Pick node with index of 0 as the Galera master

When a Galera cluster loses a member, it will recalculate the
wsrep_local_index values. As the index is zero-based, we can be certain
that in a valid cluster there will always be a node with an index of 0.

If the galeramon can't find a node with an index of 0, it means that
either the cluster hasn't stabilized and there's a pending recalculation
of the index or that there's no connectivity between MaxScale and the node
with the index value 0.

With this change and default settings, active-active MaxScale setups with
Galera clusters should always choose the same node as the master.

Documentation/Monitors/Galera-Monitor.md

@@ -62,6 +62,20 @@ Enable interaction with server priorities. This will allow the monitor to determ
 use_priority=true
 ```
 
+### `root_node_as_master`
+
+This option controls whether the write master Galera node requires a
+_wsrep_local_index_ value of 0. This option is enabled by default and was
+introduced in MaxScale 2.1.0.
+
+A Galera cluster will always have a node which has a _wsrep_local_index_ value
+of 0. Based on this information, multiple MaxScale instances can always pick the
+same node for writes.
+
+If the `root_node_as_master` option is disabled for galeramon, the node with the
+lowest index will always be chosen as the master. If it is enabled, only the
+node with a a _wsrep_local_index_ value of 0 can be chosed as the master.
+
 ## Interaction with Server Priorities
 
 If the `use_priority` option is set and a server is configured with the

Documentation/Release-Notes/MaxScale-2.1.0-Release-Notes.md

@@ -94,6 +94,15 @@ In 2.1.0, each listener has its own sub-directory in the service cache
 directory. The old caches in `/var/cache/maxscale` will need to be manually
 removed if they are no longer used by older versions of MaxScale.
 
+### Galeramon Monitoring Algorithm
+
+The galeramon monitor will only choose nodes with a _wsrep_local_index_
+value of 0 as the master. This allows multiple MaxScales to always choose
+the same node as the write master node for the cluster. The old behavior
+can be taken into use by disabling the new `root_node_as_master` option.
+
+For more details, read the [Galeramon documentation](../Monitors/Galera-Monitor.md).
+
 ## New Features
 
 ### Dynamic server configuration

server/modules/monitor/galeramon/galeramon.c

@@ -145,6 +145,7 @@ startMonitor(MONITOR *mon, const CONFIG_PARAMETER *params)
         handle->disableMasterRoleSetting = 0;
         handle->master = NULL;
         handle->script = NULL;
+        handle->root_node_as_master = true;
         handle->use_priority = false;
         memset(handle->events, false, sizeof(handle->events));
         spinlock_init(&handle->lock);
@@ -165,6 +166,10 @@ startMonitor(MONITOR *mon, const CONFIG_PARAMETER *params)
         {
             handle->disableMasterRoleSetting = config_truth_value(params->value);
         }
+        else if (!strcmp(params->name, "root_node_as_master"))
+        {
+            handle->root_node_as_master = config_truth_value(params->value);
+        }
         else if (!strcmp(params->name, "use_priority"))
         {
             handle->use_priority = config_truth_value(params->value);
@@ -547,8 +552,6 @@ monitorMain(void *arg)
         {
             const int repl_bits = (SERVER_SLAVE | SERVER_MASTER | SERVER_MASTER_STICKINESS);
             if (SERVER_IS_JOINED(ptr->server))
-            {
-                if (handle->master)
             {
                 if (ptr != handle->master)
                 {
@@ -570,7 +573,7 @@ monitorMain(void *arg)
                         server_clear_set_status(ptr->server, repl_bits, SERVER_MASTER);
                     }
                 }
-                }
+
                 is_cluster++;
             }
             else
@@ -674,6 +677,17 @@ static MONITOR_SERVERS *get_candidate_master(MONITOR* mon)
         moitor_servers = moitor_servers->next;
     }
 
+    if (!handle->use_priority && !handle->disableMasterFailback  &&
+        handle->root_node_as_master && min_id > 0)
+    {
+        /** The monitor couldn't find the node with wsrep_local_index of 0.
+         * This means that we can't connect to the root node of the cluster.
+         *
+         * If the node is down, the cluster would recalculate the index values
+         * and we would find it. In this case, we just can't connect to it.  */
+        candidate_master = NULL;
+    }
+
     return candidate_master;
 }
 

server/modules/monitor/galeramon/galeramon.h

@@ -58,6 +58,8 @@ typedef struct
     int disableMasterRoleSetting; /**< Monitor flag to disable setting master role */
     MONITOR_SERVERS *master; /**< Master server for MySQL Master/Slave replication */
     char* script;
+    bool root_node_as_master; /**< Whether we require that the Master should
+                                    * have a wsrep_local_index of 0 */
     bool use_priority; /*< Use server priorities */
     bool events[MAX_MONITOR_EVENT]; /*< enabled events */
 } GALERA_MONITOR;