Merge pull request #5149 from EnterpriseDB/BDR-4195-Docs-clarify-node-requirements-for-global-locks

Added clarifying notes to PGD locking criteria.

Author: djw-m

product_docs/docs/pgd/5/ddl.mdx

@@ -122,6 +122,8 @@ function.
 
 Two kinds of locks enforce correctness of replicated DDL with PGD.
 
+### The global DDL lock
+
 The first kind is known as a global DDL lock and is used only when `ddl_locking = on`.
 A global DDL lock prevents any other DDL from executing on the cluster while
 each DDL statement runs. This ensures full correctness in the general case but
@@ -133,7 +135,9 @@ another DDL command, even if it affects different tables.
 
 To acquire a lock on DDL operations, the PGD node executing DDL contacts the
 other nodes in a PGD group and asks them to grant it the exclusive right to
-execute DDL. The lock request is sent by the regular replication stream, and the
+execute DDL. 
+
+The lock request is sent by the regular replication stream, and the
 nodes respond by the replication stream as well. So it's important that nodes (or
 at least a majority of the nodes) run without much replication
 delay. Otherwise it might take a long time for the node to acquire the DDL
@@ -150,6 +154,11 @@ take a long time to acquire the lock. Hence it's preferable to run DDLs from a
 single node or the nodes that have nearly caught up with replication changes
 originating at other nodes.
 
+A global DDL Lock has to be granted by a majority of data and witness nodes where a majority is N/2+1
+of the eligible nodes. Subscriber-only nodes are not eligible to participate. 
+
+### The global DML lock
+
 The second kind is known as a global DML lock or relation DML lock. This kind of lock is used when
 either `ddl_locking = on` or `ddl_locking = dml`, and the DDL statement might cause
 in-flight DML statements to fail. These failures can occur when you add or modify a constraint,
@@ -177,6 +186,10 @@ can be reached and are keeping up reasonably well with the current write rate.
 If such DDL commands must be performed while a node is down, first remove the
 down node from the configuration.
 
+A global DML lock is granted when *ALL* the eligible data nodes agree to grant it.
+Witness and subscriber-only nodes are not eligible to participate.
+
+
 If a DDL statement isn't replicated, no global locks are acquired.
 
 Locking behavior is specified by the `bdr.ddl_locking` parameter, as