nghttpx: Limit # of downstream connections per host when h2 proxy is used

This commit limits the number of concurrent HTTP/1 downstream
connections to same host.  By defualt, it is limited to 8 connections.
--backend-connections-per-frontend option was replaced with
--backend-http1-connections-per-host, which changes the maximum number
of connections per host.  This limitation only kicks in when h2 proxy
is used (-s option).

src/shrpx.cc

@@ -776,7 +776,7 @@ void fill_default_config() {
   mod_config()->no_location_rewrite = false;
   mod_config()->argc = 0;
   mod_config()->argv = nullptr;
-  mod_config()->max_downstream_connections = 100;
+  mod_config()->downstream_connections_per_host = 8;
   mod_config()->listener_disable_timeout = {0, 0};
 }
 } // namespace
@@ -899,13 +899,12 @@ Performance:
                      Set  maximum number  of simultaneous  connections
                      frontend accepts.  Setting 0 means unlimited.
                      Default: 0
-  --backend-connections-per-frontend=<NUM>
+  --backend-http1-connections-per-host=<NUM>
-                     Set  maximum   number  of   backend  simultaneous
+                     Set maximum  number of backend  concurrent HTTP/1
-                     connections   per  frontend.    This  option   is
+                     connections per host.   This option is meaningful
-                     meaningful when the combination of HTTP/2 or SPDY
+                     when -s option is used.
-                     frontend and HTTP/1 backend is used.
+                     Default: )"
-                     Default: )" << get_config()->max_downstream_connections
+      << get_config()->downstream_connections_per_host << R"(
-      << R"(
 
 Timeout:
   --frontend-http2-read-timeout=<SEC>
@@ -1276,7 +1275,7 @@ int main(int argc, char **argv) {
         {"stream-read-timeout", required_argument, &flag, 60},
         {"stream-write-timeout", required_argument, &flag, 61},
         {"no-location-rewrite", no_argument, &flag, 62},
-        {"backend-connections-per-frontend", required_argument, &flag, 63},
+        {"backend-http1-connections-per-host", required_argument, &flag, 63},
         {"listener-disable-timeout", required_argument, &flag, 64},
         {"strip-incoming-x-forwarded-for", no_argument, &flag, 65},
         {"accesslog-format", required_argument, &flag, 66},
@@ -1568,8 +1567,8 @@ int main(int argc, char **argv) {
         cmdcfgs.emplace_back(SHRPX_OPT_NO_LOCATION_REWRITE, "yes");
         break;
       case 63:
-        // --backend-connections-per-frontend
+        // --backend-http1-connections-per-host
-        cmdcfgs.emplace_back(SHRPX_OPT_BACKEND_CONNECTIONS_PER_FRONTEND,
+        cmdcfgs.emplace_back(SHRPX_OPT_BACKEND_HTTP1_CONNECTIONS_PER_HOST,
                              optarg);
         break;
       case 64:

src/shrpx_config.cc

@@ -132,8 +132,8 @@ const char SHRPX_OPT_ADD_RESPONSE_HEADER[] = "add-response-header";
 const char SHRPX_OPT_WORKER_FRONTEND_CONNECTIONS[] =
     "worker-frontend-connections";
 const char SHRPX_OPT_NO_LOCATION_REWRITE[] = "no-location-rewrite";
-const char SHRPX_OPT_BACKEND_CONNECTIONS_PER_FRONTEND[] =
+const char SHRPX_OPT_BACKEND_HTTP1_CONNECTIONS_PER_HOST[] =
-    "backend-connections-per-frontend";
+    "backend-http1-connections-per-host";
 const char SHRPX_OPT_LISTENER_DISABLE_TIMEOUT[] = "listener-disable-timeout";
 
 namespace {
@@ -1006,7 +1006,7 @@ int parse_config(const char *opt, const char *optarg) {
     return 0;
   }
 
-  if (util::strieq(opt, SHRPX_OPT_BACKEND_CONNECTIONS_PER_FRONTEND)) {
+  if (util::strieq(opt, SHRPX_OPT_BACKEND_HTTP1_CONNECTIONS_PER_HOST)) {
     int n;
 
     if (parse_uint(&n, opt, optarg) != 0) {
@@ -1019,7 +1019,7 @@ int parse_config(const char *opt, const char *optarg) {
       return -1;
     }
 
-    mod_config()->max_downstream_connections = n;
+    mod_config()->downstream_connections_per_host = n;
 
     return 0;
   }

src/shrpx_config.h

@@ -123,7 +123,7 @@ extern const char SHRPX_OPT_ALTSVC[];
 extern const char SHRPX_OPT_ADD_RESPONSE_HEADER[];
 extern const char SHRPX_OPT_WORKER_FRONTEND_CONNECTIONS[];
 extern const char SHRPX_OPT_NO_LOCATION_REWRITE[];
-extern const char SHRPX_OPT_BACKEND_CONNECTIONS_PER_FRONTEND[];
+extern const char SHRPX_OPT_BACKEND_HTTP1_CONNECTIONS_PER_HOST[];
 extern const char SHRPX_OPT_LISTENER_DISABLE_TIMEOUT[];
 
 union sockaddr_union {
@@ -222,7 +222,7 @@ struct Config {
   size_t http2_downstream_window_bits;
   size_t http2_upstream_connection_window_bits;
   size_t http2_downstream_connection_window_bits;
-  size_t max_downstream_connections;
+  size_t downstream_connections_per_host;
   // actual size of downstream_http_proxy_addr
   size_t downstream_http_proxy_addrlen;
   size_t read_rate;

src/shrpx_downstream_queue.cc

@@ -25,12 +25,18 @@
 #include "shrpx_downstream_queue.h"
 
 #include <cassert>
+#include <limits>
 
 #include "shrpx_downstream.h"
 
 namespace shrpx {
 
-DownstreamQueue::DownstreamQueue() {}
+DownstreamQueue::HostEntry::HostEntry() : num_active(0) {}
+
+DownstreamQueue::DownstreamQueue(size_t conn_max_per_host)
+    : conn_max_per_host_(conn_max_per_host == 0
+                             ? std::numeric_limits<size_t>::max()
+                             : conn_max_per_host) {}
 
 DownstreamQueue::~DownstreamQueue() {}
 
@@ -44,49 +50,146 @@ void DownstreamQueue::add_failure(std::unique_ptr<Downstream> downstream) {
   failure_downstreams_[stream_id] = std::move(downstream);
 }
 
+DownstreamQueue::HostEntry &
+DownstreamQueue::find_host_entry(const std::string &host) {
+  auto itr = host_entries_.find(host);
+  if (itr == std::end(host_entries_)) {
+    std::tie(itr, std::ignore) = host_entries_.emplace(host, HostEntry());
+  }
+  return (*itr).second;
+}
+
 void DownstreamQueue::add_active(std::unique_ptr<Downstream> downstream) {
+  auto &ent = find_host_entry(downstream->get_request_http2_authority());
+  ++ent.num_active;
+
   auto stream_id = downstream->get_stream_id();
   active_downstreams_[stream_id] = std::move(downstream);
 }
 
-std::unique_ptr<Downstream> DownstreamQueue::remove(int32_t stream_id) {
+void DownstreamQueue::add_blocked(std::unique_ptr<Downstream> downstream) {
-  auto kv = pending_downstreams_.find(stream_id);
+  auto &ent = find_host_entry(downstream->get_request_http2_authority());
+  auto stream_id = downstream->get_stream_id();
+  ent.blocked.insert(stream_id);
+  blocked_downstreams_[stream_id] = std::move(downstream);
+}
 
-  if (kv != std::end(pending_downstreams_)) {
+bool DownstreamQueue::can_activate(const std::string &host) const {
-    auto downstream = std::move((*kv).second);
+  auto itr = host_entries_.find(host);
-    pending_downstreams_.erase(kv);
+  if (itr == std::end(host_entries_)) {
-    return downstream;
+    return true;
   }
+  auto &ent = (*itr).second;
+  return ent.num_active < conn_max_per_host_;
+}
 
-  kv = active_downstreams_.find(stream_id);
+namespace {
+std::unique_ptr<Downstream>
+pop_downstream(DownstreamQueue::DownstreamMap::iterator i,
+               DownstreamQueue::DownstreamMap &downstreams) {
+  auto downstream = std::move((*i).second);
+  downstreams.erase(i);
+  return downstream;
+}
+} // namespace
+
+namespace {
+bool remove_host_entry_if_empty(const DownstreamQueue::HostEntry &ent,
+                                DownstreamQueue::HostEntryMap &host_entries,
+                                const std::string &host) {
+  if (ent.blocked.empty() && ent.num_active == 0) {
+    host_entries.erase(host);
+    return true;
+  }
+  return false;
+}
+} // namespace
+
+std::unique_ptr<Downstream> DownstreamQueue::pop_pending(int32_t stream_id) {
+  auto itr = pending_downstreams_.find(stream_id);
+  if (itr == std::end(pending_downstreams_)) {
+    return nullptr;
+  }
+  return pop_downstream(itr, pending_downstreams_);
+}
+
+std::unique_ptr<Downstream>
+DownstreamQueue::remove_and_pop_blocked(int32_t stream_id) {
+  auto kv = active_downstreams_.find(stream_id);
 
   if (kv != std::end(active_downstreams_)) {
-    auto downstream = std::move((*kv).second);
+    auto downstream = pop_downstream(kv, active_downstreams_);
-    active_downstreams_.erase(kv);
+    auto &host = downstream->get_request_http2_authority();
-    return downstream;
+    auto &ent = find_host_entry(host);
+    --ent.num_active;
+
+    if (remove_host_entry_if_empty(ent, host_entries_, host)) {
+      return nullptr;
+    }
+
+    if (ent.blocked.empty() || ent.num_active >= conn_max_per_host_) {
+      return nullptr;
+    }
+
+    auto next_stream_id = *std::begin(ent.blocked);
+    ent.blocked.erase(std::begin(ent.blocked));
+
+    auto itr = blocked_downstreams_.find(next_stream_id);
+    assert(itr != std::end(blocked_downstreams_));
+
+    auto next_downstream = pop_downstream(itr, blocked_downstreams_);
+
+    remove_host_entry_if_empty(ent, host_entries_, host);
+
+    return next_downstream;
+  }
+
+  kv = blocked_downstreams_.find(stream_id);
+
+  if (kv != std::end(blocked_downstreams_)) {
+    auto downstream = pop_downstream(kv, blocked_downstreams_);
+    auto &host = downstream->get_request_http2_authority();
+    auto &ent = find_host_entry(host);
+    ent.blocked.erase(stream_id);
+
+    remove_host_entry_if_empty(ent, host_entries_, host);
+
+    return nullptr;
+  }
+
+  kv = pending_downstreams_.find(stream_id);
+
+  if (kv != std::end(pending_downstreams_)) {
+    pop_downstream(kv, pending_downstreams_);
+    return nullptr;
   }
 
   kv = failure_downstreams_.find(stream_id);
 
   if (kv != std::end(failure_downstreams_)) {
-    auto downstream = std::move((*kv).second);
+    pop_downstream(kv, failure_downstreams_);
-    failure_downstreams_.erase(kv);
+    return nullptr;
-    return downstream;
   }
 
   return nullptr;
 }
 
 Downstream *DownstreamQueue::find(int32_t stream_id) {
-  auto kv = pending_downstreams_.find(stream_id);
+  auto kv = active_downstreams_.find(stream_id);
 
-  if (kv != std::end(pending_downstreams_)) {
+  if (kv != std::end(active_downstreams_)) {
     return (*kv).second.get();
   }
 
-  kv = active_downstreams_.find(stream_id);
+  kv = blocked_downstreams_.find(stream_id);
 
-  if (kv != std::end(active_downstreams_)) {
+  if (kv != std::end(blocked_downstreams_)) {
+    return (*kv).second.get();
+  }
+
+  kv = pending_downstreams_.find(stream_id);
+
+  if (kv != std::end(pending_downstreams_)) {
     return (*kv).second.get();
   }
 
@@ -99,41 +202,14 @@ Downstream *DownstreamQueue::find(int32_t stream_id) {
   return nullptr;
 }
 
-std::unique_ptr<Downstream> DownstreamQueue::pop_pending() {
+const DownstreamQueue::DownstreamMap &
-  auto i = std::begin(pending_downstreams_);
-
-  if (i == std::end(pending_downstreams_)) {
-    return nullptr;
-  }
-
-  auto downstream = std::move((*i).second);
-
-  pending_downstreams_.erase(i);
-
-  return downstream;
-}
-
-Downstream *DownstreamQueue::pending_top() const {
-  auto i = std::begin(pending_downstreams_);
-
-  if (i == std::end(pending_downstreams_)) {
-    return nullptr;
-  }
-
-  return (*i).second.get();
-}
-
-size_t DownstreamQueue::num_active() const {
-  return active_downstreams_.size();
-}
-
-bool DownstreamQueue::pending_empty() const {
-  return pending_downstreams_.empty();
-}
-
-const std::map<int32_t, std::unique_ptr<Downstream>> &
 DownstreamQueue::get_active_downstreams() const {
   return active_downstreams_;
 }
 
+const DownstreamQueue::DownstreamMap &
+DownstreamQueue::get_blocked_downstreams() const {
+  return blocked_downstreams_;
+}
+
 } // namespace shrpx

src/shrpx_downstream_queue.h

@@ -30,6 +30,7 @@
 #include <stdint.h>
 
 #include <map>
+#include <set>
 #include <memory>
 
 namespace shrpx {
@@ -38,39 +39,67 @@ class Downstream;
 
 class DownstreamQueue {
 public:
-  DownstreamQueue();
+  typedef std::map<int32_t, std::unique_ptr<Downstream>> DownstreamMap;
+
+  struct HostEntry {
+    // Set of stream ID that blocked by conn_max_per_host_.
+    std::set<int32_t> blocked;
+    // The number of connections currently made to this host.
+    size_t num_active;
+    HostEntry();
+  };
+
+  typedef std::map<std::string, HostEntry> HostEntryMap;
+
+  // conn_max_per_host == 0 means no limit for downstream connection.
+  DownstreamQueue(size_t conn_max_per_host = 0);
   ~DownstreamQueue();
   void add_pending(std::unique_ptr<Downstream> downstream);
   void add_failure(std::unique_ptr<Downstream> downstream);
+  // Adds |downstream| to active_downstreams_, which means that
+  // downstream connection has been started.
   void add_active(std::unique_ptr<Downstream> downstream);
-  // Removes |downstream| from either pending_downstreams_,
+  // Adds |downstream| to blocked_downstreams_, which means that
-  // active_downstreams_ or failure_downstreams_ and returns it
+  // download connection was blocked because conn_max_per_host_ limit.
-  // wrapped in std::unique_ptr.
+  void add_blocked(std::unique_ptr<Downstream> downstream);
-  std::unique_ptr<Downstream> remove(int32_t stream_id);
+  // Returns true if we can make downstream connection to given
+  // |host|.
+  bool can_activate(const std::string &host) const;
+  // Removes pending Downstream object whose stream ID is |stream_id|
+  // from pending_downstreams_ and returns it.
+  std::unique_ptr<Downstream> pop_pending(int32_t stream_id);
+  // Removes Downstream object whose stream ID is |stream_id| from
+  // either pending_downstreams_, active_downstreams_,
+  // blocked_downstreams_ or failure_downstreams_.  If a Downstream
+  // object is removed from active_downstreams_, this function may
+  // return Downstream object with the same target host in
+  // blocked_downstreams_ if its connection is now not blocked by
+  // conn_max_per_host_ limit.
+  std::unique_ptr<Downstream> remove_and_pop_blocked(int32_t stream_id);
   // Finds Downstream object denoted by |stream_id| either in
-  // pending_downstreams_, active_downstreams_ or
+  // pending_downstreams_, active_downstreams_, blocked_downstreams_
-  // failure_downstreams_.
+  // or failure_downstreams_.
   Downstream *find(int32_t stream_id);
-  // Returns the number of active Downstream objects.
+  const DownstreamMap &get_active_downstreams() const;
-  size_t num_active() const;
+  const DownstreamMap &get_blocked_downstreams() const;
-  // Returns true if pending_downstreams_ is empty.
+
-  bool pending_empty() const;
+  HostEntry &find_host_entry(const std::string &host);
-  // Pops first Downstream object in pending_downstreams_ and returns
+
-  // it.
+  // Maximum number of concurrent connections to the same host.
-  std::unique_ptr<Downstream> pop_pending();
+  size_t conn_max_per_host_;
-  // Returns first Downstream object in pending_downstreams_.  This
-  // does not pop the first one.  If queue is empty, returns nullptr.
-  Downstream *pending_top() const;
-  const std::map<int32_t, std::unique_ptr<Downstream>> &
-  get_active_downstreams() const;
 
 private:
+  // Per target host structure to keep track of the number of
+  // connections to the same host.
+  std::map<std::string, HostEntry> host_entries_;
   // Downstream objects, not processed yet
-  std::map<int32_t, std::unique_ptr<Downstream>> pending_downstreams_;
+  DownstreamMap pending_downstreams_;
-  // Downstream objects in use, consuming downstream concurrency limit
-  std::map<int32_t, std::unique_ptr<Downstream>> active_downstreams_;
   // Downstream objects, failed to connect to downstream server
-  std::map<int32_t, std::unique_ptr<Downstream>> failure_downstreams_;
+  DownstreamMap failure_downstreams_;
+  // Downstream objects, downstream connection started
+  DownstreamMap active_downstreams_;
+  // Downstream objects, blocked by conn_max_per_host_
+  DownstreamMap blocked_downstreams_;
 };
 
 } // namespace shrpx

src/shrpx_http2_upstream.cc

@@ -352,35 +352,24 @@ int on_request_headers(Http2Upstream *upstream, Downstream *downstream,
     downstream->set_request_state(Downstream::MSG_COMPLETE);
   }
 
-  upstream->maintain_downstream_concurrency();
+  upstream->start_downstream(downstream);
 
   return 0;
 }
 } // namespace
 
-void Http2Upstream::maintain_downstream_concurrency() {
+void Http2Upstream::start_downstream(Downstream *downstream) {
-  while (get_config()->max_downstream_connections >
+  auto next_downstream =
-         downstream_queue_.num_active()) {
+      downstream_queue_.pop_pending(downstream->get_stream_id());
-    if (downstream_queue_.pending_empty()) {
+  assert(next_downstream);
-      break;
-    }
 
-    {
+  if (downstream_queue_.can_activate(
-      auto downstream = downstream_queue_.pending_top();
+          downstream->get_request_http2_authority())) {
-      if (downstream->get_request_state() != Downstream::HEADER_COMPLETE &&
+    initiate_downstream(std::move(next_downstream));
-          downstream->get_request_state() != Downstream::MSG_COMPLETE) {
+    return;
-        break;
-      }
-    }
-
-    auto downstream = downstream_queue_.pop_pending();
-
-    if (!downstream) {
-      break;
-    }
-
-    initiate_downstream(std::move(downstream));
   }
+
+  downstream_queue_.add_blocked(std::move(next_downstream));
 }
 
 void
@@ -610,7 +599,10 @@ uint32_t infer_upstream_rst_stream_error_code(uint32_t downstream_error_code) {
 } // namespace
 
 Http2Upstream::Http2Upstream(ClientHandler *handler)
-    : handler_(handler), session_(nullptr), settings_timerev_(nullptr) {
+    : downstream_queue_(get_config()->http2_proxy
+                            ? get_config()->downstream_connections_per_host
+                            : 0),
+      handler_(handler), session_(nullptr), settings_timerev_(nullptr) {
   reset_timeouts();
 
   int rv;
@@ -1154,9 +1146,12 @@ void Http2Upstream::remove_downstream(Downstream *downstream) {
     handler_->write_accesslog(downstream);
   }
 
-  downstream_queue_.remove(downstream->get_stream_id());
+  auto next_downstream =
+      downstream_queue_.remove_and_pop_blocked(downstream->get_stream_id());
 
-  maintain_downstream_concurrency();
+  if (next_downstream) {
+    initiate_downstream(std::move(next_downstream));
+  }
 }
 
 Downstream *Http2Upstream::find_downstream(int32_t stream_id) {
@@ -1400,6 +1395,11 @@ void Http2Upstream::on_handler_delete() {
       handler_->write_accesslog(ent.second.get());
     }
   }
+  for (auto &ent : downstream_queue_.get_blocked_downstreams()) {
+    if (ent.second->accesslog_ready()) {
+      handler_->write_accesslog(ent.second.get());
+    }
+  }
 }
 
 } // namespace shrpx

src/shrpx_http2_upstream.h

@@ -88,7 +88,7 @@ public:
   int consume(int32_t stream_id, size_t len);
   void log_response_headers(Downstream *downstream,
                             const std::vector<nghttp2_nv> &nva) const;
-  void maintain_downstream_concurrency();
+  void start_downstream(Downstream *downstream);
   void initiate_downstream(std::unique_ptr<Downstream> downstream);
 
   nghttp2::util::EvbufferBuffer sendbuf;

src/shrpx_spdy_upstream.cc

@@ -232,7 +232,7 @@ void on_ctrl_recv_callback(spdylay_session *session, spdylay_frame_type type,
       downstream->set_request_state(Downstream::MSG_COMPLETE);
     }
 
-    upstream->maintain_downstream_concurrency();
+    upstream->start_downstream(downstream);
 
     break;
   }
@@ -242,29 +242,18 @@ void on_ctrl_recv_callback(spdylay_session *session, spdylay_frame_type type,
 }
 } // namespace
 
-void SpdyUpstream::maintain_downstream_concurrency() {
+void SpdyUpstream::start_downstream(Downstream *downstream) {
-  while (get_config()->max_downstream_connections >
+  auto next_downstream =
-         downstream_queue_.num_active()) {
+      downstream_queue_.pop_pending(downstream->get_stream_id());
-    if (downstream_queue_.pending_empty()) {
+  assert(next_downstream);
-      break;
-    }
 
-    {
+  if (downstream_queue_.can_activate(
-      auto downstream = downstream_queue_.pending_top();
+          downstream->get_request_http2_authority())) {
-      if (downstream->get_request_state() != Downstream::HEADER_COMPLETE &&
+    initiate_downstream(std::move(next_downstream));
-          downstream->get_request_state() != Downstream::MSG_COMPLETE) {
+    return;
-        break;
-      }
-    }
-
-    auto downstream = downstream_queue_.pop_pending();
-
-    if (!downstream) {
-      break;
-    }
-
-    initiate_downstream(std::move(downstream));
   }
+
+  downstream_queue_.add_blocked(std::move(next_downstream));
 }
 
 void SpdyUpstream::initiate_downstream(std::unique_ptr<Downstream> downstream) {
@@ -425,7 +414,10 @@ uint32_t infer_upstream_rst_stream_status_code(uint32_t downstream_error_code) {
 } // namespace
 
 SpdyUpstream::SpdyUpstream(uint16_t version, ClientHandler *handler)
-    : handler_(handler), session_(nullptr) {
+    : downstream_queue_(get_config()->http2_proxy
+                            ? get_config()->downstream_connections_per_host
+                            : 0),
+      handler_(handler), session_(nullptr) {
   // handler->set_bev_cb(spdy_readcb, 0, spdy_eventcb);
   reset_timeouts();
 
@@ -875,9 +867,12 @@ void SpdyUpstream::remove_downstream(Downstream *downstream) {
     handler_->write_accesslog(downstream);
   }
 
-  downstream_queue_.remove(downstream->get_stream_id());
+  auto next_downstream =
+      downstream_queue_.remove_and_pop_blocked(downstream->get_stream_id());
 
-  maintain_downstream_concurrency();
+  if (next_downstream) {
+    initiate_downstream(std::move(next_downstream));
+  }
 }
 
 Downstream *SpdyUpstream::find_downstream(int32_t stream_id) {
@@ -1094,6 +1089,11 @@ void SpdyUpstream::on_handler_delete() {
       handler_->write_accesslog(ent.second.get());
     }
   }
+  for (auto &ent : downstream_queue_.get_blocked_downstreams()) {
+    if (ent.second->accesslog_ready()) {
+      handler_->write_accesslog(ent.second.get());
+    }
+  }
 }
 
 } // namespace shrpx

src/shrpx_spdy_upstream.h

@@ -81,7 +81,7 @@ public:
 
   int consume(int32_t stream_id, size_t len);
 
-  void maintain_downstream_concurrency();
+  void start_downstream(Downstream *downstream);
   void initiate_downstream(std::unique_ptr<Downstream> downstream);
 
   nghttp2::util::EvbufferBuffer sendbuf;