Add jitter to backup start time to avoid thundering herd (#135065)

homeassistant/components/backup/config.py

@@ -7,6 +7,7 @@
 from dataclasses import dataclass, field, replace
 from datetime import datetime, timedelta
 from enum import StrEnum
+import random
 from typing import TYPE_CHECKING, Self, TypedDict
 
 from cronsim import CronSim
@@ -28,6 +29,10 @@
 CRON_PATTERN_DAILY = "45 4 * * *"
 CRON_PATTERN_WEEKLY = "45 4 * * {}"
 
+# Randomize the start time of the backup by up to 60 minutes to avoid
+# all backups running at the same time.
+BACKUP_START_TIME_JITTER = 60 * 60
+
 
 class StoredBackupConfig(TypedDict):
     """Represent the stored backup config."""
@@ -329,6 +334,8 @@ async def _create_backup(now: datetime) -> None:
             except Exception:  # noqa: BLE001
                 LOGGER.exception("Unexpected error creating automatic backup")
 
+        next_time += timedelta(seconds=random.randint(0, BACKUP_START_TIME_JITTER))
+        LOGGER.debug("Scheduling next automatic backup at %s", next_time)
         manager.remove_next_backup_event = async_track_point_in_time(
             manager.hass, _create_backup, next_time
         )

tests/components/backup/test_websocket.py

@@ -1345,6 +1345,7 @@ async def test_config_update_errors(
         ),
     ],
 )
+@patch("homeassistant.components.backup.config.BACKUP_START_TIME_JITTER", 0)
 async def test_config_schedule_logic(
     hass: HomeAssistant,
     hass_ws_client: WebSocketGenerator,
@@ -1787,6 +1788,7 @@ async def test_config_schedule_logic(
         ),
     ],
 )
+@patch("homeassistant.components.backup.config.BACKUP_START_TIME_JITTER", 0)
 async def test_config_retention_copies_logic(
     hass: HomeAssistant,
     hass_ws_client: WebSocketGenerator,