We currently use the djb2 hash function in order to compute the shard a
document should go to. Unfortunately this hash function is not very
sophisticated and you can sometimes hit adversarial cases, such as numeric ids
on 33 shards.
Murmur3 generates hashes with a better distribution, which should avoid the
adversarial cases.
Here are some examples of how 100000 incremental ids are distributed to shards
using either djb2 or murmur3.
5 shards:
Murmur3: [19933, 19964, 19940, 20030, 20133]
DJB: [20000, 20000, 20000, 20000, 20000]
3 shards:
Murmur3: [33185, 33347, 33468]
DJB: [30100, 30000, 39900]
33 shards:
Murmur3: [2999, 3096, 2930, 2986, 3070, 3093, 3023, 3052, 3112, 2940, 3036, 2985, 3031, 3048, 3127, 2961, 2901, 3105, 3041, 3130, 3013, 3035, 3031, 3019, 3008, 3022, 3111, 3086, 3016, 2996, 3075, 2945, 2977]
DJB: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 900, 900, 900, 900, 1000, 1000, 10000, 10000, 10000, 10000, 9100, 9100, 9100, 9100, 9000, 9000, 0, 0, 0, 0, 0, 0]
Even if djb2 looks ideal in some cases (5 shards), the fact that the
distribution of its hashes has some patterns can raise issues with some shard
counts (eg. 3, or even worse 33).
Some tests have been modified because they relied on implementation details of
the routing hash function.
Close#7954
The MLT field query is simply replaced by a MLT query set to specififc field.
To simplify code maintenance we should deprecate it in 1.4 and remove it in
2.0.
Closes#8238
Returns information about settings, aliases, warmers, and mappings. Basically returns the IndexMetadata. This new endpoint replaces the /{index}/_alias|_aliases|_mapping|_mappings|_settings|_warmer|_warmers and /_alias|_aliases|_mapping|_mappings|_settings|_warmer|_warmers endpoints whilst maintaining the same response formats. The only exception to this is on the /_alias|_aliases|_warmer|_warmers endpoint which will now return a section for 'aliases' or 'warmers' even if no aliases or warmers exist. This backwards compatibility change is documented in the reference docs.
Closes#4069
Adrien Grand
Alex Ksikes
Colin Goodheart-Smithe
Martijn van Groningen
Clinton Gormley
Simon Willnauer
Karel Minarik
Luca Cavanna