Question:
Compare and analyze the relative merits of centralized and
hierarchical deadlock detection approaches in a distributed DBMS.
Answer:
A
centralized deadlock detection scheme is a reasonable choice if the concurrent
control algorithm is also centralized. It is better for distributed access
patterns across sites since deadlocks occurring between any can be immediately
identified. However, this benefit comes at the cost of communications between
the central location and every other site.
In hierarchical
deadlock detection algorithms, sites are arranged
hierarchically, and a site detects deadlocks involving only its descendant
sites. To efficiently detect deadlocks, hierarchical algorithms exploit access
patterns local to a cluster of sites. They tend to get the best of both worlds:
they have no single point of failure (as centralized algorithms have), and a
site is not bogged down by deadlock detection activities that it is not
concerned with (as sometimes happens in distributed algorithms). For
efficiency, most deadlocks should be localized to as few clusters as possible;
the objective of hierarchical algorithms will be defeated if most deadlocks
span several clusters.
A hierarchical
deadlock detection scheme releases the burden of one single site for deadlock
detection, and let more sites get involved. When access patterns are more
localized, perhaps by geographic area, they may likely occur among certain
sites with frequent communications. The hierarchical approach is more efficient
in that it checks for deadlocks where they most likely happen, the hierarchical
scheme splits deadlock detection efforts, thus resulting in greater efficiency.
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