[389-devel] 389-DS plugin: need some help on the design

[thierry bordaz]

On 09/25/2014 04:32 AM, Rich Megginson wrote:
> On 09/24/2014 04:33 AM, thierry bordaz wrote:
>> Hello,
>>
>>     I was investigating the alternative/impacts of a new plugin and I
>>     would like to share some thoughts and check I did not miss
>>     something important.
>>
>>     Here is the description of the problem we want to address. In MMR
>>     topology, we have an entry containing a single valued attribute.
>>     It is an integer syntax attribute. Our need is that the attribute
>>     can only be increased. So if its initial value is 5, an update
>>     MOD/REPL '6' is valid and applied, while MOD/REPL '3' is invalid
>>     and rejected/ignored. Also being in MMR, the attribute can be
>>     updated on several instances.
>>
>>     The current approach is to create a BE_PREOP or BE_TXN_PREOP
>>     plugin. This allow to retrieve the current value from the pblock
>>     (SLAPI_ENTRY_PRE_OP) and guaranties the value is exact as only
>>     one operation is processed at a time.
>>
>>     The plugin  registers a mod operation callback. It controls the
>>     new_value vs current_value to check that  new_value
>>     >current_value. The plugin will update the mods. In particular
>>     translates a MOD/REPL into a MOD/DEL(current value) +
>>     MOD/ADD(new_value).
>>
>>     Regarding the change of the MODS (mod/repl -> mod/del + mod/add),
>>     the plugin should be a BE_PREOP. This is because MODS are applied
>>     after BE_PREOP plugins, then new MODS added by BE_TXN_PREOP
>>     plugins are applied. A BE_TXN_PREOP plugin may translate mod/repl
>>     -> mod/del+mod/add but it is too late, mod/repl has already been
>>     applied after BE_PREOP plugins were called.
>>
>>     Regarding replication, for non replicated updates, it should just
>>     reject (unwilling to perform) ops with new_value < current_value.
>>     For replicated update I see the two cases ([server / csn /
>>     attribute value] ): [A/csnA/valueA], [B/csnB/valueB] and the
>>     expected final value is ValueB+csnB
>>
>>      1. csnA < csnB and ValueA < ValueB.
>>          1. When server A receives csnB/valueB, this is fine as
>>             ValueB>ValueA. But to know that ValueB will be selected 
>>             the plugin needs to check that csnB>csnA.
>>          1. When server B receives csnA/valueA it has 3 possibilities:
>>              1. reject (unwilling to perform) the update. But then
>>                 replication A->B will fail indefinitely
>>              2. erase the update. For example the plugin could erase
>>                 the mod from the set of mod.
>>              3. let the operation continue because csnA < csnB, the
>>                 kept value will be ValueB. Here again the plugin
>>                 needs to check csnA vs csnB
>>      2. csnA > csnB and ValueA < ValueB.
>>          1. When server A receives csnB/valueB, this is fine as
>>             ValueB>ValueA. But to know that ValueB will be selected 
>>             the plugin need to check that csnB>csnA.
>>          2. When server B receives csnA/valueA it has 2 possibilities:
>>              1. reject (unwilling to perform) the update. But then
>>                 replication A->B will fail indefinitely
>>              2. erase the update. For example the plugin could erase
>>                 the mod from the set of mod.
>>
>>     So I think the plugin should not rely on the new_value present in
>>     the operation but rather  computes the final_value (taking into
>>     account the CSN).
>>     If the final_value > current_value, it let the operation going on
>>     (even if the new_value in the operation < current_value). If the
>>     final_value < current_value it should remove the mod from the
>>     mods (2.2.2) and likely log a message.
>>
>
> What happens if ValueA == ValueB and csnA != csnB?  Do we want to 
> allow the same value to be issued by two different servers?  Is this a 
> case as with DNA and uidNumber, that we assign servers to have ranges?

That is a good question and so far I still need confirmation.
This is a case with OTP updating the HOTPcounter/TOTPwatermark.
If a bind happens with a given new HOTPcounter value, it will trigger 
internal mod on an entry (related to bindDN) to update this counter.
IMHO we can have parallel bind with a same counter, this on different or 
on the same server as well. In both cases, the csn will be different but 
the value identical.

thanks Rich
thierry
>
>
>>
>>     Changing MOD/REPL into MOD/DEL+ MOD/ADD is a possibility but the
>>     attribute being single valued I think it is not mandatory.
>>
>>     Thanks
>>     thierry
>>
>>
>>
>>
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>
>
>
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