# Chapter 9 Interceptors

omniORBpy has limited interceptor support. Interceptors permit the application to insert processing at various points along the call chain, as requests are processed. The Portable Interceptors API is not yet supported.

Interceptors are registered using functions in the `omniORB.interceptors` module:

addClientSendRequest() addClientReceiveReply() addServerReceiveRequest() addServerSendReply() addServerSendException()

To register an interceptor function, call the relevant registration function with a callable argument. The callable will be called with two or three arguments. The first argument is the name of the operation being invoked; the second is the set of service contexts to be retrieved or filled in. `ServerSendException` has a third argument, the repository id of the exception being thrown.

When receiving service contexts (in the `ClientReceiveReply` and `ServerReceiveRequest` interceptors), the second argument is a tuple of 2-tuples. In each 2-tuple, the first item is the service context id and the second item is the CDR encapsulation of the service context. The encapsulation can be decoded with `omniORB.cdrUnmarshal()` (but only if you know the type to decode it to).

When sending service contexts (`ClientSendRequest`, `ServerSendReply`, and `ServerSendException`), the second argument is an empty list. The interceptor function can choose to add one or more service context tuples, with the same form described above, by appending to the list. Encapsulations are created with `omniORB.cdrMarshal()`.

Interceptor registration functions may only be called before the ORB is initialised. Attempting to call them later results in a `BAD_INV_ORDER` exception.