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From Emmanuel Lecharny <elecha...@apache.org>
Subject Re: Journal first draft
Date Mon, 02 Mar 2009 19:44:03 GMT
Alex Karasulu wrote:
> OK thanks for the clarifications.
>   

This was the easy part :)

I'm now dealing with the hard part :
- how to avoid the journal to grow indefinitively
- how to use the journal with the coming replication system
- how to build the recovery system

I have some idea about #1 and #3 : Considering that the journal is just 
a representation of the pending operation,
we can rotate the file every N operations, or each time we sync(), if of 
course we have had some modifications, or evey T seconds. Once we have a 
rotated the file, we can check if all the operation has been acked, and 
if so, we can ditch them. If the server works well, we won't have 
operation pending for ever.

When the server crash, or when we stop it and restart it (it makes no 
difference), we just have to check on the journal if we have pending 
operation. If so, we will check if the pending operations have been 
applied or not (it's easy for add or del operation, a bit more 
complicated for modifications, definitively more complex for move and 
rename, as the operation can have been partially applied).

What if the base is screwed ? (ie, we can't restart the server because 
the backend files are FU). Then we have to start from a recent backup, 
and apply all the journal on this backup.

So next question : how do we get this backup ? We have to build it : the 
idea is to have a local server working as a slave, and periodically 
applying the journal on its base. When done, the server is stopped, the 
partitions are saved, and we just wait for the next backup to start. So 
if the machine crashes, even during a backup, we can still start from 
the previous saved file and reapply the journal on it. Then when the 
journal has been applied, we just have to copy the backup partition to 
the real server, and we are done.

Seems a bit complex, but don't worry : it is complex :)
> Alex
>
> On Mon, Mar 2, 2009 at 2:20 PM, Emmanuel Lecharny <elecharny@apache.org>wrote:
>
>   
>> Alex Karasulu wrote:
>>
>>     
>>> So wdyt about this first version ? What's missing ?
>>>       
>>>>
>>>>         
>>> (1) Why not make these comment attribute actual attributes in the LDIF
>>> even
>>> though they are special?
>>>
>>>
>>>       
>> I was considering this as an option, but the problem is that you need to
>> clone the entry in order to add those special attributes, ar add them and
>> remove them immediately, which is not really elegant.
>>
>>     
>>> (2) Why is rev not a regular attribute?
>>>
>>>
>>>       
>> Same reason. And we can't use the ChangeLog rev because it is not added if
>> the changeLog is not activated.
>>
>> In any case, we don't need to store those elements in the partition within
>> the entry, because they are volatile (each time you restart the server, the
>> revision is set to a new value)
>>
>>     
>>> (3) What exactly is the difference between ACK and NACK and why do we need
>>> a
>>> NACK?
>>>
>>>
>>>       
>> We will have ACK for entries which have been stored in the partition. NACK
>> is for entries that we _know_ haven't been stored for any reason. If the
>> entry has not been stored yet, or not been rejected yet, then we won't have
>> any ACK or NACK.
>>
>> It's interesting to have a NACK to distinguish between those two cases.
>>
>>
>> --
>> --
>> cordialement, regards,
>> Emmanuel L├ęcharny
>> www.iktek.com
>> directory.apache.org
>>
>>
>>
>>     
>
>   


-- 
--
cordialement, regards,
Emmanuel L├ęcharny
www.iktek.com
directory.apache.org



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