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2 <chapter id="introduction"><title>Introduction</title>
5 The <ulink url="http://www.indexdata.dk/yaz/">&yaz;</ulink>
6 toolkit offers several different levels of access to the
7 <ulink url="http://www.loc.gov/z3950/agency/">ISO23950/Z39.50</ulink>
8 and <ulink url="http://www.nlc-bnc.ca/iso/ill/">ILL</ulink> protocols.
9 The level that you need to use depends on your requirements, and
10 the role (server or client) that you want to implement.
11 If you're developing a client application you should consider the
12 <link linkend="zoom">ZOOM</link> API.
13 It is, by far, the easiest way to develop clients in C.
14 Server implementers should consider the
15 <link linkend="server">generic frontend server</link>.
16 None of those high-level APIs support the whole protocol, but
17 they do include most facilities used in existing Z39.50
21 If you're using 'exotic' functionality (meaning anything not included in
22 the high-level APIs), developing non-standard extensions to Z39.50 or you're
23 going to develop an ILL application you'll have to learn the lower
27 The basic low level modules, which are independent of the role
28 (client or server), consist of three primary interfaces:
31 <listitem><para>&asn;, which provides a C representation of the Z39.50
32 protocol packages (PDUs).
34 <listitem><para>&odr;, which encodes and decodes the packages according
35 to the BER specification.
37 <listitem><para>&comstack;, which exchanges the encoded packages with
38 a peer process over a network.
42 The &asn; module represents the ASN.1 definition of
43 the Z39.50 protocol. It establishes a set of type and
44 structure definitions, with one structure for each of the top-level
45 PDUs, and one structure or type for each of the contained ASN.1 types.
46 For primitive types, or other types that are defined by the ASN.1
47 standard itself (such as the EXTERNAL type), the C representation is
48 provided by the &odr; (Open Data Representation) subsystem.
51 &odr; is a basic mechanism for representing an
52 ASN.1 type in the C programming language, and for implementing BER
53 encoders and decoders for values of that type. The types defined in
54 the &asn; module generally have the prefix <literal>Z_</literal>, and
55 a suffix corresponding to the name of the type in the ASN.1
56 specification of the protocol (generally Z39.50-1995). In the case of
57 base types (those originating in the ASN.1 standard itself), the prefix
58 <literal>Odr_</literal> is sometimes seen. Either way, look for
59 the actual definition in either <filename>proto.h</filename> (for the types
60 from the protocol), <filename>odr.h</filename> (for the primitive ASN.1
61 types, or <filename>odr_use.h</filename> (for the ASN.1
62 <emphasis>useful</emphasis> types). The &asn; library also
63 provides functions (which are, in turn, defined using &odr;
64 primitives) for encoding and decoding data values. Their general form is
67 int z_xxx(ODR o, Z_xxx **p, int optional, const char *name);
69 (note the lower-case "z" in the function name)
74 If you are using the premade definitions of the &asn; module, and you
75 are not adding new protocol of your own, the only parts of &odr; that you
76 need to worry about are documented in section
77 <link linkend="odr-use">Using ODR</link>.
82 When you have created a BER-encoded buffer, you can use the &comstack;
83 subsystem to transmit (or receive) data over the network. The &comstack;
84 module provides simple functions for establishing a connection
85 (passively or actively, depending on the role of your application),
86 and for exchanging BER-encoded PDUs over that connection. When you
87 create a connection endpoint, you need to specify what transport to
89 For the remainder of the connection's lifetime, you don't have
90 to worry about the underlying transport protocol at all - the &comstack;
91 will ensure that the correct mechanism is used.
94 We call the combined interfaces to &odr;, &asn;, and &comstack; the service
95 level API. It's the API that most closely models the Z39.50
96 service/protocol definition, and it provides unlimited access to all
97 fields and facilities of the protocol definitions.
100 The reason that the &yaz; service-level API is a conglomerate of the
101 APIs from three different submodules is twofold. First, we wanted to allow
102 the user a choice of different options for each major task. For instance,
103 if you don't like the protocol API provided by &odr;/&asn;, you
104 can use SNACC or BERUtils instead, and still have the benefits of the
105 transparent transport approach of the &comstack; module. Secondly,
106 we realize that you may have to fit the toolkit into an existing
107 event-processing structure, in a way that
108 is incompatible with the &comstack; interface or some other part of &yaz;.
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