[Fedora-directory-users] Advantages of using FDS vs OpenLDAP?

Rich Megginson rmeggins at redhat.com
Fri Jul 8 18:37:17 UTC 2005

Sam Tran wrote:

>On 7/8/05, Kevin Myer <kevin_myer at iu13.org> wrote:
>>Quoting Sam Tran <stlist at gmail.com>:
>>>I was wondering how FDS compares with OpenLDAP in terms of performance.
>>Well, its faster, in our case (and I say that tongue-in-cheek).  Our current
>>primary directory server is running on a dual 168Mhz Ultra 2 Sun box.  And our
>>secondary is a Sparcstation 10, at a whopping 70Mhz :)
>>>If you don't mind me asking how big is your current LDAP
>>>infrastructure in terms of entries and number of connections/sec.? How
>>>well your test FDS performs compare to your current LDAP server?
>>Its a small installation, maybe 10,000 entries, but that services our own
>>organization, as well as 22 school districts, each in separate trees.  But per
>>above, there's really no way I can provide an accurate comparison between the
>>current and anything else, other than to say its faster.
>>>I would appreciate your input.
>>Factors that go into my decision (and I use Fedora loosely, to describe my
>>experience and observations with Netscape/Sun/iPlanet products generally):
>>Cost (its a wash between OpenLDAP vs. Fedora DS)
>>Manageability (hands down Fedora, simply because I'm familar with the Netscape
>>management infrastructure)
>>Scalability (my guess is Fedora, based on the size of installations
>>that are out
>>Availability (Fedora - no proven multimaster support in OpenLDAP)
>>Ease of migration (Fedora - an upgrade to OpenLDAP meant refactoring
>>all ACL's,
>>massaging attributes and objectclasses, etc.)
>>Kevin M. Myer
>>Senior Systems Administrator
>>Lancaster-Lebanon Intermediate Unit 13  http://www.iu13.org
>>Fedora-directory-users mailing list
>>Fedora-directory-users at redhat.com
>Thanks Rich and Kevin for your input.
>As far as I am concerned the management console and the multimaster
>replication would be the two main advantages of FDS.
>However according to that paper multi-master replication could lead to
>What do you think?
I think Kurt Z. is correct.  Any loosely consistent replication model 
can lead to inconsistencies - including OpenLDAP single master 
replication.  I won't go into too many details, but if you really want 
to know about different replication consistency models, read this - 

In general, the only way to ensure absolute consistency is to use 
something like two phase commit, used by some RDBMS products.  In this 
case, your write operation does not return a response until that write 
operation has been successfully propagated to all systems in the 
replication topology (or rolled back from all in the case of failure).

There is no way for any LDAP loosely coupled replication to guarantee 
"read your writes" consistency.  As an example, consider a single master 
case with one or more read only replicas.  End user clients will 
typically be pointed to one or more read only replicas to use for 
searches for load balancing or fail over purposes.  If the client makes 
a write request, it will typically be sent a referral to the master, 
where the write operation will be performed.  The write operation will 
return immediately to the client, without waiting for that write 
operation to be propagated to the replicas.  If the client immediately 
performs a search request on a replica (which it has been configured to 
do so), that data may or may not be available, depending on the 
replication schedule, speed of the master, write performance of the 
replica, etc., etc.

Any kind of loosely coupled replication breaks ACID:
Consistency - the "view" of the data is different depending on which 
server you talk to
Isolation - the update is visible on the master before it is visible on 
a consumer
Durability - it's possible that the change could be lost or refused due 
to an error condition on a replica

However, Kurt's document states the following:

>It is noted that X.500 replication (shadowing) model allows for
>  transient inconsistencies to exist between the master and shadow
>  copies of directory information.  As applications which update
>  information operate upon the master copy, any inconsistencies in
>  shadow copies are not evident to these applications.
The problem is that no one deploying LDAP follows this model.  As I 
said, people use replicas for load balancing, failover, and data 
locality (e.g. putting a copy of the corporate data in a remote 
office).  Therefore, in almost every LDAP deployment, clients _use_ the 
"shadow copies" directly.  In almost every case, load balancing, 
failover, data locality, "no single point of failure" are the most 
salient concerns of network architects, and absolute data consistency is 

Kurt then goes on to give specific examples of where multi master can 
lead to inconsistencies.  In almost every case, the MMR protocol can 
handle the inconsistency in a logical manner, or flag the inconsistency 
for operator intervention (with the operational attribute 

So, knowing that, you have to make your own trade-off between 
convenience and absolute consistency.  MMR gives you the ability to have 
data locality with writes and no single point of write failure, at the 
cost of extra administrative overhead - monitoring, looking for 
conflicts (e.g. search for nsds5ReplConflict=*), and manually resolving 
them.  MMR has been deployed for years (starting in 3/2001 with iPlanet 
DS 5.0), and in the vast majority of cases, data inconsistency just 
doesn't happen.

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