Allow to have a master/slave DB configuration

c2cgeoportal/__init__.py

@@ -564,6 +564,15 @@ def includeme(config):
     config.include(pyramid_tm.includeme)
     config.include("pyramid_closure")
 
+    if "sqlalchemy_slave.url" in settings and \
+            settings["sqlalchemy.url"] != settings["sqlalchemy_slave.url"]:
+        # Setup a slave DB connection and add a tween to switch between it and the default one.
+        log.info("Using a slave DB for reading")
+        engine_slave = sqlalchemy.engine_from_config(config.get_settings(), "sqlalchemy_slave.")
+        sqlahelper.add_engine(engine_slave, name="slave")
+        config.add_tween("c2cgeoportal.models.db_chooser_tween_factory",
+                         over="pyramid_tm.tm_tween_factory")
+
     # initialize the dbreflection module
     dbreflection.init(engine)
 

c2cgeoportal/models.py

@@ -974,3 +974,25 @@ class Shorturl(Base):
     creation = Column(DateTime)
     last_hit = Column(DateTime)
     nb_hits = Column(Integer)
+
+
+def db_chooser_tween_factory(handler, registry):
+    """
+    Tween factory to route to a slave DB for read-only queries.
+    Must be put over the pyramid_tm tween and sqlahelper must have a "slave" engine
+    configured.
+    """
+    def db_chooser_tween(request):
+        session = DBSession()
+        old = session.bind
+        if request.method in ("GET", "OPTIONS"):
+            log.debug("Using slave database for %s %s" % (request.method, request.path))
+            session.bind = sqlahelper.get_engine("slave")
+        else:
+            log.debug("Using master database for %s %s" % (request.method, request.path))
+        try:
+            return handler(request)
+        finally:
+            session.bind = old
+
+    return db_chooser_tween

doc/integrator/cluster.rst

@@ -28,3 +28,25 @@ In this case the first thing to do is to separate the services:
  * Print
  * WSGI application
  * MapServer / MapCache / Memcached
+
+To have better DB performances, one can setup multiple Postgres servers in a master/slave
+configuration. To enable this feature on GeoMapFish, you must add this to your
+``vars_<project>.yaml``:
+
+.. code:: yaml
+
+    dbhost_slave: my_db_slave_hostname
+    sqlalchemy_slave:
+        url: postgresql://{dbuser}:{dbpassword}@{dbhost_slave}:{dbport}/{db}
+
+Add in your project Makefile ``<package>.mk``:
+
+.. code:: makefile
+
+   CONFIG_VARS += sqlalchemy_slave.url
+
+Then, all the GET and OPTIONS requests will use one of the slave Postgres instances (DNS
+pointing to more than one IP if you need it) and the rest will use the master instance.
+It is assumed, here, that the views handling the GET and OPTIONS queries don't cause write
+operations to the database (not supported by slave instances). If it's not the case in your
+application (bad practice), don't use this feature.