We want copying settings to be the default behavior. This commit
deprecates not copying settings, and disallows explicitly not copying
settings. This gives users a transition path to the future default
behavior.
Today when an index is created from shrinking or splitting an existing
index, the target index inherits almost none of the source index
settings. This is surprising and a hassle for operators managing such
indices. Given this is the default behavior, we can not simply change
it. Instead, we start by introducing the ability to copy settings. This
flag can be set on the REST API or on the transport layer and it has the
behavior that it copies all settings from the source except non-copyable
settings (a property of a setting introduced in this
change). Additionally, settings on the request will always override.
This change is the first step in our adventure:
- this flag is added here in 7.0.0 and immediately deprecated
- this flag will be backported to 6.4.0 and remain deprecated
- then, we will remove the ability to set this flag to false in 7.0.0
- finally, in 8.0.0 we will remove this flag and the only behavior will
be for settings to be copied
Today when a resize operation is performed, we copy the analysis,
similarity, and sort settings from the source index. It is possible for
the resize request to include additional index settings including
analysis, similarity, and sort settings. We reject sort settings when
validating the request. However, we silently ignore analysis and
similarity settings on the request that are already set on the source
index. Since it is possible to change the analysis and similarity
settings on an existing index, this should be considered a bug and the
sort of leniency that we abhor. This commit addresses this bug by
allowing the request analysis/similarity settings to override the
existing analysis/similarity settings on the target.
I have seen this question a couple times already, most recently at
https://twitter.com/dimosr7/status/973872744965332993
I tried to keep the explanation as simple as I could, which is not always easy
as this is a matter of trade-offs.
Today we require users to prepare their indices for split operations.
Yet, we can do this automatically when an index is created which would
make the split feature a much more appealing option since it doesn't have
any 3rd party prerequisites anymore.
This change automatically sets the number of routinng shards such that
an index is guaranteed to be able to split once into twice as many shards.
The number of routing shards is scaled towards the default shard limit per index
such that indices with a smaller amount of shards can be split more often than
larger ones. For instance an index with 1 or 2 shards can be split 10x
(until it approaches 1024 shards) while an index created with 128 shards can only
be split 3x by a factor of 2. Please note this is just a default value and users
can still prepare their indices with `index.number_of_routing_shards` for custom
splitting.
NOTE: this change has an impact on the document distribution since we are changing
the hash space. Documents are still uniformly distributed across all shards but since
we are artificually changing the number of buckets in the consistent hashign space
document might be hashed into different shards compared to previous versions.
This is a 7.0 only change.
This change adds a new `_split` API that allows to split indices into a new
index with a power of two more shards that the source index. This API works
alongside the `_shrink` API but doesn't require any shard relocation before
indices can be split.
The split operation is conceptually an inverse `_shrink` operation since we
initialize the index with a _syntetic_ number of routing shards that are used
for the consistent hashing at index time. Compared to indices created with
earlier versions this might produce slightly different shard distributions but
has no impact on the per-index backwards compatibility. For now, the user is
required to prepare an index to be splittable by setting the
`index.number_of_routing_shards` at index creation time. The setting allows the
user to prepare the index to be splittable in factors of
`index.number_of_routing_shards` ie. if the index is created with
`index.number_of_routing_shards: 16` and `index.number_of_shards: 2` it can be
split into `4, 8, 16` shards. This is an intermediate step until we can make
this the default. This also allows us to safely backport this change to 6.x.
The `_split` operation is implemented internally as a DeleteByQuery on the
lucene level that is executed while the primary shards execute their initial
recovery. Subsequent merges that are triggered due to this operation will not be
executed immediately. All merges will be deferred unti the shards are started
and will then be throttled accordingly.
This change is intended for the 6.1 feature release but will not support pre-6.1
indices to be split unless these indices have been shrunk before. In that case
these indices can be split backwards into their original number of shards.
Jason Tedor
Adrien Grand
Simon Willnauer