Intercepting Invocations

CORBA brokers provide support for interception of GIOP requests and replies both in the server and client sides. Although the interception model is heavily influenced by the CORBA Portable Interceptors specification, OiL does not stick to it. Like other aspects of the OiL's API, it priories a simpler model more adequate to Lua than the defined by the CORBA standard.

Interceptors

Invocation interception is done by interceptor objects, which can be register in the broker as a client interceptor or a server interceptor. Client interceptors intercept invocations issued to remote servants, while server interceptors intercept invocations received from remote applications.

A client interceptor is an object that provides either methods sendrequest or receivereply. The first method is called before an invocation request is sent to a remote servant, and the second is called after a reply is received from a remote servant. On the other hand, a server interceptor is an object that provides either methods receiverequest or sendreply, which are called before an invocation request is processed by the server, and after a reply is sent to the corresponding invoker, respectively. Each one of these methods receives a single parameter, which is a table representing the invocation. The structure and information contained in this table varies accordingly to the method being called.

However, for a single invocation issued or received, the table received by method sendrequest and receivereply are always the same. The same is true for methods receiverequest and sendreply, which means that the parameter received by the former method is the same parameter passed to the corresponding invocation of the later. This also means that any field stored in this table during the execution of the first method will be available during the execution of the other for a single invocation. For an example, consider the code below that measures the time each invocation takes to conclude.

local socket = require "socket"

TimeProfiler = {}

function TimeProfiler:receiverequest(request)
  request.start = socket.gettime()
end

function TimeProfiler:sendreply(reply)
  local time = socket.gettime() - reply.start
  print(string.format("opreation '%s' lasted %d seconds", reply.operation, time))
end

Client Interceptors

The fields provided by table request of operation sendrequest of the client interceptor are:

If the ORB is not able to decode an IOR profile, this field is nil.

`request_id`	number	[read-only]	Number that identifies the pending request over the connection used to communicate with the remote ORB. Note that this identifier is not unique across different connections, and it may be reused by other requests after this one is completed (i.e. replied).
`response_expected`	boolean		Flag that indicates if the server must send a response for this request.
`reference`	table	[read-only]	The complete IOR structure (see `CORBA::IOP::IOR` in CORBA specs.) that identifies the servant the request is sent to.
`object_key`	string	[occasional]	Key of the object the request is addressed to.
`profile_tag`	number	[occasional, read-only]	The identification of the kind of the IOR profile used to establish a connection with the servant's ORB. If the ORB is not able to establish a connection, this field is `nil`.
`profile_data`	table	[occasional, read-only]	The decoded profile data used to establish a connection with the servant's ORB. If the ORB is not able to establish a connection, this field is `nil`. The structure of this profile depends on the value of the `profile_tag`. For IIOP profiles (`profile_tag == 0`), this table follows the structure of `CORBA::IIOP::ProfileBody_1_0`.
`interface`	object	[read-only]	Complete description of the interface of the servant being invoked.
`interface_name`	string	[read-only]	Name of the the interface of the servant being invoked.
`operation`	object		Description of the operation being invoked. To change the operation being invoked on the remote servant, the application must set a new operation description to this field, which can be obtained from the local Interface Repository (see field `types`)
`operation_name`	string	[read-only]	Name of the operation being invoked.
`parameters`	table		List of the parameters passed to the operation being invoked.
`service_context`	table	[optional]	Set this value to define the list of service context data to be marshaled in the request. See `CORBA::IOP::ServiceContextList` in CORBA specs.
`success`	boolean	[optional]	Set this value to cancel request: `true` cancels the request and indicates to the invocation was successful; `false` cancels the request and indicates to the invocation raised an exception.
`results`	table	[optional]	Set this value to provide the result values of a canceled request. This value must be a list of the values to be returned (in case of a successful invocation) or a list with a single value indicating the raised exception or error (in case of a failed invocation).
`reply_service_context`	table	[optional]	Set this value to define the list of service context to be added to the reply of a cancelled request. See `CORBA::IOP::ServiceContextList` in CORBA specs.
`forward_reference`	table	[optional]	Set this value to forward this request to another reference. This value must be a complete IOR structure (see `CORBA::IOP::IOR` in CORBA specs.).

The fields added to table request of operation receivereply of the client interceptor are (all other fields provided on the corresponding sendrequest operation are still available):

`success`	boolean	Indication of success of the invocation. `true` indicates the invocation was successful; `false` indicates the invocation raised an exception.
`results`	table	List of the values to be returned (in case of a successful invocation) or a list with a single value indicating the raised exception or error (in case of a failed invocation).
`reply_service_context`	table	The list of service context received with the reply of the request. See `CORBA::IOP::ServiceContextList` in CORBA specs.

Server Interceptors

The fields provided by table request of operation receiverequest of the client interceptor are:

`request_id`	number	[read-only]	Number that identifies the pending request over the connection used to communicate with the remote ORB. Note that this identifier is not unique across different connections, and it may be reused by other requests after this one is completed (i.e. replied).
`response_expected`	boolean	[read-only]	Flag that indicates if the server must send a response for this request.
`servant`	table	[occasional, read-only]	The implementation of the object request is addressed to. If the ORB cannot find a servant with the provided `object_key` then this field is `nil`.
`object_key`	string		Key of the object the request is addressed to.
`interface`	object	[read-only]	Complete description of the interface of the servant being invoked. If the ORB cannot find a servant with the provided `object_key` then this field is `nil`.
`interface_name`	string	[occasional, read-only]	Name of the the interface of the servant being invoked. If the ORB cannot find a servant with the provided `object_key` then this field is `nil`.
`operation`	object	[occasional]	Description of the operation being invoked. If the ORB cannot find a servant with the provided `object_key` or the operation requested is not defined in the servant's interface then this field is `nil`. To change the operation being invoked on the remote servant, the application must set a new operation description to this field, which can be obtained from the local Interface Repository (see field `types`)
`operation_name`	string	[read-only]	Name of the operation being invoked.
`parameters`	table	[occasional]	List of the parameters passed to the operation being invoked. If the ORB cannot find a servant with the provided `object_key` or the operation requested is not defined in the servant's interface then this field is `nil`.
`service_context`	table	[optional]	Set this value to define the list of service context data to be marshaled in the request. See `CORBA::IOP::ServiceContextList` in CORBA specs.
`success`	boolean	[optional]	Set this value to cancel request: `true` cancels the request and indicates to the invocation was successful; `false` cancels the request and indicates to the invocation raised an exception.
`results`	table	[optional]	Set this value to provide the result values of a canceled request. This value must be a list of the values to be returned (in case of a successful invocation) or a list with a single value indicating the raised exception or error (in case of a failed invocation).
`reply_service_context`	table	[optional]	Set this value to define the list of service context to be added to the reply of a cancelled request. See `CORBA::IOP::ServiceContextList` in CORBA specs.
`forward_reference`	table	[optional]	Set this value to forward this request to another reference. This value must be a complete IOR structure (see `CORBA::IOP::IOR` in CORBA specs.).

The fields added to table request of operation sendreply of the client interceptor are (all other fields provided on the corresponding receiverequest operation are still available):

`success`	boolean		Indication of success of the invocation. `true` indicates the invocation was successful; `false` indicates the invocation raised an exception.
`results`	table		List of the values to be returned (in case of a successful invocation) or a list with a single value indicating the raised exception or error (in case of a failed invocation).
`reply_service_context`	table		The list of service context received with the reply of the request. See `CORBA::IOP::ServiceContextList` in CORBA specs.
`forward_reference`	table	[optional]	Set this value to forward this request to another reference. This value must be a complete IOR structure (see `CORBA::IOP::IOR` in CORBA specs.).

Adding Information to Invocations

GIOP messages for requests and replies has the special field service_context, which can be used to store additional information to be send with the invocation. Basically, the service_context is a sequence of structures, each containing a tag number and a octet sequence that is used to store arbitrary data. Interceptors can define the value of field service_context in the parameter of methods sendrequest or sendreply. Similarly, the value of field service_context can be accessed through the parameter of methods receiverequest or receivereply.

To promote interoperability with other CORBA ORBs, the information contained in field service_context is usually encoded using CORBA's CDR. To help this, OiL brokers provide methods newencoder() and newdecoder(stream). The example below, illustrates how to use interceptors to add authentication information in an invocation.

Client

require "oil"

oil.main(function()
  local broker = oil.init()
  
  print("Type the your username")
  local Username = io.read()
  print("Type the password")
  local Password = io.read()
  
  local AuthID = broker:loadidl"struct AuthID { string user; string pswd; };"
  
  local encoder = broker:newencoder()
  encoder:put({user=UserName,pswd=Password}, AuthID)
  local AuthData = encoder:getdata()
  
  local Authenticator = {}
  function Authenticator:sendrequest(request)
    request.service_context[#request.service_context + 1] = {
      context_id = 404,
      context_data = AuthData,
    }
  end
  
  broker:setinterceptor(Authenticator, "client")
  
  ...
  
end)

Server

require "oil"

oil.main(function()
  local broker = oil.init()
  
  local AuthID = broker:loadidl[[
    struct AuthID {
      string user;
      string pswd;
    };
  ]]
  
  local Validator = {}
  function Validator:receiverequest(request)
    local valid
    for _, context in ipairs(request.service_context) do
      if context.context_id == 404 then
        local decoder = broker:newdecoder(context.context_data)
        local authid = decoder:get(AuthID)
        valid = (authid.user == "admin") and (authid.pswd == "admin")
      end
    end
    if not valid then
      -- cancel request with an exception
      request.success = false
      request[1] = broker:newexcept{ "CORBA::NO_PERMISSION",
        minor_code_value = 0,
      }
    end
  end
  
  broker:setinterceptor(Validator, "server")
  
  ...
  
end)

Multithreading Issues

OiL guarantees that for each single invocation both methods of the interceptor is invoked in the same thread. Moreover, this thread is the same that initiated the invocation (in case of a client interceptor) or the thread that will execute the invocation (in case of a server interceptor). However, while an invocation is being executed, others can be issued and the interceptor be invoked again, but it will execute in a different thread. Therefore, you can use the current thread (oil.tasks.current) to identify an invocation. On the server side, the ORB may reuse a thread that executed a previous invocation that finished to execute other invocations.

Multiple Interceptors

Although CORBA brokers only allow one interceptor to be defined on the client side and one on the server side, if you need to define a chain of interceptors, you can register an interceptor that delegates interception to a chain of interceptors. The following example illustrates the implementation of a interception chain that does not guarantee the order the interceptors are invoked. The implementation use class loop.object.Publisher of LOOP library.

require "oil"
require "loop.object.Publisher"

oil.main(function()
  local broker = oil.init()
  
  InterceptionChain = loop.object.Publisher{
    Interceptor_01,
    Interceptor_02,
    Interceptor_03,
    ...
  }
  
  broker:setinterceptor(InterceptionChain, "server")
  
  ...
  
end)