11.1. Setup

Everything you need to know to prepare the cluster for the installation of CouchDB.

11.1.1. Firewall

If you do not have a firewall between your servers, then you can skip this.

CouchDB in cluster mode uses the port 5984 just as standalone, but is also uses 5986 for the admin interface.

Erlang uses TCP port 4369 (EPMD) to find other nodes, so all servers must be able to speak to each other on this port. In an Erlang Cluster, all nodes are connected to all other nodes. A mesh.

Warnung

If you expose the port 4369 to the Internet or any other untrusted network, then the only thing protecting you is the cookie.

Every Erlang application then uses other ports for talking to each other. Yes, this means random ports. This will obviously not work with a firewall, but it is possible to force an Erlang application to use a specific port rage.

This documentation will use the range TCP 9100-9200. Open up those ports in your firewalls and it is time to test it.

You need 2 servers with working hostnames. Let us call them server1 and server2.

On server1:

erl -sname bus -setcookie 'brumbrum' -kernel inet_dist_listen_min 9100 -kernel inet_dist_listen_max 9200

Then on server2:

erl -sname car -setcookie 'brumbrum' -kernel inet_dist_listen_min 9100 -kernel inet_dist_listen_max 9200

An explanation to the commands:

• erl the Erlang shell.
• -sname bus the name of the Erlang node.
• -setcookie 'brumbrum' the “password” used when nodes connect to each other.
• -kernel inet_dist_listen_min 9100 the lowest port in the rage.
• -kernel inet_dist_listen_max 9200 the highest port in the rage.

This gives us 2 Erlang shells. shell1 on server1, shell2 on server2. Time to connect them. The . is to Erlang what ; is to C.

In shell1:

net_kernel:connect_node(car@server2).

This will connect to the node called car on the server called server2.

If that returns true, then you have a Erlang cluster, and the firewalls are open. If you get false or nothing at all, then you have a problem with the firewall.

First time in Erlang? Time to play!

Run in both shells:

register(shell, self()).

shell1:

{shell, car@server2} ! {hello, from, self()}.

shell2:

flush().
{shell, bus@server1} ! {"It speaks!", from, self()}.

shell1:

flush().

To close the shells, run in both:

q().

Make CouchDB use the open ports.

Open sys.config, on all nodes, and add inet_dist_listen_min, 9100 and inet_dist_listen_max, 9200 like below:

[
    {lager, [
        {error_logger_hwm, 1000},
        {error_logger_redirect, true},
        {handlers, [
            {lager_console_backend, [debug, {
                lager_default_formatter,
                [
                    date, " ", time,
                    " [", severity, "] ",
                    node, " ", pid, " ",
                    message,
                    "\n"
                ]
            }]}
        ]},
        {inet_dist_listen_min, 9100},
        {inet_dist_listen_max, 9200}
    ]}
].

11.1.2. Configuration files

Erlang Cookie

Open up vm.args and set the -setcookie to something secret. This must be identical on all nodes.

Set -name to the name the node will have. All nodes must have a unique name.

Admin

All nodes authenticates users locally, so you must add an admin user to local.ini on all nodes. Otherwise you will not be able to login on the cluster.