Don't create a new connection for each request

[Totktonada]

Since our tests are time bounded, creating a connection eats a time that can be used to give more dense workload to tarantool.

I didn't investigated the code much against this question, but I see the problem in the bank-lua test:

jepsen.tarantool/src/tarantool/bank.clj

Lines 54 to 64 in 6165e6e

	:transfer
	(let [{:keys [from to amount]} (:value op)
	con (cl/open (first (db/primaries test)) test)
	table (clojure.string/upper-case table-name)
	r (-> con
	(sql/query [(str "SELECT _WITHDRAW('" table "'," from "," to "," amount ")")])
	first
	:COLUMN_1)]
	(if (false? r)
	(assoc op :type :fail, :value {:from from :to to :amount amount})
	(assoc op :type :ok))))))

(Consider cl/open.)

Moreover, the (db/primaries test) call also creates its own connection and performs a request:

jepsen.tarantool/src/tarantool/client.clj

Lines 63 to 68 in 6165e6e

	(defn primary
	[node]
	(let [conn (open node test)
	leader (:COLUMN_1 (first (sql/query conn ["SELECT _LEADER()"])))]
	;(assert leader)
	leader))

How it should work, I think:

• We should keep the connection until it dies. In case of several instances (of a replicaset), don't know: either current one or all. The latter seems better.
• We should request leadership information using the existing connection / connection pool.
• We should NOT renew a leader each time: instead, do it when the leader is gone.
  • If the instance becomes unavailable.
  • For write requests we can catch an error about the read-only state.
  • For read requests, which should be executed on a leader (if there are such cases), we should implement some server side check, whether the node that executes a request is a leader.
• Re-create died connections or configure tarantool-java to overcome long unavailability, but still reconnect quite fast.