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socketserver Module Complexity¶

The socketserver module is the accept-and-dispatch loop behind http.server, xmlrpc.server and wsgiref.simple_server. A server waits on one listening socket and, for each request, builds an instance of your handler class; everything else is the choice of where that handler runs - in the serving thread, in a new thread, or in a forked child. The module is pure Python, and a stream server reads nothing from a connection itself: every byte of the request is left for the handler.

A request is the unit throughout. t is the handler threads a ThreadingMixIn server is tracking, c is the child processes a ForkingMixIn server is tracking, b is the bytes of one received datagram, at most max_packet_size, and r is the bytes a datagram handler writes as its reply. n is the bytes one read or write on a handler's files moves, and f is the frames in a printed traceback. Each system call - bind, listen, accept, select, fork, starting a thread - is priced O(1): the bounds count calls, not kernel time or network latency. The handler's own work is outside every bound, which is why the dispatch rows say "plus the handler".

Complexity Reference¶

BaseServer¶

Operation	Time	Space	Notes
`socketserver.BaseServer(server_address, RequestHandlerClass)`	O(1)	O(1)	Stores both; makes no socket. The base every server class below extends
`BaseServer.serve_forever(poll_interval=0.5)`	O(1) per wakeup, plus each request	O(1)	Wakes when a request is ready or `poll_interval` seconds pass, handles at most one request, then calls `service_actions()` unless shutting down; ignores `timeout`
`BaseServer.shutdown()`	O(1)	O(1)	Blocks until `serve_forever()` returns: up to one `poll_interval`, plus whatever request the loop is handling. Called from the thread running the loop, it never returns
`BaseServer.handle_request()`	O(1), plus the request	O(1)	Serves at most one request, waiting for at most `timeout` or the socket's own timeout, whichever is smaller; on expiry calls `handle_timeout()` instead
`BaseServer.service_actions()`	O(1)	O(1)	A no-op hook, called by `serve_forever()` after each wakeup
`BaseServer.fileno()`	O(1)	O(1)	The listening socket's descriptor, so a server can be passed to a selector
`BaseServer.get_request()`	O(1)	O(1)	One `accept()` on a stream server; the datagram version is under Server classes
`BaseServer.verify_request(request, client_address)`	O(1)	O(1)	Returns `True`; override it to refuse a request before any handler is built
`BaseServer.process_request(request, client_address)`	O(1), plus the handler	O(1)	`finish_request()` then `shutdown_request()`, in the serving thread; the mix-ins replace it
`BaseServer.finish_request(request, client_address)`	O(1), plus the handler	O(1)	Instantiates `RequestHandlerClass`, which runs the handler
`BaseServer.shutdown_request(request)`, `BaseServer.close_request(request)`	O(1)	O(1)	Hooks for ending a request: `TCPServer.shutdown_request()` shuts down the write side, then calls `close_request()`, which closes; a datagram server does neither
`BaseServer.handle_error(request, client_address)`	O(f)	O(f)	Prints the traceback to stderr; the server goes on to the next request
`BaseServer.handle_timeout()`	O(1)	O(1)	A no-op hook for `handle_request()` expiring
`BaseServer.server_bind()`, `BaseServer.server_activate()`	O(1)	O(1)	Hooks the `TCPServer` constructor calls: `bind()`, then `listen(request_queue_size)`
`BaseServer.server_close()`	O(1)	O(1)	A hook, also called on leaving a `with` block; `TCPServer` closes the listening socket, and the mix-ins add waiting, below
`BaseServer.address_family`, `BaseServer.socket_type`, `BaseServer.allow_reuse_address`, `BaseServer.request_queue_size`, `BaseServer.timeout`	O(1)	O(1)	Class attributes to override on a subclass; all but `timeout` are read during construction. `request_queue_size` is 5, the `listen()` backlog
`BaseServer.server_address`, `BaseServer.socket`, `BaseServer.RequestHandlerClass`	O(1)	O(1)	After binding, `server_address` is the address actually bound, so port 0 reads back as the port chosen

Server classes¶

Operation	Time	Space	Notes
`socketserver.TCPServer(server_address, RequestHandlerClass, bind_and_activate=True)`	O(1)	O(1)	Creates, binds and listens; with `bind_and_activate=False` only the socket is created
`socketserver.UDPServer(server_address, RequestHandlerClass, bind_and_activate=True)`	O(1)	O(1)	Binds; a datagram socket does not listen
`socketserver.UnixStreamServer(...)`, `socketserver.UnixDatagramServer(...)`	O(1)	O(1)	The same over `AF_UNIX`, where available; `server_close()` leaves the socket file in place
`TCPServer.get_request()`	O(1)	O(1)	One `accept()`
`UDPServer.get_request()`	O(b)	O(`max_packet_size`)	One `recvfrom(max_packet_size)`, whose buffer is the full limit before it shrinks to the datagram; the request is the datagram and the socket
`UDPServer.max_packet_size`	O(1)	O(1)	8192; on POSIX the rest of a longer datagram is discarded without an error
`TCPServer.allow_reuse_port`	O(1)	O(1)	Python 3.11+; an IP server sets `SO_REUSEPORT` on binding

ThreadingMixIn and ForkingMixIn¶

Operation	Time	Space	Notes
`ThreadingMixIn.process_request(request, client_address)`	O(t)	O(t)	Starts one thread per request, first dropping finished threads from the t it tracks
`ThreadingMixIn.process_request_thread(request, client_address)`	O(1), plus the handler	O(1)	The thread's body: `finish_request()`, `handle_error()` on an exception, then `shutdown_request()`
`ThreadingMixIn.server_close()`	O(t), plus waiting	O(t)	Closes the socket, then joins every tracked thread, so it lasts as long as the slowest handler still running
`ThreadingMixIn.daemon_threads`	O(1)	O(1)	`False`; `True` leaves threads untracked, so `process_request()` is O(1) and `server_close()` does not wait
`ThreadingMixIn.block_on_close`, `ForkingMixIn.block_on_close`	O(1)	O(1)	`True`; `False` stops `server_close()` joining threads, or waiting on children that have not exited
`ForkingMixIn.process_request(request, client_address)`	O(1)	O(1)	One `fork()`; the parent closes its copy of the request and records the pid. POSIX only
`ForkingMixIn.collect_children(*, blocking=False)`	O(c)	O(c)	One `waitpid()` per tracked child; at `max_children` it first blocks until one exits
`ForkingMixIn.service_actions()`, `ForkingMixIn.handle_timeout()`	O(c)	O(c)	Both call `collect_children()`, so every `serve_forever()` wakeup is O(c)
`ForkingMixIn.server_close()`	O(c), plus waiting	O(c)	Closes the socket, then `collect_children(blocking=block_on_close)`
`ForkingMixIn.max_children`	O(1)	O(1)	40
`ForkingMixIn.active_children`	O(1)	O(c)	The set of pids, or `None` before the first fork
`ForkingMixIn.timeout`	O(1)	O(1)	300 seconds, used by `handle_request()`

Predefined servers¶

Operation	Time	Space	Notes
`socketserver.ThreadingTCPServer`, `socketserver.ThreadingUDPServer`, `socketserver.ThreadingUnixStreamServer`, `socketserver.ThreadingUnixDatagramServer`	O(1)	O(1)	`ThreadingMixIn` over each server class; a request costs what `ThreadingMixIn.process_request()` does
`socketserver.ForkingTCPServer`, `socketserver.ForkingUDPServer`, `socketserver.ForkingUnixStreamServer`, `socketserver.ForkingUnixDatagramServer`	O(1)	O(1)	`ForkingMixIn` over each; POSIX only, and the two Unix ones are Python 3.12+

BaseRequestHandler¶

Operation	Time	Space	Notes
`socketserver.BaseRequestHandler(request, client_address, server)`	O(1), plus the handler	O(1)	Constructing it is handling the request: `setup()`, `handle()`, then `finish()`, which runs even if `handle()` raises
`BaseRequestHandler.setup()`, `BaseRequestHandler.handle()`, `BaseRequestHandler.finish()`	O(1)	O(1)	No-ops to override
`BaseRequestHandler.request`, `BaseRequestHandler.client_address`, `BaseRequestHandler.server`	O(1)	O(1)	Set before `setup()`; for a datagram server `request` is `(data, socket)`

StreamRequestHandler¶

Operation	Time	Space	Notes
`StreamRequestHandler.setup()`	O(1)	O(1)	Applies `timeout` and `disable_nagle_algorithm` to the connection, then opens `rfile` and `wfile` over it
`StreamRequestHandler.rfile`	O(n) per read	O(n)	Buffered, so small reads do not each make a system call. `readline()` without a limit holds the whole line
`StreamRequestHandler.wfile`	O(n) per write	O(1)	Unbuffered by default, so each `write()` is one `sendall()` call, made before `write()` returns
`StreamRequestHandler.rbufsize`, `StreamRequestHandler.wbufsize`	O(1)	O(1)	-1 and 0; a positive `wbufsize` batches writes into a buffer of that size, sent when it fills and at `finish()`
`StreamRequestHandler.timeout`, `StreamRequestHandler.disable_nagle_algorithm`	O(1)	O(1)	`None` and `False`; read once, in `setup()`
`StreamRequestHandler.finish()`	O(1), plus buffered writes	O(1)	Flushes what a positive `wbufsize` still holds, then closes both files

DatagramRequestHandler¶

Operation	Time	Space	Notes
`DatagramRequestHandler.setup()`	O(1)	O(1)	Wraps the datagram already received; no system call
`DatagramRequestHandler.rfile`	O(n) per read	O(n)	A `BytesIO` over the whole datagram
`DatagramRequestHandler.wfile`	O(n) per write	O(r)	A `BytesIO` holding the whole reply until `finish()`
`DatagramRequestHandler.finish()`	O(r)	O(r)	Sends everything written as one datagram

Serving Requests¶

One Request at a Time¶

The plain server classes run the handler in the thread that called handle_request() or serve_forever(), so a slow handler delays every client behind it. handle_request() serves at most one request, which makes it the easiest way to drive a server without threads.

import socket
import socketserver

class Echo(socketserver.StreamRequestHandler):
    def handle(self):
        line = self.rfile.readline(1024)  # O(n), capped at 1024 bytes
        self.wfile.write(b"echo: " + line)  # O(n), sent at once

with socketserver.TCPServer(("127.0.0.1", 0), Echo) as server:  # O(1) - bind and listen
    host, port = server.server_address  # port 0 reads back as the port chosen
    assert port != 0

    client = socket.create_connection((host, port))  # waits in the listen backlog
    client.sendall(b"hello\n")
    server.handle_request()  # O(1) plus the handler, in this thread
    with client, client.makefile("rb") as reply:
        assert reply.read() == b"echo: hello\n"  # the server closed its side

Serving Until Shutdown¶

serve_forever() wakes when a request is ready or every poll_interval seconds, whichever comes first, and checks for a shutdown request only then. shutdown() sets that flag and waits for the loop to notice, so it can block for up to one poll_interval plus any request in progress, and it must come from another thread: called by the loop's own thread, it waits for itself.

import socket
import socketserver
import threading

class Upper(socketserver.StreamRequestHandler):
    def handle(self):
        self.wfile.write(self.rfile.readline(1024).upper())

server = socketserver.TCPServer(("127.0.0.1", 0), Upper)
loop = threading.Thread(target=server.serve_forever, kwargs={"poll_interval": 0.1})
loop.start()

with socket.create_connection(server.server_address) as client:
    client.sendall(b"abc\n")
    with client.makefile("rb") as reply:
        assert reply.readline() == b"ABC\n"

server.shutdown()  # O(1), waits for the loop to exit
loop.join()
server.server_close()
assert not loop.is_alive()

Timeouts¶

handle_request() honours timeout; serve_forever() does not. When no request arrives in time, handle_request() calls handle_timeout() and returns.

import socketserver

class Quiet(socketserver.TCPServer):
    timeout = 0.05
    timed_out = 0

    def handle_timeout(self):
        self.timed_out += 1

with Quiet(("127.0.0.1", 0), socketserver.BaseRequestHandler) as server:
    server.handle_request()  # no client: returns after 0.05 s
    assert server.timed_out == 1

Refusing and Failing Requests¶

verify_request() runs before the handler is built, so refusing there costs one accept and one close. An Exception raised in a handler reaches handle_error(), which prints the traceback and lets the server continue.

import contextlib
import io
import socket
import socketserver

handled = []

class Record(socketserver.BaseRequestHandler):
    def handle(self):
        handled.append(self.client_address)
        raise ValueError("handler failed")

class Picky(socketserver.TCPServer):
    allowed = True

    def verify_request(self, request, client_address):
        return self.allowed

with Picky(("127.0.0.1", 0), Record) as server:
    server.allowed = False
    refused = socket.create_connection(server.server_address)
    server.handle_request()  # O(1) - the handler is never built
    assert handled == []
    assert refused.recv(1) == b""  # the connection was closed
    refused.close()

    server.allowed = True
    client = socket.create_connection(server.server_address)
    stderr = io.StringIO()
    with contextlib.redirect_stderr(stderr):
        server.handle_request()  # the handler raises; handle_error prints it
    assert len(handled) == 1
    assert "ValueError: handler failed" in stderr.getvalue()
    client.close()

Concurrency¶

Threads¶

ThreadingMixIn starts one thread per request. By default it also keeps a reference to each non-daemon thread so server_close() can join them, and each new request first scans that list for threads that have finished: a request costs O(t) where t is the handler threads still tracked. daemon_threads = True stops the tracking, and with it the wait on close.

import socketserver
import threading

release = threading.Event()
started = []

class Wait(socketserver.BaseRequestHandler):
    def handle(self):
        started.append(self.client_address)
        release.wait()

class Request:
    """Stands in for an accepted connection."""
    def shutdown(self, how):
        pass
    def close(self):
        pass

server = socketserver.ThreadingTCPServer(("127.0.0.1", 0), Wait, bind_and_activate=False)
for n in range(3):
    server.process_request(Request(), ("client", n))  # O(t) - scans the tracked threads

closer = threading.Thread(target=server.server_close)  # O(t), plus waiting
closer.start()
closer.join(0.1)
assert closer.is_alive()  # still waiting for the handlers
release.set()
closer.join()
assert len(started) == 3

Processes¶

ForkingMixIn forks a child per request and remembers its pid. Every serve_forever() wakeup then calls waitpid() once per tracked child to reap the ones that have exited, so the loop costs O(c) even when idle; at max_children it blocks until one exits.

import os
import socket
import socketserver

if hasattr(os, "fork"):
    class Pid(socketserver.StreamRequestHandler):
        def handle(self):
            self.wfile.write(str(os.getpid()).encode())

    with socketserver.ForkingTCPServer(("127.0.0.1", 0), Pid) as server:
        client = socket.create_connection(server.server_address)
        server.handle_request()  # O(1) - one fork; the child runs the handler
        assert len(server.active_children) == 1  # O(c) pids tracked
        with client, client.makefile("rb") as reply:
            assert int(reply.read()) != os.getpid()  # handled in the child
    # Leaving the block waited for the child: block_on_close is True
    assert not server.active_children

Request Handlers¶

Stream Handlers¶

rfile is buffered and wfile is not: each write() is one sendall(), so a handler that writes many small pieces sends each one separately. Set wbufsize to batch them; the buffer is flushed when the handler finishes. rfile.readline() holds a whole line, so pass a limit when the client is not trusted.

import socket
import socketserver

class Pieces(socketserver.StreamRequestHandler):
    wbufsize = 65536  # buffer writes; finish() flushes them

    def handle(self):
        request = self.rfile.readline(64)  # O(n), n <= 64
        assert request == b"x" * 64  # the rest of the line is left unread
        for n in range(1000):
            self.wfile.write(b"%d\n" % n)  # O(n) each, no system call

with socketserver.TCPServer(("127.0.0.1", 0), Pieces) as server:
    client = socket.create_connection(server.server_address)
    client.sendall(b"x" * 100 + b"\n")
    server.handle_request()
    with client, client.makefile("rb") as reply:
        assert reply.read().splitlines()[-1] == b"999"

Datagram Handlers¶

A datagram server receives at most max_packet_size bytes per request; on POSIX the rest is dropped without an error. DatagramRequestHandler holds the whole datagram in rfile and collects the reply in wfile, sending it as a single datagram when the handler finishes.

import socket
import socketserver

class Size(socketserver.DatagramRequestHandler):
    def handle(self):
        data = self.rfile.read()  # O(b) - the whole datagram
        self.wfile.write(b"%d" % len(data))  # O(n) - held until finish()
        self.wfile.write(b" bytes")

with socketserver.UDPServer(("127.0.0.1", 0), Size) as server:
    client = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    client.sendto(b"x" * 10000, server.server_address)
    server.handle_request()  # O(b) - one recvfrom
    reply, _ = client.recvfrom(1024)
    assert reply == b"8192 bytes"  # max_packet_size, not 10000; one datagram back
    client.close()

Common Patterns¶

A Threaded Server in the Background¶

import socket
import socketserver
import threading

class Echo(socketserver.StreamRequestHandler):
    def handle(self):
        for line in self.rfile:  # O(n) per line
            self.wfile.write(line)

class Server(socketserver.ThreadingTCPServer):
    daemon_threads = True  # untracked: process_request is O(1)

with Server(("127.0.0.1", 0), Echo) as server:
    loop = threading.Thread(target=server.serve_forever, kwargs={"poll_interval": 0.1})
    loop.start()

    clients = [socket.create_connection(server.server_address) for _ in range(3)]
    for n, client in enumerate(clients):  # all three connected at once
        client.sendall(b"%d\n" % n)
    for n, client in enumerate(clients):
        with client, client.makefile("rb") as reply:
            assert reply.readline() == b"%d\n" % n

    server.shutdown()
    loop.join()

Performance Best Practices¶

✅ Do:

Use a threading or forking server when a handler can block; the plain classes serve one client at a time
Set daemon_threads = True on a busy threading server that need not wait for handlers on close, so each request stops paying O(t)
Pass a limit to rfile.readline() so one client cannot make the handler hold an unbounded line
Set wbufsize when a handler writes many small pieces, so they are not one sendall() each
Refuse unwanted clients in verify_request(), before a handler, thread or child is created

❌ Avoid:

Calling shutdown() from a handler of a single-threaded server, or from the thread running serve_forever() - it waits for itself
A long poll_interval when a prompt shutdown() matters - a longer interval can delay it
Relying on timeout with serve_forever() - only handle_request() honours it
Datagrams larger than max_packet_size - on POSIX the excess is dropped without an error

Version Notes¶

Python 3.11+: Added allow_reuse_port
Python 3.12+: Added ForkingUnixStreamServer and ForkingUnixDatagramServer
All Python 3: ForkingMixIn and the Forking* servers exist only where os.fork does, and the Unix* servers only where AF_UNIX does

socket - the calls each server makes
selectors - many clients served from one thread, without a thread each
threading - the threads ThreadingMixIn starts
http - http.server, built on TCPServer and ThreadingMixIn
wsgiref - a WSGI server built on these classes
asyncio - many connections in one thread, with coroutines for handlers