There was some interesting discussion before I left for Nepal about Event Stores. The general question is “can you have a generic event log similar to a transaction log in a database”. A related question is what is the difference between an event log and a transaction log.
Having an event log is not a new idea, its been around for decades. Databases due something very similar inside of their own transaction log. The major difference between an event log and a transaction log is that by definition an event log also captures intent. Consider the difference between:
There are many semantic and linguistic differences between these two concepts. The first would be a transaction log and the second an event log. With a create these differences can be very subtle. Let’s try something less subtle.
Here intent is quite obviously different between the two messages. There could be a second event CustomerManuallyOverridenToGoldStatus which represents a manual override of our algorithm for dealing with customer promotions. A subscriber may care *why* the customer was promoted. This concept of intent (or “why”) is being represented in the event log.
As an important concept. If you can have two events use the same transition then you are by losing information
Things get to be a bit harry though and this is where the discussion started falling apart. I wish I could have dropped in a longer response but was travelling at the time. Can’t we model the first to be equivalent to the second? We see something in RESTful APIs.
YES you can do this! This produces a record that captures the intent as well. In fact this is how my second event store worked. There are lots of reasons you may want to do this (such as ability to use a generic state box on top in certain HA scenarios with the event store).
We can just consider this a different serialization mechanism. The key is that everything still maps directly back to a single event.
Now let’s get back to that original question of “event log vs transaction log”. An event log includes business level intent, this is not needed with a transaction log. An event log is a stricter definition of a transaction log. I don’t need to store the intent in order to be a transaction log though we can have a really interesting discussion about what the “business events” are in a database domain 🙂.
Is an Event Log a type of journal or transaction log? Yes. I like to think though that even if you use the generic update as in the third example above it requires that you specify intent. Intent is a valuable thing. Can I build a transaction log that completely captures intent and does not lose information? Sure think about a database with a “Transaction” table. I would say this is actually just a serialization mechanism with the intent of being an event log.
If I don’t store intent there are an entire series of questions I can no longer ask the data.