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.
Note: A recent addition to YAZ is SRW support. You can now make SRW ZOOM connections by specifying scheme http:// for the hostname for a connection.
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.
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.
You can prefix the host with a scheme followed by colon. The default scheme is tcp (Z39.50 protocol). The scheme http selects SRW over HTTP.
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 |
| password | 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 |
| serverImplementationId | Implementation ID of server. (The old targetImplementationId option is also supported for the benefit of old applications.) | none |
| targetImplementationName | Implementation Name of server. (The old targetImplementationName option is also supported for the benefit of old applications.) | none |
| serverImplementationVersion | Implementation Version of server. (the old targetImplementationVersion option is also supported for the benefit of old applications.) | none |
| databaseName | One or more database names separated by character plus (+), which to be used by subsequent search requests on this Connection. | Default |
| piggyback | True (1) if piggyback should be used in searches; false (0) if not. | 1 |
| smallSetUpperBound | If hits is less than or equal to this value, then target will return all records using small element set name | 0 |
| largeSetLowerBound | If hits is greater than this value, the target will return no records. | 1 |
| mediumSetPresentNumber | This value represents the number of records to be returned as part of a search when when hits is less than or equal to large set lower bound and if hits is greater than small set upper bound. | 0 |
| smallSetElementSetName | The element set name to be used for small result sets. | none |
| mediumSetElementSetName | The element set name to be for medium-sized result sets. | 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);
int ZOOM_connection_error_x (ZOOM_connection c, const char **cp,
const char **addinfo, const char **dset);
Function ZOOM_connection_error checks 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. Function
ZOOM_connection_error_x is an extended version
of ZOOM_connection_error that is capable of
returning name of diagnostic set in dset.
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).
The SRW protocol doesn't feature an Inititialize Request, so the connection phase merely establishes a TCP/IP connection with the SOAP service.
Most of the ZOOM connection options do not affect SRW and they are ignored. However, future versions of YAZ might honor implementationName and put that as part of User-Agent header for HTTP requests.
The charset is used in the Content-Type header of HTTP requests.