What does LDAP solve?

DITs are a hierarchical description scheme that lend themselves to B-Tree algos very nicely, resulting in tremendous search performance in most cases. Directory Server like OpenDS return indexed searches in micro-seconds, whereas RDBMS systems are much slower. Directory Servers (often called LDAP servers) trade resources (RAM, CPU) for fast read response. RDBMS systems provide greater functionality in terms of management of data in question. Need speed with few or zero updates, simplicity, and small network protocol? Use a Directory Server. Need data management and mining capabilities, and/or high rate-of-change of the database with relational aspects defined between data? Use an RDBMS (MySQL is your best bet here).

LDAP has O(1) read performance, in exchange for O(something worse) write performance. It's ideal for data that's accessed frequently, but changed rarely - directories of people, machine names and addresses, and so on. (hence the acronym: Lightweight Directory Access Protocol.)

LDAP is the right choice where the pain of using a database that isn't relational, in terms of decreased developer familiarity and strange performance characteristics, is less than the gain of blindingly fast read access.

What is LDAP? What are the scenarios where LDAP is the right choice?

At its core, LDAP is a protocol for accessing objects that are suitable for storage in a directory. Whether something is "suitable" is an entirely subjective determination that's left up to implementers, but typically this means collections of many objects that each have infrequently (or never) updated data, where each object has an obvious or canonical way to be looked up:

• a phone book (look up by name or by phone number)
• titles in a library (look up by title, author, etc.)
• tenants in a building (look up by floor, suite, name, etc.)

and so on.

Note that LDAP itself is just a protocol and doesn't provide any actual storage -- in much the same way, HTTP doesn't imply anything about whether you're using Apache, Jetty, Tomcat, Mongrel, et al. as a web server. (One problem with LDAP in general is the confusing reuse of names to mean different things. Wikipedia has a good section on this.)