The specifications require that when a connection pool max size is not specified, the driver imposes a default max size. The current max size in driver is 5. The size required in specification is 100.

We think 100 is excessive for typical Ruby applications. At the same time 5 is insufficient FLE may require up to 2 connections in addition to the connection used for operation against the server, thus effectively a pool of size 5 can permit only 1 concurrent operation.

We think a default max size of 20 is a good compromise between permitting FLE usage with the same level of service as non-FLE usage right now (5 concurrent operations) while not creating excessively large pools.