The many things I learned setting up external database enrollment on our moodle site...

I am a conceptual person.  I am a programmer, mostly of the past.  I am a current Site administrator for 4 moodle instances.  I am a DB analyst, DB programmer, problem solver and explorer.

When my customer said can we sync the enrollments with moodle, my brain insisted first on understanding the bigger picture before tackling the details.  I must have used about 4 notebook pages trying to figure out what was talking to who and when....

Questions like,

• Who is synching with who?  
• What exactly is synching?  
• What part of the enrollment data is synching? 
• Who will push data to who? 
• How will moodle do this? 
• Is moodle pulling or pushing?  
• Do I need to make a new DB or table to push to in moodle?  
• Did the user account have to exist in moodle?  
• Did the course have to exist in moodle?

I started my journey in the moodle docs page External Database enrollment. , which prompted me to ask my customer, I actually asserted to my customer that the user and course must already exist in moodle for the sync to work with the external enrollment DB.  He verified that to be true.

Edit 5/2/14 - My customer did not understand what I was asking and answered me *in-correctly. He verified that the record, account, was created in Genius, not moodle.  Eventually, we sorted this out.

I got a message from another of my customers clients about this,

"Hi James, you don't have to wrote code to do the synching, moodle has internal functionality for synching the enrollments with another database.  Im not an expert, but one other client we have said to set it up using the cron to run the following scripts

path_to_file\enrol_database_sync.php
path_to_file\auth_db_sync_users.php
"

Well, it was a start, but was mostly in-correct or in-complete, since those were files that used a much older version of moodle, I think 1.x.  Neither of these files were involved in the solution.

EDIT - 5/2/14 - two weeks later and I was wrong about part of what I said here.  We did also need to sync external DB Authentication.  I am using Moodle 2.2, so the file names are a little different, but the concept is accurate.

Concepts that I had to get into my mental model

1. The external DB is the Master
2. Moodle pulls from the Master.
3. The moodle pull from the master occurs when the sync.php file is executed.
4. The moodle sync.php file should be called from cron.php - since cron.php should already be set up in your moodle instance.
5. I could/should use a DOS .bat file to call the sync.php file.
6. Have the cron.php script call the DOS .bat file.
7. Moodle is the consumer of enrollment data coming from the external DB
8. The external DB is rather simple, consisting of only two tables, of which only ONE is moodle interested in
9. The table that moodle is interested in has only 4 columns (id, courseID, username and role).  
10. The course, user and role are the 3 bits of information that moodle needs to *sync to.
11. Moodle will actually grab that data from the external DB and update itself.
12. Moodle will initiate the grab by calling a specific core file called sync.php
13. The sync.php file, for me, needed to be called at the command line.
14. The sync.php file in my install is located at webroot/instance/enrol/database/cli.
15. A windows .bat file is used to call the sync.php file from the command line.
16. A trigger to call the bat file is necessary, so the cron.php script is a logical place.
17. The sync.php script is automatically called when an account logs in.

Two actions that cause moodle to sync with the external DB.  Its a pull from moodle, not a push from the Master DB.  Either action calls the same sync.php file.

Action 1 - cron.php execution
Action 2 - account logs into moodle.