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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