### Portable Interceptors

We have revised TAO's interceptor implementation so that it conforms to the Portable Interceptor specification which is now a part of the CORBA 3.0.3 specification. The purpose of this document is to provide a transition guide for those who have used our old interceptors. The old interceptors will no longer be supported now that we have the new mechanism in place. A paper that describes TAO's portable interceptors and smart proxies is available online.

### Table of Contents

- Context
- TAO's Implementation
- Transition
- Current Status
- Future Work
- Known Issues
- References

## Context

Interceptors allow you to interpose other CORBA services to the ORB and extend the ORB's functionalities. They are most commonly used in, but not limited to, Security Service, Transaction Service. They are also for accounting, auditing and debugging distributed applications.

### TAO's Implementation of "Portable Interceptors"

We have modifed TAO's interceptor interface to conform with the CORBA 2.5 spec. The current implementation of interceptors consists of support for the `Dynamic` module as well as the canonical interception points including (1) `send_request`, (2) `receive_reply`, (3) `receive_exception`, (4) `receive_other`, (5) `receive_request_service_contexts`, (6) `receive_request`, (7) `send_reply`, (8) `send_exception`, (9) `send_other`, and (10) `establish_components` (specific to `IORInterceptor`s). Each request interception point is passed a `RequestInfo` object which encapsulates the details of the operation like arguments, etc. The IOR interception point is passed an `IORInfo` object that encapsulates operations for adding tagged components to profiles in an IOR. Registration of all three types of interceptors (client and server request interceptors, and IOR interceptors) is now done using the interface provided by the standard `ORBInitInfo` object.

Details of this implementation along with benchmarking is available in the paper on Meta-programming mechanisms.

Examples on this new version of Portable Interceptors is available at `$TAO_ROOT/tests/Portable_Interceptors`.

### Transitting from TAO's Old "Portable" Interceptor APIs to the Standard Portable Interceptor APIs

Please refer to CORBA 2.5 specification for details on the proposed Portable Interceptor interfaces. Below is the old but now obsolete interceptor version in TAO.

    // -*- IDL -*- interceptors.html,v 1.28 2004/06/02 06:23:44 ossama Exp

    // This file contains the interface definitions for "Portable"
    // Interceptor support.

    // **********************************************************
    //   Notice that the Portable Interceptor specification
    //   is still under discussion in OMG and both the IDL
    //   and the implementation details in TAO will eventually
    //   change to conform with the PI spec in the future.
    //
    //   @@ Now that a working draft of the Portable Interceptors
    //      is available, we will provide a compliant implementation
    //      shortly.
    //
    //      Please see the annotation marked with "@@" in this file
    //      for hints on transitting from the temporary
    //      implementation to new APIs.
    //
    //      See $TAO_ROOT/docs/interceptors.html for more info.
    // **********************************************************

    // Author (currently): Nanbor Wang <nanbor@cs.wustl.edu>
    // @@ I will no longer be the author of this IDL file. ;-)

    #include <corba.pidl>
    #include <IOP.pidl>

    #pragma prefix "TAO"
    // The prefix should be changed to "omg.org" once the spec. gets
    // finallized.
    // @@ The prefix will be changed to "omg.org".

    module PortableInterceptor
    {
      interface Cookie
        {
          // Cookie's are used to pass information among interceptors
          // within a invocation or an upcall.
          //
          // @@ Cookie will no longer be available.
          string myname ();
        };

      typedef sequence<Cookie>  Cookies;
      // Collections of Cookie's become Cookies'es.
      //
      // @@ Cookies will no longer be available.

      interface Interceptor
        {
          // Base interface for Interceptors.
          //
          // @@ This interface will not change.
          readonly attribute string name;
        };

      interface ServerRequestInterceptor : Interceptor
        {
          // Server side request interceptor definition.
          //
          // @@ The name of the interface will not change.

          void preinvoke (in unsigned long request_id,
                          in boolean response_expected,
                          in CORBA::Object objref,
                          in string operation_name,
                          inout IOP::ServiceContextList sc,
                          inout NVList arguments,
                          inout Cookies ck);
          // Interception pointer before invoking the servant method.
          // Currently, we don't pass NVList into the interceptor because
          // I haven't figured out how to best optimize this stuff.
          // In the future, NVList will contain all in and inout arguments
          // of the operation.
          //
          // @@ This operation will map to either
          // <receive_request_service_contexts> or <receive_request> of
          // the standard APIs.  If you are not sure, use
          // <receive_request>.
          //
          // void receive_request_service_contexts (in ServerRequestInfo ri) raises (ForwardRequest);
          // void receive_request (in ServerRequestInfo ri) raises (ForwardRequest);
          //
          // @@ Note that all arguments will be accessed thru
          // <PortableInterceptor::ServerRequestInfo> interface.

          void postinvoke (in unsigned long request_id,
                           in boolean response_expected,
                           in CORBA::Object objref,
                           in string operation_name,
                           inout IOP::ServiceContextList sc,
                           inout NVList arguments,
                           inout Cookies ck);
          // Interception pointer after invoking the servant method.
          // Currently, we don't pass NVList into the interceptor because
          // I haven't figured out how to best optimize this stuff.
          // In the future, NVList will contain all out, inout arguments
          // and the return value of the operation.
          //
          // @@ This operation will map to <send_reply>.
          // It is not clear whether oneway call will invoke <send_other>
          // operation or not.
          //
          // void send_reply (in ServerRequestInfo ri);
          // void send_other (in ServerRequestInfo ri) raises (ForwardRequest);
          //
          // @@ Note that all arguments will be accessed thru
          // <PortableInterceptor::ServerRequestInfo> interface.

          void exception_occurred (in unsigned long request_id,
                                   in boolean response_expected,
                                   in CORBA::Object objref,
                                   in string operation_name,
                                   inout Cookies ck);
          // Exception interception point.
          //
          // @@ This method will map to <send_exception> method.
          //
          // void send_exception (in ServerRequestInfo ri) raises (ForwardRequest);
          //
          // @@ Note that all arguments will be accessed thru
          // <PortableInterceptor::ServerRequestInfo> interface.
        };

      interface ClientRequestInterceptor : Interceptor
        {
          // Client side interceptor.
          //
          // @@ The name of the interface will not change.

          void preinvoke (in unsigned long request_id,
                          in boolean response_expected,
                          in CORBA::Object objref,
                          in string operation_name,
                          inout IOP::ServiceContextList sc,
                          inout NVList arguments,
                          inout Cookies ck);
          // Before remote invocation.
          // Currently, we don't pass NVList into the interceptor because
          // I haven't figured out how to best optimize this stuff.
          // In the future, NVList will contain all in and inout arguments
          // of the operation.
          //
          // @@ This operation will map to <send_request> of the standard
          // APIs.
          //
          // void send_request (in ClientRequestInfo) raises (ForwardRequest);
          //
          // @@ Note that all arguments will be accessed thru
          // <PortableInterceptor::ClientRequestInfo> interface.

          void postinvoke (in unsigned long request_id,
                           in boolean response_expected,
                           in CORBA::Object objref,
                           in string operation_name,
                           inout IOP::ServiceContextList sc,
                           inout NVList arguments,
                           inout Cookies ck);
          // After returned from remote invocation.
          // Currently, we don't pass NVList into the interceptor because
          // I haven't figured out how to best optimize this stuff.
          // In the future, NVList will contain all out, inout arguments
          // and the return value of the operation.
          //
          // @@ This operation will map to either <receive_reply> or
          // <receive_other> in the standard APIs depending on whether the
          // operation is oneway or not.
          //
          // void receive_reply (in ClientRequestInfo ri);
          // void receive_other (in ClientRequestInfo ri);
          //
          // @@ Note that all arguments will be accessed thru
          // <PortableInterceptor::ClientRequestInfo> interface.

          void exception_occurred (in unsigned long request_id,
                                   in boolean response_expected,
                                   in CORBA::Object objref,
                                   in string operation_name,
                                   inout Cookies ck);
          // Exception occurred.
          //
          // @@ This method will map to <receive_exception> method as:
          //
          //    void receive_exception (in ClientRequestInfo ri) raises (ForwardRequest);
          //
          // @@ Note that all arguments will be accessed thru
          // <PortableInterceptor::ClientRequestInfo> interface.
        };
    };

    #pragma prefix ""

### Current Status

- The core infrastructure is in place as well as the canonical request and IOR interception points: `send_request`, `receive_reply`, `receive_exception`, `receive_other`, `receive_request_service_contexts`, `receive_request`, `send_reply`, `send_exception`, `send_other`, and `establish_components`. The remaining client request interception point, `send_poll`, is time independent invocation specific. Once TAO supports time independent invocations, the `send_poll` interception point will be implemented.
- `ORBInitializer` registration has been implemented, as per the spec.
- Registration of interceptors is now conformant to the spec through the `ORBInitInfo` class. Multiple interceptors may now be registered.
- IOR interceptors have been implemented. They allow an external service, for example, to add tagged components to profiles within IORs as they are being generated.
- Policy factory registration, i.e. `ORBInitInfo::register_policy_factory`, has been implemented. Corresponding policies can then be created using the `ORB::create_policy` method.
- Initial reference registration, i.e. `ORBInitInfo::register_initial_reference`, has been implemented. This is particularly useful for registering local objects with the ORB's `resolve_initial_references` mechanism since they can't be stringified and registered via `-ORBInitRef` ORB option.
- Basically, all `ORBInitInfo` methods have been implemented except `allocate_slot_id`.
- Implemented most of the remaining `ClientRequestInfo`, `ServerRequestInfo` and `IORInfo` methods.
- Added support for the `PortableInterceptor::ForwardRequest` exception on both the client and server sides.
- Implemented the `IOP::CodecFactory` and the CDR encapsulation `IOP::Codec` objects. The CDR encapsulation `Codec` is useful for embedding data in an `octet` sequence that conforms to the CDR encapsulation rules. For example, it could be used to marshal data into the `octet` sequence that is part of an `IOP::ServiceContext` or an `IOP::TaggedComponent`. This means that it could compliment the IOR interceptor support, and the service context manipulation support found in request interceptors.
- Implemented the `PortableInterceptor::ServerRequestInfo::object_id`, `PortableInterceptor::ServerRequestInfo::adapter_id` and `PortableServer::POA::id` methods.
- Greatly improved the speed of the `PortableInterceptor::RequestInfo::arguments` method for the case when a given target method has more than one parameter.
- Corrected the `Dynamic::Parameter` IDL. It now correctly uses the `CORBA::ParameterMode` enumeration in place of the `Dynamic::ParameterMode` enumeration. The latter has been removed since it was not a standard type.
- The `PortableInterceptor::ClientRequestInterceptor::send_request` interception point now occurs before a connection attempt to the target is ever made. This greatly improves the speed of client request interceptor initiated `LOCATION_FORWARD`s, in addition to making it possible to prevent connection attempts from occuring by throwing an exception, for example.
- Corrected `PortableInterceptor::ForwardRequest` exception support. It is longer possible to throw a `PortableInterceptor::ForwardRequest` exception in application code (i.e. not an interceptor) and expect it to be converted to a `LOCATION_FORWARD`. A `PortableInterceptor::ForwardRequest` exception will now only be treated as a `LOCATION_FORWARD` if it is thrown from an interception point capable of raising that exception. Otherwise it will be propagated to the client. This change also has the added benefit of reducing the stub/skeleton footprint, particularly for IDL with many interfaces.
- Implemented `PortableInterceptor::Current` interface, `ORBInitInfo::allocate_slot_id`, `ClientRequestInfo::get_slot`, `ServerRequestInfo::get_slot`, `ServerRequestInfo::set_slot`, and `ServerRequestInfo::get_server_policy` methods.
- Client interception points are now invoked for AMI calls.

## Future Work

1. Add support for the `ThruPOA` collocation optimization to the interceptor chain; the `direct` collocation optimization will not go through the interceptor chain.
2. The `send_poll` request interception point implementation will most likely be deferred until TII is supported in TAO.

### Known Issues

- Currently none.

### References

- formal/2004-03-01 -- CORBA 3.0.3 Specification -- contains the Portable Interceptors chapter