Chapter 4 Transaction Management Title: Granularity of Locks and Degrees of Consistency in a Shared Database Authors: J.N. Gray, R.A. Lorie, G. R. Putzolu and I. L. Traiger

Problem Definition Database Systems require an efficient means of providing concurrent transactions Choice of lock granularity with a two stage lock (share and exclusive modes) has drastic concurrency-overhead implications Explicitly locking a sub-tree requires a great deal of computation in the two stage lock

Contributions Reasons that lock granularity is important to be able to allow efficient concurrent transactions Provides a technique for implicitly locking an entire sub-tree Introduces an intention mode to tag ancestors to prevent explicit or implicit exclusive or share modes

Concepts Granularity of locks (Hierarchical Lock structure) Lock Modes  Exclusive Mode (X)  Share Mode (S)  Intention Share Mode (IS)  Intention Exclusive Mode (IX)  Share and Intention Exclusive mode (SIX) Database Area 1 File A Record Q Record R Record S File B Record T Record U Area 2 File C Record V Record W

Concepts Compatibility between access modes Protocol for requesting node locks – root to leaf Leaf nodes are never requested in intention mode ISIXSSIXX ISYes No IXYes No SYesNoYesNo SIXYesNo X Example of locking for record R 1. Lock data-base mode IS 2. Lock area containing Rmode IS 3. Lock file containing Rmode IS 4. Lock Record Rmode S

Concepts Implicit locking (Ancestor based locking) Directed Acyclic Graphs (DAG)– (non- hierarchical lock / indices)  Access via file (sequential)  Access via index (associative) Example of implicit locking for File F 1. Lock data-base mode IS 2. Lock area containing Fmode IS 3. Lock file Fmode S 4. All Records in F implicit mode S Example of implicit locking for Index I 1. Lock data-base mode IS 2. Lock area containing Imode IS 3. Lock Index Imode S 4. All Records in I implicit mode S

Concepts Dynamic Locks  Index interval locks – index based on value  Extends DAG adding a lock on the index intervals. (If moving from one index interval to another, it requires both) Scheduling and Granting Requests  Coexisting requests per group Conversion modes  Request higher permissions Deadlock due to higher permission requests

Validation Methodology Content primarily presents informal theory Validated while being developed and implemented on IBM Research Lab System Shows benefits of working system / Proof of concept Lacks formal proof

Assumptions Database can be broken into some fashion of a hierarchical form Assumes the use of an early database model (hierarchical or directed graph in nature).  This assumption does not allow for transcending to all DB types (relational and structured, etc)  Removing assumption allows for maintaining a basic structure, but would require other means of controlling the concurrency access.  Possible problems with more complex index based queries

Rewrite Changes Rewrite to reflect the current state of database systems (as article is from 1975) Add statistical evidence of tradeoffs between overhead and concurrency Preserve the notion of the groups and the scheduling and conversions of blocking modes.