YAZ User's Guide and Reference
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# Chapter 3. Building clients with ZOOM

ZOOM is an acronym for 'Z39.50 Object-Orientation Model' and is an initiative started by Mike Taylor (Mike is from the UK, which explains the peculiar name of the model). The goal of ZOOM is to provide a common Z39.50 client API not bound to a particular programming language or toolkit.

The lack of a simple Z39.50 client API for YAZ has become more and more apparent over time. So when the first ZOOM specification became available, an implementation for YAZ was quickly developed. For the first time, it is now as easy (or easier!) to develop clients than servers with YAZ. This chapter describes the ZOOM C binding. Before going further, please reconsider whether C is the right programming language for the job. There are other language bindings available for YAZ, and still more are in active development. See the ZOOM web-site for more information.

In order to fully understand this chapter you should read and try the example programs `zoomtst1.c`, `zoomtst2.c`, .. in the `zoom` directory.

The C language misses features found in object oriented languages such as C++, Java, etc. For example, you'll have to manually, destroy all objects you create, even though you may think of them as temporary. Most objects has a `_create` - and a `_destroy` variant. All objects are in fact pointers to internal stuff, but you don't see that because of typedefs. All destroy methods should gracefully ignore a `NULL` pointer.

In each of the sections below you'll find a sub section called protocol behavior, that describes how the API maps to the Z39.50 protocol.

# Connections

The Connection object is a session with a target.

       #include <yaz/zoom.h>

       ZOOM_connection ZOOM_connection_new (const char *host, int portnum);

       ZOOM_connection ZOOM_connection_create (ZOOM_options options);

       void ZOOM_connection_connect(ZOOM_connection c, const char *host,
                                     int portnum);
       void ZOOM_connection_destroy (ZOOM_connection c);

Connection objects are created with either function `ZOOM_connection_new` or `ZOOM_connection_create`. The former creates and automatically attempts to establish a network connection with the target. The latter doesn't establish a connection immediately, thus allowing you to specify options before establishing network connection using the function `ZOOM_connection_connect`. If the port number, `portnum`, is zero, the `host` is consulted for a port specification. If no port is given, 210 is used. A colon denotes the beginning of a port number in the host string. If the host string includes a slash, the following part specifies a database for the connection.

Connection objects should be destroyed using the function `ZOOM_connection_destroy`.

        void ZOOM_connection_option_set (ZOOM_connection c,
                                         const char *key,
                                         const char *val);

        const char *ZOOM_connection_option_get (ZOOM_connection c,
                                                const char *key);

The `ZOOM_connection_option_set` allows you to set an option given by `key` to the value `value` for the connection. Function `ZOOM_connection_option_get` returns the value for an option given by `key`.

Table 3-1. ZOOM Connection Options

Option | Description | Default
--- | --- | ---
implementationName | Name of Your client | none
user | Authentication user name | none
group | Authentication group name | none
pass | Authentication password | none
host | Target host. This setting is "read-only". It's automatically set internally when connecting to a target. | none
proxy | Proxy host | none
async | If true (1) the connection operates in asynchronous operation which means that all calls are non-blocking except `ZOOM_event`. | 0
maximumRecordSize | Maximum size of single record. | 1 MB
preferredMessageSize | Maximum size of multiple records. | 1 MB
lang | Language for negotiation. | none
charset | Character set for negotiation. | none

If either option `lang` or `charset` is set, then Character Set and Language Negotiation is in effect.

         int ZOOM_connection_error (ZOOM_connection c, const char **cp,
                                     const char **addinfo);

Use `ZOOM_connection_error` to check for errors for the last operation(s) performed. The function returns zero if no errors occurred; non-zero otherwise indicating the error. Pointers `cp` and `addinfo` holds messages for the error and additional-info if passed as non-`NULL`.

## Protocol behavior

The calls `ZOOM_connection_new` and `ZOOM_connection_connect` establishes a TCP/IP connection and sends an Initialize Request to the target if possible. In addition, the calls waits for an Initialize Response from the target and the result is inspected (OK or rejected).

If `proxy` is set then the client will establish a TCP/IP connection with the peer as specified by the `proxy` host and the hostname as part of the connect calls will be set as part of the Initialize Request. The proxy server will then "forward" the PDU's transparently to the target behind the proxy.

For the authentication parameters, if option `user` is set and both options `group` and `pass` are unset, then Open style authentication is used (Version 2/3) in which case the username is usually followed by a slash, then by a password. If either `group` or `pass` is set then idPass authentication (Version 3 only) is used. If none of the options are set, no authentication parameters are set as part of the Initialize Request (obviously).

When option `async` is 1, it really means that all network operations are postponed (and queued) until the function `ZOOM_event` is invoked. When doing so it doesn't make sense to check for errors after `ZOOM_connection_new` is called since that operation "connecting - and init" is still incomplete and the API cannot tell the outcome (yet).

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