Merge pull request #724 from metabase/less_dumb_memoization

less-dumb memoization

src/metabase/driver/generic_sql/util.clj

 (ns metabase.driver.generic-sql.util
   "Shared functions for our generic-sql query processor."
-  (:require [clojure.core.memoize :as memo]
-            [clojure.java.jdbc :as jdbc]
+  (:require [clojure.java.jdbc :as jdbc]
             [clojure.tools.logging :as log]
             [colorize.core :as color]
             [korma.core :as korma]
@@ -9,7 +8,6 @@
             [metabase.driver :as driver]
             [metabase.driver.query-processor :as qp]))
 
-;; Cache the Korma DB connections for a given Database for 60 seconds instead of creating new ones every single time
 (defn- db->connection-spec [{{:keys [short-lived?]} :details, :as database}]
   (let [driver                              (driver/engine->driver (:engine database))
         database->connection-details        (:database->connection-details driver)
@@ -23,14 +21,17 @@
 (def ^{:arglists '([database])}
   db->korma-db
   "Return a Korma database definition for DATABASE.
-   This does a little bit of smart caching (for 60 seconds) to avoid creating new connections when unneeded."
-  (let [-db->korma-db (memo/ttl (fn [database]
-                                  (log/debug (color/red "Creating a new DB connection..."))
-                                  (kdb/create-db (db->connection-spec database)))
-                                :ttl/threshold (* 60 1000))]
-    ;; only :engine and :details are needed for driver/connection so just pass those so memoization works as expected
-    (fn [database]
-      (-db->korma-db (select-keys database [:engine :details])))))
+   Since Korma/C3PO seems to be bad about cleaning up its connection pools, this function is
+   memoized and will return an existing connection pool on subsequent calls."
+  (let [db->korma-db          (fn [database]
+                                (log/debug (color/red "Creating a new DB connection..."))
+                                (kdb/create-db (db->connection-spec database)))
+        memoized-db->korma-db (memoize db->korma-db)]
+    (fn [{{:keys [short-lived?]} :details, :as database}]
+      ;; Use un-memoized version of function for so-called "short-lived" databases (i.e. temporary ones that we won't create a connection pool for)
+      ((if short-lived?
+         db->korma-db
+         memoized-db->korma-db) (select-keys database [:engine :details]))))) ; only :engine and :details are needed for driver/connection so just pass those so memoization works as expected
 
 (def ^:dynamic ^java.sql.DatabaseMetaData *jdbc-metadata*
   "JDBC metadata object for a database. This is set by `with-jdbc-metadata`."