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

[Mike Jackson]

Pierangelo Masarati wrote:
> Pete Rowley wrote:
> 
>> Balance the issues raised in that draft versus having only one master, 
>> which
>> means a single point of write failure without any write load balancing.
>>  
>>
> 
> Just curious: I see this "write load balaincing" issue coming up all 
> times; if I have, say, 2 MMasters, does it mean that each one gets 
> (roughly) half the write operations?  What about the other half to get 
> in sync within each other?

Hi Pierangelo,
  The fact that LDAP directory servers are not intended to support a 
high frequency of write operations means that the term "write load 
balancer" is not the correct term to use when describing the benefits of 
multi-master versus single-master replication - unless you are arguing 
how to support systems architects who intentionally (or perhaps out of 
ignorance) use LDAP technology in an incorrect manner in their designs. 
The correct term to use in this context, IMO, is "highly available write 
operations".

  If you want to balance operations, then of course you need to use a 
load balancer. When you use single-master replication, it means that 
even if you use a load balancer, write operations are still not highly 
available, e.g. they can not be performed if the master is down.

  The key benefit of multi-master replication is that _all_ of the LDAP 
operations _can_ be made highly available, and that applications which 
need to write something ASAP (think disabling a user's account) should 
never fail or be postponed as long as there is either a front-end load 
balancer, or clients which can use a failover list of LDAP server names.

  The key failure of the single-master replication model is that it 
(currently) does not stand up in a highly available system design. If 
there were some sort of automatic promotion of replica to master feature 
that worked well, then it would certainly be a viable alternative to 
using MM replication within a single geographical location / directory 
partition.

  IMO, the usage of MM replication is most suited for DIT designs which 
span across wide area networks with subtree replication across a set of 
"international masters", where it unfortunately has not worked very well 
at all due to it's design. The recent version of FDS/RHDS is supposed to 
address this issue via the use of a "sliding window algorithm", and 
subtree replication support was included already in NDS 6.21.

  So, how about implementing automatic promotion of slaves to master 
status, so that single-master environments can be made highly available? 
This is a generic problem, and applies to both FDS and OL. Do you think 
it's a good idea or not?

Mike

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