Proposition of the Independent Recovery Protocols

There was a lot of effort invested into research of ideal independent recovery protocol.
The results are mainly negative.

Prop. 1
Independent recovery protocols exist only for single-site failures. There exists no independent recovery
protocol which is resilient to multiple-site failures.

Prop 2.
There exists no nonblocking protocol that is resilient to a network partition if messages are lost when the
partition occurs.

Prop 3.
There exist nonblocking protocols which are resilient to a single network partition if all undeliverable messages
are sent back to the sender.

Prop. 4
There exists no nonblocking protocol which is resilient to a multiple partition.

Thus it exists no a general solution of this problem.
Practical solutions: the largest partition terminates the transaction to be not blocked.

Problem: which partition is the largest?

Primary site approach and the majority approach

There are different methods to decide which partition is largest to terminate the group level transaction.

Primary site approach:

A site is designated as primary site, and the partition containing this primary site is allowed to terminate
the transaction. It is usual to denote the role of primary site to the coordinator. In this case all
transactions within this partition are terminated correctly.

If the primary site differs from the coordinator site, then a 3PC termination protocol should be used to
terminate all transaction of the group with the primary site.

Majority approach

Only the group containing the majority of sites can terminate the transaction. The sites in the groups may
vote for aborting or for committing. The majority of sites must agree on the abort or commit before the
transaction terminates.


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