What activities write to a config server?

[MARK CALLAGHAN]

I am watching less for week 6 of M102. The content is excellent. The manual states that the only activities that write to a config server are splits & migrates, but in one of the videos this sequence is done after connecting to a mongo process and it returns the hostnames+ports used for shard servers. Does this mean that adding a shard initially or after replacing a failed server also writes to a config server.
use config

db.shards.find()

http://docs.mongodb.org/manual/core/sharded-cluster-config-servers/

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Mark Callaghan
mdca...@gmail.com

[Asya Kamsky]

I think by activities it must be talking about the changelog collection.

Metadata that's relevant in long term is saved in other collections (shards, databases, collections, etc).

Replacing a failed server should be a no-op from the config server POV unless you have renamed the server and reconfigured the replica set, but then that's just discoverable info.

Our docs have a pretty comprehensive description of the config DB contents.

Asya

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[MARK CALLAGHAN]

Quoting the manual, as it was less vague than what I wrote above. I also assume that most replacements of a failed shard server do not use the same hostname, so if the config server stores hostnames for shard servers then that must be updated on a replacement.
 
"MongoDB only writes data to the config server in the following cases:

• To create splits in existing chunks. For more information, see chunk splitting.
• To migrate a chunk between shards. For more information, see chunk migration."

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Mark Callaghan
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[Asya Kamsky]

"Only writes data ..."

Yeah, that's terrible phrase to use since immediately after it says " MongoDB also uses the config server to manage distributed locks." And immediately before it says that the config DB is where the shard metadata is kept.

I can only imagine that maybe initially some wording made it clear that these are the only writes that happen without admin intervention (I.e. all other instances of writing is because it's reflecting actions of DBA - but even that's strictly not complete).

I'll get it fixed.

Asya

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