Sam Tran wrote:
On 7/8/05, Kevin Myer <kevin_myer(a)iu13.org> wrote:
>Quoting Sam Tran <stlist(a)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
>there)
>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
>
>--
>Kevin M. Myer
>Senior Systems Administrator
>Lancaster-Lebanon Intermediate Unit 13
http://www.iu13.org
>
>
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>
>
>
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
inconsistencies:
http://www.watersprings.org/pub/id/draft-zeilenga-ldup-harmful-02.txt
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 -
http://www2.parc.com/csl/projects/bayou/pubs/uist-97/Bayou.pdf
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
secondary.
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
nsds5ReplConflict).
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.
Thanks.
Sam
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