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On 4 September 2014 23:42, Nejc Saje <nsaje at redhat.com> wrote:
>
>
> On 09/04/2014 11:51 AM, Robert Collins wrote:

> It doesn't contain that term precisely, but it does talk about replicating
> the buckets. What about using a descriptive name for this parameter, like
> 'distribution_quality', where the higher the value, higher the distribution
> evenness (and higher memory usage)?
>
>
>>
>> I've no objection talking about keys, but 'node' is an API object in
>> Ironic, so I'd rather we talk about hosts - or make it something
>> clearly not node like 'bucket' (which the 1997 paper talks about in
>> describing consistent hash functions).
>>
>> So proposal:
>>   - key - a stringifyable thing to be mapped to buckets
>
> What about using the term 'item' from the original paper as well?

Sure. Item it is.

>
>>   - bucket a worker/store that wants keys mapped to it
>>   - replicas - number of buckets a single key wants to be mapped to
>
> Can we keep this as an Ironic-internal parameter? Because it doesn't really
> affect the hash ring. If you want multiple buckets for your item, you just
> continue your journey along the ring and keep returning new buckets. Check
> out how the pypi lib does it:
> https://github.com/Doist/hash_ring/blob/master/hash_ring/ring.py#L119

That generator API is pretty bad IMO - because it means you're very
heavily dependent on gc and refcount behaviour to keep things clean -
and there isn't (IMO) a use case for walking the entire ring from the
perspective of an item. Whats the concern with having replicas a part
of the API?

>>   - partitions - number of total divisions of the hash space (power of
>> 2 required)
>
> I don't think there are any divisions of the hash space in the correct
> implementation, are there? I think that in the current Ironic implementation
> this tweaks the distribution quality, just like 'replicas' parameter in
> Ceilo implementation.

its absolutely a partition of the hash space - each spot we hash a
bucket onto is thats how consistent hashing works at all :)

-Rob

-- 
Robert Collins <rbtcollins at hp.com>
Distinguished Technologist
HP Converged Cloud