The transport layer

ElasticsearchClient exposes the Elasticsearch APIs using high level objects and data structures. It relies on a Transport implementation for lower level concerns that include:

• JSON serialization and deserialization, by delegating to a JsonpMapper.
• network communication: HTTP, TLS, load-balancing among cluster nodes, etc.

The Java API client comes with two JSON mapping implementations:

• JacksonJsonpMapper, the default implementation based on the popular Jackson library.
• JsonbJsonpMapper, based on the JakartaEE JSONP specification, which allows using any implementation of this specification such as Eclipse Parsson

Which implementation should you use?

Unless you have specific requirements, use the default Jackson implementation.

The Java API client comes with two HTTP implementations:

• REST 5 Client, the default implementation, based on the Apache http client library version 5. It's a direct port of the previous version to this new library.
• Rest Client, the legacy implementation that has existed since Elasticsearch version 7, based on the Apache http client library version 4.

Which implementation should you use?

• if you're starting a new application, use the default Rest 5 Client.
• if you're upgrading an existing application, consider upgrading to Rest 5 Client. Adapting your client creation code to the newer Apache http library should be releatively straightforward.
• if you're upgrading an application with a heavily customized Rest Client configuration, then consider staying with this legacy version. Be mindful however that it may be removed in the next major release.