Reuse connection

Tap-postgres opens a new connection every time it needs to cast a value.
This is highly inefficient as opening a connection is usually a slow and
resource-intensive operation. An easy fix would be to use something like
PgBouncer, but it's even better if we open just once connection and
reuse it for all queries.

We created a Singleton Postgres connection wrapper. This wrapper
actually holds up to two connections, since we need two different
connection factories. The `connect` method returns the connection we
need based on the arguments provided.

tap_postgres/db.py

@@ -11,6 +11,8 @@
 from typing import List
 from dateutil.parser import parse
 
+from tap_postgres.postgres import Postgres
+
 LOGGER = singer.get_logger('tap_postgres')
 
 CURSOR_ITER_SIZE = 20000
@@ -39,23 +41,8 @@ def fully_qualified_table_name(schema, table):
 
 
 def open_connection(conn_config, logical_replication=False):
-    cfg = {
-        'application_name': 'pipelinewise',
-        'host': conn_config['host'],
-        'dbname': conn_config['dbname'],
-        'user': conn_config['user'],
-        'password': conn_config['password'],
-        'port': conn_config['port'],
-        'connect_timeout': 30
-    }
-
-    if conn_config.get('sslmode'):
-        cfg['sslmode'] = conn_config['sslmode']
-
-    if logical_replication:
-        cfg['connection_factory'] = psycopg2.extras.LogicalReplicationConnection
-
-    conn = psycopg2.connect(**cfg)
+    pg = Postgres.get_instance()
+    conn = pg.open_connection(conn_config, logical_replication)
 
     return conn
 

tap_postgres/postgres.py

@@ -0,0 +1,46 @@
+import psycopg2
+import psycopg2.extras
+
+
+# pylint: disable=missing-class-docstring,missing-function-docstring
+class Postgres:
+    __instance = None
+
+    @staticmethod
+    def get_instance():
+        if Postgres.__instance is None:
+            Postgres()
+
+        return Postgres.__instance
+
+    def __init__(self):
+        if Postgres.__instance is not None:
+            raise Exception("This class is a singleton!")
+
+        Postgres.__instance = self
+        self.connections =  {"logical": None, "transactional": None}
+
+    def open_connection(self, conn_config, logical_replication):
+        connection_type = "logical" if logical_replication else "transactional"
+
+        cfg = {
+            'application_name': 'pipelinewise',
+            'host': conn_config['host'],
+            'dbname': conn_config['dbname'],
+            'user': conn_config['user'],
+            'password': conn_config['password'],
+            'port': conn_config['port'],
+            'connect_timeout': 30
+        }
+
+        if conn_config.get('sslmode'):
+            cfg['sslmode'] = conn_config['sslmode']
+
+        if logical_replication:
+            cfg['connection_factory'] = psycopg2.extras.LogicalReplicationConnection
+
+        if not self.connections[connection_type] or self.connections[connection_type].closed:
+            self.connections[connection_type] = psycopg2.connect(**cfg)
+
+        return self.connections[connection_type]
+