Tuesday, February 2, 2010

WORK doesn't work the same way

So hows that for a double entendre?

One of the biggest challenges traditional SAS users have trouble with is the fact that WORK does not work the same way in BI/Web applications. Such SAS users are used to the work library containing all sorts of data sets and catalogs that are easily accessible later in the program (either in an interactive SAS session or a long batch job). Most BI tasks however are (and should be) structured as a series of short tasks. This is exactly where some SAS users get tripped up when building BI/Web based reports - each request starts with a brand new clean/emply WORK library. This is because the Web is a stateless environment – each request knows nothing about any prior request. This creates a simple environment in that each request is completely self contained. However, quite often it is desirable and necessary to maintain certain information from one request to the next (i.e. do what the WORK library does). This is known as maintaining state.

The good news is that that there are a number of ways to deal with this. The Stored Process Server (and the SAS/IntrNet Application Dispatcher) support the creation and use of Sessions as a way to share data and macro variables from one request to the next. A Session is simply the saving of data and parameters from one request to the next. Sessions provide the ability to save library members (data sets and catalogs) and macro variables. The program explicitly specifies what to save and restore.

The mechanics of using Sessions are straightforward. Once a session has been created, a library named SAVE is created. By creating or copying data sets and catalogs to this library, the user program can rely on them being there the next time a request is made that uses this session. For global macro variables, at the completion of any request that uses sessions, all the global macro variables whose name begin with SAVE_ are saved. At the beginning of any future request, they are restored.

And sometimes SAS will create sessions automatically. For example, when ODS is used to create an HTML page containing a graph, ODS will create a lightweight session. Since a single web request can only return one file, ODS will create the additional files and save them in SAS catalog entries. For an HTML page with an embedded graph, the graph (or graphs) will be created and saved as a graphic (e.g., gif or jpeg) entry in a catalog. For a frame, the table of contents file and the body file are saved in html entries in such a catalog. These entries are returned to the user’s browser by a link that reconnects to the session to replay the entry.

Check back later for some examples on creating and using sessions:
  • the mechanics of creation a session
  • using lightweight sessions so you can generate, for example, HTML, PDF and RTF output in one request
  • creating a session to extract/summarize data once so multiple reports can be generated from it.