From 9901f3c3dd0346fb7bf2ed3a157d47fec9d2ce2c Mon Sep 17 00:00:00 2001
From: Erik Montnemery <erik@montnemery.com>
Date: Thu, 9 Jan 2025 10:53:33 +0100
Subject: [PATCH] Add jitter to backup start time to avoid thundering herd
 (#135065)

---
 homeassistant/components/backup/config.py | 7 +++++++
 tests/components/backup/test_websocket.py | 2 ++
 2 files changed, 9 insertions(+)

diff --git a/homeassistant/components/backup/config.py b/homeassistant/components/backup/config.py
index 3c5d5d39f7e..7c40792aec5 100644
--- a/homeassistant/components/backup/config.py
+++ b/homeassistant/components/backup/config.py
@@ -7,6 +7,7 @@ from collections.abc import Callable
 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 @@ if TYPE_CHECKING:
 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 @@ class BackupSchedule:
             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
         )
diff --git a/tests/components/backup/test_websocket.py b/tests/components/backup/test_websocket.py
index 307a1d79e0c..e95481373d6 100644
--- a/tests/components/backup/test_websocket.py
+++ b/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,