In case of a read-only transaction, the
client specifies an explicit freshness limit for the data accessed. Furthermore, decoupled
refresh transactions propagate updates through the cluster.
Transaction management by the coordination middleware guarantees global correctness
and consistency. We deploy a simplified two-layered open-nested transaction
model (Weikum & Schek, 1992): The queries of read-only transactions as submitted
by clients are executed and committed as separate subtransactions in the component
DBMSs. The coordination middleware also contains a global logger. It keeps
track of the update subtransactions on the master node and their decoupled refresh
subtransactions on the secondary nodes. The latter are controlled by the refresher.
This allows being globally correct without distributed commit processing as with,
for example, two-phase-commit (2PC). Avoiding distributed commit processing is
especially important for large clusters.
Physical. Design.Alternatives
A fundamental problem for OLAP with a database cluster is the physical organisation
of data that yields good performance with regard to queries and updates. The two
2 6 R?¶hm
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Idea Group Inc. is prohibited.
primitives for physical design of individual relations that are specific to distribution
are data partitioning and data replication.
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