how do I remove an extra node

Question

I have a group of erlang nodes that are replicating their data through Mnesia\'s \"extra_db_nodes\"... I need to upgrade hardware and software so I have to detach some nodes as

Answer 1

I'm extremely late to the party, but came across this info in the doc when looking for a solution to the same problem:

"The function call mnesia:del_table_copy(schema, mynode@host) deletes the node 'mynode@host' from the Mnesia system. The call fails if mnesia is running on 'mynode@host'. The other mnesia nodes will never try to connect to that node again. Note, if there is a disc resident schema on the node 'mynode@host', the entire mnesia directory should be deleted. This can be done with mnesia:delete_schema/1. If mnesia is started again on the the node 'mynode@host' and the directory has not been cleared, mnesia's behaviour is undefined." (http://www.erlang.org/doc/apps/mnesia/Mnesia_chap5.html#id74278)

I think the following might do what you desire:

AllTables = mnesia:system_info(tables),
DataTables = lists:filter(fun(Table) -> Table =/= schema end,
                          AllTables),

RemoveTableCopy = fun(Table,Node) ->
  Nodes = mnesia:table_info(Table,ram_copies) ++
          mnesia:table_info(Table,disc_copies) ++
          mnesia:table_info(Table,disc_only_copies),
  case lists:member(Node,Nodes) of
    true -> mnesia:del_table_copy(Table,Node);
    false -> ok
  end
end,

[RemoveTableCopy(Tbl,'gone@gone_host') || Tbl <- DataTables].

rpc:call('gone@gone_host',mnesia,stop,[]),
rpc:call('gone@gone_host',mnesia,delete_schema,[SchemaDir]),
RemoveTablecopy(schema,'gone@gone_host').

Though, I haven't tested it since my scenario is slightly different.

Answer 2

I've certainly used this method to perform this (supporting the mnesia:del_table_copy/2 use). See removeNode/1 below:

-module(tool_bootstrap).

-export([bootstrapNewNode/1, closedownNode/0,
     finalBootstrap/0, removeNode/1]).

-include_lib("records.hrl").

-include_lib("stdlib/include/qlc.hrl").

bootstrapNewNode(Node) ->
    %% Make the given node part of the family and start the cloud on it
    mnesia:change_config(extra_db_nodes, [Node]),
    %% Now make the other node set things up
    rpc:call(Node, tool_bootstrap, finalBootstrap, []).

removeNode(Node) ->
    rpc:call(Node, tool_bootstrap, closedownNode, []),
    mnesia:del_table_copy(schema, Node).

finalBootstrap() ->
    %% Code removed to actually copy over my tables etc...
    application:start(cloud).

closedownNode() ->
    application:stop(cloud), mnesia:stop().

Answer 3

I wish I had found this a long time ago: http://weblambdazero.blogspot.com/2008/08/erlang-tips-and-tricks-mnesia.html

basically, with a properly functioning cluster....

• login to the cluster to be removed

• stop mnesia

  mnesia:stop().
  

• login to a different node on the cluster

• delete the schema

  mnesia:del_table_copy(schema, node@host.domain).
  

Answer 4

If you have replicated the table (added table copies) on nodes other than the one you're removing, then you're already fine - just remove the node.

If you wanted to be slightly tidier you'd delete the table copies from the node you're about to remove first via mnesia:del_table_copy/2.

Generally, mnesia gracefully handles node loss and detects node rejoin (rebooted nodes obtain new table copies from nodes that kept running, nodes that didn't reboot are detected as a network partition event). Mnesia does not consume CPU or network traffic for nodes that have gone down. I think, though I haven't confirmed it in the source, mnesia won't reconnect to nodes that have gone down automatically - the node that goes down is expected to reboot (mnesia) and reconnect.

mnesia:add_table_copy/3, mnesia:move_table_copy/3 and mnesia:del_table_copy/2 are the functions you should look at for live schema management.

The extra_db_nodes parameter should only be used when initialising a new DB node - once a new node has a copy of the schema it doesn't need the extra_db_nodes parameter.