The configuration file found in the Elasticsearch config directory could previously have a .yml, .yaml or .json extension. Only elasticsearch.yml is now supported.

In previous versions of Elasticsearch, the configuration file was allowed to contain duplicate keys. For example:

node:
  name: my-node

node
  attr:
    rack: my-rack

In Elasticsearch 6.0.0, this is no longer permitted. Instead, this must be specified in a single key as:

node:
  name: my-node
  attr:
    rack: my-rack

Previously, Elasticsearch recognized the strings true, false, on, off, yes, no, 0, 1 as booleans. Elasticsearch 6.0 recognizes only true and false as boolean and will throw an error otherwise. For backwards compatibility purposes, during the 6.x series index settings on pre-6.0 indices will continue to work. Note that this does not apply to node-level settings that are stored in elasticsearch.yml.

The internal setting cluster.routing.allocation.snapshot.relocation_enabled that allowed shards with running snapshots to be reallocated to different nodes has been removed. Enabling this setting could cause allocation issues if a shard got allocated off a node and then reallocated back to this node while a snapshot was running.

Store throttling has been removed. As a consequence, the indices.store.throttle.type and indices.store.throttle.max_bytes_per_sec cluster settings and the index.store.throttle.type and index.store.throttle.max_bytes_per_sec index settings are not recognized anymore.

The default index.store.type has been removed. If you were using it, we advise that you simply remove it from your index settings and Elasticsearch will use the best store implementation for your operating system.

The blocking TCP client, blocking TCP server, and blocking HTTP server have been removed. As a consequence, the network.tcp.blocking_server, network.tcp.blocking_client, network.tcp.blocking,transport.tcp.blocking_client, transport.tcp.blocking_server, and http.tcp.blocking_server settings are not recognized anymore.

The previously unused settings transport.netty.max_cumulation_buffer_capacity, transport.netty.max_composite_buffer_components and http.netty.max_cumulation_buffer_capacity have been removed.

The base similarity is now ignored as coords and query normalization have been removed. If provided, this setting will be ignored and issue a deprecation warning.

All of the existing scripting security settings have been removed. Instead they are replaced with script.allowed_types and script.allowed_contexts.

The discovery.type settings no longer supports the values gce, aws and ec2. Integration with these platforms should be done by setting the discovery.zen.hosts_provider setting to one of those values.