Pazpar2 offers programmers a simple Web Service protocol that can be used (queried in a request/response fashion) from any, server- or client-side, programming language with an XML support. However, when programming a Web-based client to Pazpar2, to achieve certain level of interactivity and instant notification of latest changes in the result set, Ajax (Asynchronous JavaScript and XML) technology may be used. An Ajax client allows user to browse the results before the lengthy process of information retrieval from the back-end targets is finished. Blocking and waiting for usually slow back-end targets is one of the biggest functionality issues in a federated search engine.

Pazpar2 comes with a small JavaScript library called pz2.js. This library is designed to simplify development of an Ajax-based pazpar2 client and alleviate the programmer from the low-level details like polling the web service, fetching and parsing returned XML output or managing timers, sessions and basic state variables.

The library supports most major browsers including Firefox 1.5+, IE 6+, Safari 2+, Opera 9+ and Konqueror.

The library can work in two modes: a session-aware mode and a session-less mode.

In the session-aware mode, the library assumes that the pazpar2 daemon is contacted directly (preferably via Apache proxy to avoid security breaches) and tracks the session IDs internally.

In the session-less, mode the library assumes that the client is identified on the server and the session IDs are not managed directly. This way of operation requires a more sophisticated pazpar2 proxy (preferably a wrapper written in a server-side scripting language like PHP that can identify clients and relate them to open pazpar2 sessions).

Using pz2.js

Client development with the pz2.js is strongly event based and the style should be familiar to most JavaScript developers. A simple client (jsdemo) is distributed with Pazpar2's source code and shows how to set-up and use pz2.js.

In short, the programmer starts by instantiating the pz2 object and passing an array of parameters to the constructor. The parameter array specifies callbacks used for handling responses to the pazpar2 commands. Additionally, the parameter array is used to configure run-time parameters of the pz2.js like polling timer time-outs, session-mode and XSLT stylesheets.

Command callbacks

Callback naming is simple, and follows “on” prefix plus command name scheme (like onsearch, onshow, onrecord, ... etc.). When the programmer calls a function like show or record on the pz2 object, pz2.js will keep on polling pazpar2 (until the backend targets are idle) and with each command's response an assigned callback will be called. In case of Pazpar2's internal error, an error callback is called.

  my_paz = new pz2 (
  {
   "pazpar2path": "/pazpar2/search.pz2",
   "usesessions" : true,

   // assigning command handler, turns on automatic polling
   "onshow": my_onshow,
   // polling period for each command can be specified
   "showtime": 500,

   "onterm": my_onterm,
   // facet terms are specified as a comma separated list
   "termlist": "subject,author",

   "onrecord": my_onrecord
  }
  );
 

Each command callback is a user-defined function that takes a hash object as a parameter. The hash object contains parsed pazpar2 responses (hash members that correspond to the elements in the response XML document). Within the handler, the programmer further processes the data and updates the viewed document.

  function my_onstat(data) {
   var stat = document.getElementById("stat");
   stat.innerHTML = '<span>Active clients: '+ data.activeclients
    + '/' + data.clients + ' | </span>'
    + '<span>Retrieved records: ' + data.records
    + '/' + data.hits + '</span>';
  }

  function my_onshow(data) {
   // data contains parsed show response
   for (var i = 0; i < data.hits[0].length; i++)
    // update page with the hits
  }

  function on_record(data) {
   // if detailsstylesheet parameter was set data array
   // will contain raw xml and xsl data
   Element_appendTransformResult(someDiv, data.xmlDoc, data.xslDoc);
  }
 

pz2.js on runtime

The search process is initiated by calling the search method on the instantiated pz2 object. To initiate short status reports and per-target status information, methods stat and bytarget are called accordingly.

  my_paz.search (query, recPergPage, 'relevance');
 

Managing the results (keeping track of the browsed results page and sorting) is up to the client's programmer. At any point the show method may be called to bring up the latest result set with a different sorting criteria or range, without re-executing the search on the back-end.

  my_paz.show (1, 10, 'relevance');
 

To retrieve a detailed record, the record command is called. When calling the record command, one may temporarily override its default callback by specifying the handler parameter. This might be useful when retrieving raw records that need to be processed differently.

  my_paz.record (recId, 2, 'opac', { “callback”: temp_callback, “args”, caller_args});
 

pz2.js comes with a set of cross-browser helper classes and functions.