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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