Add request.db_replica property connecting to a DB replica

This commit only adds the setup necessary and a /_status check for the
new property.

Follow up commits will start using the new property in endpoints that
potentially make long queries.

h/config.py

@@ -57,6 +57,9 @@ def configure(environ=None, settings=None):  # pylint: disable=too-many-statemen
     settings_manager.set("mail.host", "MAIL_HOST")
     settings_manager.set("mail.port", "MAIL_PORT", type_=int)
     settings_manager.set("sqlalchemy.url", "DATABASE_URL", required=True)
+    settings_manager.set(
+        "sqlalchemy.replica.url", "REPLICA_DATABASE_URL", required=False
+    )
 
     # Configuration for Pyramid
     settings_manager.set("secret_key", "SECRET_KEY", type_=_to_utf8, required=True)

h/db/__init__.py

@@ -101,6 +101,25 @@ def close_the_sqlalchemy_session(_request):
     return session
 
 
+def _replica_session(request):  # pragma: no cover
+    engine = create_engine(request.registry.settings["sqlalchemy.replica.url"])
+    session = Session(bind=engine)
+
+    # While this is superflux when using a real replica it guarantees that usage of request.db_replica
+    # in the codebase never expects to be able to write to the DB, useful on the dev and tests environments.
+    session.execute(text("SET SESSION CHARACTERISTICS AS TRANSACTION READ ONLY;"))
+
+    @request.add_finished_callback
+    def close_the_sqlalchemy_session(_request):
+        # Close any unclosed DB connections.
+        # It's okay to call `session.close()` even if the session does not need to
+        # be closed, so just call it so that there's no chance
+        # of leaking any unclosed DB connections.
+        session.close()
+
+    return session
+
+
 def _maybe_create_default_organization(engine, authority):  # pragma: no cover
     from h.services.organization import OrganizationService
 
@@ -144,3 +163,4 @@ def includeme(config):  # pragma: no cover
     # that view functions need only refer to `request.db` in order to retrieve
     # the current database session.
     config.add_request_method(_session, name="db", reify=True)
+    config.add_request_method(_replica_session, name="db_replica", reify=True)

h/views/status.py

@@ -17,6 +17,13 @@ def status(request):
         log.exception(err)
         raise HTTPInternalServerError("Database connection failed") from err
 
+    if "replica" in request.params:
+        try:
+            request.db_replica.execute(text("SELECT 1"))
+        except Exception as err:
+            log.exception(err)
+            raise HTTPInternalServerError("Replica database connection failed") from err
+
     if "sentry" in request.params:
         capture_message("Test message from h's status view")
 

tests/unit/h/views/status_test.py

@@ -28,12 +28,27 @@ def test_it_sends_test_messages_to_sentry(self, pyramid_request, capture_message
 
         capture_message.assert_called_once_with("Test message from h's status view")
 
+    def test_it_access_the_replica(self, pyramid_request, db_replica):
+        pyramid_request.params["replica"] = ""
+        db_replica.execute.side_effect = Exception("explode!")
+
+        with pytest.raises(HTTPInternalServerError) as exc:
+            status(pyramid_request)
+
+        assert "Replica database connection failed" in str(exc.value)
+
     @pytest.fixture
     def db(self, pyramid_request):
         db = mock.Mock()
         pyramid_request.db = db
         return db
 
+    @pytest.fixture
+    def db_replica(self, pyramid_request):
+        db = mock.Mock()
+        pyramid_request.db_replica = db
+        return db
+
 
 @pytest.fixture(autouse=True)
 def capture_message(patch):

tox.ini

@@ -101,6 +101,7 @@ setenv =
     dev: WEB_CONCURRENCY = {env:WEB_CONCURRENCY:2}
     tests: COVERAGE_FILE = {env:COVERAGE_FILE:.coverage.{envname}}
     dev: DATABASE_URL = {env:DATABASE_URL:postgresql://postgres@localhost/postgres}
+    dev: REPLICA_DATABASE_URL = {env:DATABASE_URL:postgresql://postgres@localhost/postgres}
     tests: DATABASE_URL = {env:DATABASE_URL:postgresql://postgres@localhost/h_tests}
     functests: DATABASE_URL = {env:DATABASE_URL:postgresql://postgres@localhost/h_functests}
     OBJC_DISABLE_INITIALIZE_FORK_SAFETY = YES