# of connection strings

# to neighbor nodes

</programlisting>

<para>The <literal>multimaster.max_nodes</literal> variable defines the maximum cluster size. If you plan to add new nodes to your cluster, the <literal>multimaster.max_nodes</literal> value should exceed the initial number of nodes. In this case, you can add new nodes without restarting <productname>&productname;</productname>.

<para>The <literal>multimaster.max_nodes</literal> variable defines the maximum cluster size. If you plan to add new nodes to your cluster, the <literal>multimaster.max_nodes</literal> value should exceed the initial number of nodes. In this case, you can add new nodes without restarting <productname>&productname;</productname> until the specified number of nodes is reached.

</para>

<para>In most cases, three cluster nodes are enough to ensure high availability. Since the data on all cluster nodes is the same, you do not typically need more than five cluster nodes.

</para>

<para>If you would like to change the default connection settings for a cluster node, you can add other <link linkend="libpq-paramkeywords"> connection parameters</link> to the corresponding connection string in the <varname>multimaster.conn_strings</varname> variable.Additionally, youcan specifythe<literal>arbiter_process</literal> parameter to change theport used by the arbiter process to connect tothenodes. For details, see <xref linkend="multimaster-conn-strings"> and <xref linkend="multimaster-arbiter-port">.</para>

<para>If you would like to change the default connection settings for a cluster node, you can add other <link linkend="libpq-paramkeywords"> connection parameters</link> to the corresponding connection string in the <varname>multimaster.conn_strings</varname> variable.If youchangethedefault port on which thearbiter process listens for connections, you must specify this port inthe<literal>arbiter_port</literal> parameter. For details, see <xref linkend="multimaster-arbiter-port"> and <xref linkend="multimaster-conn-strings">.</para>

<important><para>The

<literal>multimaster.node_id</literal> variable takes natural

numbers starting from 1, without any gaps in numbering.

the <structname>mtm.get_cluster_state()</structname> view:

</para>

<programlisting>

SELECT * FROMmtm.get_cluster_state();

mtm.get_cluster_state();

</programlisting>

<para>

If <literal>liveNodes</literal> is equal to

To check node-specific information, use <literal>mtm.get_nodes_state()</literal>:

</para>

<programlisting>

SELECT * FROMmtm.get_nodes_state();

mtm.get_nodes_state();

</programlisting>

<para>To check the status of the whole cluster, use the

<literal>mtm.get_cluster_state()</literal> view:

</para>

<programlisting>

SELECT * FROMmtm.get_cluster_state();

mtm.get_cluster_state();

</programlisting>

<para>For details on all the returned information, see <xref linkend="multimaster-functions">.

</para>

</sect3>

<sect3 id="multimaster-adding-new-nodes-to-the-cluster">

<title>Adding New Nodes to the Cluster</title>

<para>With the <filename>multimaster</filename> extension, you can add or drop cluster nodes without

stopping the database service.

</para>

<para>

With multimaster, you can add or drop cluster nodes without a

restart. To add a new node, you need to change the cluster

To add a new node, you need to change the cluster

configuration on alive nodes, load all the data to the new node using

<application>pg_basebackup</application>, and start the node.

</para>

<literal>node4</literal>, follow these steps:

</para>

<orderedlist>

<listitem>

<para>Check whether the current number of cluster nodes has reached the value specified in the

<varname>multimaster.max_nodes</varname> variable. If this value is reached, increase

the <varname>multimaster.max_nodes</varname> value on each node and restart all nodes.

You can restart the nodes one by one, without stopping the database.

If the maximum number of nodes is not reached, proceed to the next step.

</para>

</listitem>

<listitem>

<para>

Figure out the required connection string to

access the new node. For example, for the database

<literal>mydb</literal>, user <literal>myuser</literal>, and

the new node <literal>node4</literal>, the connection string

is <literal>&quot;dbname=mydb user=myuser host=node4&quot;</literal>.

can be <literal>&quot;dbname=mydb user=myuser host=node4&quot;</literal>.

For details, see <xref linkend="multimaster-conn-strings">.

</para>

</listitem>

<listitem>

<para>

In <literal>psql</literal> connected to any alive node, run:

</para>

<programlisting>

SELECT * FROMmtm.add_node('dbname=mydb user=myuser host=node4');

mtm.add_node('dbname=mydb user=myuser host=node4');

</programlisting>

<para>

This command changes the cluster configuration on all nodes

In <literal>psql</literal> connected to any alive node, create a new replication slot for the disconnected node with the following command:

</para>

<programlisting>

SELECT * FROMmtm.recover_node(2);

mtm.recover_node(2);

</programlisting>

<para>where 2 is the ID of the disconnected node specified in the <varname>multimaster.node_id</varname> variable.</para>

</listitem>

Connection strings must appear in the order of the node IDs

specified in the <varname>multimaster.node_id</varname> variable.

Connection string for the i-th node must be on the i-th position.

You can also specify a

custom port for the arbiter to listen on using the <literal>arbiter_port</literal>

parameter. The provided value must be the same as the <varname>multimaster.arbiter_port</varname>

value specified for this node.

If you specify a custom port in the <varname>multimaster.arbiter_port</varname>,

you must provide this value in the <literal>arbiter_port</literal>

parameter in the connection string for the corresponding node.

</para></listitem></varlistentry>

<varlistentry><term><varname>multimaster.max_nodes</varname><indexterm><primary><varname>multimaster.max_nodes</varname></primary></indexterm></term><listitem><para>

The maximum number of nodes allowed in the cluster. If you plan to add new nodes to your cluster, the <literal>multimaster.max_nodes</literal> value should exceed the initial number of nodes. In this case, you can add new nodes without restarting <productname>&productname;</productname>. In most cases, three cluster nodes are enough to ensure high availability. Since the data on all cluster nodes is the same, you do not typically need more than five cluster nodes. The maximum possible number of nodes is limited to 64.</para>

The maximum number of nodes allowed in the cluster. If you plan to add new nodes to your cluster, the <literal>multimaster.max_nodes</literal> value should exceed the initial number of nodes. In this case, you can add new nodes without restarting <productname>&productname;</productname> until the specified number of nodes is reached. In most cases, three cluster nodes are enough to ensure high availability. Since the data on all cluster nodes is the same, you do not typically need more than five cluster nodes. The maximum possible number of nodes is limited to 64.</para>

<para>Default: the number of nodes specified in the <varname>multimaster.conn_strings</varname> variable

</para></listitem></varlistentry>

<varlistentry id="multimaster-arbiter-port">

<term><varname>multimaster.arbiter_port</varname><indexterm><primary><varname>multimaster.arbiter_port</varname></primary></indexterm></term><listitem><para>

Port for the arbiter process to listen on. If you change the default value, you must specify this value in the <literal>arbiter_port</literal>

parameter in the connection stringof the corresponding node.</para>

parameter in the connection stringfor the corresponding node.</para>

<para>Default: 5433

</para></listitem></varlistentry>

<varlistentry><term><varname>multimaster.heartbeat_send_timeout</varname><indexterm><primary><varname>multimaster.heartbeat_send_timeout</varname></primary></indexterm></term><listitem><para>

</para></listitem></varlistentry>

<varlistentry><term><varname>multimaster.cluster_name</varname><indexterm><primary><varname>multimaster.cluster_name</varname></primary></indexterm></term><listitem><para>

Name of the cluster. If

youset this variable, <filename>multimaster</filename> checks that

youdefine this variable when setting up the cluster, <filename>multimaster</filename> checks that

the cluster name is the same for all the cluster nodes.

</para></listitem></varlistentry>

<varlistentry>