We have reviewed the development of middleware in this survey. Our main focus has been on the DOC architecture, which is regarded as the state-of-art in field. As the current stage of middleware development is concentrated on distribution middleware layer, we described CORBA, RMI, and DCOM in certain level of details, and compared them horizontally. CORBA structure is specified in Section 2.2.1 from a generally point of view to understand today's middleware architecture. RMI and DCOM are regarded as two implementations, which are reviewed from a practical point of view in Section 2.2.2.
Middleware has been wildly used so far, and will keep increasingly popular in the later stage. People from different communities have made many contributions to take the middleware architecture to a new generation. We realize that the whole evolution process is eventually an approach to build ``systems of systems'', which is more or less a model-driven development. Up to this writing, DOC layer structure has not been fulfilled in practise. Some development like CORBAservices is still under going. But, just as the popular saying: single CPU era has gone, and middleware is everywhere now.