tree eaaa1ab7ddb6db6ef010847cc454cde58cdb8cb8
parent 192e002f6cb79ae84a207270db9539a30f2d8cea
author Olav Loite <koloite@gmail.com> 1555836736 +0200
committer Knut Olav Løite <koloite@gmail.com> 1572250496 +0000

spanner: keep better track of max sessions

The session pool did not keep proper track of the max number of sessions in use within one
maintenance window. This change ensures that the max number of sessions in use is actually kept
track of per window, and prevents the maintainer from deleting sessions too eagerly. This will
change the behavior of the session pool and lead to the pool keeping more sessions in the pool for
a longer time, and to less deletion and creation of sessions.

The logic is now:
1. Keep track of the max number of sessions in use when a session is taken from the pool, instead
of probing this only at the moment that the maintainer is running. This prevents the session pool
from missing spikes in session usage.
2. Store the max number of sessions during a health check cycle in a maintenance window struct
containing one value for each cycle in an entire window of 10 cycles.
3. Only delete sessions after at least one full maintenance window has passed. After that,
the maintainer will delete sessions based on a rolling set of maxSessionsInUse for the last
10 minutes.
4. The shrinkPool and replenishPool both take a max number of sessions to delete/create as an
argument in order to prevent the methods from deleting/creating more sessions than intended if the
number of sessions in the pool change as a result of other operations on the pool.
5. If the shrinkPool method notices that the session pool has started creating new sessions,
the method will stop deleting any further sessions.

Fixes #1303, #1382, #1398 and #1406.

Change-Id: I20c0821a45b8ce02cb09a9dc492d389da03cccab
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/40150
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Shanika Kuruppu <skuruppu@google.com>
Reviewed-by: Jean de Klerk <deklerk@google.com>