SbloodyS commented on code in PR #18549:
URL: 
https://github.com/apache/dolphinscheduler/pull/18549#discussion_r4215796359


##########
docs/docs/en/guide/upgrade/upgrade.md:
##########
@@ -75,6 +75,14 @@ Execution result:
 - After version 3.3.X and later, we only support upgrading from 3.0.0. For 
versions lower than this, please download the historical version and upgrade to 
3.0.0.
 - After version 3.3.X and later, binary packages no longer provide plugins 
dependencies by default, so when you use them for the first time, you need to 
download and install them yourself. For more information, please refer to 
[Pseudo-Cluster](../installation/pseudo-cluster.md).
 
+#### Rolling Upgrade Order
+
+When performing a rolling upgrade, **the Alert Server must be upgraded before 
Master/Worker**.
+
+Starting from 3.5.0, the Master may persist new `alert_type` enum values (e.g. 
`TASK_RESULT`) into the `t_ds_alert` table. If the Alert Server has not yet 
been upgraded to a version that includes the new enum value, MyBatis will 
deserialize the unknown value as `null`, causing `AlertSender` to throw a 
`NullPointerException` while building the alert data. The alert will remain 
stuck in `WAIT_EXECUTION` and never be delivered.

Review Comment:
   If the upgrade fails, users need to be provided with specific operation 
steps.



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