Immediate feedback has a positive effect on the performance of a student practising a procedural skill in exercises. Giving feedback to a number of students is labour-intensive for a teacher. To alleviate this, many electronic exercise assistants have been developed. However, many of the exercise assistants have some limitations in the feedback they offer. We have a feedback engine that gives semantically rich feedback for several domains (like logic, linear algebra, arithmetic), and that can be relatively easy extended with new domains. Our feedback engine needs to have knowledge about the domain, how to reason with that knowledge (i.e. a set of rules), and a specified strategy. We offer the following types of feedback: correct/incorrect statements, distance to the solution, rulebased feedback, buggy rules, and strategy feedback. We offer the feedback functionality in the form of light-weight web services. These services are offered using different protocols, for example