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From Emmanuel Lecharny <elecha...@gmail.com>
Subject Re: TXN work update
Date Tue, 01 Nov 2011 14:09:50 GMT
Hi !

comments inline...

On 11/1/11 12:19 PM, Selcuk AYA wrote:
> thanks for the feedback! Please see inline:

<snip/>
>
>
> I guess the xdbm-partition Maven module is now going to be pretty damn thin
> or non-existent. Was this module destroyed and if not what actually remains?
> I think some util classes for tree based implementations remain there.
> Also there some avl related classes.

We can review this project and eventually move the remaining classes 
elsewhere.

>
>>> The todo is the following:
>>> *add changes to keep track of dn space changes.
>>> *test txn manager services
>>> *move the modification code path in AbstractBTreePartition in xdbm to
>>> high up to core. Probably modifications should be done in
>>> partitionNexus and partition nexus should handle preparing txn log
>>> edits and wal them. The overall flow for modifications will be:
>>>    -DefaultDirectoryService:
>>>           -begintxn
>>>                             -execute interceptor chain
>>>           - handle txn abort, or conflict.
>>>
>> Moving this into the PartitionNexus might not be a good idea but no problem
>> for now we can move it later. Let me explain why:
>> Eventually we're going to enable a root Partition with Partition nesting and
>> so when this happens the PartitionNexus will just be another nestable
>> Partition since these will have to handle routing based on DN to other
>> partitions residing/nested under it.
>> I see two possible locations for this functionality:
>> (1) Let the InterceptorChain itself handle this since it can demarcate the
>> start and end of calls into the chain with Txn begin and abort/commit calls.
>> It does this by making calls against the TxnManager which I guess is the
>> entire facade for the transaction subsystem.
>> ---OR---
>> (2) Handle Txn demarcation within the CoreSession. However this might not be
>> optimal due to the need to handle additional logic which might be required
>> for handling chain re-entry concerns.
>> NOTE: I've not actually looked at the code after these major moves so my
>> advice might not be very dependable. I will try to setup my environment to
>> get a better idea of these matters.
>> However for the time being do whatever actually makes this thing work. Let's
>> follow an agile methodology. This thing is big. So let's get it working with
>> solid test coverage then we can actually look at shuffling things around to
>> optimal positions. Not saying what you've chosen is wrong ... it might just
>> present the need for some additional refactoring when other features might
>> need to be introduced.
>> These are some of the biggest changes to the architecture to have taken
>> place in years and you're doing a great job.
> You are right for txn demarcation. I wrote it wrong in the email.
> Demarcation has to be done either at defaultcoresession or
> defaultoperationmanager.

OperationManager is probably the right place.

For inner operations, I'm almost 100% sure they aren't modifiying 
anything (the only modification done as a inner operation has been 
removed a while ago : it was dealing with the addition of the 
ModifyTimeStamp and ModifiersName attribute, and it' snow done in the 
main operation). So we are safe if they are done in the current external 
transaction.

>
> as for where to handle the change logic, as you mentioned we need a
> place where all interceptor chain routes end up for modification and
> ideally we should handle modification logic above partitions using
> master table and index interfaces so that we have a common place to
> prepare and apply txn log edits. PartitionNexus seems to fit this
> requirement for now. If we add another layer above PartitionNexus
> which can get master and index table from below layers and work with
> them, we should be able to move the change logic up there.
>
>>> *move xdbm-search to core as well. Making search transactional will
>>> mostly be mostly mechanica after this point I think(hope). It should
>>> just use the wrappers the txn manager provides for index, master and
>>> cursors it gets from the partitions.
>>>
>>> *handle caches various interceptors keep. I am thinking of handling
>>> this with a common read-write lock.
>>>
>> This was the latest issue for which I see some more threads. Will look at
>> that as well.
> This is the issue I talked you about briefly. These are mostly admin
> caches that change infrequently and that are read mostly. The
> difficulty with them is that they are not always entry caches. They
> might map Dns to some logical property of the entry. A simple and good
> example is notaliascache (whether we really need this is open to
> debate but we have it now).
This is a very interesting point. The notAliasCache is definitively 
questionable. The question though is to find a better way to deal with 
it. An option would be to not support alias (yes, I know, pretty 
drastic, but if it allows us to speed up the delivery of a production 
ready server, I think this would be a good tradeoff. Also note that once 
we have a solid base, we can reintroduce alias handling in a future 
version).

> This maps a Dn to whether the entry is an
> alias. If we followed the normal way of merging what is read from
> partitions with the txn log, we would have to a way of merging what is
> read from this notaliascache with the log. This is not very difficult
> but as we have quite a number of these caches, having a separate merge
> and update logic for each of them is a pain and error prone.
>
> Instead, what I thought was to use a single system wide read-write
> lock for these caches. Since they are read mostly, a txn might update
> its lock to exclusive when it needs to change one of these caches and
> might release it when it actually commits/aborts.

We may start a new thread to discuss further those cache issues. We have 
many caches, for may different thinsg, and many different approaches to 
deal with the (MRU, EhCache, etc...). It's probably a good timing for 
clarifying this item...

-- 
Regards,
Cordialement,
Emmanuel L├ęcharny
www.iktek.com


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