I began writing this a long time ago after I viewed a talk on Event Sourcing by Greg Young. It was just a simple thought on maintaining the current snapshot of a projection of an event stream as events are generated. I eventually heard a talk called “Turning the database inside-out with Apache Samza” by Martin Kleppmann, http://www.confluent.io/blog/turning-the-database-inside-out-with-apache-samza/. The talk was awesome, as I mentioned in a previous post. It provided structure, understanding and coherence to the thoughts I had.
It still took a while to finish this post after seeing the talk (because I have too much going on), but I would probably still be stuck on this post for a long time if I hadn’t heard the talk and looked further into stream processing.
Event sourcing is the storing of a stream of facts that have occurred in a system. Facts are immutable. Once a fact is stored it can’t be changed or removed. Facts are captured as events. An event is a representation of an action that occurred in the past. An event is usually an abstraction of some business intent and can have other properties or related data. To get the state for a point in time we have to process all of the previous events to build up the state to the point in time.
In our system we want to store events that happen in the system. We will use these events to figure out the current state of the system. When we need to know the current state of the system we calculate a left fold of the previous facts we have stored from the beginning of time to the last fact stored. We iterate over each fact, starting with the first one, calculating the change in state at each iteration. This produces a projection of the current transient state of the system. Projections of current state are transient because they don’t last long. As new facts are stored new projections have to be produced to get the new current state.
Snapshots are sometimes shown in examples of event sourcing. Snapshots are a type of memoization used to help optimize rebuilding state. If we have to rebuild state from a large stream of facts, it can be cumbersome and slow. This is a problem when you want your system to be fast and responsive. So we take snapshots of a projection taken at various points in the stream so that we can begin rebuilding state from a snapshot instead of having to replay the entire stream. So, a snapshot is a cache of a projection of state at some point in time.
Snapshots in traditional event sourcing examples have a problem because the snapshot is a version of state for some version of a projection. A projection is a representation of state based on current understanding. When the understanding changes there are problems.
Let’s say we have an application and we understand the state as a Contact object containing a name and address property. Let’s also say we have a couple facts that alter state. One fact is a new contact was created and is captured by a “Create Contact” event containing “Name” and “Address” data that is the name and address of a contact. Another fact is a contact changed their address and is captured by a “Change Contact Address” event containing “Address” data with the new address for a contact.
When a new contact is added a “Create Contact” event is stored. When a contact’s address is changed a “Change Contact Address” event is stored. To project the current state of a contact that has a “Create Contact” and “Change Contact Address” event stored, we first create a new Contact object, then get the first event, “Create Contact”, from the event store and update the Contact object from the event data. Then we get the “Change Contact Address’ event and update the Contact object with the new address from the event.
That was a lot of words, but very simple concept. We created a projection of state in the form of a Contact object and changed the state of the projection from stored events. What happens when we change the structure of the projection? Instead of a Contact object with Name and Address, we now have a Contact object with Name, Address1, Address2, City, State, and Zip. We now have a new version of the projection and previous snapshots made with other versions of the projection are invalid. To get a valid projection for the current state with the new projection we have to recalculate from the beginning of time.
Sometimes we don’t want the current state. What if we want to see state at some other point in time instead of the head of our event stream. We could optimized rebuilding a new projection by using some clever mapping to transform an old snapshot version to the new version of the projection. If there are many versions, we would have to make sure all supported versions are accounted for.
We could use a CQRS architecture with event sourcing. Commands write events to the event store and queries read state from a projection snapshot. Queries would target a specific version of a projection. The application would be as consistent as the time it takes to take a new snapshot from the previous snapshot which was only one event earlier (fast).
A snapshot is like a cache of state and you know how difficult it is to invalidate state. If we instead create a real-time snapshot as facts are produced, we always have the current snapshot for a version of the projection. To maintain backwards compatibility we can have real-time snapshots for various versions of projections that we want to maintain. When we have a new version of a projection we start rebuilding state from the beginning of time. When the rebuilding has caught up with current state we start real-time snapshots. So, there will be a period of time where new versions of projections aren’t ready for consumption as they are being built. With real-time snapshots we don’t have to worry about running funky code to invalidate or rebuild state, just read the snapshot for the version of the projection that we want. When we don’t want to support a version of a projection, just take the endpoint that points to it offline. When we have a new version that is ready for consumption we bring a new endpoint online. When we want to upgrade or downgrade we just point to the endpoint we want.
Storage may be a concern if we are storing every snapshot of state. We could have a strategy to purge older snapshots. Deleting a snapshot is not a bad thing. We can always rebuild a projection from the event store. As long as we keep the events stored we can always create new projections or rebuild projections.
Well, this was just me trying to clean out a backlog of old posts and finishing some thoughts I had on real-time state snapshots from an event stream. If you want to read or see a much better examination of this subject visit “Turning the database inside-out with Apache Samza” by Martin Kleppmann, http://www.confluent.io/blog/turning-the-database-inside-out-with-apache-samza/. You can also check out implementations of the concepts with Apache Samza or something like it with Azure Steam Analytics.