Out of order commands

In the previous posts a simple mechanism of storing information needed for operation idempotence was introduced. A simple hash table, which state is transactionally saved with the state of object onto which the send operation was applied. How about receiving operations out of order? What if infrastructure (for instance, messaging system) will pass one operation earlier than the second, which in reality occurred earlier?

It’s time to make it explicit and start calling elements in the DDD manner. So for sake of reference, the object considered as the subject of an operation is an aggregate root. The operation is of course a message. The modeling assumes using the event sourcing as a storage for aggregates’ states.

Assume, that the aggregate, which the command is sent to, has a property called Version, incremented with each event applied on. Assume then, each command contains a version number, which is supposed to be equal to the aggregate’s version. If, during dispatch, these two values are different, an exception is thrown and command do not change the state of the aggregate. It’s a simple optimistic concurrency implementation, allowing discarding out of order commands sent to an object.

To make it more interesting, consider a sharded system, where specific aggregates are stored by different nodes (but for each aggregate there is one node where it is stored). An aggregate’s events (state changes) have to be propagated across all the nodes/shards in the same idempotent manner as commands are sent to aggregates. It’s easy to apply hashtable for each node and with using the very same key: aggregateId with version but it would mean storing all the pairs of aggregate identifiers with their versions, which could possibly bring down each of your nodes (or make you use GBs of memory). Can the trivial fact, that version is increased with every event on the aggregate, could be used for some optimization? You’ll see in the next entry.

Idempotence, pt. 2

In the previous post a few operations were taken into consideration, whether there are (not) idempotent. For the sake of reference, here there are:

  • Marked as default
  • Money transfer ’500$’ ordered to ‘x’ account
  • Label ‘leave sth for the future month’ added

If we consider ‘idempotent’ as an operation which can be applied multiple times in a row, then all the operations overriding previous values of some properties are idempotent. Having some entity marked as default 5 times does not change the fact that it is default. That’s for sure. What about provisioning ‘x’ account with 500$? Can this type of operation can be reapplied multiple times? Of course not, because it does not override any property, it changes the state, by interacting with a previous one. The same goes for ‘labeling’, of course if there is no compensation introduced (select only unique labels before saving, which would allow reapplying).

What if you want your system to be resistant to operations resend multiple times? The simplest solution is to add unique identifier for each operation and storing them is a lookup (hashtable). Each time the operation arrives, the lookup is checked whether there this operation was already processed. If so, skip it.

There is one additional condition is to have the lookup transactional with a storage you save the states. This condition is a simple ‘all-or-none’ for storing the result of operation with the fact, that this specific operation was already applied. Otherwise, if lookup would be updated in the first place and storing the state after the operation failed, there would be no change saved. The same applies to a situation, where the lookup is updated at the very end. The operation result is saved, adding info about operation to lookup fails and the next time the same operation arrives it is applied one more time. Having that said, lookup must be transactional with the medium where state is saved.

Composition vs derivation

Assume you’re writing some reports in your application. You’ve just created the third controller covering some kind of reporting and it seems to be, that all the three controllers have a very similar code, modulo type passed as the entity type to your NH ISession.QueryOver() or another data source. It seems that, if the method creating report base was generic, it could be used in all the cases. You want to extract it, make it clearer and to stop repeating yourself. What would you do?

The very first thing is to extract method in each of the controllers. Now they seem almost the same. A place is needed where the method can be easily moved. How about a super type? Let’s create a controller, call it in a fashionable way, for instance: ReportControllerBase, and move the method in there. Now you can easily remove the methods in the deriving controllers. Yeah! It’s reusable, everyone writing his/her report can derive from the ReportControllerBase and use its methods to speed up his/her task.

The very first step is exactly the same: the extract method must be done, to see the common code. Once it’s done, you notice that the whole method has only a few dependencies which can be easily pushed to parameters, for instance: isession, the entity type passed to query, the value used in some complex where clause, etc. You change all the field and properties usages to parameters which allows this method to be static. You create a static method and turn it into an extension method of a session. The refactorization is done, you can easily call this method in all the controllers, by simply calling an extension onto a session.

What’s the difference and why composition should be preferred
If you use C# or Java you should always be aware of one limitation: you can derive from only one type. Spending this ‘once in a lifetime’ for a simple functionality extraction, for me – that’s the wrong way. Using composition, and deriving only when you truly see that one type is another type, that’s the right way to go. In the next post I’ll write about ASP MVC Detached Actions, a simple mechanism you can use, to derive your controllers only, when it is needed and delegating common actions, without it.