[rabbitmq-discuss] Unexpected Behavior When Using the "X-Consistent-Hash" Exchange Type

Richard Raseley richard at raseley.com
Thu Oct 17 17:56:37 BST 2013

As a final question, can anyone on the engineering team provide a
recommendation as to the best way to generate a header value for hashing
use in terms of what will implicitly provide the most even distribution?

For example, if a random integer is the best way to go, should the range of
possible integers be scoped from 1 to Q*B where Q is equal to the number of
queues you have bound to the x-consistent-hash exchange and B is equal to
the sum of all the integers used as routing keys when binding the queues to
the x-consistent-hash exchange?



On Wed, Oct 16, 2013 at 4:13 PM, Richard Raseley <richard at raseley.com>wrote:

> An interesting update. In re-reading the x-consistent-hash type exchange
> documentation, the following passage:
> "The more points in the hash space each binding has, the closer the actual
> distribution will be to the desired distribution (as indicated by the ratio
> of points by binding). However, large numbers of points (many thousands)
> will substantially decrease performance of the exchange type."
> It appears that my initial understanding of the integer used to bind
> queues to the x-consistent-hash exchange as a "weight" was incorrect, but
> rather it represents the number of points along the overall hash continuum
> where that particular queue is represented, with the points being
> (seemingly) pseudorandomly distributed across the overall hash space. So I
> updated the bindings on each of the queues from 1 to 100 and re-ran my
> tests with 50K messages this time (in the interests of expediency).
> The results are as follows:
> **FIRST TEST (Using UUID as the Hash Header)**
> Queue Name,Message Count,Percent of Total,Percent of Equality
> error01,3505,7.01%,89.16%
> error02,2919,5.84%,107.06%
> error03,3418,6.84%,91.43%
> error04,2912,5.82%,107.31%
> error05,2945,5.89%,106.11%
> error06,3146,6.29%,99.33%
> error07,3057,6.11%,102.22%
> error08,2868,5.74%,108.96%
> error09,2968,5.94%,105.29%
> error10,2866,5.73%,109.04%
> error11,3513,7.03%,88.96%
> error12,4077,8.15%,76.65%
> error13,3054,6.11%,102.32%
> error14,2648,5.30%,118.01%
> error15,2718,5.44%,114.97%
> error16,3386,6.77%,92.29%
> ALL,50000,100.00%,N/A
> **SECOND TEST (Using Random Int Between 1 and 1000000000 as the Hash
> Header)**
> Queue Name,Message Count,Percent of Total,Percent of Equality
> error01,3313,6.63%,94.33%
> error02,3255,6.51%,96.01%
> error03,3077,6.15%,101.56%
> error04,3048,6.10%,102.53%
> error05,2384,4.77%,131.08%
> error06,2954,5.91%,105.79%
> error07,3586,7.17%,87.14%
> error08,3055,6.11%,102.29%
> error09,3030,6.06%,103.14%
> error10,3517,7.03%,88.85%
> error11,2954,5.91%,105.79%
> error12,2921,5.84%,106.98%
> error13,3046,6.09%,102.59%
> error14,3710,7.42%,84.23%
> error15,3024,6.05%,103.34%
> error16,3126,6.25%,99.97%
> ALL,50000,100.00%,N/A
> So we now have a much more even distribution. I then changed the bindings
> of the queues from 100 to 1000 and reran the test. I saw significant
> increase in CPU usage and flow-control started kicking in (running this VM
> on a single core on my laptop so not unexpected). This is in line with the
> warning in the excerpt above and makes sense.
> This leaves me with the following questions:
> 1) Is it true to say that it is now just a matter of choosing what
> tradeoff we want to make in terms of performance vs. uniformity of
> distribution?
> 2) Can anyone comment on Michael's previous comment on how using a UUID
> would be handled as the value in the hashed header? The numbers above don't
> seem to show a large difference between using a random integer and a UUID.
> Regards,
> Richard Raseley
> On Wed, Oct 16, 2013 at 3:34 PM, Michael Klishin <mklishin at gopivotal.com>wrote:
>> On oct 17, 2013, at 2:33 a.m., Richard Raseley <richard at raseley.com>
>> wrote:
>> > Do you mean to say that those "voided" messages didn't make it to a
>> queue or weren't hashed but simply put to an arbitrary queue?
>> The former but it was just a guess.
>> --
>> MK
>> Software Engineer, Pivotal/RabbitMQ
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