# Finding optimum number of shards for custom routing **URL:** \ **Category:** Elasticsearch\ **Created:** [July 11, 2018, 3:11pm UTC](https://discuss.elastic.co/t/finding-optimum-number-of-shards-for-custom-routing/139606 "2018-07-11T15:11:49Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![pokaleshrey](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/pokaleshrey/32/48680_2.png) [@pokaleshrey](https://discuss.elastic.co/u/pokaleshrey)\ **Post date:** [July 11, 2018, 3:11pm UTC](https://discuss.elastic.co/t/finding-optimum-number-of-shards-for-custom-routing/139606/1 "2018-07-11T15:11:49Z")
Hi, I am trying to find optimum number of shards for my data. I have custom routing value in my application. I know the formula: hash(routing) % num of primary shards. **Question** : Can anyone point me to the hashing function above ? Is there a utility or source code which i can use ? ---
**Author:** ![pokaleshrey](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/pokaleshrey/32/48680_2.png) [@pokaleshrey](https://discuss.elastic.co/u/pokaleshrey)\ **Post date:** [July 12, 2018, 5:35am UTC](https://discuss.elastic.co/t/finding-optimum-number-of-shards-for-custom-routing/139606/2 "2018-07-12T05:35:28Z")
I found it out myself from the ES code. Anyone who is looking for the hash function please use this from ES codebase: > Math.floorMod(Murmur3HashFunction.hash(\_routing), numberOfShards) This output a number which is a shard number to which your \_routing value will be allocated. I hope this saves someones day, especially if they don't get replies on this forum. ---
**Author:** ![Christian\_Dahlqvist](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/christian_dahlqvist/32/4617_2.png) [@Christian\_Dahlqvist](https://discuss.elastic.co/u/Christian_Dahlqvist)\ **Post date:** [July 12, 2018, 5:57am UTC](https://discuss.elastic.co/t/finding-optimum-number-of-shards-for-custom-routing/139606/3 "2018-07-12T05:57:40Z")
What are you going to use this for? ---
**Author:** ![pokaleshrey](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/pokaleshrey/32/48680_2.png) [@pokaleshrey](https://discuss.elastic.co/u/pokaleshrey)\ **Post date:** [July 12, 2018, 6:24am UTC](https://discuss.elastic.co/t/finding-optimum-number-of-shards-for-custom-routing/139606/4 "2018-07-12T06:24:49Z")
To decide the number of primary shards for my use case. I am using custom \_routing. I need to choose a number of primary shards so that all my content is evenly distributed across shards. My current issue: when i am indexing data using multiple threads and all my data which has same \_routing, is getting shoved into the same shard, i am loosing data. ---
**Author:** ![Christian\_Dahlqvist](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/christian_dahlqvist/32/4617_2.png) [@Christian\_Dahlqvist](https://discuss.elastic.co/u/Christian_Dahlqvist)\ **Post date:** [July 12, 2018, 7:08am UTC](https://discuss.elastic.co/t/finding-optimum-number-of-shards-for-custom-routing/139606/5 "2018-07-12T07:08:01Z")
What are you using routing for. Is it based on a low cardinality field? ---
**Author:** ![pokaleshrey](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/pokaleshrey/32/48680_2.png) [@pokaleshrey](https://discuss.elastic.co/u/pokaleshrey)\ **Post date:** [July 12, 2018, 7:16am UTC](https://discuss.elastic.co/t/finding-optimum-number-of-shards-for-custom-routing/139606/6 "2018-07-12T07:16:54Z")
I have data divided into several groups. Each group has a two character identifier. My \_routing is not based on any field in my document. I expect each group to be stored on a single shard. Now my two character identifier serves as my "\_routing" while indexing. At search time i know my identifier, so it is easy to search. I expect my groups to be evenly divided across my shards. Hence i need to decided optimum number of primary shards which will be useful while indexing and also beneficial in searching. ---
**Author:** ![system](https://us1.discourse-cdn.com/elastic/original/3X/1/a/1ac57faf039f6b580b3f104ef42a2a89e41014de.png) [@system](https://discuss.elastic.co/u/system)\ **Post date:** [August 9, 2018, 7:17am UTC](https://discuss.elastic.co/t/finding-optimum-number-of-shards-for-custom-routing/139606/7 "2018-08-09T07:17:01Z")
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