The formula for calculating the shard which a document is routed in reference is wrong.

[ViggoC]

The formula for calculating the shard which a document is routed in reference is wrong. It does not match the actual calculation process in the code.

Formula in reference

shard_num = hash(_routing) % num_primary_shards

Code

indexMetaData.routingFactor = routingNumShards / numberOfShards;
Math.floorMod(hash, indexMetaData.getRoutingNumShards()) / indexMetaData.getRoutingFactor();

[elasticmachine]

Pinging @elastic/es-docs (Team:Docs)

[elasticmachine]

Pinging @elastic/es-search (Team:Search)

[jrodewig]

Thanks for raising this @ViggoC. I'm not sure we need the formulae here at all. It's probably enough to say that we use a hash of the document _id to route documents by default and that this evenly distributes documents among an index's shards.

[ViggoC]

@jrodewig Thank you for your response, I agree with your opinion that it's enough for some users. But I believe that a link to more detail about how the shard num is calculated will be helpful for senior user and developer. Maybe the information can be appended to where index.number_of_routing_shards is mentioned.

[jrodewig]

Thanks for the feedback @ViggoC. I'll revise #76203 to update the formulas and cross-reference the settings involved. Thanks again for raising this.

[ViggoC]

👍