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As seen in Table 5-1, we need at least a multiple cluster topology to achieve a major release update while keeping the application available during the update. This is because a major release update involves changes in the EIS tier, either the database or in a back-end application.
An upgrade type application update can be done in a single cell environment if the application is able to handle changes to the persistence layer. If this is not the case, then we need at least a multiple cluster topology to stay online with your application while updating it.
Another issue for these two update types might be that you have to wait for active sessions to expire. This is true for the multiple cells and multiple clusters topologies because there is no failover between cells or clusters. Therefore the first step during the update is to make sure no new requests arrive at the cell or cluster.
