ejabberD cluster

WARNING: Erlang considers nodes as completely-friends or completely-non-friends. When you give access to a node you give FULL access to it. This access includes executing any command, and even closing the node completely. If you are connecting to a production node don’t use any command if you are not completely sure about the effects.

  1. Cookie

    1. Set the same cookie on all the nodes. There are different ways to set the cookie in a node:

      • When starting Erlang, use this argument in the command line:

        erl -setcookie SFEWRG34AFDSGAFG35235 -name nodex
      • Store the cookie in the file $HOME/.erlang.cookie:

      • Or, if you started Erlang with option -sname or -name, you can change the cookie in the erl console:

        erlang:set_cookie(node(), 'SFEWRG34AFDSGAFG35235').
    2. Check that all the Erlang nodes use the same cookie. In the Erlang console:

  2. Nodes names

    1. Set a different node name for every node when you start them with command line argument:

      erl -name node1
    2. Once a node is started, you can check the node name in the erlang console:

  3. Hosts file

    1. Create a file named $HOME/.hosts.erlang (alternatively, it can be placed in code:root_dir().). In machine1 it contains:

    2. Check if it is correctly loaded:

  4. Interconnect the Erlang nodes

    1. Start all the nodes and run this in the first one:

    2. Check what nodes is this node connected to:

    3. You can manually try to connect to other nodes. If the connection is successfull, answer will be ‘pong’. If unsuccessful, ‘pang’:

    4. Make a call to all nodes (including the local one) to retrieve their localtime:

      rpc:multicall([node()| nodes()], erlang, localtime, []).
  5. Start an erlang shell on a remote node

    1. In the erlang shell in node1, press Control+G to enter the jobs control mode.

      User switch command
    2. You can ask for help:

      User switch command
       --> h
        c [nn]   - connect to job
        i [nn]   - interrupt job
        k [nn]   - kill job
        j        - list all jobs
        s        - start local shell
        r [node] - start remote shell
        q        - quit erlang
        ? | h    - this message
    3. Show the current jobs:

       --> j
         1* {shell,start,[]}
    4. Create a new console in node2 and see how the new job is added:

       --> r 'node2@machine2.example.com'
       --> j
         1 {shell,start,[]}
         2* {node2@machine2.example.com,shell,start,[]}
    5. Connect to the new job:

       --> c 2
      Eshell V5.4 (abort with ^G)
  6. Play with your remote Erlang shell

    • Remark: In the remote Erlang shell you can do anything you can already do in the local Erlang shell. So take care with that you do!

    • Get memory information:

    • Get the number of Erlang processes running:

    • Get list of running processes in the Erlang node:

      Pid                   Initial Call                          Heap     Reds Msgs
      Registered            Current Function                     Stack
      <0.0.0>               otp_ring0:start/2                      377     3633    0
      init                  init:loop/1                              2
      <0.2.0>               erlang:apply/2                        2584    74621    0
      erl_prim_loader       erl_prim_loader:loop/3                   5
      <0.4.0>               gen_event:init_it/6                    377      277    0
    • Number of vCards published:

      mnesia:table_info(vcard, size).
    • Number of roster items in database:

      mnesia:table_info(roster, size).
    • Total MUC rooms:

      ets:info(muc_online_room, size).
    • Ask for help on shell commands:

      ** shell internal commands **
      b()        -- display all variable bindings
      e(N)       -- repeat the expression in query <N>
      f()        -- forget all variable bindings
      f(X)       -- forget the binding of variable X
      h()        -- history
  7. Close the remote Erlang shell safely

    1. Once you finish your work with the remote node Erlang shell, do not forget to close it. Press Control+G again:

      User switch command
    2. Display the list of current jobs. You want to know the number that identifies the remote shell:

       --> j
         1 {shell,start,[]}
         2* {node2@machine2.example.com,shell,start,[]}
    3. The remote shell is number 2, so now you can kill it:

       --> k 2
    4. Since no success message is shown, you will want to verify you actually closed the remote shell:

       --> j
         1 {shell,start,[]}

There is a quick way to start an Erlang node, connect to another node and start a shell there. Just use the ‘-remsh’ command line option when starting the node:

erl -sname node1 -remsh node2@machine2.example.com
Erlang (BEAM) emulator version 5.4.8 [source][/source] [hipe]

Eshell V5.4.8  (abort with ^G)

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