Add CordonedServersExistedDuringAllocation notification (#86) (#99)

https://trello.com/c/1ymz1Qiq/975-create-a-notification-when-doing-allocations-while-some-servers-are-cordoned

This PR proposes the introduction of the
`CordonedServersExistedDuringAllocation` notification. This will be
returned whenever a command that triggers a database allocation decision
is executed. This includes `CREATE DATABASE`, `ALTER DATABASE`,
`DEALLOCATE DATABASES FROM SERVER`, `ALTER SERVER`. But not `REALLOCATE
DATABASES` as this command has the requirement for no cordoned servers
and errors if there is such a server.

This is useful to remind the person managing the cluster that some
servers are cordoned and this impacts the allocation decision (databases
can't be allocated to cordoned servers). So this should help the user
not get confused by the allocation decisions made.

I have included this as a `WARNING` initially but maybe `INFORMATION`
would be better suited, not sure on the criteria to be made a `WARNING`.
Cordoned servers aren't necessarily a bad thing in themselves but their
existence could lead to "worse" allocation decision than if they were
uncordoned. So, the warning could be seen as a prompt the user to do
something about the cordoned server.

Co-authored-by: Jack Waudby <[email protected]>

Author: NataliaIvakina

modules/ROOT/pages/changelogs.adoc

@@ -1,6 +1,15 @@
 :description: This page lists all changes to status codes per Neo4j version.
 = Changes to status codes per Neo4j version
 
+== Neo4j 5.15
+
+**New:**
+
+[source, status codes, role="noheader"]
+-----
+Neo.ClientNotification.Cluster.CordonedServersExistedDuringAllocation
+-----
+
 == Neo4j 5.14
 
 **New:**

modules/ROOT/pages/notifications/all-notifications.adoc

@@ -1233,6 +1233,44 @@ Use `DATABASE *` without the parameter to revoke the privilege on all databases.
 
 Topology notifications provide additional information related to managing databases and servers.
 
+[#_neo_clientnotification_cluster_cordonedserversexistedduringallocation]
+=== CordonedServersExistedDuringAllocation
+
+.When is this notification returned?
+[TIP]
+====
+When a Cypher administration command triggers an allocation decision and some of the servers are cordoned.
+For example, `CREATE DATABASE`, `ALTER DATABASE`, `DEALLOCATE DATABASES FROM SERVER[S]`, and `ALTER DATABASE` return this notification. However, `REALLOCATE DATABASES` requires that there are no cordoned servers and, therefore, does not return it.
+====
+
+.Notification details
+[cols="<1s,<4"]
+|===
+|Code
+m|Neo.ClientNotification.Cluster.CordonedServersExistedDuringAllocation
+|Title
+a| Cordoned servers existed when making an allocation decision.
+|Severity
+m|INFORMATION
+|Category
+m|TOPOLOGY
+|===
+
+.Cordoned servers existed during an allocation decision
+====
+The example assumes that you have a cluster with three servers, of which server `123e4567-e89b-12d3-a456-426614174000` is cordoned using the `dbms.cluster.cordonServer` procedure. Then the below command will return this notification.
+
+Command::
++
+[source, cypher]
+----
+CREATE DATABASE foo TOPOLOGY 2 PRIMARIES
+----
+
+Description of the returned code::
+Server(s) `123e4567-e89b-12d3-a456-426614174000` are cordoned. This can impact allocation decisions.
+====
+
 [#_neo_clientnotification_cluster_nodatabasesreallocated]
 === NoDatabasesReallocated