Frontier Tutorials / Working With Threads / An Example

An Example

As an example of a threaded application, let's look at a web search CGI that searches multiple commercial search engines and combines the results. (I'll not provide an entire CGI; that's left as an exercise for the reader. <evil laugh>)

First, we need a main thread, or executive. It takes the search terms from the paramTable and starts a separate thread for each search engine to be called.

local queryThreadInfo = ThreadSafeGetUnique( tableType, "MultiQuery", @system.temp ) tempThreadID « Create Yahoo subtable new( tableType, @queryThreadInfo^.yahoo ) « Copy params table for Yahoo query queryThreadInfo^.yahoo.params = searchParams^ « Start the Yahoo query thread queryThreadInfo^.yahoo.threadID = thread.easyCall( "queryYahoo", @queryThreadInfo^.yahoo ) « Create AltaVista subtable new( tableType, @queryThreadInfo^.altaVista ) « Copy params table for AltaVista query queryThreadInfo^.altaVista.params = searchParams^ « Start the AltaVista query thread queryThreadInfo^.altaVista.threadID = thread.easyCall( "queryAltaVista", @queryThreadInfo^.altaVista )

(The ThreadSafeGetUnique script is documented on the Thread Utilities page.)

Then, for each search engine known to the CGI, we need a routine that constructs the query from parameters provided by the executive, calls tcp.httpClient to query the search engine, extracts the search results from the HTML page it received from the search engine into a table for use by the executive, and exits. These search-engine-specific routines are called queryYahoo and queryAltaVista in the code fragment above.

While the separate query threads are running, the executive has to bide its time, waiting for them to complete. The simple way to do this is for it to monitor a completion flag in the query threads' parameter tables.

local yahooQueryCompleted = false altaVistaQueryCompleted = false queriesCompleted = false   while ( ! queriesCompleted ) thread.sleepFor( 1 ) if ( ! yahooQueryCompleted ) yahooQueryCompleted = queryThreadInfo^.yahoo.queryCompleted if ( ! altaVistaQueryCompleted ) altaVistaQueryCompleted = queryThreadInfo^.altaVista.queryCompleted queriesCompleted = yahooQueryCompleted && altaVistaQueryCompleted

The call to thread.sleepFor results in this loop executing once every second. We could simply call thread.exists to determine whether each thread has completed, but the queryCompleted flags allow the query threads to indicate when they have completed successfully. We should actually check both, so we will know if the thread terminates for some other reason, so we don't wait forever for the flag to be set. Also, since Web interactions can take a long time, we should have time-out logic that will kill a query thread if it takes longer than a user-specified time.

The final stage, then, will be to combine the results received and extracted from the various search engines into a single report page, and return it to the user.

The processing timeline then looks something like this:

This is, of course, a greatly simplified example. A complete multiple-search CGI would need at least the following additions:

• handle scriptErrors in all phases
• allow the user to select from a number of search engines
• process multiple pages of query results from each search engine
• time out in the Monitoring phase if the queries take too long