Distributed LDAP Features
 
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Many LDAP directories are really interfaces to existing X.500 directory servers. When the market for X.500 directories turned out to be disappointingly small, vendors who had built X.500 products quickly retrofitted them for LDAP use. One of the main goals of X.500 directory services was its distributed nature, whereby a hierarchy of servers would pass queries around transparently, just as they would replicate directory information across servers. As an access method, LDAP doesn't have the built-in richness of X.500 when it comes to distributed features. A directory based on X.500 like Critical Path Global Directory Server, Siemens DirX, and Computer Associates eTrust Directory will generally have good capabilities in this area.
Generic directory distributed features can generally be divided into three areas: referrals, chaining and replication. Referrals - which is the only distributed feature currently supported by LDAP -- and chaining are query-based behaviors. When a query comes into a directory server and cannot satisfy the request directly, the server may have information about a different server that does indeed have the answer. In that case, it can return to the client a referral, which says "go ask this other server for the answer." Referrals are defined in LDAP beginning with version 3, the version that most products support today.
Alternatively, the server that doesn't have the answer on the behalf of the client can go ask the server that does. This server-to-server chaining is an integral part of X.500, but is not defined in the core LDAP specifications---LDAP only deals with how a client talks to a server, not how servers talk to each other. Directory servers that include both LDAP and X.500 access methods generally support chaining, because chaining is an integral part of X.500.
Replication is a different distribution feature, in which directory information is reproduced and synchronized between two directory servers. Replication can be master/slave, under which a single directory is updated and propagates the changes to one or more slaves; or multi-master, under which any copy of the directory can be updated, automatically or periodically updating all the other copies. Because replication can be critical to performance and reliability of applications, most directories support one form of replication. However, if replication is going to be an important aspect of your directory project, you should determine very early what replication features are needed, as support for replication varies widely between products.
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