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2002). As shown there, performance is quite reasonable when protocols are executed over
a reduced set of nodes, which is the most typical situation for a highly-available Web application
setting.
The.Request.Manager
This component is possibly the most visible component of the highly-available architecture
we propose. Visibility is important because this is the component that directly interacts with
both clients and Web applications, and it will be the component that should be configured
for optimizations.
Its goal is to ensure that client requests are processed, and that service replies reach clients
even if failures occur. To ensure it, besides parsing and analyzing HTTP requests, it implements
a reliable invocation protocol.
Reliable invocation protocols (Rodrigues, Siegel, & Ver?ssimo, 1994) are required by every
replicated system when replicas have distinct addresses. In these scenarios, clients contact
one particular replica to request a service, and in case that replica fails, clients have to
reconnect to another replica to retry the service request. This scheme is different to the replication
scheme that results when all replicas share the same address (Damani et al.
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