Thanks for your answer.
I understand that ISPConfig requires a SQL Database to store information like service configuration (Apache, Mysql, Postfix). I think LDAP could became only a "account backend".
Do you think that is possible to "modify" ISPConfig with a core plugin to store users, clients and ressellers informations in LDAP. For me, this information must be linked with database information like apache sites affect to this client. Primary Key used for identify user in sql database could be store in a field in LDAP object.
It could be possible to "choice", during ISPConfig install, the "account backend" : LDAP ou MySQL.
Possibility to duplicate users informations store in mysql to ldap could be a first step.
Number of services, like postfix and dovecot, could be configure to use LDAP as authentification backend. Using LDAP could be usefull to retrieve user informations to authenticate third party application like CMS (Joomla), ERP (OpenERP) , and other... For me, ISPConfig could became more expandable with an LDAP "accout backend".
With a replicate solution, if user update his data from third party application, ISPConfig override it after an update and data store by user will be lost.
In case of it will be possible to create a core plugin for that, could you give me documentation to devellop core plugin (I've just found interface plugin documentation).