Read-Only, Read-Write and Partition Ops transactions now use a single multiplexed session, instead of a session from the session pool. Multiplexed sessions can handle any number of queries of read-only, read-write and partition ops transactions at the same time.

The use of multiplexed sessions can be disabled for read-only, read-write and partition-ops transactions by setting the environment variables.

GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS=false
GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS_FOR_RW=false
GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS_PARTITIONED_OPS=false

See https://cloud.google.com/spanner/docs/sessions#multiplexed_sessions for more background information about multiplexed sessions.