The YAZ toolkit offers several different levels of access to the Z39.50 and SR protocols. The level that you need to use depends on your requirements, and the role (server or client) that you want to implement.
The basic level, which is independent of the role, consists of three primary interfaces:
The ASN module represents the ASN.1 definition of the SR/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.
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 Z_, 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
Odr_ is sometimes seen. Either way, look for
the actual definition in either proto.h (for the types from the
protocol), odr.h (for the primitive ASN.1 types, or odr_use.h (for the
ASN.1 useful 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
int z_xxx(ODR o, Z_xxx **p, int optional, const char *name);
(note the lower-case "z" in the function name)
NOTE: 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 Using ODR.
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 (OSI or TCP/IP), and which protocol you want to use (SR or Z39.50). For the remainer 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.
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/SR service/protocol definition, and it provides unlimited access to all fields and facilities of the protocol definitions.
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 realise 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.