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My problem with having a tree is that it implies that at every level - including at the root - there's a single point of failure. I get the error kernel concept

My problem with having a tree is that it implies that at every level - including at the root - there's a single point of failure. I get the error kernel concept, I just don't necessarily trust the hardware.

mononcqc 16y ago
What you need to do then is look into a few different options. The first one is distributed Applications. This is part of the standard OTP stuff. What this does is declare a node as a master, and then failover nodes for your OTP application. When your master goes down, the other nodes take over it. Whenever the master is back, the failover nodes drop their applications and restarts it on the master. This loses all of the state if it hasn't been kept in a safe place. Another one is to look at gen_leader. gen_leader is not part of OTP, but has been written by Ulf Wiger and is available on github or through agner. A quick description of it would simply be to say that it's a boosted gen_server that will find a master of itself somewhere and automatically replicate its data everywhere else. If the master dies, a leader election occurs and a new one takes over. A third option is to have a masterless concept. This is what happens in Riak. Data has to be shared in clever ways and code organized to support such a design. Nobody's a master, everyone's a follower. You add redundancy here and there, push important state to a safe spot, etc. In the standard library, modules such as 'global' handles this kind of thing. Supervisors are not exactly made to cross node boundaries. They protect your local processes. If you need something more complex to keep your state alive, this usually happens at a higher level for OTP.
- regularfry 16y ago
  Yeah, it sounds like either distributed applications (more likely 'cos it's in OTP) or gen_leader (maybe interesting technically) would be good to look at. That being said, riak_core has been on my List Of Things To Have A Really Good Look At for a while now...

mononcqc 16y ago

What you need to do then is look into a few different options. The first one is distributed Applications. This is part of the standard OTP stuff. What this does is declare a node as a master, and then failover nodes for your OTP application. When your master goes down, the other nodes take over it. Whenever the master is back, the failover nodes drop their applications and restarts it on the master. This loses all of the state if it hasn't been kept in a safe place. Another one is to look at gen_leader. gen_leader is not part of OTP, but has been written by Ulf Wiger and is available on github or through agner. A quick description of it would simply be to say that it's a boosted gen_server that will find a master of itself somewhere and automatically replicate its data everywhere else. If the master dies, a leader election occurs and a new one takes over. A third option is to have a masterless concept. This is what happens in Riak. Data has to be shared in clever ways and code organized to support such a design. Nobody's a master, everyone's a follower. You add redundancy here and there, push important state to a safe spot, etc. In the standard library, modules such as 'global' handles this kind of thing. Supervisors are not exactly made to cross node boundaries. They protect your local processes. If you need something more complex to keep your state alive, this usually happens at a higher level for OTP.

regularfry 16y ago
Yeah, it sounds like either distributed applications (more likely 'cos it's in OTP) or gen_leader (maybe interesting technically) would be good to look at. That being said, riak_core has been on my List Of Things To Have A Really Good Look At for a while now...