Systems

The internal building blocks of an Elasticsearch node — architectural components that don't map cleanly to a single feature or plugin. They live primarily under server/src/main/java/org/elasticsearch/.

System	Source root	What it does
Action layer	action/, rest/	Request/response routing, REST <-> transport translation
Cluster coordination	cluster/coordination/, cluster/service/	Master election, cluster state publication, shard allocation
Indices and shards	indices/, index/	Per-shard lifecycle, engine, translog, mapping
Search	search/	Two-phase distributed search, aggregations dispatch
Transport	transport/, http/	Binary RPC between nodes; HTTP server
Snapshots and repositories	repositories/, snapshots/	Backup/restore, repository abstraction
Plugins	plugins/	Extension API and plugin loader
Telemetry and health	telemetry/, health/, monitor/	Metrics, traces, diagnostic health framework

The lens here is "internal infrastructure". For user-visible capabilities (search, ESQL, ML, security, ingest pipelines) see Features. For optional code packaged separately see Modules, Plugins, and X-Pack.