Transfer Mnesia Backup To Smaller Cluster

Question

I have a backup from a 3 node Mnesia cluster (a production cluster) that I want to restore to a 2 node cluster (which I use for development purposes to reproduce bugs). If the cluster had the same number of nodes, one would follow the recipe outlined here. How does one remove a node from the backup? Is this possible?

I tried changing the clause ({schema, db_nodes, Nodes}, Acc) to return a smaller list of nodes, but that doesn't seem to work (or maybe it works, but is insufficient).

Answer 1

Turns out this can be done pretty easily:

#!/usr/bin/env escript

change_node_name(Mod, Source, Target) ->

    % Change this to whatever you want the new node list to be,
    % with however many nodes, whatever names, etc.
    ChangeNodes =
        fun(_Nodes) ->
            ['node1@jamie.kingsbery.net','node2@jamie.kingsbery.net']
        end,

    Convert =
        fun({schema, db_nodes, Nodes}, Acc) ->
                {[{schema, db_nodes, ChangeNodes(Nodes)}], Acc};
           ({schema, version, Version}, Acc) ->
                {[{schema, version, Version}], Acc};
           ({schema, cookie, Cookie}, Acc) ->
                {[{schema, cookie, Cookie}], Acc};
           ({schema, Tab, CreateList}, Acc) ->
                Keys = [ram_copies, disc_copies, disc_only_copies],
                OptSwitch =
                    fun({Key, Val}) ->
                            case lists:member(Key, Keys) of
                                true -> {Key, ChangeNodes(Val)};
                                false-> {Key, Val}
                            end
                    end,
                {[{schema, Tab, lists:map(OptSwitch, CreateList)}], Acc};
           (Other, Acc) ->
                {[Other], Acc}
        end,
    mnesia:traverse_backup(Source, Mod, Target, Mod, Convert, switched).

main(_Args)->
    change_node_name(mnesia_backup,"/tmp/source-backup","dest-backup").