B/c, by design, you want the archived stream of events to include everything.

e.g. a lot of these systems have a "replay" node that can be used by components that just restarted. You want the replay to include ALL of the messages seen so you can rebuild the state at any given point.

(There are, of course, tradeoffs to this so I'm just commenting on the "single sequencer" design philosophy)

by definition: an exchange doesn't need any reference to outside market data

even for systems built on a sequencer which do (e.g. an OMS), the volume is too large

the usual strategy is for processes which require it, is to sample it and them stamp it on commands

which maintains the invariants

(my background: I have been a developer on one of Mike Blum's original sequencers)

How would an exchange enforce the trade through rule without any outside market data?

Fair point that you don't actually need the market data you send out if you have the original order book data internally.