Thank you for the excellent feedback. You raise some important points.
As you said, MySQL will work well for now (it solves the problem for the near future, 2+years), after that we've been told that MySQL will start to struggle with the log, exactly as you described. Eventstore is a solid option, we're looking into it and other technologies better suited to massive event streams.
As for the one table for all events, we've had no issues with it. Now, this doesn't mean there's one event log for ALL events, just for the events produced by a service. We're currently indexing by aggregate ID and aggregate Type, so we can easily select the subset we want. We may move to a per aggregate event store, but I'm not happy with this, as it makes it harder to change aggregate boundaries. We have metrics in place to monitor performance, so once it starts becoming problematic we'll be warned and can prepare a solution.
For projection replaying, rather than connecting to the log, we plan for the projections to connect to a copy of the log, optimised for projection reads. We're thinking of using Kafka for this. It will keep the event log indefinitely (if we want it to) and it will at least ensure ordering. This will give us more life out of our MySQL log and also speed up projection rebuilding.
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