Flash5/flash/docs/http2/DECISIONS.md

Flash HTTP/2 — Decision Log

This is the living record of every non-obvious choice made while implementing flash/docs/http2/IMPLEMENTATION-PLAN.md. It is not a changelog of what was built — the git history is that — it is a record of why, for choices that were not forced by the RFC and that a future reader would otherwise have to re-derive or, worse, silently re-litigate.

Every entry: Context / Options / Decision / Consequence / Revisit when.

Seeded at Phase 0 with DEC-01…DEC-10 (the decisions already implied by the plan itself, per Appendix A). Every subsequent non-obvious choice appends a new entry with the next free number. Numbers are never reused, even if a decision is later reversed — the reversal gets its own entry that supersedes the earlier one and says so explicitly.

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DEC-01 — HTTP/2 lives in flash core, package dev.relism.flash.h2, not an extension

Context. Flash has an extension mechanism (flash-ext-* modules) for optional functionality. HTTP/2 could in principle be shipped as flash-ext-h2.

Options.

1. Ship as an extension, loaded optionally.
2. Ship in flash core, alongside HTTP/1.1.

Decision. Core (option 2).

Consequence. The protocol decision (h1 vs h2) is made once, immediately after ALPN/preface detection, inside the transport layer. HttpServer (and its Phase 2 replacement) is package-private to flash core; an extension cannot hook into ALPN negotiation or the accept loop without core exposing seams it does not otherwise need. HTTP/2 is a transport concern in the same sense HTTP/1.1 is — it cannot be optional in the way, say, an OpenAPI generator is.

Revisit when. Never, absent a restructuring of the extension mechanism itself to support transport-level extensions (not currently planned).

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DEC-02 — h1 and h2 are peers behind a ConnectionProtocol seam, never flags in shared code

Context. The obvious shortcut is if (isHttp2) { ... } else { ... } scattered through the existing HTTP/1.1 code paths.

Options.

1. Flag-branch inside shared code.
2. A ConnectionProtocol interface with two implementations (Http1Connection, Http2Connection), selected once per connection.

Decision. Option 2 (R1).

Consequence. Shared code (byte scanning, the writer discipline, Request/Response) is extracted upward into protocol-neutral components (dev.relism.flash.bytes, ResponseSerializer), never pushed sideways with a protocol flag. This is enforced by an architecture test (Phase 2) asserting dev.relism.flash.http1 never references dev.relism.flash.h2 and vice versa. The cost is more up-front extraction work in Phase 2 and Phase 6; the benefit is that h1 throughput cannot regress from an if that the JIT fails to eliminate, and that either implementation can be read in isolation.

Revisit when. Never — this is a structural invariant, not a tunable.

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DEC-03 — ReentrantLock everywhere, never synchronized around blocking I/O

Context. Java 21 (this project's baseline) has virtual threads (JEP 444) but not JEP 491 (which removes synchronized carrier-pinning); JEP 491 lands in JDK 24. A virtual thread that blocks inside a synchronized block pins its carrier platform thread for the duration of the block, including any blocking I/O inside it.

Options.

1. Keep synchronized where it already exists (WebSocketSession, EX-01) and accept the pinning risk.
2. Replace every synchronized block that can block on I/O with java.util.concurrent.locks .ReentrantLock, which unmounts a blocked virtual thread instead of pinning its carrier.

Decision. Option 2, applied retroactively to the existing WebSocket code (Phase 2) and as a standing rule for every future connection-writer path, most importantly Http2FrameWriter (Phase 3).

Consequence. One virtual thread blocking on a slow write no longer starves the carrier pool for every other connection scheduled onto that carrier. The cost is that ReentrantLock is slightly more expensive than an uncontended synchronized monitor in theplatform-thread case — irrelevant here, since every request-serving thread in this codebase is virtual.

Revisit when. The project's Java baseline moves to JDK 24+ and JEP 491 is confirmed to remove pinning for synchronized. Even then, ReentrantLock's explicit tryLock() — which synchronized cannot offer — is load-bearing for Phase 3's writer design, so this decision would only partially reverse.

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DEC-04 — The HPACK encoder uses the static table only; no dynamic table

Context. RFC 7541's dynamic table is optional for an encoder (a decoder must always support the peer using one; nothing requires the encoder to use one itself). Using it on the encode side would save bytes on repeated headers (e.g. a constant server value) but requires mutable, connection-shared state: an insertion changes indices for every subsequent encode on that connection.

Options.

1. Encoder uses the dynamic table, saving bytes on repeated custom headers.
2. Encoder emits only Indexed (static) and Literal-Without-Indexing representations; no dynamic table, no mutable encoder state.

Decision. Option 2.

Consequence. The write path — already the project's largest architectural risk (Phase 3) — needs no shared-table lock and no invalidation protocol across concurrently-writing streams. The cost is a few extra bytes per response for headers that do not already have a static-table entry (i.e. everything except the ~30 header names RFC 7541 Appendix A knows about). The encoder still honours the peer's SETTINGS_HEADER_TABLE_SIZE by sending a Dynamic Table Size Update of 0 at the start of the first header block, declaring "I will never use this table" — a correctness detail, not optional politeness (Phase 9 task 1).

Revisit when. Benchmark evidence (Phase 17) shows the extra wire bytes materially hurt throughput or latency on a realistic workload — not before. A shared dynamic table is a non-trivial correctness surface (see DEC-06's discussion of the analogous decode-side hazard) and should only be taken on with a measured reason.

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DEC-05 — Huffman-encode constants at boot; emit runtime values as raw literals

Context. HPACK lets the encoder Huffman-code any string at its option. Constants (status lines, content-type values) are a closed, known set and can be Huffman-encoded once, at class initialization, for free at runtime. Runtime-generated values (a dynamic ETag, a user-set custom header) would need to be Huffman-encoded on every response.

Options.

1. Huffman-encode everything, including runtime values, on every write.
2. Huffman-encode only boot-time constants; emit runtime values as raw (uncompressed) literals.

Decision. Option 2, with FlashConfiguration.h2HuffmanDynamicValues (default false) so option 1's cost/benefit can actually be measured on real traffic rather than argued about in the abstract.

Consequence. The response write path's critical section has no per-byte Huffman encode loop for the common case. The cost is a few extra bytes on the wire for runtime header values, which HPACK's other mechanisms (indexing on the receive side, if the receiver chooses to use its dynamic table) can still partially recover.

Revisit when. Phase 17 benchmarks the flag both ways on a representative response shape.

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DEC-06 — Decoded headers are copied into a per-stream arena, not referenced in the dynamic table

Context. A ByteView into the HPACK dynamic table's arena is valid only while its entry is still live. Under HTTP/1.1 this is trivially safe (one thread, one request at a time). Under HTTP/2, the demux thread can decode a second stream's HEADERS — evicting and overwriting dynamic-table arena bytes — while a handler on a different virtual thread is still reading a view produced by an earlier decode. This is a genuine, silent data race: it does not manifest in any test that decodes one block at a time, only under real multiplexed load.

Options.

1. Reference dynamic-table entries directly from decoded ByteViews, and protect them with an epoch or reference-count scheme so an entry cannot be evicted while still referenced.
2. Copy every decoded header (name and value) into an arena owned by the stream being assembled, at decode time. One ~30-byte-average memcpy per header; correctness by construction, no cross-thread coordination.

Decision. Option 2.

Consequence. Header decode is not zero-copy relative to the dynamic table (R3's "honest naming" clause applies: HTTP/2 copies each novel header once per connection and references it by index thereafter — the per-stream arena copy is that one copy). In exchange, no handler can ever observe a torn or evicted header value, and the demux thread never needs to coordinate with a handler thread to decode the next block. Per-stream arenas are pooled (returned on stream close) so this is zero allocation at steady state despite the copy.

Revisit when. Profiling (Phase 17) shows the per-header copy is a measurable cost on a realistic HPACK-heavy workload. Even then, option 1's concurrent bookkeeping is a large correctness surface to take on to avoid a small memcpy, and should not be revisited casually.

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DEC-07 — :authority is exposed to user code as both :authority and host

Context. HTTP/2 requests carry authority information in the :authority pseudo-header (RFC 9113 §8.3.1), not a Host header — host may optionally also be present and, if so, must match :authority, but is not required. Existing Flash middleware (and most middleware in the wild) reads Host by convention, inherited from HTTP/1.1.

Options.

1. Expose only :authority, under whatever name the h2 header map uses for pseudo-headers. Middleware written against Host silently breaks on h2.
2. Expose :authority's value under both keys: the literal :authority and host.

Decision. Option 2.

Consequence. A single small duplication (one extra index entry into the same per-stream arena bytes — no extra copy) buys behavioural parity for existing and future middleware that reads Host, without requiring every middleware author to special-case h2. Documented in flash/docs/http2/STREAMS.md.

Revisit when. Not planned to be revisited; this is a compatibility shim with negligible cost, not a design compromise under pressure.

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DEC-08 — Flash ships HTTP/2, not a gRPC codec

Context. gRPC is one of the strongest motivations for HTTP/2 support (Pathway's upstream use case), and it is tempting to let that motivation expand scope into shipping gRPC framing, proto codecs, or a service-definition layer.

Options.

1. Ship a gRPC codec/framework alongside HTTP/2 transport support.
2. Ship HTTP/2 transport only; validate gRPC compatibility with an interop test, not a feature.

Decision. Option 2.

Consequence. Phase 12's GrpcInteropTest proves that the protocol features gRPC actually needs — trailers, content-type: application/grpc, te: trailers, half-close, streaming — are present and correct, using a real gRPC client against a hand-written Flash handler that speaks the wire format directly. Flash does not gain a dependency on any gRPC/protobuf library, and users who want a gRPC service framework build it on top of Flash rather than being handed one.

Revisit when. Not planned to be revisited; this is a scope boundary, not a temporary limitation.

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DEC-09 — The chosen Http2FrameWriter design, with its benchmark numbers

Status. Not yet decided — this entry is a placeholder until Phase 3 runs its gate. Phase 3 benchmarks three candidate writer designs ((a) plain ReentrantLock.lock() per frame, (b) tryLock() + intrusive MPSC, (c) a dedicated writer virtual thread fed by an MPSC queue) against the numeric gate criteria in the plan (0 B/op and <50 ns overhead at N=1; ≥60% of the N=1 per-thread aggregate throughput and <1 ms p999 at N=64; no carrier pinning). This entry is filled in with the winning design and the raw numbers when Phase 3 completes, or with the failure and the redesign taken if no candidate meets the gate.

Revisit when. N/A until Phase 3 lands.

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DEC-10 — Upgrade: h2c is deliberately not implemented

Context. RFC 7540 §3.2 (the original HTTP/2 RFC) defined an Upgrade: h2c mechanism to move a plaintext HTTP/1.1 connection to HTTP/2 mid-connection. RFC 9113 (which obsoletes RFC 7540) §3.1 removes this mechanism entirely from the current specification.

Options.

1. Implement Upgrade: h2c for compatibility with any client that still relies on it.
2. Do not implement it; support cleartext HTTP/2 only via prior knowledge (RFC 9113 §3.4).

Decision. Option 2.

Consequence. Every h2c client that matters for Flash's use case (gRPC, and every modern h2c implementation) uses prior knowledge, not the upgrade dance, so nothing is lost in practice. Recorded explicitly so a future contributor who notices Upgrade: h2c is unhandled does not assume it was an oversight and add it back.

Revisit when. A concrete client that requires Upgrade: h2c and cannot be changed is identified. Not anticipated.

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DEC-11 — Commit scope stays core; h2 is not added to AGENTS.md's allowed-scope list

Context. AGENTS.md (§Commit Messages) enumerates the allowed Conventional Commits scopes. h2 is not among them. R9 leaves the choice open: either add h2 as a new scope via a docs: commit, or use core and record the decision here.

Options.

1. Add h2 as a new allowed scope, so h2-specific commits are distinguishable in history from other core work at a glance.
2. Use the existing core scope for all HTTP/2 work.

Decision. Option 2.

Consequence. All HTTP/2 commits use feat(core): ... / fix(core): ... / refactor(core): ..., consistent with the branch name (feature/core/http2) and with DEC-01 (HTTP/2 is core, not a separate concern). A reader can still find every h2-related commit via the file paths touched (dev.relism.flash.h2/**, flash/docs/http2/**) or via the commit body, which is no worse than a scope label and avoids growing the scope list for what is, by DEC-01, not actually a separate module.

Revisit when. The h2 package's commit volume makes core too coarse to navigate in git log — not expected before Phase 10 at the earliest, if ever.