Release notes: Cluster-internal connectivity checks (#256)

* add release notes

* introduced in 3.11.5

* apply review suggestions

Author: nerpaula

site/content/3.11/release-notes/version-3.11/whats-new-in-3-11.md

@@ -1011,6 +1011,30 @@ attempt to create an additional database fails with error
 if other databases are dropped first. The default value for this option is
 unlimited, so an arbitrary amount of databases can be created.
 
+### Cluster-internal connectivity checks
+
+<small>Introduced in: v3.11.5</small>
+
+This feature makes Coordinators and DB-Servers in a cluster periodically send
+check requests to each other, in order to see if all nodes can connect to
+each other.
+If a cluster-internal connection to another Coordinator or DB-Server cannot
+be established within 10 seconds, a warning is now logged.
+
+The new `--cluster.connectivity-check-interval` startup option can be used
+to control the frequency of the connectivity check, in seconds.
+If set to a value greater than zero, the initial connectivity check is
+performed approximately 15 seconds after the instance start, and subsequent
+connectivity checks are executed with the specified frequency.
+If set to `0`, connectivity checks are disabled.
+
+You can also use the following metrics to monitor and detect temporary or
+permanent connectivity issues:
+- `arangodb_network_connectivity_failures_coordinators`: Number of failed
+  connectivity check requests sent by this instance to Coordinators.
+- `arangodb_network_connectivity_failures_dbservers_total`: Number of failed
+  connectivity check requests sent to DB-Servers.
+
 ## Miscellaneous changes
 
 ### Write-write conflict improvements

site/content/3.12/release-notes/version-3.11/whats-new-in-3-11.md

@@ -1011,6 +1011,30 @@ attempt to create an additional database fails with error
 if other databases are dropped first. The default value for this option is
 unlimited, so an arbitrary amount of databases can be created.
 
+### Cluster-internal connectivity checks
+
+<small>Introduced in: v3.11.5</small>
+
+This feature makes Coordinators and DB-Servers in a cluster periodically send
+check requests to each other, in order to see if all nodes can connect to
+each other.
+If a cluster-internal connection to another Coordinator or DB-Server cannot
+be established within 10 seconds, a warning is now logged.
+
+The new `--cluster.connectivity-check-interval` startup option can be used
+to control the frequency of the connectivity check, in seconds.
+If set to a value greater than zero, the initial connectivity check is
+performed approximately 15 seconds after the instance start, and subsequent
+connectivity checks are executed with the specified frequency.
+If set to `0`, connectivity checks are disabled.
+
+You can also use the following metrics to monitor and detect temporary or
+permanent connectivity issues:
+- `arangodb_network_connectivity_failures_coordinators`: Number of failed
+  connectivity check requests sent by this instance to Coordinators.
+- `arangodb_network_connectivity_failures_dbservers_total`: Number of failed
+  connectivity check requests sent to DB-Servers.
+
 ## Miscellaneous changes
 
 ### Write-write conflict improvements

site/content/3.12/release-notes/version-3.12/whats-new-in-3-12.md

@@ -125,6 +125,30 @@ attempt to create an additional database fails with error
 if other databases are dropped first. The default value for this option is
 unlimited, so an arbitrary amount of databases can be created.
 
+### Cluster-internal connectivity checks
+
+<small>Introduced in: v3.11.5, v.3.12.0</small>
+
+This feature makes Coordinators and DB-Servers in a cluster periodically send
+check requests to each other, in order to see if all nodes can connect to
+each other.
+If a cluster-internal connection to another Coordinator or DB-Server cannot
+be established within 10 seconds, a warning is now logged.
+
+The new `--cluster.connectivity-check-interval` startup option can be used
+to control the frequency of the connectivity check, in seconds.
+If set to a value greater than zero, the initial connectivity check is
+performed approximately 15 seconds after the instance start, and subsequent
+connectivity checks are executed with the specified frequency.
+If set to `0`, connectivity checks are disabled.
+
+You can also use the following metrics to monitor and detect temporary or
+permanent connectivity issues:
+- `arangodb_network_connectivity_failures_coordinators`: Number of failed
+  connectivity check requests sent by this instance to Coordinators.
+- `arangodb_network_connectivity_failures_dbservers_total`: Number of failed
+  connectivity check requests sent to DB-Servers.
+
 ## Miscellaneous changes
 
 ### In-memory edge cache startup options and metrics