Resource Managers

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

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

3. 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. Begin Begin Action Action Action Action Save Save Action Action Save Save Action Action Action Action Action Action Save Save Action Action Action Rollback Commit Whirlwind Tour Types Of Transaction Executions • Shaded stuff is “undone” A Simple A Simple A Persistent Transaction A Partial Commit Abort Surviving A System Restart Rollback Begin Begin Action Action Action Action Save Persistent Save Action Action Action Save Save Action Action Action Action Save Action Restart Action Action Action Commit Save Save Action Action Rollback Commit

5. 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) Transaction Application Application Servers Servers Application Resource Resource Managers Managers

6. Whirlwind tour Normal (no failure) Transaction Execution • TM generates the TRID at Begin_Work(). • Coordinates Commit, • RM joins work, generates log records, allows commit

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

8. WW tour: The Resource manager view • Boolean Savepoint(LSN *); /* invoked at tran Save_Work(). Returns RM vote */ • Boolean Prepare(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 */ • Boolean UNDO_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. 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. 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.

11. WW tour: the Transaction Managerat 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.

12. End of Whirl-Wind Tour

13. Resource Manager Concepts:Undo Redo Protocol

14. 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. 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. 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 undo log record New State redo log record

17. 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)}

18. 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.

19. Example: Communications Session RM • Ops are idempotent (sequence numbers) • and testable (sequence numbers)

20. 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. Sample Physical LOG RECORD 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 */ }; /* */ • 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.

22. 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. 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. 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. 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

26. 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. 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. Logical vs Physio-logical Logging Note: physical log records would be bigger for sorted pages.

29. 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. Physiological Logging RulesOnline 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. Physiological Logging RulesRestart 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 pageOnly write action-consistent pages • Force-Log-At-CommitMake transaction log records durable at commit.

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

34. 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. 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. Page and Log Consistency for 1-Bit RM • Datadirty 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. 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. 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. 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)

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

47. Examples • 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. Transitions in Case of Restart 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. 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)

50. 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