add

doc/design/db_rules

+
+Here is a more formal statement of the important database design
+rules.  Each rule is given a 5 character mnemonic for referral
+convenience.
+
+
+Reference Safety						[DBREF]
+
+	1)	Any node reference retrieved from the database shall remain
+		valid until discarded by the caller.
+
+	2)	Any rdataset retrieved from the database shall remain valid,
+		with its rdata contents unaltered, until it has been discarded
+		by the caller.
+
+
+Basic Versioning						[DBVER]
+
+	The rdata contents of a committed version do not change.
+
+
+Database Future Version						[DBFUV]
+
+	1)	Zone databases may have at most one open future version.
+
+	2)	When committed, the future version becomes the current version.
+
+	3)	Until it is committed, the future version changes may be
+		rolled back.
+
+
+Node Lookups							[DBNLU]
+
+	XXX
+
+
+Zone Node Lookups						[DBZNL]
+
+	A node zone lookup returns the requested node, if it exists, in the
+	version specified, or DNS_R_NONEXISTENT otherwise.
+
+
+Zone Rdataset Lookups						[DBZRL]
+
+	A zone lookup returns the requested data, if any, in the
+	version specified, or DNS_R_NONEXISTENT otherwise.
+
+
+Cache Node Lookups						[DBCNL]
+
+	1)	A node lookup shall return a handle to the desired node,
+		if it exists.
+
+	2)	If there is a negative caching entry for the desired node,
+		DNS_R_NONEXISTENT shall be returned.
+	
+	3)	If the desired node does not exist, and there is not a
+		negative caching entry, DNS_R_UNKNOWN shall be returned.
+
+
+Cache Rdataset Lookups						[DBCRL]
+
+	1)	A cache lookup returns the most recently written data
+		(if any).
+
+	2)	If there is a negative caching entry for the desired data,
+		DNS_R_NONEXISTENT shall be returned.
+
+	3)	If the requested data does not exist, and there is not a
+		negative caching entry, DNS_R_UNKNOWN shall be returned.
+
+
+Cache Updates							[DBCUP]
+
+	Concurrent attempts to update the same rdataset type of a given
+	node must appear to execute in some order.
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