This means that in a cluster environment, one has to propagate updates asynchronously
without two-phase commit protocol.
An interesting approach is using group communication primitives (Kemme, 2000;
Wu & Kemme, 2005): a client submits its update to one database server which then
broadcasts the update to all other cluster nodes using an atomic broadcast protocol.
The use of appropriate network primitives makes a concluding two phase commit
OLAP with a Database Cluster 24
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unnecessary, because those protocols guarantee the ordered delivery of the messages
so that the global serialisation order can be determined locally by each node.
However, group communication protocols must determine this global order first,
and hence add a relatively high overhead. Approaches such as by Wu and Kemme
(2005) furthermore require direct support by the database management system,
which violates the component-oriented nature of a cluster of databases.
There are a variety of approaches to asynchronous replication. Most restrict transactions
to access only a single node in order to be able to ensure serialisability
for certain restricted cluster configurations (e.g., Breitbart, Komondoor, Rastogi,
Seshadri, & Silberschatz, 1999).
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