Interoperability in action: the Aladin experience

Since its first version created 8 years ago, Aladin has been continuously developed, and has been recently used as the main component of the Astrophysical Virtual Observatory prototype, demonstrating the power of VO interoperability standards for enabling real science. But interoperability is not just a set of standards used to describe accurately the data - it is also a means by which programs and tools can interact and cooperate. Several ways of communicating with and from Aladin have been developed, including a scripting language, Java interfaces, and plug-ins. The paper focuses on the lessons learned during the Aladin experience in several aspects: the strengths and weaknesses of the programming language (Java); the importance of the standards and protocols, their surprises and their challenges; the difficulties of actual interoperation; and the necessity of maintaining compatibility with the users' material.