12c ADG 延迟告警，延迟消失后，告警自动消失。

DGMGRL> show configuration;

Configuration - dg_config

  Protection Mode: MaxPerformance
  Members:
  orcldg - Primary database
    orcl   - Physical standby database 
      Warning: ORA-16857: standby disconnected from redo source for longer than specified threshold

Fast-Start Failover: DISABLED

Configuration Status:
WARNING   (status updated 41 seconds ago)

DGMGRL> exit
$oerr ora 16857
16857,0000, "standby disconnected from redo source for longer than specified threshold"
// *Cause: The amount of time the standby was disconnected from the 
//         redo source database exceeded the value specified by the 
//         'TransportDisconnectedThreshold' database property. It is caused by 
//         no network connectivity between the redo source and the standby 
//         databases.
// *Action: Ensure that there is network connectivity between the redo source
//          and standby databases, and the redo source is working properly.
$dgmgrl /
DGMGRL for Linux: Version 12.1.0.2.0 - 64bit Production

Copyright (c) 2000, 2013, Oracle. All rights reserved.

Welcome to DGMGRL, type "help" for information.
Connected as SYSDG.
DGMGRL> show configuration;

Configuration - dg_config

  Protection Mode: MaxPerformance
  Members:
  orcldg - Primary database
    orcl   - Physical standby database 

Fast-Start Failover: DISABLED

Configuration Status:
SUCCESS   (status updated 20 seconds ago)

DGMGRL>

说明：

The Data Guard broker also has configurable database properties that can be used to generate warnings when a transport or apply lag exceed a user defined value:

The ApplyLagThreshold configurable database property generates a warning status for a logical or physical standby when the database's apply lag exceeds the value specified by the property. The property value is expressed in seconds. A value of 0 seconds results in no warnings being generated when an apply lag exists.  As a best practice, Oracle recommends setting ApplyLagThreshold to at least 15 minutes.
The TransportLagThreshold configurable database property can be used to generate a warning status for a logical, physical, or snapshot standby when the database's transport lag exceeds the value specified by the property. The property value is expressed in seconds. A value of 0 seconds results in no warnings being generated when a transport lag exists. As a best practice, Oracle recommends setting TransportLagThreshold to at least 15 minutes.