Support backlog configuration

docs/deployment.md

@@ -69,6 +69,8 @@ Options:
   --limit-concurrency INTEGER     Maximum number of concurrent connections or
                                   tasks to allow, before issuing HTTP 503
                                   responses.
+  --backlog INTEGER               Maximum number of connections to hold in
+                                  backlog
   --limit-max-requests INTEGER    Maximum number of requests to service before
                                   terminating the process.
   --timeout-keep-alive INTEGER    Close Keep-Alive connections if no new data

docs/index.md

@@ -111,6 +111,8 @@ Options:
   --limit-concurrency INTEGER     Maximum number of concurrent connections or
                                   tasks to allow, before issuing HTTP 503
                                   responses.
+  --backlog INTEGER               Maximum number of connections to hold in
+                                  backlog
   --limit-max-requests INTEGER    Maximum number of requests to service before
                                   terminating the process.
   --timeout-keep-alive INTEGER    Close Keep-Alive connections if no new data

docs/settings.md

@@ -65,6 +65,7 @@ connecting IPs in the `forwarded-allow-ips` configuration.
 
 * `--limit-concurrency <int>` - Maximum number of concurrent connections or tasks to allow, before issuing HTTP 503 responses. Useful for ensuring known memory usage patterns even under over-resourced loads.
 * `--limit-max-requests <int>` - Maximum number of requests to service before terminating the process. Useful when running together with a process manager, for preventing memory leaks from impacting long-running processes.
+* `--backlog <int>` - Maximum number of connections to hold in backlog. Relevant for heavy incoming traffic.
 
 ## Timeouts
 

uvicorn/config.py

@@ -130,6 +130,7 @@ def __init__(
         root_path="",
         limit_concurrency=None,
         limit_max_requests=None,
+        backlog=2048,
         timeout_keep_alive=5,
         timeout_notify=30,
         callback_notify=None,
@@ -162,6 +163,7 @@ def __init__(
         self.root_path = root_path
         self.limit_concurrency = limit_concurrency
         self.limit_max_requests = limit_max_requests
+        self.backlog = backlog
         self.timeout_keep_alive = timeout_keep_alive
         self.timeout_notify = timeout_notify
         self.callback_notify = callback_notify

uvicorn/main.py

@@ -185,6 +185,12 @@ def print_version(ctx, param, value):
     default=None,
     help="Maximum number of concurrent connections or tasks to allow, before issuing HTTP 503 responses.",
 )
+@click.option(
+    "--backlog",
+    type=int,
+    default=2048,
+    help="Maximum number of connections to hold in backlog",
+)
 @click.option(
     "--limit-max-requests",
     type=int,
@@ -273,6 +279,7 @@ def main(
     forwarded_allow_ips: str,
     root_path: str,
     limit_concurrency: int,
+    backlog: int,
     limit_max_requests: int,
     timeout_keep_alive: int,
     ssl_keyfile: str,
@@ -309,6 +316,7 @@ def main(
         "forwarded_allow_ips": forwarded_allow_ips,
         "root_path": root_path,
         "limit_concurrency": limit_concurrency,
+        "backlog": backlog,
         "limit_max_requests": limit_max_requests,
         "timeout_keep_alive": timeout_keep_alive,
         "ssl_keyfile": ssl_keyfile,
@@ -417,15 +425,15 @@ async def startup(self, sockets=None):
             self.servers = []
             for sock in sockets:
                 server = await loop.create_server(
-                    create_protocol, sock=sock, ssl=config.ssl
+                    create_protocol, sock=sock, ssl=config.ssl, backlog=config.backlog
                 )
                 self.servers.append(server)
 
         elif config.fd is not None:
             # Use an existing socket, from a file descriptor.
             sock = socket.fromfd(config.fd, socket.AF_UNIX, socket.SOCK_STREAM)
             server = await loop.create_server(
-                create_protocol, sock=sock, ssl=config.ssl
+                create_protocol, sock=sock, ssl=config.ssl, backlog=config.backlog
             )
             message = "Uvicorn running on socket %s (Press CTRL+C to quit)"
             logger.info(message % str(sock.getsockname()))
@@ -437,7 +445,7 @@ async def startup(self, sockets=None):
             if os.path.exists(config.uds):
                 uds_perms = os.stat(config.uds).st_mode
             server = await loop.create_unix_server(
-                create_protocol, path=config.uds, ssl=config.ssl
+                create_protocol, path=config.uds, ssl=config.ssl, backlog=config.backlog
             )
             os.chmod(config.uds, uds_perms)
             message = "Uvicorn running on unix socket %s (Press CTRL+C to quit)"
@@ -448,7 +456,11 @@ async def startup(self, sockets=None):
             # Standard case. Create a socket from a host/port pair.
             try:
                 server = await loop.create_server(
-                    create_protocol, host=config.host, port=config.port, ssl=config.ssl
+                    create_protocol,
+                    host=config.host,
+                    port=config.port,
+                    ssl=config.ssl,
+                    backlog=config.backlog,
                 )
             except OSError as exc:
                 logger.error(exc)

uvicorn/workers.py

@@ -2,7 +2,6 @@
 import logging
 
 from gunicorn.workers.base import Worker
-
 from uvicorn.config import Config
 from uvicorn.main import Server
 
@@ -46,6 +45,9 @@ def __init__(self, *args, **kwargs):
             }
             config_kwargs.update(ssl_kwargs)
 
+        if self.cfg.settings["backlog"].value:
+            config_kwargs["backlog"] = self.cfg.settings["backlog"].value
+
         config_kwargs.update(self.CONFIG_KWARGS)
 
         self.config = Config(**config_kwargs)