git-annex/doc/design/assistant/blog/day_46__notification_pools.mdwn

Focus today was writing a notification broadcaster library. This is a way to
send a notification to a set of clients, any of which can be blocked
waiting for a new notification to arrive. A complication is that any number
of clients may be be dead, and we don't want stale notifications for those
clients to pile up and leak memory.

It took me 3 tries to find the solution, which turns out to be head-smackingly
simple: An array of SampleVars, one per client.

Using SampleVars means that clients only see the most recent notification,
but when the notification is just "the assistant's state changed somehow;
display a refreshed rendering of it", that's sufficient.

----

First use of that was to make the thread that woke up every 10 minutes
and checkpointed the daemon status to disk also wait for a notification
that it changed. So that'll be more current, and use less IO.

----

Second use, of course, was to make the WebApp block long polling clients
until there is really a change since the last time the client polled.

To do that, I made one change to my Yesod routes:

[[!format diff """
	-/status StatusR GET
	+/status/#NotificationId StatusR GET
"""]]

Now I find another reason to love Yesod, because after doing that,
I hit "make".. and fixed the type error. And hit make.. and fixed
the type error. And then it just freaking worked! Html was generated with
all urls to /status including a `NotificationId`, and the handler for
that route got it and was able to use it:

[[!format haskell """
	{- Block until there is an updated status to display. -}
	b <- liftIO $ getNotificationBroadcaster webapp
	liftIO $ waitNotification $ notificationHandleFromId b nid
"""]]

And now the WebApp is able to display transfers in realtime!
When I have both the WebApp and `git annex get` running on the same screen,
the WebApp displays files that git-annex is transferring about as fast
as the terminal updates.

The [[progressbars]] still need to be sorted out, but otherwise
the WebApp is a nice live view of file transfers.

---

I also had some fun with Software Transactional Memory. Now when the
assistant moves a transfer from its queue of transfers to do, to its map of
transfers that are currently running, it does so in an atomic transaction.
This will avoid the transfer seeming to go missing (or be listed twice) if
the webapp refreshes at just the wrong point in time. I'm really starting
to get into STM.

----

Next up, I will be making the WebApp maintain a list of notices, displayed
on its sidebar, scrolling new notices into view, and removing ones the user
closes, and ones that expire. This will be used for displaying errors, as
well as other communication with the user (such as displaying a notice
while a git sync is in progress with a remote, etc). Seems worth doing now,
so the basic UI of the WebApp is complete with no placeholders.