The OpenLDAP attribute olcBackend is used to specify the backend type for a database entry in the cn=config DIT. The backend type determines how the data is stored and accessed by the OpenLDAP server. There are several backend types available, but not all of them can be used with the olcBackend attribute. The backend types that can be used with the olcBackend attribute are:

bdb: The Berkeley Database backend, which uses the Berkeley DB library to store the data in files on disk. This backend supports transactions, indexing, replication, and caching. It is the default backend for OpenLDAP and is recommended for most applications.

ldap: The LDAP backend, which uses another LDAP server as the data source. This backend allows the OpenLDAP server to act as a proxy or a gateway to another LDAP server. It can be used to merge data from multiple LDAP servers, to provide access control or schema mapping, or to implement load balancing or failover.

text: The text backend, which uses plain text files to store the data. This backend is mainly used for testing purposes or for simple applications that do not require advanced features. It does not support indexing, replication, or caching.

The backend types that cannot be used with the olcBackend attribute are:

xml: The XML backend, which uses XML files to store the data. This backend is experimental and not recommended for production use. It does not support indexing, replication, or caching.

passwd: The passwd backend, which uses the /etc/passwd and /etc/group files as the data source. This backend is deprecated and should not be used. It does not support indexing, replication, or caching.

References:

How To Configure OpenLDAP and Perform Administrative LDAP Tasks

OpenLDAP Online Configuration Reference - Tyler’s Guides