--- loncom/lonsql	2003/02/03 05:08:06	1.54
+++ loncom/lonsql	2004/05/03 16:20:57	1.58
@@ -3,7 +3,7 @@
 # The LearningOnline Network
 # lonsql - LON TCP-MySQL-Server Daemon for handling database requests.
 #
-# $Id: lonsql,v 1.54 2003/02/03 05:08:06 harris41 Exp $
+# $Id: lonsql,v 1.58 2004/05/03 16:20:57 matthew Exp $
 #
 # Copyright Michigan State University Board of Trustees
 #
@@ -39,9 +39,59 @@ lonsql - LON TCP-MySQL-Server Daemon for
 This script should be run as user=www.  
 Note that a lonsql.pid file contains the pid of the parent process.
 
-=head1 DESCRIPTION
+=head1 OVERVIEW
 
-lonsql is currently mutilated.
+=head2 Purpose within LON-CAPA
+
+LON-CAPA is meant to distribute A LOT of educational content to A LOT
+of people. It is ineffective to directly rely on contents within the
+ext2 filesystem to be speedily scanned for on-the-fly searches of
+content descriptions. (Simply put, it takes a cumbersome amount of
+time to open, read, analyze, and close thousands of files.)
+
+The solution is to index various data fields that are descriptive of
+the educational resources on a LON-CAPA server machine in a
+database. Descriptive data fields are referred to as "metadata". The
+question then arises as to how this metadata is handled in terms of
+the rest of the LON-CAPA network without burdening client and daemon
+processes.
+
+The obvious solution, using lonc to send a query to a lond process,
+doesn't work so well in general as you can see in the following
+example:
+
+    lonc= loncapa client process    A-lonc= a lonc process on Server A
+    lond= loncapa daemon process
+
+                 database command
+    A-lonc  --------TCP/IP----------------> B-lond
+
+The problem emerges that A-lonc and B-lond are kept waiting for the
+MySQL server to "do its stuff", or in other words, perform the
+conceivably sophisticated, data-intensive, time-sucking database
+transaction.  By tying up a lonc and lond process, this significantly
+cripples the capabilities of LON-CAPA servers.
+
+The solution is to offload the work onto another process, and use
+lonc and lond just for requests and notifications of completed
+processing:
+
+                database command
+
+  A-lonc  ---------TCP/IP-----------------> B-lond =====> B-lonsql
+         <---------------------------------/                |
+           "ok, I'll get back to you..."                    |
+                                                            |
+                                                            /
+  A-lond  <-------------------------------  B-lonc   <======
+           "Guess what? I have the result!"
+
+Of course, depending on success or failure, the messages may vary, but
+the principle remains the same where a separate pool of children
+processes (lonsql's) handle the MySQL database manipulations.
+
+Thus, lonc and lond spend effectively no time waiting on results from
+the database.
 
 =head1 Internals
 
@@ -53,6 +103,7 @@ use strict;
 
 use lib '/home/httpd/lib/perl/';
 use LONCAPA::Configuration;
+use LONCAPA::lonmetadata();
 
 use IO::Socket;
 use Symbol;
@@ -165,6 +216,11 @@ unless ($dbh = DBI->connect("DBI:mysql:l
     my $subj="LON: $perlvar{'lonHostID'} Cannot connect to database!";
     system("echo 'Cannot connect to MySQL database!' |".
            " mailto $emailto -s '$subj' > /dev/null");
+
+    open(SMP,'>/home/httpd/html/lon-status/mysql.txt');
+    print SMP 'time='.time.'&mysql=defunct'."\n";
+    close(SMP);
+
     exit 1;
 } else {
     $dbh->disconnect;
@@ -420,8 +476,9 @@ sub do_sql_query {
         #prepare and execute the query
         my $sth = $dbh->prepare($query);
         unless ($sth->execute()) {
-            &logthis("<font color=blue>WARNING: ".
-                     "Could not retrieve from database: $@</font>");
+            &logthis('<font color=blue>'.
+                     'WARNING: Could not retrieve from database:'.
+                     $sth->errstr().'</font>');
         } else {
             my $aref=$sth->fetchall_arrayref;
             foreach my $row (@$aref) {
@@ -880,17 +937,6 @@ sub DISCONNECT {
 }
 
 
-
-
-
-
-
-
-
-
-
-# ----------------------------------- POD (plain old documentation, CPAN style)
-
 =pod
 
 =back