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  Distributed transaction management

Distributed transaction management:

A distributed system is presumed to consist of a collection of autonomous nodes that are tied together with a distributed data communication system in the style of high level ARPANET, DECNET or SNA protocols. Resources are presumed to be partitioned in the sense that a resource is owned by only one node. The system must be:

a) Inhomogeneous nodes are ours, theirs, small, medium, large,...
b) Unchanged by the loss of messages.
c) Unaffected by means of the loss of nodes (that is  requests to that node wait for the node to return ether nodes continue working.)

Every node may implement whatever data management and transaction management system it wants to.

We only necessitates that it obey the network protocols some node might be a minicomputer running a fairly simple data management system and using an old-master new-master recovery protocol. One more node might be running a very sophisticated data management system with many concurrent transactions and fancy recovery.

If one transaction may perhaps access resources in many nodes of a network then a part of the transaction must ‘run’ in each node. We by now have an entity that represents transaction instances- processes.

Every node will want to:

a) Authorize local activities of the process (transaction).
b) Build an implementation environment for the process (transaction).
c) Follow local resources supposed by the process (transaction).
d) Begin a recovery mechanism to undo the local updates of that process (see recovery section).
e) Examine the two-phase commit protocol (in cooperation with its cohorts (see section on recovery)).

Thus the structure needed for a process in a distributed system is almost identical to the structure needed by a transaction in a centralized system.

This latter examination is key. That’s why I advocate viewing every node as a transaction processor. This is a minority view to install a distributed transaction one should install prototypes for its cohorts in the various nodes. This permits each node to control access by distributed transactions in the same way it controls access by terminals. If a node desires to give away the keys to its kingdom it is able to install a universal cohort (transaction) which has access to all data and which performs all requests.

If a transaction desires to initiate a process (cohort) in a new node a few process of the transaction must request that the node construct a cohort and that the cohort go into session with the requesting process (see data communications section for a discussion of sessions). The picture below demonstrates this.

91_distributed transaction.jpg

Two groups of a distributed transaction in session

A process carries together the transaction name T1 and 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 now 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. Therefore they need to:

a) Follow the lock protocol of holding locks to end of transaction (see section on locking).
b) Examine the two-phase commit protocol (see recovery section).

These comments apparently generalize to transactions of more than two charts.

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