4.2 Defining the Schema

The first step in implementing a directory is determining what information to store in the directory. The naming context of your server has already been defined as:

dc=plainjoe,dc=org

Store contact information for employees in the people organzational unit:

ou=people,dc=plainjoe,dc=org

There are several ways to identify the data that should be placed in an employee's entry. Information stored in an existing Human Resources database can provide a good starting point. Of course, you may not want to place all of this information in your directory. As a general rule, I prefer not to put information in a directory if that data probably won't be used. If it turns out that the data is actually necessary, you can always add it later. Eliminating unnecessary data at the start means that there's less to worry about when you start thinking about protecting the directory against unauthorized access.

An alternative to starting with an existing database is to determine which employee attributes you wish to make available and define a schema to match that list. The reverse also works: you can select a standard schema and use the attributes already defined. I prefer this approach because it makes it easy to change from one server vendor to another. Widely used, standard schemas are more likely to be supported by a wide range of vendors. Custom, in-house schemas may need to be redesigned to adapt to a new vendor (or even a new version from the same vendor).

For your directory, the inetOrgPerson schema defined in RFC 2798 is more than adequate. From Section 3.5.1 in Chapter 3, we know that this object class and associated attributes are defined in OpenLDAP's inetorgperson.schema file. As shown in Figure 4-1, an inetOrgPerson is a descendant of the organizationalPerson, which was itself derived from the person object class.

Figure 4-1. Hierarchy of the inetOrgPerson object class

The union of these object classes defines the set of required and optional attributes that are available. This means that the only required attributes in an inetOrgPerson object are the cn and sn attributes derived from the person object class.

From this point on, diagrams of an object class will not include RFC 2252-style schema definitions. If you wish to study the exact syntax of any object class, refer to the schema files included with OpenLDAP or the relevant RFC (or Internet-Draft).

Your directory will use the cn attribute as the RDN for each entry. Remember that the RDN of an entry must be unique among siblings of a common parent. In larger organizations, two people may have the same first and last name. In these cases, using a more specific value for the cn, such as including a middle name (or initial), can alleviate name collisions.

Another way to reduce the number of name collisions is to redesign the directory layout to reduce the total number of user entries sharing a common parent. In other words, group employees in some type of logical container, such as a departmental organizational unit. Figure 4-2 illustrates how this design avoids namespace conflicts. In this directory the "John Arbuckle" in sales is different from the "John Arbuckle" in engineering because the entries possess different parent nodes.

Figure 4-2. Using organizational unit to avoid collisions of common names (cn)

For our example, going with a single container of ou=people is fine; furthermore, our employee base is small enough to use an employee's common name (cn) without fear of conflict. Figure 4-3 shows the directory namespace developed so far.

Figure 4-3. Directory namespace for company address book

Here is an employee entry that contains the attributes needed for our directory. Notice that the two required attributes outlined in Figure 4-1, cn and sn, are present in addition to several optional attributes.

## LDIF entry for employee "Gerald W. Carter"
dn: cn=Gerald W. Carter,ou=people,dc=plainjoe,dc=org
objectClass: inetOrgPerson
cn: Gerald W. Carter
sn: Carter
mail: jerry@plainjoe.org
mail: gcarter@valinux.com
labeledURI: http://www.plainjoe.org/
roomNumber: 1234 Dudley Hall
departmentNumber: Engineering
telephoneNumber: 222-555-2345
pager: 222-555-6789
mobile: 222-555-1011

Deep or Wide? Is it better to maintain a shallow (and wide) tree or a deep (and narrow) directory? The best structure for your directory depends on two factors. First, how likely is it for a change to force an entry (in our case, a person) to be moved from one organizational unit to another? The answer to this question is based on a solid understanding of your organization and its needs. Deeper directory trees imply that an entry must meet more requirements in order to be placed in a certain container. For example, rather than placing all employees under the ou=people, using characteristics such as departments, job description, and geographic location makes for a more defined grouping. However, if these characteristics are likely to change frequently, you will only be creating more work for yourself in the long term. It is also good to note that deep directories require longer DNs to reference entries. This can become an annoyance over time. Second, does the implementation of your LDAP directory server favor one design over another? For OpenLDAP, this answer depends on your needs. The determining factor will be the number of updates, or writes, that will be made to the directory. To update an entry, the slapd server obtains a lock on the parent entry for the requesting client. Now suppose that you have a very shallow directory tree with 10,000 entries under a single parent. If many updates occur at the same time, the contention for the lock on the parent entry will be very high. The end result will be slower updates because processes will block waiting for the lock. A deeper tree means that you can often make searches more efficient by giving a more detailed search base. For more information on designing LDAP namespaces, you may wish to read Howes, et al., Understanding and Deploying LDAP Directory Services (MacMillan Technical Press).