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 <chapter id="introduction"><title>Introduction</title>
  
  <para>
   The <ulink url="http://www.indexdata.dk/yaz/">&yaz;</ulink>
   toolkit offers several different levels of access to the
   <ulink url="http://www.loc.gov/z3950/agency/">ISO23950/Z39.50</ulink>
   and <ulink url="http://www.nlc-bnc.ca/iso/ill/">ILL</ulink> protocols.
   The level that you need to use depends on your requirements, and
   the role (server or client) that you want to implement.
   If you're developing a client application you should consider the
   <link linkend="zoom">ZOOM</link> API.
   It is, by far, the easiest way to develop clients in C.
   Server implementers should consider the 
   <link linkend="server">generic frontend server</link>.
   None of those high-level APIs support the whole protocol, but
   they do include most facilities used in existing Z39.50
   applications.
  </para>
  <para>
   If you're using 'exotic' functionality (meaning anything not included in
   the high-level APIs), developing non-standard extensions to Z39.50 or you're
   going to develop an ILL application you'll have to learn the lower
   level APIs of &yaz;.
  </para>
  <para>
   The basic low level modules, which are independent of the role (client or server),
   consist of three primary interfaces:
   
   <itemizedlist>
    <listitem><para>&asn;, which provides a C representation of the Z39.50
      protocol packages (PDUs).
     </para></listitem>
    <listitem><para>&odr;, which encodes and decodes the packages according
      to the BER specification.
     </para></listitem>
    <listitem><para>&comstack;, which exchanges the encoded packages with
      a peer process over a network.
     </para></listitem>
   </itemizedlist>
   
   The &asn; module represents the ASN.1 definition of
   the Z39.50 protocol. It establishes a set of type and
   structure definitions, with one structure for each of the top-level
   PDUs, and one structure or type for each of the contained ASN.1 types.
   For primitive types, or other types that are defined by the ASN.1
   standard itself (such as the EXTERNAL type), the C representation is
   provided by the &odr; (Open Data Representation) subsystem.
  </para>
  <para>
   &odr; is a basic mechanism for representing an
   ASN.1 type in the C programming language, and for implementing BER
   encoders and decoders for values of that type. The types defined in
   the &asn; module generally have the prefix <literal>Z_</literal>, and
   a suffix corresponding to the name of the type in the ASN.1
   specification of the protocol (generally Z39.50-1995). In the case of
   base types (those originating in the ASN.1 standard itself), the prefix
   <literal>Odr_</literal> is sometimes seen. Either way, look for
   the actual definition in either <filename>proto.h</filename> (for the types
   from the protocol), <filename>odr.h</filename> (for the primitive ASN.1
   types, or <filename>odr_use.h</filename> (for the ASN.1
   <emphasis>useful</emphasis> types). The &asn; library also
   provides functions (which are, in turn, defined using &odr;
   primitives) for encoding and decoding data values. Their general form is

   <synopsis>
    int z_xxx(ODR o, Z_xxx **p, int optional, const char *name);
   </synopsis>
   (note the lower-case &quot;z&quot; in the function name)
  </para>

  <note>
   <para>
    If you are using the premade definitions of the &asn; module, and you
    are not adding new protocol of your own, the only parts of &odr; that you
    need to worry about are documented in section 
    <link linkend="odr-use">Using ODR</link>.
   </para>
  </note>

  <para>
   When you have created a BER-encoded buffer, you can use the &comstack;
   subsystem to transmit (or receive) data over the network. The &comstack;
   module provides simple functions for establishing a connection
   (passively or actively, depending on the role of your application),
   and for exchanging BER-encoded PDUs over that connection. When you
   create a connection endpoint, you need to specify what transport to
   use (TCP/IP or SSL).
   For the remainder of the connection's lifetime, you don't have
   to worry about the underlying transport protocol at all - the &comstack;
   will ensure that the correct mechanism is used.
  </para>
  <para>
   We call the combined interfaces to &odr;, &asn;, and &comstack; the service
   level API. It's the API that most closely models the Z39.50
   service/protocol definition, and it provides unlimited access to all
   fields and facilities of the protocol definitions.
  </para>
  <para>
   The reason that the &yaz; service-level API is a conglomerate of the
   APIs from three different submodules is twofold. First, we wanted to allow
   the user a choice of different options for each major task. For instance,
   if you don't like the protocol API provided by &odr;/&asn;, you
   can use SNACC or BERUtils instead, and still have the benefits of the
   transparent transport approach of the &comstack; module. Secondly,
   we realize that you may have to fit the toolkit into an existing
   event-processing structure, in a way that
   is incompatible with the &comstack; interface or some other part of &yaz;.
  </para>
 </chapter>

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