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[7.x] [DOCS] Update routing formulas (#76203) #76250

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Aug 9, 2021
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[7.x] [DOCS] Update routing formulas (#76203) #76250

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4 changes: 3 additions & 1 deletion docs/reference/index-modules.asciidoc
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Original file line number Diff line number Diff line change
Expand Up @@ -47,8 +47,10 @@ NOTE: The number of shards are limited to `1024` per index. This limitation is a
`index.number_of_routing_shards`::
+
====
Number of routing shards used to <<indices-split-index,split>> an index.
Integer value used with <<index-number-of-shards,`index.number_of_shards`>> to
route documents to a primary shard. See <<mapping-routing-field>>.

{es} uses this value when <<indices-split-index,splitting>> an index.
For example, a 5 shard index with `number_of_routing_shards` set to `30` (`5 x
2 x 3`) could be split by a factor of `2` or `3`. In other words, it could be
split as follows:
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18 changes: 12 additions & 6 deletions docs/reference/mapping/fields/routing-field.asciidoc
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Expand Up @@ -2,12 +2,17 @@
=== `_routing` field

A document is routed to a particular shard in an index using the following
formula:
formulas:

routing_factor = num_routing_shards / num_primary_shards
shard_num = (hash(_routing) % num_routing_shards) / routing_factor

shard_num = hash(_routing) % num_primary_shards

The default value used for `_routing` is the document's <<mapping-id-field,`_id`>>.
`num_routing_shards` is the value of the
<<index-number-of-routing-shards,`index.number_of_routing_shards`>> index
setting. `num_primary_shards` is the value of the
<<index-number-of-shards,`index.number_of_shards`>> index setting.

The default `_routing` value is the document's <<mapping-id-field,`_id`>>.
Custom routing patterns can be implemented by specifying a custom `routing`
value per document. For instance:

Expand Down Expand Up @@ -118,9 +123,10 @@ This is done by providing the index level setting <<routing-partition-size,`inde
As the partition size increases, the more evenly distributed the data will become at the
expense of having to search more shards per request.

When this setting is present, the formula for calculating the shard becomes:
When this setting is present, the formulas for calculating the shard become:

shard_num = (hash(_routing) + hash(_id) % routing_partition_size) % num_primary_shards
routing_value = hash(_routing) + hash(_id) % routing_partition_size
shard_num = (routing_value % num_routing_shards) / routing_factor

That is, the `_routing` field is used to calculate a set of shards within the index and then the
`_id` is used to pick a shard within that set.
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