AEM CS & Mongo exceptions

If you are an avid log checker on your AEM CS environments you might have come across messages like this in your authoring logs:

02.04.2024 13:37:42:1234 INFO [cluster-ClusterId{value='6628de4fc6c9efa', description='MongoConnection for Oak DocumentMK'}-cmp57428e1324330cluster-shard-00-02.2rgq1.mongodb.net:27017] org.mongodb.driver.cluster Exception in monitor thread while connecting to server cmp57428e1324330cluster-shard-00-02.2rgq1.mongodb.net:27017 com.mongodb.MongoSocketException: cmp57428e1324330cluster-shard-00-02.2rgq1.mongodb.net 
at com.mongodb.ServerAddress.getSocketAddresses(ServerAddress.java:211) [org.mongodb.mongo-java-driver:3.12.7] 
at com.mongodb.internal.connection.SocketStream.initializeSocket(SocketStream.java:75) [org.mongodb.mongo-java-driver:3.12.7] 
...
Caused by: java.net.UnknownHostException: cmp57428e1324330cluster-shard-00-02.2rgq1.mongodb.net

And you might wonder what is going on. I get this question every now and then, often assuming that this something problematic. Because we have all learned that stacktraces normally indicate problems. And on first sight this indicates a problem, that a specific hostname cannot be resolved. Is there a DNS problem in AEM CS?

Actually this message does not indicate any problem. The reason behind this is the way how mongodb implemented scaling operations. If you up- or downscale the mongo cluster, this does not happen in-place, but you get actually a new mongo cluster of the new size and of course the same content. And this new cluster comes with a new hostname.

So in this situation there was a scaling operation, and AEM CS connected to the new cluster and now looses connection to the old cluster, because the older cluster is stopped and its DNS entry is removed. Which is of course expected. And for that reason you can also see that this is logged on level INFO, and not as an ERROR.

Unfortunately this is a log message created by the mongo-driver itself, so this cannot be changed on the Oak level by removing the stacktrace from this message and changing the message itself. And for that reason you will continue to see it in the AEM CS logs, until a new improved mongo driver changes that.