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supervisorring

Supervisorring exposes the same module and behaviour as an erlang OTP supervisor but distributed into several nodes monitored using (https://github.com/awetzel/nano_ring ):

  • each child is started on different up nodes according to the mapping of the child id on a consistent hashing DHT
  • when the up node cluster change (NanoRing event), on each node, according to the new DHT : moved in processes are started, moved out processes are killed, and a state migration happened before the kill if possible
  • when child die repeatedly, a local supervisor is restarted to try a local state restoration, if this local supervisor die repeatedly, then all the supervisors on all nodes are killed , then let the parent supervisor of the supervisorring supervisor handle the global state restoration (like with a classical otp supervisor)

Usage

The supervisorring module and behaviour can be use nearly the same way as the OTP/erlang :supervisor module and behaviour with the following differences :

  • terminate_child only stops child locally, if the child is moved on another node then a terminated child will be restarted.
  • children cannot be transient or temporary, they have to be permanent for the reason above.
  • children are not necessary started in the given order.
  • the supervisor has to have locally registered name, so the start_link/2 does not exist and start_link/3 needs {:local,NAME} as first parameter.
  • in addition to the supervisor init/1 function, :supervisorring behaviour needs you to implement a migrate/3 function which will be called if possible before to kill a child on a node where it doesnot belong anymore.
  • in order to allow the use of dynamic child management (start_child /delete_child), you need to maintain a global list of children. To allow that, a new child_spec can be used in init/1 child list : {:dyn_child_handler,YourModule}. YourModule has to implement the behaviour :dyn_child_handler :
    • add(::childspec)->:ok,del(::childid)->:ok callbacks will be used respectively after a successful start_child and delete_child
    • get_all()->[::childspec] callback will be called during ring migration in order to get the current global list of children to determine which one has to be started locally.
    • match(::childid)->boolean allows you to use multiple :dyn_child_handler in childspecs and during addition and deletion, the correct handler will be selected if the child id match according to this function.
  • in addition to supervisor function, supervisorring exposes two necessary new functions :
    • find(supname,childid) allows you to find the node where a child belong. Indeed, as a supervisor, a supervisorring does not give you a particular way to locate your process among its children, you have to use your own mechanism to locate your process : register, pid communication, etc... BUT if you just use locally registered pid, you need to know the node where the child run for instance if the registered name is also the child id in sup : :gen_server.call({ChildName,:supervisorring.find(ChildSup,ChildName)},:you_call)
    • the local ring may be unaware of the last ring update and :supervisorring.find can give you a down node if this one just crashed. To ensure that your code executes himself on the node where a given child is currently running, you can use exec(supname,childid,youfun)
  • finally, the local supervisor is registered with the name given to the supervisorring, so you can use for instance : :supervisorring.which_children(MySup) to get all the children of the distributed supervisor and :supervisor.which_children(MySup) to get the local children associate with this supervisor.

The :dyn_child_handler can maintain the child list globally with an external database shared by every cluster nodes (network fs,mnesia,riak,etc.).

Example :

defmodule MySup do
  @behaviour :supervisorring
  def migrate({_id,_type,_modules},old_server,new_server), do:
    :gen_server.cast(new_server,{:set_state,:gen_server.call(old_server,:get_state)})
  def init(_arg) do
    {:ok,{{:one_for_one,2,3},[
      {:dyn_child_handler,NetFSChildHandler},
      {MySup.C1,{:gen_server,:start_link,[{:local,MySup.C1},GenericServer,nil,[]]},:permanent,2,:worker,[GenericServer]},
      {MySup.C2,{:gen_server,:start_link,[{:local,MySup.C2},GenericServer,nil,[]]},:permanent,2,:worker,[GenericServer]}
    ]}}
  end
end
# if childs file is shared on every node with a shared fs :
defmodule NetFSChildHandler do
  @behaviour :dyn_child_handler
  def match(_), do: true
  def get_all, do:
    File.read!("childs")|>binary_to_term
  def add(childspec), do:
    File.write!("childs",File.read!("childs") |> binary_to_term |> List.insert_at(0,childspec) |> term_to_binary)
  def del(childid), do:
    File.write!("childs",File.read!("childs") |> binary_to_term |> List.keydelete(childid,0) |> term_to_binary)
end

:supervisorring.start_link({:local,MySup},MySup,nil)
c3 = {MySup.C3,{:gen_server,:start_link,[{:local,MySup.C3},GenericServer,nil,[]]},:permanent,2,:worker,[GenericServer]}
:supervisorring.start_child(MySup,c3)
:gen_server.call({MySup.C3,:supervisorring.find(MySup,MySup.C3)},:youcall)
# get all childs
:supervisorring.which_children(MySup)
# get local childs
:supervisor.which_children(MySup)

How does it work ?

Supervisorring.App.Sup-> .Events
                      -> .SuperSup -> NodesListener
your_app -> your_sup_tree -> your_supervisorring -> your_childs
actually starts the following supervision tree :
your_app -> your_sup_tree -> global_sup -> local_sup(children=your_childs in dht(localnode)))
                                        -> child_manager -> ring_event_handler

The application SuperSup gen_server listens nanoring events to update its consistant hashing dht, then notify all the local Supervisiorring Global Supervisor with a gen_event.

The ChildManager gen_server start or kill the local supervisor children according to the new DHT.

The local children are supervised by 2 parent supervisor, global_sup -> local_sup -> children so that :

  • if a child die it is restarted by local_sup as a classical supervised process
  • if the child die repeatedly (according to defined MaxR,MaxT), local_sup will die
  • if local_sup die, it is restarted by global_sup to "test" if local children state restoration is sufficient to ensure the viability of the children
  • if local_sup die, repeatedly, according to MaxR=2 MaxT=LocalMaxT*2, then global_sup will die
  • super_sup monitors global_sup so that when global_sup exits for anormal reason, it send exit(global_sup_ref,kill) to all nodes in order to kill the supervisor "globally" (as the supervisor maintains the list of children globally, the working child state may need a global children restart)

This way children supervised by supervisorring can be supervised globally in a similar fashion as local_supervision locally.

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