Realistically, you never store summaries in the log. Instead, you store what it took to calculate them. So you won't store "we have 10 customers on this date wi

Realistically, you never store summaries in the log. Instead, you store what it took to calculate them. So you won't store "we have 10 customers on this date with this range" but instead store "we found these 10 customers on this date with this range". This assumes you can store infinite sized lists in your log, but realistically, this is never a concern if you can keep your time windows small enough. Then, you periodically do a reconciliation and log corrections (look for entries not summarized -- easily done via a bloom filter which can tell you what entries are definitely NOT in your set) over a longer period. For example, we had a 28-day reconciliation period at one company I worked at (and handled over 120 million events per day). If you appended an event earlier than 28 days prior, it was simply ignored. This very rarely happened, but allowed us to fix bugs with events for up to 28 days.