Glossary

Envoy has a lot of acronyms. This page is a fast lookup for the ones that appear repeatedly in the codebase. Each entry points at the file that introduces or implements the concept.

Core architecture

Term	Expansion / meaning
Listener	A named entity that owns a socket on a specific address and runs a chain of filters on each accepted connection. Public interface: envoy/network/listener.h. Implementation: source/common/listener_manager/.
Cluster	A logical group of upstream hosts plus the policy for sending traffic to them (load balancing, circuit breaking, health checking, transport socket). Interface: envoy/upstream/cluster_manager.h.
Endpoint	A single network address belonging to a cluster, possibly with locality and weight metadata.
Filter chain	An ordered list of filters that runs over either a connection (network/listener filters) or a request (HTTP filters).
HCM	HTTP Connection Manager. The network filter that owns the HTTP/1, HTTP/2, or HTTP/3 codec and runs the L7 filter chain. See source/extensions/filters/network/http_connection_manager/.
Route	A match-and-action rule that maps a request to a cluster (or redirect/direct response). Routes are grouped into virtual hosts inside a RouteConfiguration.
Connection pool	Per-host pool of upstream connections, keyed by protocol and transport socket. Implementation in source/common/conn_pool/ and source/common/http/conn_pool_*.
Transport socket	The pluggable layer that wraps raw bytes (raw TCP, TLS, ALTS, proxy_protocol, …). See envoy/network/transport_socket.h.
Worker	A thread that owns a slice of the listeners and runs their filter chains. See source/server/worker_impl.h.
Dispatcher	The libevent/io_uring-based event loop running on each thread. Interface: envoy/event/dispatcher.h.
TLS slot	"Thread-local storage" — a copy-on-write piece of state replicated to every worker for lock-free reads. See envoy/thread_local/thread_local.h.

xDS — discovery services

xDS is the family of gRPC APIs Envoy uses to receive dynamic configuration from a control plane. Every "x" is a kind of resource:

API	Resource	Wire definition
LDS	Listener Discovery Service	api/envoy/service/listener/v3/lds.proto
CDS	Cluster Discovery Service	api/envoy/service/cluster/v3/cds.proto
RDS	Route Discovery Service	api/envoy/service/route/v3/rds.proto
EDS	Endpoint Discovery Service	api/envoy/service/endpoint/v3/eds.proto
SDS	Secret Discovery Service (TLS certs, JWTs)	api/envoy/service/secret/v3/sds.proto
RTDS	Runtime Discovery Service	api/envoy/service/runtime/v3/rtds.proto
ECDS	Extension Config Discovery Service (per-filter config)	api/envoy/service/extension/v3/config_discovery.proto
VHDS	Virtual Host Discovery Service (incremental RDS)	Embedded in RDS
ADS	Aggregated Discovery Service — multiplexes the above onto a single bidi gRPC stream	api/envoy/service/discovery/v3/ads.proto
Delta xDS	Incremental variant of every xDS API; only changed resources are sent	Same proto package, different RPC method

The client implementations are in source/extensions/config_subscription/.

Protocols

Term	Meaning
HTTP/1.1	Implemented by source/common/http/http1/ on top of http-parser.
HTTP/2	Implemented by source/common/http/http2/ on top of nghttp2 (legacy) and the oghttp2 codec.
HTTP/3	QUIC + HTTP/3 via google's QUICHE. Code in source/common/quic/ and source/extensions/quic/.
gRPC	Two client implementations: Envoy-native ("Envoy gRPC") and Google C++ core ("Google gRPC"). See source/common/grpc/.
QUIC	Datagram protocol underlying HTTP/3.

Process lifecycle

Term	Meaning
Hot restart	Live binary upgrade: a new Envoy starts, takes over listeners from the old one over a Unix domain socket, then the old one drains and exits. See source/server/hot_restart_impl.cc and the hot restart page.
Drain manager	Tracks the multi-stage drain sequence applied during shutdown or listener removal. source/server/drain_manager_impl.cc.
GuardDog	Watchdog thread that kills the process if a worker's event loop is stuck longer than miss/megamiss thresholds. source/server/guarddog_impl.cc.
Overload manager	Reads pressure from resource monitors and applies actions (e.g. shedding load, stopping accepting). source/server/overload_manager_impl.cc.
Init manager	Tracks initialization targets that must complete before a server, listener, or cluster can serve traffic. source/common/init/manager_impl.cc.

Observability

Term	Meaning
Stat sink	Pluggable destination for periodically-flushed stats (statsd, dog_statsd, metrics_service, hystrix, …).
Symbol table	Compact, deduped storage for stat names. source/common/stats/symbol_table.cc.
Tracer	Pluggable span exporter (Datadog, OpenTelemetry, Zipkin, SkyWalking, X-Ray, Fluentd).
Access log	Per-request log line emitted by listeners, HCM, TCP proxy, and most filters.
Stream info	Per-request metadata bag exposed to filters and access log formatters. envoy/stream_info/stream_info.h.

Build and project terms

Term	Meaning
Bootstrap	The root protobuf config. api/envoy/config/bootstrap/v3/bootstrap.proto.
Contrib	Extensions held to a looser maintenance bar; built into a separate envoy-contrib binary. See EXTENSION_POLICY.md.
Wasm	WebAssembly — runtime for sandboxed plugins; runtimes in source/extensions/wasm_runtime/.
Dynamic module	C ABI extension dlopen'd at runtime. See source/extensions/dynamic_modules/ and the dynamic modules feature page.
API alpha (v3alpha etc.)	New API version under design. See api/API_VERSIONING.md.
Runtime	Key/value flag store consulted at request-time for feature flags and gradual rollout. See source/common/runtime/runtime_impl.cc.
DCP	Data plane / control plane (in conversation, not in code).
OWNERS / CODEOWNERS	Contributor governance. OWNERS.md lists humans, CODEOWNERS maps directories to GitHub teams.

Related reading

The user-facing version of this glossary is at envoyproxy.io/docs/envoy/latest/intro/arch_overview/intro/terminology. It explains operator-facing concepts (rate limiting, circuit breakers, retries) that are out of scope for the source-tree wiki.