Connection Pool

[sbellware]

• Raw PG connections are pooled
• Session checks out a connection

Consumer

• Prior to a consumer fetching a batch, a connection is checked out from the pool
• Prior to every DB interaction, a new connection is checked out
• After processing the batch [sic] check it back in

Session

• Session has a reference to the connection pool
• Move most of implementation to Connection class

Connection

• Egress adapter of PG's raw connection
• Session's implementation that pertains to connection moves to MessageStore::Connection

Connection Pool

• When a connection is checked out, it's tested
• If the connection is inactive, discard it, and as the pool for a NEW connection
• Connection pool's list of connections is process-global (ie: Ruby process-wide singleton; an instance var in class context)
• Connection pool does checkout based on Thread.current
• If there is a connection is in thread local storage, use it (still test it, though)
• If there is no connection in thread local storage, get it from the pool's list
• Use shift and unshift on the list. Threat it as a queue.

Reaper

• Connection pool may need a reaper for threads

Connection Pool Interface

• get
• get_new [sic]
• checkout

Connection Pool Checkout

• get connection from list
• remove it from list
• test it
• if inactive, get a new one (and check it in)
• return the connection
• If connection is associated with a thread, it doesn't have to be tested (it's assumed to be active because threads are short-lived)
• A connection associated with a thread is not ALSO in the pool's connection list
  So, connection doesn't have any "reconnect" ability. The pool does the "reconnect" by disposing of an inactive connection, and then constructing a new, active connection

Connection Pool Checkin

• remove it from thread local storage to disassociate it from the thread

• add it to the list

• Every time consumer fetches a batch, the connection is tested, and possibly a new raw PG connection is created (if the previous one was inactive)

• Need abstraction for raw PG connection that has the "re-connect" implementation

Reference

• Rails connection pool: https://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/ConnectionPool.html
• Rails connection (AbstractAdapter): https://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/AbstractAdapter.html