Lock management in a distributed system:
To replicate the discussion in the section OIL distributed transaction management, if a transaction wants to do work at a new node, some process of the transaction must request that the node construct a cohort as well as that the cohort go into session with the requesting process (see section on data communications for a discussion of sessions.) The picture below illustrates this.
A cohort carries both the transaction name T1 as well as the process name (in NODE1 the cohort of T1 is process P2 and in NODE2 the cohort of T1 is process P6.)
The two processes are able to nor converse and carry out the work of the transaction. If one process aborts, they must both abort, as well as if one process commits, they must both commit.
The lock manager of every node can keep its lock tables in any form it desires. Additional deadlock detectors running in every node may use any technique they like to detect deadlocks among transactions that run exclusively in that node. We call such deadlocks as local deadlocks. Nevertheless, just for the reason that there are no cycles in the local wait-for graph, doesn’t mean that there are no cycles. Gluing acyclic local graphs collectively might produce a graph with cycles (See the illustration bellow.) Therefore the deadlock detectors of every node will have to agree on a common protocol in order to handle deadlocks involving distributed transactions. We call such deadlocks as global deadlocks.
Inspection of the following figure may perhaps help to understand the nature of global deadlocks. Note that transaction T1 has two processes Pl as well as P2 in nodes 1 and 2 respectively. Pl is session-waiting for its cohort P2 to do few works. P2 in the process of doing this work needed access to FILE2 in NODE2. However FILE2 is locked exclusive by another process (P4 of NODE2) thus P2 is in lock wait state. Therefore the transaction T1 is waiting for FILE2. Now Transaction T2 is in a alike state one of its cohorts is session waiting for the other which in turn is lock waiting for FILE1. Indeed transaction T1 is waiting for FILE2 which is granted to transaction T2 that is waiting for file FILE1 that is granted to transaction T1. A global deadlock if you forever saw one.
The notion of wait-for graph should be generalized to handle global deadlock. The nodes of the graph are processes as well as resources (sessions are resources). The edges of the graph are constructed as go after:
• Draw a directed edge empty a process to a resource if - the process is in lock wait for the resource - or the process is in session-wait for the resource (session)-
• Depict a directed edge from a resource to a process if - the resource is lock granted to the process - or it is a session of the process as well as the process is not in session-wait on it.
A local deadlock is alock wait ->. . . .-> lockwait cycle.
A global deadlock is alockwait->... -> sessionwait -> lockwait ->...-> sessionwait cycle
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