On Thu, 2015-08-06 at 14:25 -0700, Noriko Hosoi wrote:
Very interesting plug-in!
Thanks. As a plugin, it's value is quite useless due to the nsDS5ReplicaType
flags. But it's a nice simple exercise to get ones head around how the plugin
architecture works from scratch. It's one thing to patch a plugin, compared to
writing one from nothing.
Regarding betxn plug-in, it is for putting the entire operation -- the
primary update + associated updates by the enabled plug-ins -- in one
transaction. By doing so, the entire updates are committed to the DB if
and only if all of the updates are successful. Otherwise, all of them
are rolled back. That guarantees there will be no consistency among
Okay, so if I can be a pain, how to betxn handle reads? Do reads come from
within the transaction? Or is there a way to read from the database outside the
Say for example:
add some object Y
Does read Y see the object within the transaction? Is there a way to make the
search happen so that it occurs outside the transaction, IE it doesn't see Y?
In that sense, your read-only plug-in is not a good example for betxn
since it does not do any updates. :) Considering the purpose of the
"read-only" plug-in, invoking it at the pre-op timing (before the
transaction) would be the best.
Very true! I kind of knew what betxn did, but I wanted to confirm more
completely in my mind. So I think what my read-only plugin does at the moment
works quite nicely then outside of betxn.
Is there a piece of documentation (perhaps the plugin guide) that lists the
order in which these operations are called?
Since MEP requires the updates on the DB, it's supposed to be called in
betxn. That way, what was done in the MEP plug-in is committed or
rolled back together with the primary updates.
The toughest part is the deadlock prevention. At the start transaction,
it holds a DB lock. And most plug-ins maintain its own mutex to protect
its resource. It'd easily cause deadlock situation especially when
multiple plug-ins are enabled (which is common :). So, please be careful
not to acquire/free locks in the wrong order...
Of course. This is always an issue in multi-threaded code and anything with
locking. Stress tests are probably good to find these deadlocks, no?
About your commented out code in read_only.c, I guess you copied the
part from mep.c and are wondering what it is for?
There are various type of plug-ins.
$ egrep nsslapd-pluginType dse.ldif | sort | uniq
The reason why slapi_register_plugin and slapi_register_plugin_ext were
* Allows a plugin to register a plugin.
* This was added so that 'object' plugins could register all
* the plugin interfaces that it supports.
On the other hand, MEP has this type.
The type is not "object", but the MEP plug-in is implemented as having
the type. Originally, it might have been "object"... Then, we
introduced the support for "betxn". To make the transition to
smoothly, we put the code to check "betxn" is in the type. If there is
"betxn" as in "betxnpreoperation", call the plug-in in betxn,
call them outside of the transaction. Having the switch in the
configuration, we could go back to the original position without
rebuilding the plug-in.
Since we do not go back to pre-betxn era, the switch may not be too
important. But keeping it would be a good idea for the consistency with
the other plug-ins.
Does this answer you question? Please feel free to let us know if it
That answers some of my question. I guess the larger part of the question is how
the plugin subsystem treats each pluginType differently and the value of having
a plugin register to more than one pluginType. Are there some documents you can
point me to about this?
Additionally, with betxn, this seems quite black-or-white. It's either on a ds
instance that has betxn support, so every update will be betxn capable, or it's
not on such a system so you fall back to other methods. Is this correct? With
new plugins is it even worth writing them without betxn support?
I'm sure our team is interested in your idea and work, so let me
your test plug-in with them.
Sure. It's not really that useful, like I said, nsDS5ReplicaType already does
this job. It was just an exercise to get my head more into the framework before
I work on http://directory.fedoraproject.org/docs/389ds/design/mep-rework.html
Thanks for the review, and for answering my questions. Your advice has helped a