Andy Hedges' Blog

Events And Service Oriented Architecture (SOA)

When people start to service orient their organisation they often focus on exposing APIs and those APIs invariably solely or mostly focus on method calls, what I and others often refer to as RPC. This is great and brings huge benefit but it does miss a huge opportunity and that is being able to observe and react to what’s happening in your organisation.

In order to be able to observe and therefore react to whats happening in the services that make up your organisation you need to add events to your services. What do I mean by events? To start with let’s leave technology aside and think of the business problem you might be trying to solve. As an example let’s take a retail bank that offers current (checking) accounts. To model this account appropriately there are things that should be modelled as RPC and things that should be modelled as events. If a customer uses an ATM to check their balance this should be RPC, the ATM will call the account service to get the current balance to display it to the customer. There is little point in doing this as an business event because you need the output of the customer asking for their balance to continue.

Now suppose the customer wants to make a withdrawal, this would cause an RPC type invocation (i.e debit £100) and an event (i.e. user withdrawal occured on account id 5123). The RPC call allows us to perform a blocking operation to check there is sufficient funds, make the deduction and inform the ATM that it can dispense, the event will be published for interested parties to be informed that something they might be interested in has happened. Who might be interested in this event? Well it could be an analytics package that wants to keep track of which ATMs are popular or maybe a complex anti-fraud system figuring out suspicious patterns of withdrawals.

The great thing about events is that the systems raising them doesn’t need to understand how they are used they can simply raise them and go about their business. In the example above suppose you were asked to add a feature where customers could have details of their withdrawals emailed to them. Rather than go in and change the mission critical code around financial transactions you could set up a service that listens to these events and when it sees one email the customer. The team that looks after the account service need not even know.

Events start to get really interesting when you combine them, what some folks call this Complex Event Processing (CEP) but I prefer to consider a fairly logical part of of Event Driven SOA.The ‘complex’ in CEP refers to the fact that multiple events are combined to infer or derive something more interesting has happened. This is all a bit theoretical so let’s revisit the anti-fraud example from earlier. A security analyst has identified that when a customer makes withdrawals from ATMs in two different counties within the space of a day then this is suspicious but not impossible, it might raise an event such as “Customer Crossed Border” event. If the customer goes on to make high value transactions in the first country then the matter needs investigating as another “Customer Crossed Border” event occured on the same day as the first. The fact that this has happened would raise an “Suspicious Occurance” event which the account system listens to and locks the account. When the account is locked an “Account Locked” event is raised with a reason code; the customer support centre service listens on this event. When one is received a task is added to one of the call centre operatives list of work (i.e. call customer and verify transactions) and so on.

Please don’t get me wrong you shouldn’t expose all data and behaviour solely via events, this would be ridiculous but I’ve seen horror stories of people doing so, indeed I’ve worked to put them right. What I’m advocating is using a balance of RPC and events to best represent your organisation in an SOA fashion.

I’ll talk more in future blog posts about patterns for adding and utilising events in your SOA.

Andy Hedges

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