Deprecate `maxIdleSessions`

[skuruppu]

From a quick search of the repo, it seems that we're no longer using this field. We should mark it as deprecated since otherwise users try to set the field.

[olavloite]

It is actually still being used, however the usefulness of it might be up for discussion:

java-spanner/google-cloud-spanner/src/main/java/com/google/cloud/spanner/SessionPool.java

Line 1684 in 54cfa0f

if (numSessionsInUse >= (options.getMinSessions() + options.getMaxIdleSessions())) {

The basic usage of it in the session pool is as follows:

1. Always create MinSessions sessions, and never go below this value, regardless of how many sessions are actually used.
2. If more sessions than MinSessions are needed, the pool will create additional sessions, as long as this does not exceed MaxSessions.
3. When keeping sessions alive (the ping query) the session pool will try to keep MinSessions + MaxIdleSessions alive.

So setting a value for MaxIdleSessions will prevent additional sessions above MinSessions from expiring.

[thiagotnunes]

I think this is still useful if we want to keep min and max sessions configurable (if we don't want to have just a single num sessions). I would keep this configuration, but would like to know if you agree @skuruppu .

[skuruppu]

It seems that the intention is to keep X number of sessions alive in the pool. In that case, couldn't the user increase MinSessions to X so that they don't need to configure three different parameters?

This is just my opinion but I think we should minimize the number of configurable parameters, especially if the parameters don't directly help to optimize the performance of the client.

[thiagotnunes]

Yes, I think you are right, the only difference I can see is if the user does not want all the maxIdleSessions created before they are necessary, but that would be a very niche case.

[olavloite]

It seems that the intention is to keep X number of sessions alive in the pool. In that case, couldn't the user increase MinSessions to X so that they don't need to configure three different parameters?

Yes, I think you are right, the only difference I can see is if the user does not want all the maxIdleSessions created before they are necessary, but that would be a very niche case.

Correct, this setting will prevent the excess sessions from being removed from the session pool, but they will only be created if the application actually needs more than MinSessions sessions. So setting MinSessions to a higher value would achieve almost the same. I think MaxIdleSessions made sense before BatchCreateSessions was introduced, as setting a (very) high value for MinSessions would cause a large burst of CreateSession RPCs when a session pool was created. That is no longer the case, and I think that this setting does not really add any value any more. Users should instead increase the value of MinSessions if they are currently using MaxIdleSessions in combination with a lower value for MinSessions.