I was studying the difference between Sequel's and ActiveRecord's
connection pool, and I was curious why doesn't ActiveRecord automatically
release connections back into the pool after it was used for a query?
Currently it stays checked out and assigned to the original thread, so
other threads cannot use it, even when the original thread died.
To illustrate, the following script executes 10 queries in parallel in 10
threads, but because the pool size is 5, half of the threads will fail
because they couldn't acquire a connection:
require "active_record"
ActiveRecord::Base.establish_connection(
adapter: "sqlite3",
database: "database.sqlite3",
)
threads = []
10.times do
threads << Thread.new do
ActiveRecord::Base.connection.execute "SELECT 1"
end
end
threads.each(&:join)
ActiveRecord::ConnectionTimeoutError: could not obtain a connection from
the pool within 5.000 seconds (waited 5.003 seconds); all pooled
connections were in use
Compare that to Sequel, where everything goes just fine, since it releases
connections back into the pool as soon as the query finished:
require "sequel"
DB = Sequel.sqlite("database.sqlite3", max_connections: 5)
threads = []
10.times do
threads << Thread.new do
DB.run "SELECT 1"
end
end
threads.each(&:join)
ActiveRecord works around this limitation by requiring users to call
`ActiveRecord::Base.clear_active_connections!` at the appropriate place
(end of request in a web app). In Rails this is handled automatically for
you (though at a surprising place – in
`ActiveRecord::QueryCache.complete`), but you need to remember to do it
when using ActiveRecord in other web frameworks. In Rails this started as a
specialized Rack middleware called ConnectionManagement, but has since been
moved into ActionDispatch::Executor middleware.
My question is whether the current connection pool behaviour is intentional
(maybe there is some advantage I'm not seeing?), or it's just that no one
volunteered to fix it yet?
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