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<chapter xml:id='chap-attribute-uniqueness'

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<title>Implementing Attribute Value Uniqueness</title>

<para>Some attribute values ought to remain unique. If you are using

<literal>uid</literal> values as RDNs to distinguish between millions of

user entries stored under <literal>ou=People</literal>, then you do not

want your directory to contain two or more identical

<literal>uid</literal> 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.</para>

<indexterm><primary>Unique attribute values</primary></indexterm>

<para>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 outages for directory

applications. Yet, write access during a network outage 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 outage

goes away and replication resumes.</para>

<para>This chapter shows you how to set up attribute value uniqueness

in your directory environment.</para>

<procedure xml:id="enable-unique-uids">

<title>To Enable Unique UIDs</title>

<para>OpenDJ provides a unique attribute plugin that you configure by using

the <command>dsconfig</command> command. By default, the plugin is prepared

to ensure attribute values are unique for <literal>uid</literal>

attributes.</para>

<step>

<para>Set the base DN where <literal>uid</literal> should have unique

values, and enable the plugin.</para>

<screen>$ dsconfig

set-plugin-prop

--port 4444

--hostname opendj.example.com

--bindDN "cn=Directory Manager"

--bindPassword password

--plugin-name "UID Unique Attribute"

--set base-dn:ou=people,dc=example,dc=com

--set enabled:true

--trustAll

--no-prompt</screen>

<para>Alternatively, you can specify multiple base DNs for unique values

across multiple suffixes.</para>

<screen>$ dsconfig

set-plugin-prop

--port 4444

--hostname opendj.example.com

--bindDn "cn=Directory Manager"

--bindPassword password

--plugin-name "UID Unique Attribute"

--set enabled:true

--add base-dn:ou=people,dc=example,dc=com

--add base-dn:ou=people,dc=example,dc=org

--trustAll

--no-prompt</screen>

</step>

<step>

<para>Check that the plugin is working correctly.</para>

<screen>$ cat bjensen.ldif

dn: uid=ajensen,ou=People,dc=example,dc=com

changetype: modify

add: uid

uid: bjensen

$ ldapmodify

--defaultAdd

--port 1389

--bindDN "cn=Directory Manager"

--bindPassword password

--filename bjensen.ldif

Processing MODIFY request for uid=ajensen,ou=People,dc=example,dc=com

MODIFY operation failed

Result Code: 19 (Constraint Violation)

Additional Information: A unique attribute conflict was detected for \

attribute uid: value bjensen already exists in entry

uid=bjensen,ou=People,dc=example,dc=com</screen>

<para>If you have set up multiple suffixes, you might try something like

this.</para>

<screen>$ cat bjensen.ldif

dn: uid=bjensen,ou=People,dc=example,dc=org

objectClass: top

objectClass: person

objectClass: organizationalPerson

objectClass: inetOrgPerson

cn: Babs

sn: Jensen

uid: bjensen

$ ldapmodify

--port 1389

--bindDN "cn=Directory Manager"

--bindPassword password

--defaultAdd

--filename bjensen.ldif

Processing ADD request for uid=bjensen,ou=People,dc=example,dc=org

ADD operation failed

Result Code: 19 (Constraint Violation)

Additional Information: A unique attribute conflict was detected for attribute

uid: value bjensen already exists in entry

uid=bjensen,ou=People,dc=example,dc=com</screen>

</step>

</procedure>

<procedure xml:id="enable-unique-attributes">

<title>To Enable Unique Values For Other Attributes</title>

<para>You can also configure the unique attribute plugin for use with

other attributes, such as <literal>mail</literal>, <literal>mobile</literal>,

or attributes you define, for example <literal>cardNumber</literal>.</para>

<step>

<para>Before you set up the plugin, index the attribute for equality.</para>

</step>

<step>

<para>Set up the plugin configuration for your attribute.</para>

<screen>$ dsconfig

create-plugin

--port 4444

--hostname opendj.example.com

--bindDN "cn=Directory Manager"

--bindPassword password

--plugin-name "Unique mobile numbers"

--type unique-attribute

--set enabled:true

--set base-dn:ou=people,dc=example,dc=com

--set type:mobile

--trustAll

--no-prompt</screen>

</step>

<step>

<para>Check that the plugin is working correctly.</para>

<screen>$ cat mobile.ldif

dn: uid=ajensen,ou=People,dc=example,dc=com

changetype: modify

add: mobile

mobile: +1 828 555 1212

dn: uid=bjensen,ou=People,dc=example,dc=com

changetype: modify

add: mobile

mobile: +1 828 555 1212

$ ldapmodify

--defaultAdd

--port 1389

--bindDN "cn=Directory Manager"

--bindPassword password

--filename mobile.ldif

Processing MODIFY request for uid=ajensen,ou=People,dc=example,dc=com

MODIFY operation successful for DN uid=ajensen,ou=People,dc=example,dc=com

Processing MODIFY request for uid=bjensen,ou=People,dc=example,dc=com

MODIFY operation failed

Result Code: 19 (Constraint Violation)

Additional Information: A unique attribute conflict was detected for

attribute mobile: value +1 828 555 1212 already exists in entry

uid=ajensen,ou=People,dc=example,dc=com</screen>

</step>

</procedure>

<procedure xml:id="unique-attributes-repl">

<title>To Ensure Unique Attribute Values With Replication</title>

<indexterm>

<primary>Replication</primary>

<secondary>Unique attributes</secondary>

</indexterm>

<para>The unique attribute plugin ensures unique attribute values on the

directory server where the attribute value is updated. If client applications

separately write the same attribute value at the same time on different

directory replicas, it is possible that both servers consider the duplicate

value unique, especially if the network is down between the replicas.</para>

<step>

<para>Enable the plugin identically on all replicas.</para>

</step>

<step>

<para>To avoid duplicate values where possible, try one of the following

solutions.</para>

<stepalternatives>

<step>

<para>Use a load balancer or proxy technology to direct all updates

to the unique attribute to the same directory server.</para>

<para>The drawback here is the need for an additional component to

direct the updates to the same server, and to manage failover should that

server go down.</para>

</step>

<step>

<para>Configure safe read mode assured replication between replicas

storing the unique attribute.</para>

<para>The drawbacks here are the cost of safe read assured replication,

and the likelihood that assured replication can enter degraded mode during

a network outage, thus continuing to allow updates during the

outage.</para>

</step>

</stepalternatives>

</step>

</procedure>

</chapter>