[rabbitmq-discuss] Unexpected Behavior When Using the "X-Consistent-Hash" Exchange Type
simon at rabbitmq.com
Mon Oct 21 16:30:44 BST 2013
On 17/10/2013 12:13AM, Richard Raseley wrote:
> 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
> 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.
No, they don't. The actual hashing is done by the Erlang function
phash2, which will hash a UUID or an integer or any Erlang term.
> 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
It doesn't matter.
Yeah, this confused me too when I first started looking at the hashing
exchange. As long as you use something that tends towards unique-ish
values then any unevenness in the routing displayed by the CHX is caused
by the distribution of the buckets in its routing table - this is
influenced by the number of points each binding has (as you have already
noted) *and nothing else*.
You can observe this easily - create some queues and bindings with 1
point each. Then publish some messages with random integers as the
routing keys. Observe the uneven distribution. Then purge all the queues
(but don't delete and start again) and re-run the test with UUIDs or
message sequence IDs or whatever. Not only will the distribution still
be uneven, but it will be uneven in exactly the same way.
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