Re: Implementing Referential Integrity by using a queue instead of a file

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On 07/06/2017 08:13 PM, William Brown wrote:
On Thu, 2017-07-06 at 14:33 -0400, Mark Reynolds wrote:
On 07/06/2017 01:07 PM, Ilias Stamatis wrote:
Hello,

A desire had been expressed to get rid of referint plugin's logfile:
https://pagure.io/389-ds-base/issue/49202

It finally turns out that this file is used for other purposes than
real logging.

The referint plugin currently works like that; When the delay update
is set to be more than 0 a new thread is created executing referential
integrity code every x seconds (set by delay update). When a delete or
modrdn operation happens, the plugin will write that down to its
logfile. So, every x seconds the plugin will check the logfile, see
what happened and apply the changes. Finally, it deletes the file,
thus clearing the state for the next time it reads from it.

After discussing this with William he suggested it's better to replace
the file with a queue, since the fileinvolves excess fsync / sync, and
has all kinds of potential state/race issues. Using a queue will be
much faster as well.

William went even further and suggested that we could get rid of the
async referint update completely. This probably wouldn't happen soon
though, since likely customers are using it. For now we could provide
a warning such as "we recommend you set delay to 0".

Finally, the referint-logchanges attribute does absolutely nothing. It
seems to be completely ignored by the plugin, so we could remove this
as well.

I'll start working on these changes soon.

Any thoughts or objections on the above would be welcome.
The only problem with going to a queue is if the server goes down
unexpectedly.  In such a case those RI updates would be lost. 
We already have this issue because there is a delay between the change
to the object and the log being sync() to disk. So we can already lose
changes here. TBH the only fix is ot remove the async model. I actually
question why we still need async/delay processing of the refint
plugin ...
Historically speaking, a long time ago, we used to see high CPU when the RI plugin was engaged.  Setting the delay to 1 second, and allowing the log thread to do the work, improved performance.  Of course this is now obsolete with the betxn plugin model and other improvements, but I wanted to share why the feature even existed. 

      
This also brings up a different point...  the RI plugin is a backend txn
plugin.  If we write changes to a log, and those changes end up failing
for some reason, then there is no way to rollback the original
transaction --> breaking the backend txn plugin model.

Perhaps the log/delay should just be removed?  Or ignore the log/delay
settings if the plugin is set as a backend txn plugin? 
Completely agree. Because of the delay, if we roll back the txn we still
do the refint check. 

I would be fully in support of removing the delay option and going betxn
for the plugin only. This delay behaviour is the reason we advise you
only run refint on one master in a topology, where if we remove this and
go betxn, we can run on all masters correctly. I think we would need to
make the plugin ignore replicated ops then too. 

My only concern would be what version to have this change land in - as
much as I'm excited to make the change we should be careful.

Perhaps we remove the delay processing, and have the "delay" process
flag act as a switch to check incoming repl ops? Because today if you
have delay > 0, you likely have refint on one master, so we need to
refint incoming repl ops. If you have delay 0, you ignore repl ops
because you assume all masters have refint?

No matter what, it's not a smooth upgrade process here, but I think long
term it's nicer to just have it on "all masters". 



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