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Internals ​

Read from the source at commit 6a45c7d. Every claim here points at a file and line.

Code map ​

PathResponsibility
source/exe/Process entry point and startup. main.cc:16 defines main().
source/server/Server instance, listeners, worker management, lifecycle.
source/common/http/HTTP Connection Manager, filter manager, codecs.
source/common/thread_local/Thread-local slot machinery for lock-free config distribution.
source/common/buffer/The I/O buffer abstraction used across the stack.
source/extensions/Pluggable filters, codecs, transport sockets, access loggers.
api/Protobuf definitions for the xDS configuration APIs (API v3).

Core data structures ​

ConnectionManagerImpl is the network read filter that speaks HTTP; one downstream connection maps to one instance (source/common/http/conn_manager_impl.h). Its nested ActiveStream (source/common/http/conn_manager_impl.h:145) holds all state for a single request/response: request_headers_ (:457), the DownstreamFilterManager filter_manager_ (:473), and the resolved cached_route_ (:512). ActiveStream is a LinkedObject, so live streams form an intrusive list on the connection.

FilterManager (source/common/http/filter_manager.h:692) and its downstream subclass DownstreamFilterManager (source/common/http/filter_manager.h:1192) run the decoder/encoder filter chains, control iteration, and apply watermark backpressure.

Buffer::OwnedImpl (source/common/buffer/buffer_impl.h:643) is the I/O buffer. Internally it chains Slice objects (source/common/buffer/buffer_impl.h:37) in a SliceDeque (source/common/buffer/buffer_impl.h:426) so append and drain stay close to zero-copy.

ThreadLocal::InstanceImpl and its SlotImpl (source/common/thread_local/thread_local_impl.h:20, :37) distribute state across worker threads.

A path worth tracing ​

Follow a downstream HTTP request from bytes on the wire to a resolved route, all in source/common/http/conn_manager_impl.cc.

onData is the network read filter entry point. It creates the codec lazily, then dispatches bytes into it:

text
ConnectionManagerImpl::onData (:515)
  createCodec(data)            (:525)   // first call only
  codec_->dispatch(data)       (:546)   // parses frames, calls newStream
ConnectionManagerImpl::newStream (:410)
  make_unique<ActiveStream>    (:430)
  LinkedList::moveIntoList     (:469)   // track on streams_
ActiveStream::decodeHeaders   (:1354)
  request_headers_ = move(headers)      (:1366)
  refreshCachedRoute()                  (:1553) -> (:1811)
    snapped_route_config_->route(...)   (:1827)
  filter_manager_.createDownstreamFilterChain() (:1564)
  filter_manager_.decodeHeaders(...)    (:1600)

The codec return value is a Status that the manager inspects after dispatch to classify buffer flood, protocol error, and overload before continuing. Route resolution happens once and is cached in cached_route_, so later filters reuse the same decision.

Things that surprised me ​

The thread-local model is built around an explicit hazard. A slot is destroyed immediately on the main thread and its index is recycled, so a callback must never capture the slot object directly; the index is captured by hand instead. The code says so at source/common/thread_local/thread_local_impl.h:60-65. To stop a callback posted just before destruction from dereferencing freed state, the slot holds a std::shared_ptr<bool> still_alive_guard_ (source/common/thread_local/thread_local_impl.h:69) that workers weak-reference. This is the cost of choosing snapshot-swap over locks: concurrency is solved by single-thread ownership plus immutable snapshots, and the few remaining races are handled by hand.