Rack::Session::Abstract::Persisted https://github.com/rack/rack/blob/master/lib/rack/session/abstract/id.rb

def call(env)
  context(env)
end

def context(env, app = @app)
  req = make_request env
  prepare_session(req)
  status, headers, body = app.call(req.env)
  res = Rack::Response::Raw.new status, headers
  commit_session(req, res)
  [status, headers, body]
end

....

def commit_session?(req, session, options)
  if options[:skip]
    false
  else
    has_session = loaded_session?(session) || forced_session_update?(session, options)
    has_session && security_matches?(req, options)
  end
end

....

def commit_session(req, res)
  session = req.get_header RACK_SESSION
  options = session.options

  if options[:drop] || options[:renew]
    session_id = delete_session(req, session.id || generate_sid, options)
    return unless session_id
  end

  return unless commit_session?(req, session, options)

  session.send(:load!) unless loaded_session?(session)
  session_id ||= session.id
  session_data = session.to_hash.delete_if { |k, v| v.nil? }

  if not data = write_session(req, session_id, session_data, options)
    req.get_header(RACK_ERRORS).puts("Warning! #{self.class.name} failed to save session. Content dropped.")
  elsif options[:defer] and not options[:renew]
    req.get_header(RACK_ERRORS).puts("Deferring cookie for #{session_id}") if $VERBOSE
  else
    cookie = Hash.new
    cookie[:value] = data
    cookie[:expires] = Time.now + options[:expire_after] if options[:expire_after]
    cookie[:expires] = Time.now + options[:max_age] if options[:max_age]
    set_cookie(req, res, cookie.merge!(options))
  end
end

ActiveRecordStore implementation (https://github.com/rails/activerecord-session_store/blob/master/lib/action_dispatch/session/active_record_store.rb)

def write_session(request, sid, session_data, options)
  logger.silence_logger do
    record = get_session_model(request, sid)
    record.data = session_data
    return false unless record.save

    session_data = record.data
    if session_data && session_data.respond_to?(:each_value)
      session_data.each_value do |obj|
        obj.clear_association_cache if obj.respond_to?(:clear_association_cache)
      end
    end

    sid
  end
end