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<chapter>
  <header>
    <copyright>
      <year>2003</year><year>2016</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.

    </legalnotice>

    <title>Error handling</title>
    <prepared>Ingela Anderton</prepared>
    <responsible></responsible>
    <docno></docno>
    <approved></approved>
    <checked></checked>
    <date></date>
    <rev></rev>
    <file>error_handling.xml</file>
  </header>

  <section>
    <title>Strategy </title>
    <p>On a conceptual level starting a database connection using the
      Erlang ODBC API is a basic client server application. The client
      process uses the API to start and communicate with the server
      process that manages the connection. The strategy of the Erlang
      ODBC application is that programming faults in the application
      itself will cause the connection process to terminate
      abnormally.(When a process terminates abnormally its supervisor
      will log relevant error reports.) Calls to API functions during or
      after termination of the connection process, will return <c>{error, connection_closed}</c>. Contextual errors on the other
      hand will not terminate the connection it will only return
      <c>{error, Reason} </c> to the client, where <c>Reason</c> may be
      any erlang term.</p>

    <section>
      <title>Clients </title>
      <p>The connection is associated with the process that created it
        and can only be accessed through it. The reason for this is to
        preserve the semantics of result sets and transactions when
        select_count/[2,3] is called or auto_commit is turned off.
        Attempts to use the connection from another process will
        fail. This will not effect the connection. On the other hand, if
        the client process dies the connection will be terminated.</p>
    </section>

    <section>
      <title>Timeouts </title>
      <p>All request made by the client to the connection are
        synchronous. If the timeout is used and expires the client
        process will exit with reason timeout. Probably the right thing
        to do is let the client die and perhaps be restarted by its
        supervisor. But if the client chooses to catch this timeout,
        it is a good idea to wait a little while before trying
        again. If there are too many consecutive timeouts that are
        caught the connection process will conclude that there is
        something radically wrong and terminate the connection.</p>
    </section>

    <section>
      <title>Guards </title>
      <p>All API-functions are guarded and if you pass an argument of
        the wrong type a runtime error will occur. All input parameters
        to internal functions are trusted to be correct. It is a good
        programming practise to only distrust input from truly external
        sources. You are not supposed to catch these errors, it will
        only make the code very messy and much more complex, which
        introduces more bugs and in the worst case also covers up the
        actual faults. Put your effort on testing instead, you should
        trust your own input.</p>
    </section>
  </section>

  <section>
    <title>The whole picture </title>
    <p>As the Erlang ODBC application relies on third party products
      and communicates with a database that probably runs on another
      computer in the network there are plenty of things that might go
      wrong. To fully understand the things that might happen it
      facilitate to know the design of the Erlang ODBC application,
      hence here follows a short description of the current design.</p>
    <note>
      <p>Please note that design is something, that not
        necessarily will, but might change in future releases. While the
        semantics of the API will not change as it is independent of the
        implementation.</p>
    </note>
    <image file="odbc_app_arc.gif">
      <icaption>Architecture of the Erlang odbc application</icaption>
    </image>
    <p>When you do application:start(odbc) the only thing that
      happens is that a supervisor process is started. For each call
      to the API function connect/2 a process is spawned and added as
      a child to the Erlang ODBC supervisor. The supervisors only
      tasks are to provide error-log reports, if a child process should
      die abnormally, and the possibility to do a code change. Only
      the client process has the knowledge to decide if this
      connection managing process should be restarted.</p>
    <p>The erlang connection process spawned by connect/2, will open a
      port to a c-process that handles the communication with the
      database through Microsoft's ODBC API. The erlang port will be
      kept open for exit signal propagation, if something goes wrong
      in the c-process and it exits we want know as mush as possible
      about the reason. The main communication with the c-process is
      done through sockets. The C-process consists of two threads,
      the supervisor thread and the database handler thread. The
      supervisor thread checks for shutdown messages on the supervisor
      socket and the database handler thread receives requests and sends
      answers on the database socket. If the database thread seems to
      hang on some database call, the erlang control process will send
      a shutdown message on the supervisor socket, in this case the
      c-process will exit. If the c-process crashes/exits it will
      bring the erlang-process down too and vice versa i.e. the
      connection is terminated.</p>
   
    <section>
      <title>Error types</title>
      <p>The types of errors that may occur can be divide into the
        following categories.</p>
      <list type="bulleted">
        <item>Configuration problems - Everything from that the
         database was not set up right to that the c-program that
         should be run through the erlang port was not compiled for
         your platform.</item>
        <item>Errors discovered by the ODBC driver - If calls to the
         ODBC-driver fails due to circumstances that cannot be
         controlled by the Erlang ODBC application programmer, an
         error string will be dug up from the driver.  This string
         will be the <c>Reason</c> in the <c>{error, Reason} </c>
         return value. How good this error message is will of course
         be driver dependent. Examples of such circumstances are
         trying to insert the same key twice, invalid SQL-queries and
         that the database has gone off line.</item>
        <item>Connection termination - If a connection is terminated
         in an abnormal way, or if you try to use a connection that
         you have already terminated in a normal way by calling
         disconnect/1, the return value will be<c>{error, connection_closed}</c>. A connection could end abnormally
         because of an programming error in the Erlang ODBC
         application, but also if the ODBC driver crashes.</item>
        <item>Contextual errors - If API functions are used in the
         wrong context, the <c>Reason</c> in the error tuple will
         be a descriptive atom. For instance if you try to call the
         function <c>last/[1,2] </c> without first calling <c>select_count/[2,3] </c> to associate a result set with the
         connection.  If the ODBC-driver does not support some
         functions, or if you disabled some functionality for a
         connection and then try to use it.</item>
      </list>
    </section>
  </section>
</chapter>


