feat: Add support for replicas and cross region replicas

api/app/routers.py

@@ -1,15 +1,70 @@
+import logging
 import random
 
 from django.conf import settings
+from django.core.cache import cache
+from django.db import connections
+
+logger = logging.getLogger(__name__)
+CONNECTION_CHECK_CACHE_TTL = 2
+
+
+def connection_check(database: str) -> bool:
+    try:
+        conn = connections.create_connection(database)
+        conn.connect()
+        usable = conn.is_usable()
+        if not usable:
+            logger.warning(
+                f"Unable to access database {database} during connection check"
+            )
+    except Exception:
+        usable = False
+        logger.error(
+            "Encountered exception during connection",
+            exc_info=True,
+        )
+
+    if usable:
+        cache.set(
+            f"db_connection_active.{database}", "online", CONNECTION_CHECK_CACHE_TTL
+        )
+    else:
+        cache.set(
+            f"db_connection_active.{database}", "offline", CONNECTION_CHECK_CACHE_TTL
+        )
+
+    return usable
 
 
 class PrimaryReplicaRouter:
     def db_for_read(self, model, **hints):
         if settings.NUM_DB_REPLICAS == 0:
             return "default"
-        return random.choice(
-            [f"replica_{i}" for i in range(1, settings.NUM_DB_REPLICAS + 1)]
+
+        replicas = [f"replica_{i}" for i in range(1, settings.NUM_DB_REPLICAS + 1)]
+        replica = self._get_replica(replicas)
+        if replica:
+            # This return is the most likely as replicas should be
+            # online and properly functioning.
+            return replica
+
+        # Since no replicas are available, fall back to the cross
+        # region replicas which have worse availability.
+        cross_region_replicas = [
+            f"cross_region_replica_{i}"
+            for i in range(1, settings.NUM_CROSS_REGION_DB_REPLICAS + 1)
+        ]
+
+        cross_region_replica = self._get_replica(cross_region_replicas)
+        if cross_region_replica:
+            return cross_region_replica
+
+        # No available replicas, so fallback to the default.
+        logger.warning(
+            "Unable to serve any available replicas, falling back to default database"
         )
+        return "default"
 
     def db_for_write(self, model, **hints):
         return "default"
@@ -30,6 +85,27 @@ def allow_relation(self, obj1, obj2, **hints):
     def allow_migrate(self, db, app_label, model_name=None, **hints):
         return db == "default"
 
+    def _get_replica(self, replicas: list[str]) -> None | str:
+        while replicas:
+            if settings.REPLICA_READ_STRATEGY == "DISTRIBUTED":
+                database = random.choice(replicas)
+            elif settings.REPLICA_READ_STRATEGY == "SEQUENTIAL":
+                database = replicas[0]
+            else:
+                assert (
+                    False
+                ), f"Unknown REPLICA_READ_STRATEGY {settings.REPLICA_READ_STRATEGY}"
+
+            replicas.remove(database)
+            db_cache = cache.get(f"db_connection_active.{database}")
+            if db_cache == "online":
+                return database
+            if db_cache == "offline":
+                continue
+
+            if connection_check(database):
+                return database
+
 
 class AnalyticsRouter:
     route_app_labels = ["app_analytics"]

api/app/settings/common.py

@@ -165,6 +165,7 @@
 
 DATABASE_ROUTERS = ["app.routers.PrimaryReplicaRouter"]
 NUM_DB_REPLICAS = 0
+NUM_CROSS_REGION_DB_REPLICAS = 0
 # Allows collectstatic to run without a database, mainly for Docker builds to collectstatic at build time
 if "DATABASE_URL" in os.environ:
     DATABASES = {
@@ -177,11 +178,33 @@
         "REPLICA_DATABASE_URLS", default=[], delimiter=REPLICA_DATABASE_URLS_DELIMITER
     )
     NUM_DB_REPLICAS = len(REPLICA_DATABASE_URLS)
+
+    # Cross region replica databases are used as fallbacks if the
+    # primary replica set becomes unavailable.
+    CROSS_REGION_REPLICA_DATABASE_URLS_DELIMITER = env(
+        "CROSS_REGION_REPLICA_DATABASE_URLS_DELIMITER", ","
+    )
+    CROSS_REGION_REPLICA_DATABASE_URLS = env.list(
+        "CROSS_REGION_REPLICA_DATABASE_URLS",
+        default=[],
+        delimiter=CROSS_REGION_REPLICA_DATABASE_URLS_DELIMITER,
+    )
+    NUM_CROSS_REGION_DB_REPLICAS = len(CROSS_REGION_REPLICA_DATABASE_URLS)
+
+    # DISTRIBUTED spreads the load out across replicas while
+    # SEQUENTIAL only falls back once the first replica connection is faulty
+    REPLICA_READ_STRATEGY = env("REPLICA_READ_STRATEGY", "DISTRIBUTED")
+
     for i, db_url in enumerate(REPLICA_DATABASE_URLS, start=1):
         DATABASES[f"replica_{i}"] = dj_database_url.parse(
             db_url, conn_max_age=DJANGO_DB_CONN_MAX_AGE
         )
 
+    for i, db_url in enumerate(CROSS_REGION_REPLICA_DATABASE_URLS, start=1):
+        DATABASES[f"cross_region_replica_{i}"] = dj_database_url.parse(
+            db_url, conn_max_age=DJANGO_DB_CONN_MAX_AGE
+        )
+
     if "ANALYTICS_DATABASE_URL" in os.environ:
         DATABASES["analytics"] = dj_database_url.parse(
             env("ANALYTICS_DATABASE_URL"), conn_max_age=DJANGO_DB_CONN_MAX_AGE

api/tests/unit/app/test_unit_app_routers.py

@@ -0,0 +1,188 @@
+from pytest_django.fixtures import SettingsWrapper
+from pytest_mock import MockerFixture
+
+from app.routers import PrimaryReplicaRouter, connection_check
+from users.models import FFAdminUser
+
+
+def test_connection_check_to_default_database(db: None, reset_cache: None) -> None:
+    # When
+    connection_check_works = connection_check("default")
+
+    # Then
+    assert connection_check_works is True
+
+
+def test_replica_router_db_for_read_with_one_offline_replica(
+    settings: SettingsWrapper,
+    mocker: MockerFixture,
+    reset_cache: None,
+) -> None:
+    # Given
+    settings.NUM_DB_REPLICAS = 4
+
+    # Set unused cross regional db for testing non-inclusion.
+    settings.NUM_CROSS_REGION_DB_REPLICAS = 2
+    settings.REPLICA_READ_STRATEGY = "DISTRIBUTED"
+
+    conn_patch = mocker.MagicMock()
+    conn_patch.is_usable.side_effect = (False, True)
+    create_connection_patch = mocker.patch(
+        "app.routers.connections.create_connection", return_value=conn_patch
+    )
+
+    router = PrimaryReplicaRouter()
+
+    # When
+    result = router.db_for_read(FFAdminUser)
+
+    # Then
+    # Read strategy DISTRIBUTED is random, so just this is a check
+    # against loading the primary or one of the cross region replicas
+    assert result.startswith("replica_")
+
+    # Check that the number of replica call counts is as expected.
+    conn_call_count = 2
+    assert create_connection_patch.call_count == conn_call_count
+    assert conn_patch.is_usable.call_count == conn_call_count
+
+
+def test_replica_router_db_for_read_with_local_offline_replicas(
+    settings: SettingsWrapper,
+    mocker: MockerFixture,
+    reset_cache: None,
+) -> None:
+    # Given
+    settings.NUM_DB_REPLICAS = 4
+
+    # Use cross regional db for fallback after replicas.
+    settings.NUM_CROSS_REGION_DB_REPLICAS = 2
+    settings.REPLICA_READ_STRATEGY = "DISTRIBUTED"
+
+    conn_patch = mocker.MagicMock()
+
+    # All four replicas go offline and so does one of the cross
+    # regional replica as well, before finally the last cross
+    # region replica is finally connected to.
+    conn_patch.is_usable.side_effect = (
+        False,
+        False,
+        False,
+        False,
+        False,
+        True,
+    )
+    create_connection_patch = mocker.patch(
+        "app.routers.connections.create_connection", return_value=conn_patch
+    )
+
+    router = PrimaryReplicaRouter()
+
+    # When
+    result = router.db_for_read(FFAdminUser)
+
+    # Then
+    # Read strategy DISTRIBUTED is random, so just this is a check
+    # against loading the primary or one of the cross region replicas
+    assert result.startswith("cross_region_replica_")
+
+    # Check that the number of replica call counts is as expected.
+    conn_call_count = 6
+    assert create_connection_patch.call_count == conn_call_count
+    assert conn_patch.is_usable.call_count == conn_call_count
+
+
+def test_replica_router_db_for_read_with_all_offline_replicas(
+    settings: SettingsWrapper,
+    mocker: MockerFixture,
+    reset_cache: None,
+) -> None:
+    # Given
+    settings.NUM_DB_REPLICAS = 4
+    settings.NUM_CROSS_REGION_DB_REPLICAS = 2
+    settings.REPLICA_READ_STRATEGY = "DISTRIBUTED"
+
+    conn_patch = mocker.MagicMock()
+
+    # All replicas go offline.
+    conn_patch.is_usable.return_value = False
+    create_connection_patch = mocker.patch(
+        "app.routers.connections.create_connection", return_value=conn_patch
+    )
+
+    router = PrimaryReplicaRouter()
+
+    # When
+    result = router.db_for_read(FFAdminUser)
+
+    # Then
+    # Fallback to primary database if all replicas are offline.
+    assert result == "default"
+
+    # Check that the number of replica call counts is as expected.
+    conn_call_count = 6
+    assert create_connection_patch.call_count == conn_call_count
+    assert conn_patch.is_usable.call_count == conn_call_count
+
+
+def test_replica_router_db_with_sequential_read(
+    settings: SettingsWrapper,
+    mocker: MockerFixture,
+    reset_cache: None,
+) -> None:
+    # Given
+    settings.NUM_DB_REPLICAS = 100
+    settings.NUM_CROSS_REGION_DB_REPLICAS = 2
+    settings.REPLICA_READ_STRATEGY = "SEQUENTIAL"
+
+    conn_patch = mocker.MagicMock()
+
+    # First replica is offline, so must fall back to second one.
+    conn_patch.is_usable.side_effect = (False, True)
+    create_connection_patch = mocker.patch(
+        "app.routers.connections.create_connection", return_value=conn_patch
+    )
+
+    router = PrimaryReplicaRouter()
+
+    # When
+    result = router.db_for_read(FFAdminUser)
+
+    # Then
+    # Fallback from first replica to second one.
+    assert result == "replica_2"
+
+    # Check that the number of replica call counts is as expected.
+    conn_call_count = 2
+    assert create_connection_patch.call_count == conn_call_count
+    assert conn_patch.is_usable.call_count == conn_call_count
+
+
+def test_replica_router_db_no_replicas(
+    settings: SettingsWrapper,
+    mocker: MockerFixture,
+    reset_cache: None,
+) -> None:
+    # Given
+    settings.NUM_DB_REPLICAS = 0
+    settings.NUM_CROSS_REGION_DB_REPLICAS = 0
+
+    conn_patch = mocker.MagicMock()
+
+    # All replicas should be ignored.
+    conn_patch.is_usable.return_value = True
+    create_connection_patch = mocker.patch(
+        "app.routers.connections.create_connection", return_value=conn_patch
+    )
+
+    router = PrimaryReplicaRouter()
+
+    # When
+    result = router.db_for_read(FFAdminUser)
+
+    # Then
+    # Should always use primary database.
+    assert result == "default"
+    conn_call_count = 0
+    assert create_connection_patch.call_count == conn_call_count
+    assert conn_patch.is_usable.call_count == conn_call_count