
}; /* return to caller */
• If UNDO to savepoint , the UNDO stops at desired savepoint

15. 15Gray & Reuter: Resource Manager
Resource Manager Concepts:
Restart REDO Protocol
Note: REDO forwards, UNDO backwards
void log_redo(void) /* */
{declare cursor for the_log /* declare cursor from log start forward */
select rmid, lsn /* gets RM id and log record id (lsn) */
from log /* of all log records. */
ascending lsn; /* in FIFO order */
open cursor the_log; /* open an sql cursor on the log table */
while (TRUE) /* Scan log forward& redo each record. */
{ fetch the_log into :rmid, :lsn; /* fetch the next log record */
if (sqlcode != 0) break; /* if no more, then all redone, end loop */
rmid.redo(lsn);} /* tell RM to redo that record */
close cursor the_log; /* Redo scan complete, close cursor */
}; /* return to caller */

16. 16Gray & Reuter: Resource Manager
Idempotence
F(F(X)) == F(X): Needed in case restart fails (and restarts)
Redo(Redo(old_state,log), log) = Redo(new_state,log) = new_state
Undo(Undo(new_state,log), log) = Undo(old_state,log) = old_state
Old State
New State
log record
log record
undo
redo

17. 17Gray & Reuter: Resource Manager
Testable State: Can Tell If It Happened.
IF operation not idempotent AND state not testable
THEN recovery is impossible
ELSE for F in {UNDO, REDO}:
not testable: WHILE (! ACK) F(F(X))
testable: WHILE ( not desired state) {F(x)}
New State
Old State
test
Unknown
State

18. 18Gray & Reuter: Resource Manager
Real Operations: Can Not Be Undone
Defer operations until commit is assured.
Perform as part of Phase 2 of commit
If must undo for some reason,
generate compensation log record
to be processed by some higher authority.
UNDO
REDO
New State
log record
Old State
DO
Old State
log record
Commit
New State
log record
Old State
Old State Old State
log record Compensation log record
Old State

19. 19Gray & Reuter: Resource Manager
Example: Communications Session RM
Ops are idempotent (sequence numbers)
and testable (sequence numbers)
log cancellation message
return to savepoint
acknowledge
if not duplicate
<normal DO processing>
else just acknowledge.
Sender Receiver
DO
UNDO
REDO
COMMIT
log message & seqno
send
send cancellation
(generates log record)
resend message
send any deferred (real)
messages
establish savepoint.
log message & seqno
acknowledge
Session And Message Recovery Actions
do it

20. 20Gray & Reuter: Resource Manager
Kinds of Logging
Physical:
Keep old and new value of container (page, file,...)
Pro: Simple
Allows recovery of physical object (e.g. broken page)
Con: Generates LOTS of log data
Logical:
Keep call params such that you can compute F(x), F
-1
(x)
Pro: Sounds simple
Compact log.
Con: Doesn't work (wrong failure model).
Operations do not fail cleanly.

21. 21Gray & Reuter: Resource Manager
Sample Physical LOG RECORD
Ordinary sequential insert is OK.
Update of sorted (B-tree) page:
update LSN
update page space map
update pointer to record
insert record at correct spot (move 1/2 the others)
Essentially writes whole page (old and new).
16KB log records for 100-byte updates.
struct compressed_log_record_for_page_update /* */
{ int opcode; /* opcode will say compressed page update*/
filename fname; /* name of file that was updated */
long pageno; /* page that was updated */
long offset; /* offset within page that was updated */
long length; /* length of field that was updated */
char old_value[length]; /* old value of field */
char new_value[length]; /* new value of field */
}; /* */

22. 22Gray & Reuter: Resource Manager
Sample Physical LOG RECORD
Very compact.
Implies page update(s) for record (may be many pages long).
Implies index updates (many be many indices on base table)
struct logical_log_record_for_insert /* */
{ int opcode; /* opcode will says insert */
filename fname; /* name of file that was updated */
long length; /* length of record that was updated */
char record[length]; /* value record */
}; /* */

23. 23Gray & Reuter: Resource Manager
The trouble with Logical Logging
Logical logging needs to start UNDO/REDO with an action-consistent state.
No half completed operations.
for example: insert (table, record)
ALL or NONE of the indices should be updated
when logical UNDO/REDO is invoked.
Problem:
Failure model is Page & Message action consistency
(Lampson /Sturgis model of Chapter 3).
Actions can fail due to:
Logic: e.g. duplicate key.
Limit: ran out of space
Contention: deadlock
Media: broken page or session
System: computer failure/restart

24. 24Gray & Reuter: Resource Manager
Making Logical Logging Work: Shadows
Keep old copy of each page
Reset page to old copy at abort (no undo log)
Discard old copy at commit.
Handles all online failures due to:
Logic: e.g. duplicate key.
Limit: ran out of space
Contention: deadlock
Problem: forces page locking, only one updater per page.
What about restart?
Need to atomically write out all changed pages.

25. 25Gray & Reuter: Resource Manager
Making Logical Logging Work: Shadows
Perform same shadow trick at disc level.
Keep shadow copy of old pages.
Write out new pages.
In one careful write, write out new page root.
Makes update atomic
Free Space
Bit MapDirectory
Free Space
Bit MapDirectory
Data
Old New
A Shadow Update
A B C A BC

26. 26Gray & Reuter: Resource Manager
Shadows
Pro: Simple
Not such a bad deal with non-volatile ram
Con: page locking
extra space
extra overhead (for page maps)
extra IO
declusters sequential data

27. 27Gray & Reuter: Resource Manager
Compromise Physio-Logical Logging
Physio-Logical Logging
Physical to a "page" (physical container)
Logical within a "page".
Keep old and new value of container (page, file,...)
Pro: Simple
Allows recovery of physical object (e.g. broken page)
Con: Generates LOTS of log data

28. 28Gray & Reuter: Resource Manager
Logical vs Physio-logical Logging
Insert recordrintotableA
TableA
IndexB
IndexC
insert, A,r
Logical logrecord
TableA
IndexB
IndexC
insert, A,page508,r
Physiological logrecords
insert, B,page72,s
insert, C,page94,t
Note: physical log records would be bigger for sorted pages.

29. 29Gray & Reuter: Resource Manager
Physiological Logging Rules
Complex operations are a sequence of simple operations on pages and
messages.
Each operation is constructed as a mini-transaction:
lock the object in exclusive mode
transform the object
generate an UNDO-REDO log record
record log LSN in object
unlock the object.
Action Consistent Object:
When object semaphore free, no ops in progress.
Log-Consistency:
contains log records of all complete page/msg actions.

30. 30Gray & Reuter: Resource Manager
Physiological Logging Rules
Online Operation - Only Need the Fix Rule
Each operation is structured as a mini-transaction.
Each operation generates an UNDO record.
No page operation fails with the semaphore set.
(exception handler must clean up state
and UNFIX any pages).
Then Rollback can be
physical to a page/session/container and
logical within page/session/container.

31. 31Gray & Reuter: Resource Manager
Physiological Logging Rules
Restart Operation - Need WAL and F@C
Need Page-Action consistent disc state.
Pages are action consistent.
Committed actions can be redone from log.
Uncommitted actions can be undone from log.
WAL: Write Ahead Log
Write undo/redo log records before overwriting disc page
Only write action-consistent pages
Force-Log-At-Commit
Make transaction log records durable at commit.

32. 32Gray & Reuter: Resource Manager
Physiological Logging Rules
WAL and F@C
WAL: Write Ahead Log
write page:
get page semaphore
copy page
give page semaphore /* avoids holding semaphore during IO */
Force_log(Page(LSN)) /*WAL logic, probably already flushed*/
Write copy to disc.
WAL gives idempotence and testability.
Force-Log-At-Commit
At commit phase 1:
Force_log(transaction.max_lsn)

33. 33Gray & Reuter: Resource Manager
WAL & F@C in PicturesWAL & F@C in Pictures
VVlsn
Volatile Page
Versions
Volatile Log
Records
VLlsn
PVlsn
Persistent Page
Versions
Durable Log
Records
DLlsn
Time
online:VVlsn = VLlsn
restart: DLlsn <= VVlsn
PVlsn <= DLlsn
Commit:
commit_lsn <= DLlsn
At restart all volatile memory is reset and must be
reconstructed from persistent memory.
restart:
PVlsn <= DLlsn
commit_lsn <= DLlsn
PVlsn
DLlsn
FIX, WAL and F@C assure these assertions

34. 34Gray & Reuter: Resource Manager
The One Bit Resource Manager
Manages an array of transactional bits (the free space bit map).
i = get_bit(); /* gets a free bit and sets it */
give_bit(i); /* returns a free bit (when transaction commits) */

35. 35Gray & Reuter: Resource Manager
The Bitmap and Its Log Records
The Data Structure
struct { /* layout of the one-bit RM data structure */
LSN lsn; /* page LSN for WAL protocol */
xsemaphore sem; /* semaphore regulates access to the page */
Boolean bit[BITS]; /* page.bit[i] = TRUE => bit[i] is free */
} page; /* allocates the page structure */
The Log Records
struct /* log record format for the one-bit RM */
{ int index; /* index of bit that was updated */
Boolean value; /* new value of bit[index] */
} log_rec; /* log record used by the one-bit RM */
const int rec_size = sizeof(log_rec); /*size of the log record body. */

36. 36Gray & Reuter: Resource Manager
Page and Log Consistency for 1-Bit RM
Data dirty if reflects an uncommitted transaction update
Otherwise, data is clean.
Page Consistency:
• No clean free bit has been given to any transaction.
• Every clean busy bit was given to exactly one transaction.
• Dirty bits locked in X mode by updating transactions .
• The page.lsn reflects most recent log record for page.
Log Consistency:
• Log contains a record for every completed
mini-transaction update to the page.

37. 37Gray & Reuter: Resource Manager
give_bit()
get_bit() & give_bit(i) temporarily violate page consistency.
Mini-transaction holds semaphore while violating consistency.
Makes page & log mutually consistent before releasing sem.
=> each mini-transaction observes a consistent page state.
void give_bit(int i) /* free a bit */
{ if (LOCK_GRANTED==lock(i,LOCK_X,LOCK_LONG,0)) /* Lock bit */
{ Xsem_get(&page.sem); /* get page sem */
page.bit[i] = TRUE; /* free the bit */
log_rec.index = i; /* generate log rec */
log_rec.value = TRUE; /*saying bit is free */
page.lsn = log_insert(log_rec,rec_size); /*write log rec&update lsn */
Xsem_give(&page.sem);} /* page consistent */
else /* if lock failed, caller doesn't own bit,
*/
Abort_Work(); /* in that case abort caller's trans */
return; }; /* */

38. 38Gray & Reuter: Resource Manager
get_bit()
int get_bit(void) /* allocate a bit to and returns bit index */
{ int i; /* loop variable */
Xsem_get(&page.sem); /* get the page semaphore */
for ( i = 0; i<BITS; i++); /* loop looking for a free bit */
{if (page.bit[i]) /* if bit is free, may be dirty (so locked)
*/
{if (LOCK_GRANTED =lock(i,LOCK_X,LOCK_LONG,0));/* lock bit */
{ page.bit[i] =FALSE; /* got lock on it, so it was free */
log_rec.value = FALSE; /* generate log rec describing update */
log_rec.index = i; /* */
page.lsn = log_insert(log_rec,rec_size); /* write log rec&update lsn */
Xsem_give(&page.sem); /* page now consistent, give up sem */
return i; } /* return to caller */
}; /* else lock bounce so bit dirty */
}; /* try next free bit, */
Xsem_give(&page.sem); /* if no free bits, give up semaphore */
Abort_Work(); /* abort transaction
*/
return -1;}; /* returns -1 if no bits are available. */

39. 39Gray & Reuter: Resource Manager
Compensation Logging
Undo may generate a log record recording undo step
Makes Page LSN monotonic
Similar technique was used for Communication Manager
(session sequence number was monotonic)
New State Logical Old State
UNDO
log record com pensation log record

40. 40Gray & Reuter: Resource Manager
1-bit RM UNDO Callback
void undo(LSN lsn) /* undo a one-bit RM operation */
{ int i; /* bit index */
Boolean value; /* old bit value from log rec to be undone*/
log_rec_header header; /* buffer to hold log record header */
rec_size = log_read_lsn(lsn,header,0,log_rec,big); /* read log rec */
Xsem_get(&page.sem); /* get the page semaphore */
i = log_rec.index; /* get bit index from log record */
value = ! log_rec.value; /* get complement of new bit value */
page.bit[i] = value; /* update bit to old value */
log_rec.value= value; /* make a compensation log record */
page.lsn = log_insert(log_rec,rec_size); /* log it and bump page lsn */
Xsem_give(&page.sem); /* free the page semaphore */
return; } /* */

41. 41Gray & Reuter: Resource Manager
1-bit RM Checkpoint Callback
LSN checkpoint(LSN * low_water) /* copy 1-page RM state to persistent store*/
{ Xsem_get(&page.sem); /* get the page semaphore */
*low_water = log_flush(page.lsn); /* WAL force up to page lsn, and */
/* set low water mark */
write(file,page,0,sizeof(page)); /* write page to persistent memory */
Xsem_give(&page.sem); /* give page semaphore */
return NULLlsn; } /* return checkpoint lsn (none needed) */

42. 42Gray & Reuter: Resource Manager
1-bit RM REDO Callback
void redo( LSN lsn) /* redo an free space operation */
{ int i; /* bit index */
Boolean value; /* new bit value from log rec to be redone*/
log_rec_header header; /* buffer to hold log record header */
rec_size = log_read_lsn(lsn,header,0,log_rec,big); /* read log record */
i = log_rec.index; /* Get bit index */
lock(i,LOCK_X,LOCK_LONG,0); /* get lock on the bit (often not needed)*/
Xsem_get(&page.sem); /* get the page semaphore */
if (page.lsn < lsn) /* if bit version older than log record */
{ value= log_rec.value; /* then redo the op. get new bit value */
page.bit[i] = value; /* apply new bit value to bit */
page.lsn = lsn; } /* advance the page lsn */
Xsem_give(&page.sem); /* free the page semaphore */
return; }; /* */

43. 43Gray & Reuter: Resource Manager
1-BIT Rm Noise Callbacks
Boolean prepare(LSN * lsn) /* 1-bit RM has no phase 1 work */
{*lsn = NULLlsn; return TRUE ;}; /* */
void Commit(void ) /* Commit release locks & */
{ unlock_class(LOCK_LONG, TRUE, MyRMID()); }; /* return */
void Abort(void ) /* Abort release all locks & */
{ unlock_class(LOCK_LONG, TRUE, MyRMID()); }; /* return */
Boolean savepoint((LSN * lsn) /* no work to do at savepoint */
{*lsn = NULLlsn; return TRUE ;}; /* */
void UNDO_savepoint(LSN lsn) /* rollback work or abort transaction */
{if (savepoint == 0) /* if at savepoint zero (abort) */
unlock_class(LOCK_LONG, TRUE, MyRMID()); /* release all locks */
}; /* */

44. 44Gray & Reuter: Resource Manager
Summary
Model: Complex actions are a page/message action sequence.
LSN: Each page carries an LSN and a semaphore.
ReadFix: Read acts semaphore in shared mode.
WriteFix: Update actions get semaphore in exclusive mode,
generate one or more log records covering the page,
advance the page LSN to match highest LSN
give semaphore
WAL: log_flush(page.LSN) before overwriting persistent page
F@C: force all log records up to the commit LSN at commit
Compensation Logging: Invalidate undone log record with a
compensating log record.
Idempotence via LSN: page LSN makes REDO idempotent

45. 45Gray & Reuter: Resource Manager
Two Phase Commit
Getting two or more logs to agree
Getting two or more RMs to agree
Atomically and Durably
Even in case one of them fails and restarts.
The TM phases
Prepare. Invoke each joined RM asking for its vote.
Decide. If all vote yes, durably write commit log record.
Commit. Invoke each joined RM, telling it commit
decision.
Complete. Write commit completion when all RM ACK.

46. 46Gray & Reuter: Resource Manager
Centralized Case of Two Phase Commit
Each participant: (TM &RM) goes through a
sequence of states
These generate log records
Null Active
Aborting Aborted
Prepared Committing Committed

47. 47Gray & Reuter: Resource Manager
ExamplesExamples
Committed Aborted
begin begin
DO rm1 DO rm1
DO rm2 DO rm2
DO rm2 DO rm2
prepare rm2 {locks} UNDO rm2
commit { rm1, rm2} UNDO rm2
complete UNDO rm1
UNDO begin { rm1, rm2}
complete

48. 48Gray & Reuter: Resource Manager
Transitions in Case of Restart
Null Active
Aborting Aborted
Prepared Committing Committed
Active state not persistent, others are persistent
For both TM and RM.
Log records make them persistent (redo)
TM tries to drive states to the right. (to committed, aborted)

49. 49Gray & Reuter: Resource Manager
Successful two phase commit
Message/Call flow from TM to each RM joined to transaction
If TM and RM share the same log,
the RM FORCE can piggyback on the TM FORCE
One IO to commit a transaction (less if commit is grouped)
Prepare
LocalPrepare
WritePrepareRecord
InLog(force)
yes
LocalPrepare
(lazy)
WriteCommit
RecordInLog
(force)
Commit
Ack
LocalCommitWork
WriteCompletionRecord
InLog(lazy)
Ackwhen durable.
Coordinator Participant
WriteCompletion
RecordInLog
(lazy)
State
Active
Prepared
Committing
LocalCommit
Work
(lazy)
Committed
State
Active
Prepared
Committing
Committed

50. 50Gray & Reuter: Resource Manager
Abort Two Phase Commit
If RM sends "NO" or no response (timeout), TM starts abort.
Calls UNDO of each trans log record
May stop at a savepoint.
At begin_trans it calls ABORT() callback of each joined RM

51. 51Gray & Reuter: Resource Manager
Distributed two phase commit
Tracking joined TMs -- the communications manager helps
Much as TRPC helps in the local case.
Root TM owes a Prepare/Commit/Abort message to each joined TM.
Joined TM does "local" commit.
call
first time?
Transaction
Manager A
tridis
outgoingtoB
Communications
Manager
first time?
Transaction
Manager
tridis
incomingfromA
Communications
ManagerSession calleetrid, data
trid, data

52. 52Gray & Reuter: Resource Manager
Full Transaction State Diagram
Next section explains how these states are implemented.
null
persistent save point n
=save point 0
Begun
=save point 1
save point n active
prepared
committing
committed
aborting
aborted
Durable
States
Persistent
States
Volatile
States
livestates
completestates

53. 53Gray & Reuter: Resource Manager
Summary of Resource Manager Concepts
DO/UNDO/REDO
Idempotent, Testable, Real operations
Logical vs Physical logging
Shadows to make logical logging work
Physiological logging
Fix, WAL, Force-at-commit
Page/Message/Log consistency
RM callbacks (the 1-bit resource manager)
Join, Prepare, Commit, Abort, UNDO, REDO, ....
Restart REDO/UNDO
Two phase commit (RM story is simple).