feat: implemenet connection pooling

pg-pool have removed `min` option (brianc/node-pg-pool#135). Our implementation partially implements the original pro-active pooling logic. However, unlike the original implementation, we only pool new queries when another query is complete. The benefit of this approach is that if the process is otherwise idle, there are no unnecessary sockets being kept. Equally, the moment a single query is issued, we ensure that we bring the pool size to the minimum number of connections expected.

src/factories/createPool.js

@@ -17,6 +17,54 @@ import Logger from '../Logger';
 import bindPool from '../binders/bindPool';
 import createClientConfiguration from './createClientConfiguration';
 
+const createIdleConnection = (log, pool) => {
+  pool
+    .connect()
+    .then((connection) => {
+      return connection.release();
+    })
+    .catch((error) => {
+      log.error({
+        error: serializeError(error),
+      }, 'connection could not be added to the pool');
+    });
+};
+
+/**
+ * Attempts to keep a minimum of `minimumPoolSize` connections.
+ *
+ * We do not pro-actively create connections (e.g. using setInterval); instead we listen
+ * for when connections are removed and if there are less than `minimumPoolSize` already
+ * started connections, then we create new connections.
+ *
+ * As this process relies on pool being actively used to make new connections, (as opposed to using `setInterval`)
+ * we do not implement an explicit method of killing the connection pooling logic either;
+ * connections are killed after `idleTimeout`.
+ */
+const managePoolSize = (log, pool, minimumPoolSize: number) => {
+  let connectionCount = 0;
+
+  const provision = () => {
+    let missingConnectionCount = Math.max(0, minimumPoolSize - connectionCount);
+
+    while (missingConnectionCount-- > 0) {
+      createIdleConnection(log, pool);
+    }
+  };
+
+  pool.on('connect', () => {
+    connectionCount++;
+
+    provision();
+  });
+
+  pool.on('remove', () => {
+    connectionCount--;
+
+    provision();
+  });
+};
+
 /**
  * @param connectionUri PostgreSQL [Connection URI](https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-CONNSTRING).
  */
@@ -37,7 +85,6 @@ export default (
   poolConfiguration.connectionTimeoutMillis = clientConfiguration.connectionTimeout;
   poolConfiguration.idleTimeoutMillis = clientConfiguration.idleTimeout;
   poolConfiguration.max = clientConfiguration.maximumPoolSize;
-  poolConfiguration.min = clientConfiguration.minimumPoolSize;
 
   if (clientConfiguration.connectionTimeout === 'DISABLE_TIMEOUT') {
     poolConfiguration.connectionTimeout = 0;
@@ -91,6 +138,14 @@ export default (
 
   const pool = new pg.Pool(poolConfiguration);
 
+  if (clientConfiguration.minimumPoolSize > 0) {
+    managePoolSize(
+      poolLog,
+      pool,
+      clientConfiguration.minimumPoolSize,
+    );
+  }
+
   pool.slonik = {
     poolId,
     typeOverrides: null,

test/helpers/createPool.js

@@ -37,6 +37,7 @@ export default (clientConfiguration: ClientConfigurationInputType = defaultConfi
   };
 
   const internalPool = {
+    _pulseQueue: () => {},
     _remove: () => {},
     connect: () => {
       return connection;