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how do I remove an extra node

Answer 1

A:

I am new to mnesia, so no idea, but this might be of some help:

How to add a node to an mnesia cluster?

Toby Hede 2009-06-02 23:15:03

-1 for phishing for points. Adding was never the issue and the other post never made mention.

Richard 2009-06-03 10:08:49

I was just figuring that this had been open for a while, I was reading up on mnesia last night, noticed this post about thought that *removing* a node might be similar to *adding* a node. But you know, whatevs.

Toby Hede 2009-06-03 12:23:38

Answer 2

A:

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.

archaelus 2009-06-03 22:06:10

I'm on the fence with this answer. I like the general information, however, it's not current. The three methods you mention are not included in the R13B release. A search of the R13A code did not reveal any similar methods.

Richard 2009-06-03 22:52:52

Continuing my search of the source I found some indication that there is a call to mnesia_controller:add_list/2 that is used when adding the extra node. There is a comment that suggests calling mnesia_recover:disconnect_nodes/1, however, that method does not exist anywhere and might simply be a typo; mnesia_recover:disconnect/1 exists.

Richard 2009-06-03 22:56:33

I said delete_table_copy instead of del_table copy, but apart from that those methods are present, documented and current.You shouldn't have to disconnect nodes by hand - mnesia handles node disconnection by itself. Just turn off the unwanted nodes. Or use net_kernel:disconnect/1 to do it forcibly.

archaelus 2009-06-08 17:53:09

del_table_copy does not remove the node from the extra_db_nodes list.

Richard 2009-06-09 12:28:59

Sure, but why are they in the extra_db_nodes list in the first place? You only need extra_db_nodes while joining the cluster - after that it's more of a hinderance than anything else. You can change_config to alter extra_db_nodes at runtime, but why do that? You shouldn't be specifying extra_db_nodes in normal operation, so your problem is not how to delete thing from extra_db_nodes when removing nodes, but how to avoid using extra_db_nodes at any point after a new node joins a cluster.

archaelus 2009-06-13 20:47:31

Answer 3

+1 A:

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().

Alan Moore 2009-06-08 23:11:06

while this code may have appeared to work it does not cleanup all of the data. del_table_copy does not remove the node from the extra_db_node list. In fact there is no code in the source that completely removes the node.

Richard 2009-06-09 12:30:30

Yes you're right. I removed all of the code specific to my application for clarity...

Alan Moore 2009-06-09 14:14:03

The source code I was referring to is in the Mnesia library.

Richard 2009-06-09 15:02:09

Ah - yes. I should read the original question. My solution certainly had the effect I desired - that of no longer having my (deleted) node host a replicated copy of the data. But I never checked what extra_db_nodes was set to after the change...

Alan Moore 2009-06-09 15:49:49

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