I'm putting this together to help the next version of civicrm. We needed a waiting list for events once they are full. Our needs will perhaps differ from a generic version but you have to start somewhere.

We started by detecting the state of the event with eventFull(). If it's full and the user is not already a participant OR in the waitinglist table, we give them a simple link which will add them to the waitinglist table.

We use a first in first out system so when a participant is deleted, removed (status) or otherwise changed, we trigger a checking script that checks the maxParticipants and total to see if there is now a space. If there is we take the top one from the waiting list and add it to the participant list as a specific status (we used 'added by waiting list'). Our system isn't quite finished yet but we're thinking of auto removing people from the civicrm_participant table when they've not confirmed for a while.

At the point mentioned above, we email the user with a link to the page to confirm or cancel their registration to the event.

Speaking of the confirmation of their place, we added a multipage form (wizard) that had:

page 1: 2 buttons - cancel or confirm place.
page 2: The register form slightly tweaked.
page 3: The confirm form, postProcess() altered to set the new status to registered.
page 4: Pretty much unaltered thankyou page.

StateMachine has this:
$this->_pages = array( 'CRM_Event_Form_WaitingList_Prompt' => null,
'CRM_Event_Form_Registration_Register' => null,
'CRM_Event_Form_Registration_Confirm' => null,
'CRM_Event_Form_Registration_ThankYou' => null,
);

At the moment, the waitinglist table includes only a few fields:

• id - this is the id that will be used in the link in the email
• contact_id - self explanatory
• event_id - self explanatory
• signup_date - so we can pick from the top of the list.

So for now things are quite simple. I almost forgot to mention, I make sure that the multiple registrations option is forced off for the event registration when we get to the confirmation pages.

I can post code if needed but right now the concept is a start.