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Relay backend sets up a relay virtual database that allows
to access other databases in the same instance of slapd
through different naming contexts and remapping attribute 
values.

The DN rewrite, filter rewrite and attributeType/objectClass
mapping is done by means of the rewrite-remap overlay.

The database containing the real naming context can be
explicitly selected by means of the "relay" directive,
which must contain the naming context of the target 
database.  This also causes the rewrite-remap overlay 
to be automatically instantiated.  If the optional keyword 
"massage" is present, the rewrite-remap overlay is 
automatically configured to map the virtual to the real 
naming context and vice-versa.

Otherwise, the rewrite-remap overlay must be explicitly
instantiated, by using the "overlay" directive, as 
illustrated below.  This allows much more freedom in target 
database selection and DN rewriting.

If the "relay" directive is not present, the backend is
not bound to a single target database; on the contrary,
the target database is selected on a per-operation basis.

This allows, for instance, to relay one database for 
authentication and anotheir for search/modify, or allows
to use one target for persons and another for groups
and so on.

To summarize: the "relay" directive:
- explicitly bounds the database to a single database 
  holding the real naming context;
- automatically instantiates the rewrite-remap overlay;
- automatically configures the naming context massaging
  if the optional "massage" keyword is added

If the "relay" directive is not used, the rewrite-remap
overlay must be explicitly instantiated and the massaging
must be configured, either by using the "suffixmassage"
directive, or by issuing more sophisticate rewrite 
instructions.

AttributeType/objectClass mapping must be explicitly
required.

Note that the rewrite-remap overlay is not complete nor 
production- ready yet.
Examples are given of all the suggested usages. 

# automatically massage from virtual to real naming context
database	relay
suffix		"dc=virtual,dc=naming,dc=context"
relay		"dc=real,dc=naming,dc=context" massage

# explicitly massage (same as above)
database	relay
suffix		"dc=virtual,dc=naming,dc=context"
relay		"dc=real,dc=naming,dc=context"
suffixmassage	"dc=virtual,dc=naming,dc=context" \
			"dc=real,dc=naming,dc=context"

# explicitly massage (same as above, but dynamic backend resolution)
database	relay
suffix		"dc=virtual,dc=naming,dc=context"
overlay		rewrite-remap
suffixmassage	"dc=virtual,dc=naming,dc=context" \
			"dc=real,dc=naming,dc=context"

# old fashioned suffixalias, applied also to DN-valued attributes
# from virtual to real naming context, but not the reverse...
database	relay
suffix		"dc=virtual,dc=naming,dc=context"
relay		"dc=real,dc=naming,dc=context"
rewriteContext	default
rewriteRule	"(.*)dc=virtual,dc=naming,dc=context$" \
			"$1dc=real,dc=naming,dc=context"
rewriteContext	searchFilter
rewriteContext	searchResult
rewriteContext	searchResultAttrDN
rewriteContext	matchedDN