<page xmlns="http://projectmallard.org/1.0/" xmlns:its="http://www.w3.org/2005/11/its" xmlns:xi="http://www.w3.org/2003/XInclude" type="topic" id="databases" xml:lang="el">

  <info>

    <link type="guide" xref="index#specific-how-tos"/>

    <credit type="author copyright">

      <name>Philip Withnall</name>

      <email its:translate="no">philip.withnall@collabora.co.uk</email>

      <years>2015</years>

    </credit>

    <include xmlns="http://www.w3.org/2001/XInclude" href="cc-by-sa-3-0.xml"/>

    <desc>Simple persistent object stores</desc>

    <mal:credit xmlns:mal="http://projectmallard.org/1.0/" type="translator copyright">

      <mal:name>Ελληνική μεταφραστική ομάδα GNOME</mal:name>

      <mal:email>team@gnome.gr</mal:email>

      <mal:years>2016</mal:years>

    </mal:credit>

    <mal:credit xmlns:mal="http://projectmallard.org/1.0/" type="translator copyright">

      <mal:name>Θάνος Τρυφωνίδης</mal:name>

      <mal:email>tomtryf@gnome.org</mal:email>

      <mal:years>2016</mal:years>

    </mal:credit>

  </info>

  <title>Βάσεις δεδομένων</title>

  <synopsis>

    <title>Σύνοψη</title>

    <list>

      <item>

        Use databases for appropriate use cases: not configuration data (use

        GSettings). (<link xref="#when-to-use-databases"/>)

      
</item>

      <item>

        Choose between GOM and GVDB based on whether indexing is required.

        (<link xref="#when-to-use-databases"/>)

      
</item>

      <item>

        Consider your vacuuming policy before committing to using GOM.

        (<link xref="#when-to-use-databases"/>)

      
</item>

      <item>

        Avoid SQL injection vulnerabilities by using prepared statements.

        (<link xref="#sql-injection"/>)

      
</item>

    </list>

  </synopsis>

  <section id="when-to-use-databases">

    <title>Πότε να χρησιμοποιήσετε βάσεις δεδομένων</title>

      Configuration data should be stored in

      <link href="https://developer.gnome.org/gio/stable/GSettings.html">GSettings</link>.

      As a rule of thumb, if some data needs to be persistent and affects how an

      application behaves, it is configuration data. If it could potentially be

      subject to policies imposed by the system administrator (such as proxy or

      lockdown settings), it is configuration data. If it contains user created

      content, it is not configuration data, and should not be stored in

      GSettings.

      For such situations where user data is highly structured, storing it in a

      database is sensible. There are two main databases suggested for use

      within GNOME: GOM and GVDB. GOM is a wrapper around SQLite, and hence

      implements indexing of fields and SQL-style queries. GVDB is a much

      simpler object store, supporting fast serialization of a dictionary of

      objects to disk.

      GOM should be used if you need advanced features, especially indexing.

      GVDB should be used otherwise.

      Before deciding to use GOM (and hence SQLite), you must consider a

      vacuuming policy for the database, and whether your use case will interact

      well with SQLite’s vacuuming system. Vacuuming is effectively SQLite’s

      term for defragmenting the database — if a database is not vacuumed

      appropriately, performance will degrade and the database size will

      increase indefinitely. Read

      <link href="http://blogs.gnome.org/jnelson/2015/01/06/sqlite-vacuum-and-auto_vacuum/">this

      article</link> on vacuuming for more information; please consider it

      before choosing to use GOM.

      GNOME has another database library: GNOME Data Access (GDA). This is

      targeted at abstracting access to various types of relational database,

      for use in a database utility program or office program, for example. It

      is not suitable for storing

      <link href="https://developer.gnome.org/gio/stable/GSettings.html">user

      settings</link>.

  </section>

  <section id="gom">

    <title>Χρήση του GOM</title>

      Providing a GOM tutorial is beyond the scope of this document, but a

      <link href="https://developer.gnome.org/gom/">reference manual is

      available</link>.

    <section id="sql-injection">

      <title>SQL Injection</title>

        GOM does allow access to the lower level SQLite query APIs. When using

        them, queries must be constructed using

        SQLite’s <link href="https://www.sqlite.org/c3ref/stmt.html">prepared

        statement</link> and

        <link href="https://www.sqlite.org/c3ref/bind_blob.html">value

        binding</link> API, rather than by constructing SQL strings then passing

        them to SQLite to parse. Constructing strings makes

        <link href="http://en.wikipedia.org/wiki/SQL_injection">SQL

        injection</link> vulnerabilities very likely, which can give attackers

        access to arbitrary user data from the database.

    </section>

  </section>

  <section id="gvdb">

    <title>Χρήση του GVDB</title>

      GVDB has a simple API which mirrors a conventional hash table. Presently,

      GVDB is only available as a copy-and-paste library; fetch the most recent

      copy of the code from

      <link href="https://git.gnome.org/browse/gvdb">GVDB git</link> and copy

      it into your project. It is licenced under LGPLv2.1+.

      A full GVDB tutorial is beyond the scope of this document.

  </section>

</page>