Can a service call another service inside its code?

Question

Following is a point mentioned in a presentation slide related to SOA, and it confuses me with the concepts of service orchestration and service choreography. To enable service choreography, shouldn't a web service be able to call another web service?

SOA builds applications out of software services. Services comprise intrinsically
unassociated, loosely coupled units of functionality that have no calls to
each other embedded in them.

Answer 1

In theory, a service can do anything it needs to do to accomplish its job. So there doesn't seem to be a good reason to forbid using a second service to do your work. Why reinvent the wheel?

In practice, the issue is more complicated. If you start calling other services on your own web server, then you'll eventually starve it of resources. At best, "real" clients will have to wait a bit longer for their answers while your web service server plays with itself.

Another issue is recursive loops: Service A calls B calls C calls A calls B ... you get the idea. A small change in one service can introduce such a loop without anyone noticing and it can sit there for a long time until it suddenly kills your server.

That is why you should build micro services in a hierarchy inside the server (i.e. below the web service layer - this is not exposed to clients). Those micro services can use each other in a top-down manner (to avoid the loops). Unit tests then make sure they behave properly.

Lastly, such reuse is very slow. Each HTTP request takes a lot of resources to create, send, parse and process. Calling an internal method directly can be 10 - 10000 times faster.

These are the main reasons why the services exposed by a single server shouldn't reuse each other via the "public client API".

Note: There are web services which build new services by using existing ones. IFTTT - "If This Then That" is one such beast.