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7.1: Using WebSphere Application Server in a multimachine environment >
7.1.6: Managing state >
7.1.6.3: Server affinity
7.1.6.3: Server affinity
A load distribution facility (such as the workload management service) is not always
free to pick any available server when it redirects client requests.
- For stateful session beans or entity beans within the context of a transaction, there is
only one valid server. An entity bean is instantiated on a single server in a single
container during the context of a transaction. Subsequent client requests must be directed
to that server. The workload management service always directs client requests to a
stateful session bean to the single server instance containing the bean. In either case,
directing the request to the wrong server either causes the request to fail or forces the
server to forward it to the correct server at a high performance cost.
- For clustered HTTP sessions or entity beans between transactions, the underlying shared
database ensures that any available server can be used to process client requests.
- For stateless session beans, any available server can be used because each bean instance
is identical.
Server affinity refers to the characteristics of each load distribution facility that
take these constraints into account. The load distribution facility recognizes that
multiple servers can be acceptable targets for a request. However, it also
recognizes that each request can be directed to a particular server where it is handled
better or faster.
Server affinity can be weak or strong.
- In weak server affinity, the system attempts to enforce the desired affinity
for the majority of requests, but does not always guarantee that this affinity will be
respected.
- In strong server affinity, the system guarantees that affinity is always
respected and generates an error when it cannot direct a request to the appropriate
server.
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Peer topics |
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| 7.1.6.1: HTTP sessions, servlets, and the session manager |
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| 7.1.6.2: EJB sessions and transaction affinity |
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