## 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`) do not.

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/jinterface) 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, Java 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

| 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, Java, 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).

| 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 Erlang, C and Java Sessions

These sessions are faster and more flexible variants than the CORBA session. The protocol implemented by the generated stubs is Erlang native external communication protocol, there is no ORB and IIOP engaged. Due to these facts the clients will run 10 to 20 times faster than in CORBA session case, depending on the amount of data stored and the mnesia record seeking times.

### 2.6 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.

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