libhttpserver/docs/architecture/request-flow.html at master · etr/libhttpserver · GitHub

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<!doctype html>
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<title>libhttpserver v2 — request lifecycle &amp; routing</title>
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<div class="wrap">
  <header class="top">
    <p class="kicker">libhttpserver v2.0 · request lifecycle</p>
    <h1>How a request flows — from the <span class="mono">MHD</span> callback to the wire</h1>
    <p class="lede">One HTTP request is not one function call. libmicrohttpd drives the exchange through a fixed sequence of <b>C-ABI callbacks</b> (the purple <span class="mono">webserver_impl</span> trampolines), each forwarding into a <b>behavior service</b>. This traces that journey end-to-end: the callback spine, the <b>body-accumulation loop</b>, the <b>WebSocket</b> branch, the four-tier <b>route resolution</b>, and where each of the <b>11 hook phases</b> fires. Companion to the <span class="mono">class &amp; filesystem map</span>; same colour language.</p>

    <div class="glance" aria-label="entry points at a glance">
      <div class="gcard"><span class="gk">accept</span><span class="gv">policy_callback</span><span class="gs">IP ACL · per connection</span></div>
      <div class="garrow">→</div>
      <div class="gcard"><span class="gk">allocate</span><span class="gv">connection_notify · uri_log</span><span class="gs">arena + con_cls</span></div>
      <div class="garrow">→</div>
      <div class="gcard"><span class="gk">drive</span><span class="gv">answer_to_connection</span><span class="gs">called 1..N times</span></div>
      <div class="garrow">→</div>
      <div class="gcard"><span class="gk">finish</span><span class="gv">request_completed</span><span class="gs">fire + free</span></div>
    </div>

    <div class="legend" aria-label="legend">
      <span class="lg"><span class="sw" style="background:var(--adapter)"></span>MHD C-ABI trampoline</span>
      <span class="lg"><span class="sw" style="background:var(--behavior)"></span>behavior service</span>
      <span class="lg"><span class="sw" style="background:var(--state)"></span>state collaborator</span>
      <span class="lg"><span class="sw" style="background:var(--domain)"></span>domain / value object</span>
      <span class="sep" aria-hidden="true"></span>
      <span class="lg"><span class="pill" style="color:var(--hook);border-color:color-mix(in srgb,var(--hook) 40%,transparent);background:color-mix(in srgb,var(--hook) 12%,transparent)"></span>hook phase fires</span>
      <span class="lg"><span class="pill" style="border-style:dashed;color:var(--flag);border-color:color-mix(in srgb,var(--flag) 45%,transparent);background:color-mix(in srgb,var(--flag) 10%,transparent)"></span>branch / short-circuit</span>
    </div>
  </header>

  <!-- ============ 1. LIFECYCLE SPINE ============ -->
  <section>
    <div class="sec-head"><span class="sec-num">01</span><h2>The callback spine — a normal request, top to bottom</h2></div>
    <p class="sec-sub">Each band is one libmicrohttpd callback (all <code>static</code> members of <code>webserver_impl</code>, defined in <code>webserver_callbacks.cpp</code>). The trampoline marshals the C ABI and delegates to a service. A <code>GET</code> runs straight down; <code>◈ hook</code> pills mark where a phase fires; dashed chips mark branch points. The shared blackboard threaded through every band is <code>connection_context</code> (<code>*con_cls</code>).</p>

    <!-- BAND: policy_callback -->
    <div class="band" style="--accent:var(--adapter)">
      <div class="band-head"><span class="band-cb">policy_callback</span><span class="band-tag">MHD adapter</span><span class="band-when">once · per TCP connection · before accept</span></div>
      <p class="band-note">Only runs when <code>ip_access_control_enabled</code>. Decides whether to accept the peer at all.</p>
      <div class="steps">
        <div class="step"><span class="sn">1</span><div class="sbody">
          <div class="sline"><span class="actor a-state">ip_access_control::classify</span><span class="txt">build <code>ip_representation(addr)</code> → <code>membership{denied, allowed}</code> under shared locks</span></div></div></div>
        <div class="step"><span class="sn">2</span><div class="sbody">
          <div class="sline"><span class="actor a-mhd">classify_decision</span><span class="txt">apply <code>default_policy</code> → <code>(accepted, reason)</code></span></div>
          <div class="sline"><span class="branch"><b>DENY</b> → return <code>MHD_NO</code>, connection refused</span></div></div></div>
        <div class="step"><span class="sn">3</span><div class="sbody">
          <div class="sline"><span class="hook">accept_decision</span><span class="txt">fired via <code>hooks_dispatch_.fire_accept_decision</code> → return <code>MHD_YES</code></span></div></div></div>
      </div>
    </div>
    <div class="flow-arrow">▼</div>

    <!-- BAND: connection_notify STARTED -->
    <div class="band" style="--accent:var(--adapter)">
      <div class="band-head"><span class="band-cb">connection_notify</span><span class="band-tag">MHD adapter</span><span class="band-when">STARTED · per connection</span></div>
      <div class="steps">
        <div class="step"><span class="sn">4</span><div class="sbody">
          <div class="sline"><span class="actor a-dom">new connection_state</span><span class="txt">per-connection PMR arena (8 KB inline buffer) → stored in MHD <code>socket_context</code>; copies <code>max_args_count/bytes</code></span></div></div></div>
        <div class="step"><span class="sn">5</span><div class="sbody">
          <div class="sline"><span class="hook">connection_opened</span><span class="txt"><code>fire_connection_opened(ctx)</code></span></div></div></div>
      </div>
    </div>
    <div class="flow-arrow">▼</div>

    <!-- BAND: uri_log -->
    <div class="band" style="--accent:var(--adapter)">
      <div class="band-head"><span class="band-cb">uri_log</span><span class="band-tag">MHD adapter</span><span class="band-when">once · per request</span></div>
      <div class="steps">
        <div class="step"><span class="sn">6</span><div class="sbody">
          <div class="sline"><span class="actor a-dom">make_unique&lt;connection_context&gt;</span><span class="txt">stashes <code>complete_uri</code>; the pointer becomes MHD's <code>*con_cls</code>, handed back to every later callback for this request</span></div></div></div>
      </div>
    </div>
    <div class="flow-arrow">▼</div>

    <!-- BAND: answer_to_connection FIRST -->
    <div class="band" style="--accent:var(--adapter)">
      <div class="band-head"><span class="band-cb">answer_to_connection</span><span class="band-tag">MHD adapter → request_pipeline</span><span class="band-when">first call · <code style="font-family:var(--mono)">request == nullptr</code></span></div>
      <p class="band-note">First invocation for the request: parse the head, resolve the verb, build the <code>http_request</code>.</p>
      <div class="steps">
        <div class="step"><span class="sn">7</span><div class="sbody">
          <div class="sline"><span class="actor a-mhd">answer_to_connection</span><span class="txt">stamp <code>start_time</code>, <code>ws=parent</code>; <code>base_unescaper</code>; <b>canonicalize</b>: <code>standardized_url = normalize_path(standardize_url(url))</code> — the single security-critical dot-segment collapse</span></div></div></div>
        <div class="step"><span class="sn">8</span><div class="sbody">
          <div class="sline"><span class="actor a-mhd">resolve_method_callback</span><span class="txt">verb → <code>callback = &amp;http_resource::render_get</code>, <code>method_enum</code>, sets <code>has_body</code></span></div>
          <div class="sline"><span class="branch">unknown verb → <code>callback=nullptr</code>, <b>405</b> path</span></div></div></div>
        <div class="step"><span class="sn">9</span><div class="sbody">
          <div class="sline"><span class="actor a-beh">request_pipeline::requests_answer_first_step</span><span class="txt">build <code>http_request</code> in the connection arena (<code>pick_resource</code>)</span></div></div></div>
        <div class="step"><span class="sn">10</span><div class="sbody">
          <div class="sline"><span class="hook sc">request_received</span><span class="txt"><code>hooks_.fire_request_received</code></span><span class="branch"><b>respond_with</b> → set <code>response</code>, <code>skip_handler=true</code></span></div></div></div>
        <div class="step"><span class="sn">11</span><div class="sbody">
          <div class="sline"><span class="branch"><b>has_body?</b></span><span class="txt"><b>GET / no body</b> → return <code>MHD_YES</code>, no post-processor. <b>POST / PUT</b> → set <code>content_size_limit</code>, create <code>MHD_PostProcessor</code> for form types → enters the loop below</span></div></div></div>
      </div>

      <!-- INSET: body accumulation loop -->
      <div class="inset">
        <span class="inset-tag">⟳ body-accumulation loop · POST / PUT only</span>
        <p class="band-note" style="margin-top:6px">MHD calls <code>answer_to_connection</code> <b>repeatedly</b>, each with a non-empty chunk, routing to <code>request_pipeline::requests_answer_second_step</code>. Loops until MHD delivers a zero-size chunk.</p>
        <div class="steps">
          <div class="step"><span class="sn">a</span><div class="sbody">
            <div class="sline"><span class="hook sc">body_chunk</span><span class="txt">fired <b>per chunk</b> (<code>body_bytes_seen</code> offset)</span><span class="branch">respond_with → <code>skip_handler</code>, drain</span></div></div></div>
          <div class="step"><span class="sn">b</span><div class="sbody">
            <div class="sline"><span class="actor a-beh">request_pipeline</span><span class="txt">accumulate: <code>grow_content</code> (raw / <code>put_processed_data_to_content</code>) <b>or</b> <code>MHD_post_process</code> → drives <code>post_iterator</code></span></div></div></div>
          <div class="step"><span class="sn">c</span><div class="sbody">
            <div class="sline"><span class="actor a-mhd">post_iterator</span><span class="txt">per form field →</span><span class="actor a-beh">upload_pipeline::iterate_file</span><span class="txt">memory → <code>set_arg_flat</code>; disk → <code>manage_upload_stream</code> + <code>write</code> into <code>file_info</code></span></div></div></div>
        </div>
        <p class="loopnote">Each iteration ends with <code>*upload_data_size = 0</code> to signal the chunk consumed. <b>No handler runs yet.</b></p>
      </div>
    </div>
    <div class="flow-arrow">▼</div>

    <!-- BAND: answer_to_connection TERMINAL -->
    <div class="band" style="--accent:var(--adapter)">
      <div class="band-head"><span class="band-cb">answer_to_connection</span><span class="band-tag">MHD adapter → request_pipeline</span><span class="band-when">terminal call · <code style="font-family:var(--mono)">*upload_data_size == 0</code></span></div>
      <div class="steps">
        <div class="step"><span class="sn">12</span><div class="sbody">
          <div class="sline"><span class="actor a-beh">requests_answer_second_step → complete_request</span><span class="txt"><code>set_path(standardized_url)</code>, <code>set_method</code>, <code>set_version</code> → hand off to the dispatcher</span></div></div></div>
      </div>
    </div>
    <div class="flow-arrow">▼</div>

    <!-- BAND: finalize_answer -->
    <div class="band" style="--accent:var(--behavior)">
      <div class="band-head"><span class="band-cb">request_dispatcher::finalize_answer</span><span class="band-tag">behavior service</span><span class="band-when">route · auth · dispatch</span></div>
      <p class="band-note">The heart of the exchange: try to upgrade, resolve the route, run the gate hooks, invoke the handler, catch what it throws.</p>
      <div class="steps">
        <div class="step"><span class="sn">13</span><div class="sbody">
          <div class="sline"><span class="actor a-beh">try_ws_upgrade</span><span class="txt">→ <code>websocket_upgrader::try_handle</code></span><span class="branch"><b>Upgrade: websocket</b> → 101, bypass everything below ↘ (see §3)</span></div></div></div>
        <div class="step"><span class="sn">14</span><div class="sbody">
          <div class="sline"><span class="branch"><b>skip_handler?</b> → jump straight to materialize (step 20)</span></div></div></div>
        <div class="step"><span class="sn">15</span><div class="sbody">
          <div class="sline"><span class="actor a-beh">resolve_resource_for_request</span><span class="txt">→ <code>route_table::lookup_v2</code> (see §2); replay <code>captured_params</code> via <code>set_arg</code></span></div>
          <div class="sline"><span class="branch"><b>not found</b> → <code>errors_.not_found_page</code> · <b>404</b></span></div></div></div>
        <div class="step"><span class="sn">16</span><div class="sbody">
          <div class="sline"><span class="hook">route_resolved</span><span class="txt">gated; builds <code>route_descriptor{template, methods, is_prefix}</code></span></div></div></div>
        <div class="step"><span class="sn">17</span><div class="sbody">
          <div class="sline"><span class="hook sc">before_handler</span><span class="scope">server + <span class="both">per-route</span></span><span class="txt"><b>auth &amp; 405-alias hooks run here</b></span><span class="branch">respond_with → skip handler</span></div></div></div>
        <div class="step"><span class="sn">18</span><div class="sbody">
          <div class="sline"><span class="actor a-beh">dispatch_resource_handler</span><span class="txt"><code>is_allowed(method_enum)</code> →</span><span class="branch">no → <b>405</b> + <code>Allow:</code></span></div>
          <div class="sline"><span class="txt"><code>(*hrm.*callback)(*request)</code> — <b>pointer-to-member dispatch</b> → your <code>render_get(...)</code> (or <code>lambda_resource::invoke_</code>)</span></div>
          <div class="sline"><span class="hook sc">handler_exception</span><span class="scope">server + <span class="both">per-route</span></span><span class="branch">threw &amp; unhandled → <code>internal_error_page</code> · <b>500</b></span></div></div></div>
        <div class="step"><span class="sn">19</span><div class="sbody">
          <div class="sline"><span class="hook sc">after_handler</span><span class="scope">server + <span class="both">per-route</span></span><span class="txt">a hook may <b>replace</b> the response</span></div></div></div>
      </div>
    </div>
    <div class="flow-arrow">▼</div>

    <!-- BAND: materialize -->
    <div class="band" style="--accent:var(--behavior)">
      <div class="band-head"><span class="band-cb">response_materializer::materialize_and_queue_response</span><span class="band-tag">behavior service</span><span class="band-when">http_response → the wire</span></div>
      <div class="steps">
        <div class="step"><span class="sn">20</span><div class="sbody">
          <div class="sline"><span class="actor a-beh">materialize_response</span><span class="txt"><code>response_body::materialize()</code> — polymorphic per <code>body_kind</code> → <code>MHD_Response*</code> (empty · string · file · iovec · pipe · deferred · digest); null/throw → 404/500 fallback</span></div></div></div>
        <div class="step"><span class="sn">21</span><div class="sbody">
          <div class="sline"><span class="actor a-beh">decorate_mhd_response</span><span class="txt">walk headers / footers / parsed cookies → <code>MHD_add_response_header</code></span></div></div></div>
        <div class="step"><span class="sn">22</span><div class="sbody">
          <div class="sline"><span class="actor a-beh" style="--role:var(--send)">queue_response_dispatching_kind</span><span class="txt"><b style="color:var(--send)">◀ THE SEND</b> — <code>MHD_queue_response</code> (or <code>MHD_queue_auth_required_response3</code> for a digest challenge)</span></div></div></div>
        <div class="step"><span class="sn">23</span><div class="sbody">
          <div class="sline"><span class="hook">response_sent</span><span class="scope">server + <span class="both">per-route</span> + log_access alias</span><span class="txt">after queue, before destroy; computes <code>bytes_queued</code> + <code>elapsed</code></span></div></div></div>
        <div class="step"><span class="sn">24</span><div class="sbody">
          <div class="sline"><span class="actor a-mhd">MHD_destroy_response</span><span class="txt">releases the caller ref; MHD keeps streaming bodies alive</span></div></div></div>
      </div>
    </div>
    <div class="flow-arrow">▼</div>

    <!-- BAND: request_completed -->
    <div class="band" style="--accent:var(--adapter)">
      <div class="band-head"><span class="band-cb">request_completed</span><span class="band-tag">MHD adapter</span><span class="band-when">MHD completion callback</span></div>
      <div class="steps">
        <div class="step"><span class="sn">25</span><div class="sbody">
          <div class="sline"><span class="hook">request_completed</span><span class="scope">server + <span class="both">per-route</span> via <code>resource_weak_</code></span></div></div></div>
        <div class="step"><span class="sn">26</span><div class="sbody">
          <div class="sline"><span class="actor a-dom">delete connection_context</span><span class="txt">runs <code>~http_request</code> + arena destructors</span></div></div></div>
        <div class="step"><span class="sn">27</span><div class="sbody">
          <div class="sline"><span class="actor a-dom">connection_state::reset_arena</span><span class="txt">release + <code>secure_zero</code> for keep-alive reuse</span></div></div></div>
      </div>
    </div>
    <div class="flow-arrow">▼</div>

    <!-- BAND: connection_notify CLOSED -->
    <div class="band" style="--accent:var(--adapter)">
      <div class="band-head"><span class="band-cb">connection_notify</span><span class="band-tag">MHD adapter</span><span class="band-when">CLOSED · per connection</span></div>
      <div class="steps">
        <div class="step"><span class="sn">28</span><div class="sbody">
          <div class="sline"><span class="hook">connection_closed</span><span class="txt">then <code>delete connection_state</code></span></div></div></div>
      </div>
    </div>
  </section>

  <!-- ============ 2. ROUTE RESOLUTION ============ -->
  <section>
    <div class="sec-head"><span class="sec-num">02</span><h2>Route resolution — <span class="mono" style="font-size:16px">route_table::lookup_v2</span></h2></div>
    <p class="sec-sub">Step 15 above. Four tiers, cheapest first; the first hit wins and returns a <code>lookup_result{found, tier, entry, captured_params}</code>. Lookup keys off the raw <code>standardized_url</code> string — <code>http_endpoint</code> parsing is a <b>registration-time</b> concern, not per-request. The entry is returned <b>regardless of method</b> so the 405 path still sees it.</p>
    <div class="cascade">
      <div class="tier"><div class="tlabel"><span>canon</span><span class="tn">pre</span></div>
        <div class="tbox"><div class="tt">canonicalize_lookup_path</div><div class="td">ensure leading <code>/</code>, strip trailing <code>/</code>; zero-alloc when already canonical</div></div></div>
      <div class="tier"><div class="tlabel"><span>Tier 1</span><span class="tn">exact</span></div>
        <div class="tbox"><div class="tt">exact_routes_ <span class="hit">hit → bypass cache</span></div><div class="td"><code>std::map</code> transparent <code>find</code> under <code>shared_lock</code> — the hottest path, deliberately skips the LRU</div></div></div>
      <div class="miss"></div>
      <div class="tier"><div class="tlabel"><span>Tier 2</span><span class="tn">cache</span></div>
        <div class="tbox"><div class="tt">route_lru_cache.find_by_view <span class="hit">hit → promote</span></div><div class="td">LRU over <b>parameter / regex</b> results only; <code>{method, path}</code> key, moves out <code>entry</code> + <code>captured_params</code></div></div></div>
      <div class="miss"></div>
      <div class="tier"><div class="tlabel"><span>Tier 3</span><span class="tn">radix</span></div>
        <div class="tbox"><div class="tt">param_and_prefix_routes_ · segment_trie <span class="hit">hit → radix</span></div><div class="td">walk path segments: literal children, then <code>{name}</code> wildcards (captured), applying any <code>{id|regex}</code> constraint; records best prefix terminus for <code>register_prefix</code></div></div></div>
      <div class="miss"></div>
      <div class="tier"><div class="tlabel"><span>Tier 4</span><span class="tn">regex</span></div>
        <div class="tbox"><div class="tt">regex_routes_ · linear scan <span class="hit">hit → regex</span></div><div class="td"><code>std::regex_match</code> over pre-compiled patterns; last resort</div></div></div>
      <div class="tier"><div class="tlabel" style="color:var(--behavior)"><span>install</span><span class="tn">post</span></div>
        <div class="tbox" style="border-left-color:var(--behavior)"><div class="tt" style="color:var(--behavior)">route_lru_cache.insert</div><div class="td">on any param/regex hit, memoize <code>{entry, captured_params}</code> for next time</div></div></div>
    </div>
    <p class="sec-sub" style="margin-top:20px"><b style="color:var(--text)">Method → handler.</b> <code>lookup_v2</code> never filters by method. The verb was already turned into a <b>pointer-to-member</b> (<code>render_get</code>/<code>render_post</code>/…) back in <code>resolve_method_callback</code> (step 8); <code>dispatch_resource_handler</code> then checks <code>http_resource::is_allowed(method_enum)</code> — mismatch yields <b>405</b> with the resource's <code>Allow:</code> header.</p>
  </section>

  <!-- ============ 3. WEBSOCKET BRANCH ============ -->
  <section>
    <div class="sec-head"><span class="sec-num">03</span><h2>The WebSocket branch — diverges at step 13</h2></div>
    <p class="sec-sub">Before any routing or handler dispatch, <code>finalize_answer</code> offers the request to the upgrader. A genuine upgrade queues <b>101 Switching Protocols</b> and <b>bypasses routing, all handler hooks, and the response materializer entirely</b>.</p>
    <div class="inset ws-inset" style="margin-left:0">
      <span class="inset-tag">↘ websocket_upgrader::try_handle → std::optional&lt;MHD_Result&gt;</span>
      <div class="steps" style="margin-top:8px">
        <div class="step"><span class="sn">1</span><div class="sbody">
          <div class="sline"><span class="actor a-beh">try_handle</span><span class="txt">no <code>Upgrade: websocket</code> header → <code>std::nullopt</code> → normal HTTP flow resumes</span></div></div></div>
        <div class="step"><span class="sn">2</span><div class="sbody">
          <div class="sline"><span class="actor a-beh">validate_websocket_handshake</span><span class="txt">check <code>Connection: Upgrade</code>, <code>Sec-WebSocket-Version: 13</code>, non-empty key</span><span class="branch">invalid → <b>400</b></span></div></div></div>
        <div class="step"><span class="sn">3</span><div class="sbody">
          <div class="sline"><span class="actor a-state">ws_registry::find</span><span class="txt"><code>shared_ptr&lt;websocket_handler&gt;</code> for the URL</span><span class="branch">none → fall back to normal HTTP</span></div></div></div>
        <div class="step"><span class="sn">4</span><div class="sbody">
          <div class="sline"><span class="actor a-beh">complete_websocket_upgrade</span><span class="txt"><code>Sec-WebSocket-Accept</code> + <code>MHD_create_response_for_upgrade</code> → <b>101</b> via <code>MHD_queue_response</code></span></div></div></div>
        <div class="step"><span class="sn">5</span><div class="sbody">
          <div class="sline"><span class="actor a-beh">upgrade_handler</span><span class="txt">on the raw socket: <code>on_open</code>, then a <code>recv()</code> loop → decode frames → <code>on_message</code> / <code>on_binary</code> / <code>on_ping</code> / <code>on_close</code></span></div></div></div>
      </div>
    </div>
  </section>

  <!-- ============ 4. HOOK RAIL ============ -->
  <section>
    <div class="sec-head"><span class="sec-num">04</span><h2>The 11 hook phases, in firing order</h2></div>
    <p class="sec-sub">Server-wide hooks live on <code>hook_bus</code>; five phases are also <b>per-resource</b> (<code>resource_hook_table</code>, registered via <code>http_resource::add_hook</code>). <code>hook_dispatcher</code> fires both chains. Every phase is guarded by a relaxed-atomic <code>has_hooks_for</code> check — <b>zero cost when unused</b>. Short-circuit phases can return <code>respond_with(...)</code> to end the request early.</p>
    <div class="railwrap">
      <table class="rail">
        <thead><tr><th>Phase</th><th>Fires in</th><th>Scope</th><th>Kind</th></tr></thead>
        <tbody>
          <tr><td class="ph"><span class="idx">1</span>connection_opened</td><td class="where"><code>connection_notify</code> STARTED</td><td class="scope">server</td><td><span class="kind-tag kind-ob">observe</span></td></tr>
          <tr><td class="ph"><span class="idx">2</span>accept_decision</td><td class="where"><code>policy_callback</code>, after ACL classify</td><td class="scope">server</td><td><span class="kind-tag kind-ob">observe</span></td></tr>
          <tr><td class="ph"><span class="idx">3</span>request_received</td><td class="where"><code>requests_answer_first_step</code></td><td class="scope">server</td><td><span class="kind-tag kind-sc">short-circuit</span></td></tr>
          <tr><td class="ph"><span class="idx">4</span>body_chunk</td><td class="where"><code>requests_answer_second_step</code> · <b>per chunk</b></td><td class="scope">server</td><td><span class="kind-tag kind-sc">short-circuit</span></td></tr>
          <tr><td class="ph"><span class="idx">5</span>route_resolved</td><td class="where"><code>finalize_answer</code>, after lookup</td><td class="scope">server</td><td><span class="kind-tag kind-ob">observe</span></td></tr>
          <tr><td class="ph"><span class="idx">6</span>before_handler</td><td class="where"><code>finalize_answer</code>, pre-dispatch · <b>auth</b></td><td class="scope"><span class="both">server + route</span></td><td><span class="kind-tag kind-sc">short-circuit</span></td></tr>
          <tr><td class="ph"><span class="idx">7</span>handler_exception</td><td class="where"><code>dispatch_resource_handler</code> catch arms</td><td class="scope"><span class="both">server + route</span></td><td><span class="kind-tag kind-sc">short-circuit</span></td></tr>
          <tr><td class="ph"><span class="idx">8</span>after_handler</td><td class="where"><code>finalize_answer</code>, post-handler</td><td class="scope"><span class="both">server + route</span></td><td><span class="kind-tag kind-sc">replace resp</span></td></tr>
          <tr><td class="ph"><span class="idx">9</span>response_sent</td><td class="where"><code>materialize_and_queue</code>, after queue</td><td class="scope"><span class="both">server + route</span> · log_access alias</td><td><span class="kind-tag kind-ob">observe</span></td></tr>
          <tr><td class="ph"><span class="idx">10</span>request_completed</td><td class="where"><code>request_completed</code> callback</td><td class="scope"><span class="both">server + route</span></td><td><span class="kind-tag kind-ob">observe</span></td></tr>
          <tr><td class="ph"><span class="idx">11</span>connection_closed</td><td class="where"><code>connection_notify</code> CLOSED</td><td class="scope">server</td><td><span class="kind-tag kind-ob">observe</span></td></tr>
        </tbody>
      </table>
    </div>
  </section>

  <footer>
    <p class="note"><strong>Reading the flow.</strong> Purple bands are the fixed libmicrohttpd callback sequence — <code>webserver_impl</code>'s static C-ABI trampolines, the only code touching MHD's ABI. Everything real happens in the teal <strong>behavior services</strong> they delegate to. The three branch axes that reshape the path: <code>has_body</code> (whether the body loop runs), <code>skip_handler</code> (a request_received/body_chunk short-circuit that skips routing straight to materialize), and the <strong>WebSocket upgrade</strong> (which bypasses the HTTP tail entirely). One request drives <code>answer_to_connection</code> <strong>1..N times</strong> — once for a bodyless GET, many times while a POST body streams in — but resolves to exactly one <code>finalize_answer</code>. Companion to the class &amp; filesystem map; both share the state / behavior / adapter colour language.</p>
  </footer>
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