Elasticsearch Legacy Completion Plugin

This is a "hack" to allow usage of the legacy Elasticsearch 1.x/2.x completion suggester in Elasticsearch 5.x. This legacy plugin supports deduplication and is not document based.

This fixes the following Elasticsearch issue by a workaround: elastic/elasticsearch#22912

Installation and Usage

To restore the old behaviour of the old completion suggester for new indexes created with Elasticsearch 5.x use the following process:

To compile the plugin you need to install Apache Maven and a JDK 8 in your environment.

The plugin compiles with Elasticsearch 5.3.0, but should also work with earlier or later versions. Just edit the POM file by changing the plugin version number to match the Elasticsearch version, then compile and create the package:

git clone https://github.com/uschindler/es-legacy-completion-plugin
cd es-legacy-completion-plugin
mvn clean install

After that you can install es-legacy-completion-plugin-5.X.Y.zip from the target/ directory using Elasticsearch elasticsearch-plugin install command.

After installation you can change your ES 2.x mapping to use the new field type legacy_completion instead of completion (which unfortunately triggers new behaviour). When you then query the index using the usual suggestion endpoint, it will return results in the same way as Elasticsearch 2.x did.

How it works

Elasticsearch 5.x still has the old suggester code inside to support Elasticsearch 2.x indexes, but for new indexes it uses the new suggester. The completion suggester endpoint just checks the Lucene Codec that was used to create the suggestion field and then uses the old or new one.

This plugin just adds a new field type that internally enforces the use of the old Lucene Codec. This way the suggester endpoint automatically falls back to old behaviour. It does this by some hacks in the field mapper and (unfortunately), lots of code duplication, but this was kept minimal.

Caution: This is likely to break with Elasticsearch 6!