Some attribute values ought to remain unique. If you are using uid values as RDNs to distinguish between millions of user entries stored under ou=People, then you do not want your directory too contain two or more identical uid values. If your credit card or mobile number is stored as an attribute value on your directory entry, you certainly do not want to share that credit card or mobile number with another customer. The same is true for your email address. The difficulty for you as directory administrator lies in implementing attribute value uniqueness without sacrificing the high availability that comes from using OpenDJ's loosely consistent, multi-master data replication. Indeed OpenDJ's replication model lets you maintain write access during network partitions for directory applications. Yet, write access during a network partition can result in the same, theoretically unique attribute value getting assigned to two different entries at once. You do not notice the problem until the network partition goes away and replication resumes. This chapter shows you how to set up attribute value uniqueness in your directory environment.