### Purpose
If you operate an Exchange Database Availability Group (DAG) with 2 or more servers, you may need to do maintenance to one of the members, and during that maintenance, it's possible that one of the databases of the server that was rebooted etc will be out-of-date.  In case this happens, it may suspend the database replication to one of the DAG's member servers.

### Checking DAG Database Replication Status
You will want to first log into one of the DAG servers and open the *"Exchange Management Shell"*.  From there, run the following command to get the status of database replication.  An example of the kind of output you would see is below the command.

```powershell
Get-MailboxDatabaseCopyStatus * | Format-Table Name, Status, CopyQueueLength, ReplayQueueLength, ContentIndexState
```

| **Name** | **Status** | **CopyQueueLength** | **ReplayQueueLength** | **ContentIndexState** |
| :--- | ---: | ---: | ---: | ---: |
| DB01\MX-DAG-01 | Mounted | 0 | 0 | Healthy |
| DB01\MX-DAG-02 | Healthy | 0 | 0 | Healthy |

!!! note "Example Output Breakdown"
    In the above example output, you can see that there are two member servers in the DAG, `MX-DAG-01` and `MX-DAG-02`.  Then you will see that there is a status of `Mounted`, this means that `MX-DAG-01` is the active production server; this means that it is handling all mailflow and web requests / webmail.  

    **CopyQueueLength**: This is a number of database "*transaction logs*" that have taken place since a replica database stopped getting updates.  This is the queue of all database transactions that are being copied from the production (mounted) database to replica databases.  This data is not immediately written to the replica database(s).

    **CopyReplayLength**: This represents the queue of all data that was successfully copied from the production database to the replica database on the given DAG member that still needs to process on the replica database.  The "**CopyQueueLength**" will need to reach zero before the "**CopyReplayLength**" will start making meaningful progress to reaching zero. 
    
    When both the "**CopyQueueLength**" and "**CopyReplayLength**" queues have reached zero, the replica database(s) will have reached 100% parity with the production (active/mounted) database.

### Troubleshooting
You may run into issues where either the `Status` or `ContentIndexState` are either Unhealthy, Suspended, or Failed.  If this happens, you need to resume replication of the database from the production active/mounted server to the server that is having issues.  In the worst-case, you would re-seed the replica database from-scratch.

### If `Status` is Unhealthy
If one of the DAG members has a status of "**Unhealthy**", you can run the following command to attempt to resume replication.
```powershell
Resume-MailboxDatabaseCopy -Identity "DB01\MX-DAG-02"
```

If this fails to cause replication to resume, you can try telling the database to just focus on replication, which tells it to copy the queues and replay them on the replica database, while avoiding interacting with the "**ContentIndexState**" which can be individually fixed in the commands below:
```powershell
Resume-MailboxDatabaseCopy -Identity "DB01\MX-DAG-02" -ReplicationOnly
Update-MailboxDatabaseCopy -Identity "DB01\MX-DAG-02" -CatalogOnly -BeginSeed