Mon, Nov 8, 1999
------------------------

- Return Homework

- Back to 2PL

- Introduction to "U" locks
	- look like "S" when you get them
	- only can be later upgraded and used as an "X"

- Introduction to "I" locks
	- satisfy commutativity

- Compatibility Matrices
	- for S and X locks
	- for S, X and U locks (assymmetric)
	- for S, X, U and I locks

- Example of inserting l and u operations
	- for S and X locks : Two ways:
		- Insert S before all reads and X before all writes
		- Insert S before all reads not followed by a write
		  Insert X before all reads followed by a write
		  Insert X before all writes

	- for S, X and U locks
		- Insert S before all reads not followed by a write
		  Insert U before all reads followed by a write
		  Insert X before all writes

Wed, Nov 10, 1999
------------------------

- How to look at compatibility matrices 
  and tell if one lock mode completely 
  dominates another

- S and X locks: X dominates S

- S, X and U locks: U dominates S, X dominates S and U

- The Three Aspects of a Transaction Scheduler
	- Scheduler I: Inserts l and u operations
	- Scheduler II: Makes some Txs wait if they cannot proceed,
          maintain lock data structures
	- Scheduler III: Check for special conditions

- Example of how a lock data structure looks like

- Locking a Hierarchy of DB Elements
	- IS and IX locks

- Compatibility matrix for IS, IX, S, X locks
	- notice that neither of S and IX dominates the other

- Solution: Introduce a new lock mode "SIX"

- SIX row/column contains the intersections of S and IX row/column

- What is the use of SIX?
	- can be viewed as a computational artifact
          that is an intermediary for two lock modes
	- is useful in its own right: you might
          need to change one particular tuple after 
          looking at the whole relation first (SIX mode)

- Final Compatibility Matrix

		IS  IX  S  X  SIX
	--------------------------
	IS	y   y   y  n   y
	IX      y   y   n  n   n
	S       y   n   y  n   n
	X       n   n   n  n   n
	SIX     y   n   n  n   n
	

- Notice that SIX is not compatible with SIX

- To get IS or S -> must have on all ancestors: IS or IX
- To get IX, SIX or X -> must have on all ancestors: IX or SIX

- Problems with locking hierarchies: The Phantom Menace

- Solution to Phantom Menace
	- View Adding New Tuples an "X" on the whole relation

Fri, Nov 12, 1999
------------------------

- So, far we have dealt with concurrency control
	- satisfies I
	- How do you detect deadlocks?

- Who deals with A&D? 
	- Recovery

- Who deals with C? 
	- Active and Rule-Based Elements

- Basics of Recovery
	- Commit
	- Abort

- Why would we need to abort?
	- when deadlock happens
	- or some other irreversible situation, like divide by zero

- For ensuring A: UNDO Operation
- For ensuring D: REDO Operation

- UNDO Policies
	- STEAL vs. NO-STEAL

- REDO Policies
	- FORCE vs. NO-FORCE

- Easiest to Implement
	- NO-STEAL/FORCE

- In real life
	- STEAL/NO-FORCE
	- First modeled in the ARIES Recovery Manager
	- WAL: Write-Ahead Logging: One implementation of STEAL/NO-FORCE

- OLTP Benchmarks

- Active and Rule-Based Elements
	- ECA paradigm

- Limitations of ACID Model