A few years ago, concurrency and scalability were a hype. Today, it’s a must. But how do you write applications that scale painlessly?
Command and Query Responsibility Segregation (CQRS) is an architectural pattern to address these problems. In his talk, Allard Buijze gave a good introduction. First, some of the problems of the standard approach. Your database, everyone says, must be normalized.
That can lead to a couple of problems:
- Historic data changes
- The data model is neither optimized for writes nor for queries
The first problem can result in a scenario like this. Imagine you have a report that tells you the annual turnover. You run the report for 2009 in January, 2010. You run the same report again in 2011 and 2012 and each time, the annual turnover of 2009 gets bigger. What is going on?
The data model is in third normal form. This is great, no data duplication. It’s not so great when data can change over time. So if your invoices point to the products and the products point to the prices, any change of a price will also change all the existing invoices. Or when customers move, all the addresses on the invoices change. There is no way to tell where you sent something.
The solution is to add “valid time range” to each price, address, …, which makes your SQL hideous and helps to keep your bug tracker filled.
It will also make your queries slow since you will need lots and lots of joins. These joins will eventually get in conflict with your updates. Deadlocks occur.
On the architectural side, some problems will be much easier to solve if you ignore the layer boundaries. You will end up business logic in the persistence layer.
Don’t get me wrong. All these problems can be solved but the question here is: Is this amount of pain really necessary?
CQRS to the rescue. The basic idea is to use two domain models instead of one. Sounds like more work? That depends.
With CQRS, you will have more code to maintain but the code will be much more simple. There will be more tables and data will be duplicated in the database but there will never be deadlocks, queries won’t need joins in the usual case (you could get rid of all joins if you wanted). So you trade bugs for code.
How does it work? Split your application into two main parts. One part takes user input and turns that into events which are published. Listeners will then process the events.
Some listeners will write the events into the database. If you need to, you will be able to replay these later. Imagine your customer calls you because of some bug. Instead of asking your customer to explain what happened, you go to the database, copy the events into a test system and replay them. It might take a few minutes but eventually, you will have a system which is in the exact same state as when the bug happened.
Some other listeners will process the events and generate more events (which will also be written to the database). Imagine the event “checkout”. It will contain the current content of the shopping cart. You write that into the database. You need to know what was in the shopping basket? Look for this event.
The trick here is that the event is “independent”. It doesn’t contain foreign keys but immutables or value objects. The value objects are written into a new table. That makes sure that when you come back 10 years later, you will see the exact same shopping cart as the customer saw when she ordered.
When you need to display the shopping cart, you won’t need to join 8 tables. Instead, you’ll need to query 1-2 tables for the ID of the shopping cart. One table will have the header with the customer address, the order number, the date, the total and the second table will contain the items. If you wanted, you could add the foreign keys to the product definition tables but you don’t have to. If that’s enough for you, those two tables could be completely independent of any other table in your database.
The code to fill the database gets the event as input (no database access to read anything from anywhere) and it will only write to those two tables. Minimum amount of dependencies.
The code to display the cart will only need to read those two tables. No deadlocks possible.
The code will be incredibly simple.
If you make a mistake somewhere, you can always replay all the events with the fixed code.
For tests, you can replay the events. No need to a human to click buttons in a web browser (not more than once, anyway).
Since you don’t need foreign keys unless you want to, you can spread the data model over different databases, computers, data centers. Some data would be better in a NoSQL repository? No problem.
Something crashes? Fix the problem, replay the events which got lost.
Instead of developing one huge monster model where each change possibly dirties some existing feature, you can imagine CQRS as developing thousands of mini-applications that work together.
And the best feature: It allows you to retroactively add features. Imagine you want to give users credits for some action. The idea is born one year after the action was added. In a traditional application, it will be hard to assign credit to the existing users. With CQRS, you simply implement the feature, set up the listeners, disable the listeners which already ran (so the action isn’t executed again) and replay the events. Presto, all the existing users will have their credit.
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