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When running in a cluster the sessions are read, deserialized, serialized and then written to the Redis on each request. This introduces overhead when compared to storing sessions in the Runtime memory. On the other hand, given a limited amount of memory, storing sessions in Redis has the advantage that not all sessions have to be present in the Runtime memory at the same time, only sessions associated with the requests executing at a particular moment. The following scatter plot illustrates the expected performance as average response time depending on the number of concurrent users (requests):

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The actual numbers for response time and number of users will vary from application to application. The plot highlights the following key points:

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Comparing the performance of memory vs Redis session stores using APDEX under the same conditions yields the following scatter plot:

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The key takeaway from this plot is that the performance decrease due to storing sessions in Redis is almost imperceptible by the user.

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