In
those replicas, request processing could be deferred until failures occur or they could be
executed lazily.
Another issue relevant to the automatic replication scheme consists of the HTTP analysis
which the HA-module makes. Its operation is similar, to some extent, to level-7 cluster
front-ends. It also provides replication transparency to the service clients. Analogously,
Web applications interact with the HA-module as it if was the actual client, so that the Web
application is also unaware about replication.
Additionally, HTTP analysis allows several optimizations to be included into the HA-module.
For instance, it will be possible to detect several read-only operations, so that they are
not multicast to the replica set, but only processed by one replica. Examples of read-only
operations would be, for instance, image downloads, or any URL that could be configured
into the HA-module as a read operation.
Replication.Examples.and.Replication.Scenarios
As an example of Web-based application, let us consider an online personals contact service.
Write operations that need high-availability support could be, besides online payment,
profile editing, and person-to-person contact messages. Additionally, a set of cumulative
statistics could be recorded.
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