title: What happens when a connection pool is exhausted?
body: <p>I'm reading about SQLAlchemy's connection pooling, which has a default of 5 connections and will by default overflow to 10.</p>
  
  <p>If the number of cached connections is exceeded, what happens? Are subsequent requests queued until a free connection becomes available or will a new connection that doesn't enter the pool be created?</p>
  
  <p>Also, what happens to unused connections when the pool has "overflowed" to its default max of 10? Do those connections disconnect after the default time (as with the standard pool), or are they released more aggressively than the standard pool?</p>