I have a chat phoenix channel that when a socket connect or disconnect, I set this on my postgres database (update users set online = true/false where id = ?)
How can I monitor Channel server to set all users on database to offline when it is starting (due a crash, server offline, etc) before accepts socket connects again?
thanks!
Is there any reason why You don’t want to use Presence for this?
Otherwise a simple solution would be to store this inside ETS table, and when server goes down, table is reset too.
I use Presence.
But rest of my application need to know who is online in database queries level.
For example, when a user connect to chat, I sent a list of friends online,
with database:
=> select users that is friend and online --> result a list of 100 users
without
=> select users that is friend --> result a list of 1000 users
=> find who from this 1000 friends is online using Presence.list (another 10k random users)
=> Repeat this to every connected socket
Ok I see your use case better.
It seemed strange to use a persistant storage for volatil information.
Still, I would rather take a list of ids from Presence.list, and then only query users where id in […]
Maybe you can have a start script that does an update like this
UPDATE users SET online=false;
Yes, I know that is a volatil data on database…
But I have more or less 10k online users… so my selects will be:
select * from users where is_friend and id in (10k ids send through network to database many times per second).
My presence (a using my impl of Tracker) is sync online/offline, my question is when all server is down and come back, or presence crash … where is the better place do monitor and reset this database info?
basically, what I want is:
see if something like in this article can’t get you closer to a solution using Presence with no DB…
if you trap_exit in the channel you should be able to catch all the crashes/proper shutdowns - however for the catastrophic failures (eg pulled power plug) you will be needing a solution on startup (in the application callback module - but what about distributed deployment?) - but try and see if the article doesn’t work for you - I smell that what you are trying to do with the DB might be a bit of an anti-pattern perhaps, but let’s see…
If you mean running DB operations on app startup, this post has a nice summary of two methods you may use. Using the Task module seems to be the easiest.
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