2 Overview

2.1 Interfaces

The interfaces of the Mnesia Session application are defined in IDL (Interface Definition Language). The IDL module mnesia consists of two interfaces:

• The connector (and possibly corba_connector) is started statically when starting the Mnesia Session application.
  
• The session (and corba_session) which is started dynamically at the request of a client.
  

When a Mnesia Session client needs to access the Mnesia DBMS it locates a connector on some Erlang node and starts a session there by applying the connect function on the connector.

Once the client has started a session it may use it to perform various Mnesia functions. The functions are performed locally on the Erlang node where the session resides. Most of Mnesia's functions have location transparent behavior, but some of them (e.g. start/0) don't.

The Mnesia Session interfaces makes it possible to access the administration and dirty functionality of Mnesia. To use transactions and/or Mnemosyne queries it is up to the user to define the needed interface in IDL. Implementing a user specified interface allows an opportunity to skip the general handling of erlang::term, which can sometimes be troublesome when using foreign languages, for both mnesia_session and the mnesia_corba_session.

See the IDL specification for functions which are available. The specification resembles the Mnesia API as much as possible. Please, read the Mnesia documentation set regarding the semantics used in the interface.

All the functions in the session ( and corba_session) API return Status which indicates if the operation was successful or not. Most of the functions have an out parameter with a string reason describing the error, if one occurs.

The return value Status should be checked after each call, and it should be matched against the Status enum ok. For some functions, the end_of_table also means that the operation was successful.

2.2 Communication protocols

The IDL specification of Mnesia Session has two alternatives when compiling. These can be found in the mnesia_session/include directory:

• mnesia_session.idl
  
• mnesia_corba_session.idl
  

The mnesia_session.idl file must be compiled with IC (OTP's own IDL compiler). The generated stub files use the proprietary distribution protocol of Erlang (erl_interface) to carry out the communication between clients and servers of connectors and sessions. On the server side Mnesia Session is implemented in Erlang using Mnesia's public API. On the client side Erlang or C may be used.

The mnesia_corba_session.idl file may be compiled with any Corba compliant IDL compiler (e.g. Orbix, JacORB, TelORB, IC, ...) . The generated stub files uses IIOP (a protocol standardized by OMG) to carry out the communication between clients and servers of connectors and sessions. On the server side Mnesia Session is implemented in Erlang using Mnesia's public API. On the client side a wide range of programming languages are available: Java, Smalltalk, C++, Erlang etc.

2.3 Sessions

When the Mnesia Session application is started, an Erlang process with the registered name mnesia_connector is created. The following example illustrates how a session is started:

      % erl

      1> application:start(mnesia_session).
      ok
      2> Name = mnesia_connector,
      mnesia_connector
      3> Connector = erlang:whereis(Name).
      <0.34.0>
      4> Session = mnesia_connector:connect(Connector).
      <0.35.0>
      5> ok = mnesia_connector:disconnect(Connector, Session).
      ok
    

Note: In the example given, both the client and server reside (in Erlang) on the same node.

See the Orber and IC documentation about the language mapping between Erlang, C and IDL.

2.4 Corba Sessions

If the Mnesia Session application has been started with the configuration parameter enable_corba set to true, a mnesia_corba_connector object is also created (in addition to the mandatory mnesia_connector process), and registered in Orber. The following simplified example illustrates how a corba_session can be started:

      % erl -mnesia_session enable_corba true
      
      1> application:start(mnesia_session).
      
      2> NS = corba:resolve_initial_references("NameService").
      3> NC = lname_component:set_id(lname_component:create(), 
                                     "mnesia_corba_connector").
      4> Name = lname:insert_component(lname:create(), 1, NC).
      5> Connector = 'CosNaming_NamingContext':resolve(NS, Name).
      
      6> Session = mnesia_corba_connector:connect(Connector).
      7> mnesia_corba_connector:disconnect(Connector, Session).
    

Note: In the example given, both the client and server reside (in Erlang) on the same node.

More information about Corba conventions and usage can be found in the Orber and IC documentation.

Since Orber uses Mnesia internally, some of the functions in the Mnesia API are not available via IIOP. Examples of such functions are:

• start;
  
• stop;
  
• create_schema; and,
  
• delete_schema.
  

See the IDL specification for the exact specification.

Some other functions are not supported due to the problem of representing void objects of unknown types. The dirty_[index_]match_object functionality has been replaced with the simpler function dirty_match_all which returns all records in a table.

2.5 User defined interfaces

To be able to send records over the IIOP protocol, the records must be defined as structures in an IDL specification, and compiled with IC in order to enable registering of the types in Orber's InterFace Repository (IFR). The records are mapped to the type any in Corba.

We recommend that all records are defined as IDL structures. This also applies when the erl_interface protocol is used (even though it may work without it). By including the header files produced in the code generation, several useful type definitions are made available for the application.

The generic dirty access functions in the API of Mnesia Session is merely included for the convenience of application developers and it may be tempting to organize the application code around these functions. The application interface between its clients and servers, should however be carefully designed according to the needs of the application, regardless of the Mnesia Session interface.

Instead of sending records back and forth between the server and client nodes as in the generic get-/put-oriented interface of Mnesia Session, it may (in many cases) be a better application design, to perform the application logic on the same (Erlang) node as the residing data. Besides the obvious performance advantage, it makes the applications more independent of future changes in the data model of the application.