DPE-4613 Test for multi-relation scale in/out

src/relations/mysql_provider.py

@@ -222,11 +222,13 @@ def _configure_endpoints(self, _) -> None:
 
         relation_data = self.database.fetch_relation_data()
         for relation in relations:
-            # only update endpoints if on_database_requested has executed
-            if relation.id not in relation_data:
-                continue
+            # only update endpoints if on_database_requested on any
+            # relation
+            if relation.id in relation_data:
+                break
+            return
 
-            self.charm._mysql.update_endpoints()
+        self.charm._mysql.update_endpoints()
 
     def _on_update_status(self, _) -> None:
         """Handle the update status event.

tests/integration/test_multi_relations.py

@@ -0,0 +1,99 @@
+#!/usr/bin/env python3
+# Copyright 2024 Canonical Ltd.
+# See LICENSE file for licensing details.
+
+
+from pathlib import Path
+
+import pytest
+import yaml
+from pytest_operator.plugin import OpsTest
+
+DB_METADATA = yaml.safe_load(Path("./metadata.yaml").read_text())
+SCALE_APPS = 7
+SCALE_UNITS = 3
+
+
+@pytest.mark.group(1)
+@pytest.mark.abort_on_fail
+async def test_build_and_deploy(ops_test: OpsTest):
+    """Build the charm and deploy 1 units to ensure a cluster is formed."""
+    # Build and deploy charm from local source folder
+    db_charm = await ops_test.build_charm(".")
+
+    config = {"profile": "testing"}
+    resources = {"mysql-image": DB_METADATA["resources"]["mysql-image"]["upstream-source"]}
+
+    await ops_test.model.deploy(
+        db_charm,
+        application_name="mysql",
+        config=config,
+        num_units=1,
+        resources=resources,
+        series="jammy",
+        trust=True,
+    )
+
+    for i in range(SCALE_APPS):
+        config = {"database_name": f"database{i}", "sleep_interval": "2000"}
+        await ops_test.model.deploy(
+            "mysql-test-app",
+            application_name=f"app{i}",
+            num_units=1,
+            channel="latest/edge",
+            config=config,
+        )
+        await ops_test.model.deploy(
+            "mysql-router-k8s",
+            application_name=f"router{i}",
+            num_units=1,
+            channel="8.0/edge",
+            trust=True,
+        )
+
+
+@pytest.mark.group(1)
+@pytest.mark.abort_on_fail
+async def test_relate_all(ops_test: OpsTest):
+    """Relate all the applications to the database."""
+    for i in range(SCALE_APPS):
+        await ops_test.model.relate("mysql:database", f"router{i}:backend-database")
+        await ops_test.model.relate(f"app{i}:database", f"router{i}:database")
+
+    await ops_test.model.block_until(
+        lambda: all(unit.workload_status == "active" for unit in ops_test.model.units.values()),
+        timeout=60 * 25,
+        wait_period=5,
+    )
+
+
+@pytest.mark.group(1)
+@pytest.mark.abort_on_fail
+async def test_scale_out(ops_test: OpsTest):
+    """Scale database and routers."""
+    await ops_test.model.applications["mysql"].scale(SCALE_UNITS)
+    for i in range(SCALE_APPS):
+        await ops_test.model.applications[f"router{i}"].scale(SCALE_UNITS)
+    expected_unit_sum = SCALE_UNITS + 4 * SCALE_APPS
+    await ops_test.model.block_until(
+        lambda: all(unit.workload_status == "active" for unit in ops_test.model.units.values())
+        and len(ops_test.model.units) == expected_unit_sum,
+        timeout=60 * 30,
+        wait_period=5,
+    )
+
+
+@pytest.mark.group(1)
+@pytest.mark.abort_on_fail
+async def test_scale_in(ops_test: OpsTest):
+    """Scale database and routers."""
+    await ops_test.model.applications["mysql"].scale(1)
+    for i in range(SCALE_APPS):
+        await ops_test.model.applications[f"router{i}"].scale(1)
+    expected_unit_sum = 1 + 2 * SCALE_APPS
+    await ops_test.model.block_until(
+        lambda: all(unit.workload_status == "active" for unit in ops_test.model.units.values())
+        and len(ops_test.model.units) == expected_unit_sum,
+        timeout=60 * 15,
+        wait_period=5,
+    )