So… content indexing the passive node. Whassat all about, ‘en?

I surprised an architect at one of my customers, today. i told him that in a DAG, the search service on a server indexes the active copy of the database. he didn’t believe me. i asked him how he thought it worked, and he said “replication”. uh-uh.

it’s pretty easy to get that impression. after all, it’s sort of what the official documentation says is going on:

during the seeding process, DAG members with a passive mailbox database copy replicate the content index catalog from the DAG member that has the active mailbox database copy

but that’s during the *seeding* process. what about during normal operations?

After initial seeding, the server with the passive database copy gets message data from the server with the active database and performs content indexing locally.

What does that even mean? I’ll tell you… it means the server with the passive database makes a connection over the network to the *active* database, because the database has to be *mounted* for any MAPI activities to take place. That’s right, it makes a MAPI connection. This also has ramifications for your network, because a MAPI connection is a… anybody? That’s right; it’s a *client* connection, so the traffic is carried over the CLIENT network, not the REPL network.


But it isn’t going to be much traffic is it? I mean, it’s just a bunch of indexing, right?


Hmmm. You’da thunk, but no. Microsoft claim in their documentation for 2016 that indexing the local copy of a database, as opposed to the active copy, will save approximately 40% traffic. The ever-awesome Rhoderick Milne says in this thread it’s about equivalent to the total of REPL traffic.


The official documentation does carry a community contribution at the bottom stating in plain English how things work, and there’s a bunch more detail here. That last article while awesome, is most impressive for its tone of surprise.

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