# Confluence incremental synchronisation failed and falls back to a full sync when connecting to LDAP

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Platform Notice: Server and Data Center Only - This article only applies to Atlassian products on the server and data center platforms.

## Problem

Incremental synchronisation with LDAP directory fails while full synchronisation works.

The following ERROR appears in atlassian-confluence.log during an incremental sync.

ERROR [scheduler_Worker-8] [atlassian.crowd.directory.DbCachingRemoteDirectory] synchroniseCache Incremental synchronisation for directory [ 129400833 ] was unexpectedly interrupted, falling back to a full synchronisation
com.atlassian.crowd.directory.ldap.cache.UsnChangedCacheRefresherIncSyncException: Cache returned different number of guids and users (possible reason is overlapping guids in cache, most likely null/empty values).


## Diagnosis

Environment

• A non-cluster AD with more than one domain controller
• If the Confluence is connected to Oracle 12c Database, it might be affected by  CONFSERVER-55585 - Getting issue details... STATUS

Diagnostic Steps

• No result returned from the following database query:

SELECT id, user_name, directory_id, external_id FROM cwd_user
WHERE (external_id IN (SELECT external_id FROM cwd_user GROUP BY external_id HAVING count(external_id) > 1) OR external_id IS NULL)
AND directory_id = '<user_directory_id>'; 
• The number of users in the cwd_user table is equal to the number of users returned by the user filter in the LDAP connector, however, Confluence is calling for less numbers of users in a full synchronization:

2015-12-17 06:55:10,058 INFO [scheduler_Worker-5] [atlassian.crowd.directory.DbCachingRemoteChangeOperations] getUsersToAddAndUpdate scanning [ xxxx ] users to add or update

## Cause

Confluence is comparing the number of GUID which results in either a mismatch or un-readable GUID which causes the incremental sync to fail.

## Workaround

1. Stop Confluence

2. Add the following system property into Confluence:

-Dcrowd.use.legacy.ad.incremental.sync=true

Please refer to the documentation on Configuring system properties for details on adding a system property.

Always perform the necessary backup to your Confluence instance before performing any modification. If possible, try your modifications on a test server.

3. Start Confluence