Precedence Graph 편집하기

'''Precedence Graph''', also known as a '''Serializability Graph''', is a directed graph used in database systems to determine the serializability of a transaction schedule. The graph represents the order of operations across transactions and helps identify potential conflicts that could prevent a schedule from being equivalent to a serial schedule.
==Key Concepts==
*'''Nodes:''' Represent individual transactions in the schedule.
*'''Edges:''' Indicate a precedence (or dependency) between transactions due to conflicting operations.
*'''Cycle Detection:''' The presence of a cycle in the precedence graph implies that the schedule is not serializable.
==Construction of a Precedence Graph==
To construct a precedence graph:
#Represent each transaction as a node.
#Add a directed edge from Transaction T1 to Transaction T2 if:
#*T1 performs an operation on a data item before T2 and both operations conflict.
#*A conflicting operation involves:
#**Read-Write (RW) conflict.
#**Write-Read (WR) conflict.
#**Write-Write (WW) conflict.
#Check for cycles:
#*If there is a cycle in the graph, the schedule is not serializable.
#*If no cycles exist, the schedule is serializable.
==Example==
Consider the following schedule:
*T1: Read(A), Write(A)
*T2: Read(A), Write(B)
*T3: Write(A), Read(B)
===Steps to Construct the Graph===
{| class="wikitable"
!Step!!Action!!Explanation
|-
|1||Add nodes T1, T2, T3||Each transaction is represented as a node.
|-
|2||Add edge T1 → T3||T1 writes to A, and T3 reads A (WR conflict).
|-
|3||Add edge T3 → T2||T3 writes to A, and T2 reads A (WR conflict).
|-
|4||Add edge T2 → T1||T2 writes to B, and T1 reads B (WR conflict).
|}
===Result===
The precedence graph contains the following edges:
*T1 → T3
*T3 → T2
*T2 → T1
Since there is a cycle (T1 → T3 → T2 → T1), the schedule is not serializable.
==Applications of Precedence Graphs==
Precedence graphs are widely used in:
*'''Concurrency Control:''' Verifying serializability of transaction schedules.
*'''Deadlock Detection:''' Identifying cycles in wait-for graphs, a variant of precedence graphs.
*'''Database Design:''' Ensuring consistency and preventing anomalies in concurrent transactions.
==Advantages==
*'''Clarity:''' Provides a clear visualization of transaction dependencies.
*'''Cycle Detection:''' Offers an effective way to check serializability.
*'''Conflict Resolution:''' Helps identify and resolve transaction conflicts.
==Limitations==
*'''Scalability:''' Constructing and analyzing precedence graphs can become complex for schedules with a large number of transactions.
*'''Static Nature:''' Only applicable to fixed schedules, not dynamic transaction processing.
==See Also==
*[[Transaction Management]]
*[[Serializability]]
*[[Conflict Serializability]]
*[[Dependency Graph]]
*[[Concurrency Control]]
*[[Deadlock Detection]]
[[Category:Database]]