Shrapnel is a scalable, high-performance cooperative threading library for Python.
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This Python library was evolved at IronPort Systems and has been provided as open source by Cisco Systems under an MIT license.


Shrapnel is a library for high-performance concurrency. It uses coroutines to implement user threads on top of either kqueue (FreeBSD, OS X) or /dev/epoll (linux), and is written mostly in Pyrex/Cython, supporting both 32-bit and 64-bit platforms. It is the culmination of about 8 years of work at IronPort Systems, a provider of high-speed mail appliances. It was open-sourced by Cisco Systems in late 2011.


  • Lightweight threads, event-driven scheduler.
  • Underneath: non-blocking operations on descriptors, like sockets and pipes.
  • On top, synchronous API for straight-line, simple code.
  • Highly scalable - tens or hundreds of thousands of connections/threads.
  • Thread synchronization primitives, like mutexes, semaphores, etc...
  • with_timeout(): wrap any funcall with a timeout.
  • Wait on kqueue events like file/directory changes, signals, processes, etc... [kqueue only]
  • DNS stub resolver (full-fledged resolver may be forthcoming)
  • HTTP server and client (plus WebSocket, RFC6455 & hixie-76)
  • Support for TLS via tlslite (openssl interface may be forthcoming)
  • other protocols/codecs: ldap, asn1, ftp, mysql, postgres, AMQP.
  • MIT License.


Compared to other concurrency packages available for Python, Shrapnel gives you:

  • Speed and Efficiency: the entire scheduler, poller, socket layer, synchronization objects, etc... are written in Cython, with an emphasis on performance and low memory usage.
  • Stock Python: Shrapnel works with out-of-the-box CPython [2.X]. No special variants of Python are needed, it will even work with your OS's OEM python installation. So you can use all the external libraries/modules you've come to rely on.
  • No Callbacks: no need to cuisinart your application into a thousand callbacks. No need to decompose every action into a state machine. Write simple, performant code now without having to send your programmers to class.
  • Drop to Cython for speed: all the capabilities of the system are available from Cython, so you can e.g. write a server entirely in Cython for speed. You can interface with external libraries, and do thread switches from Cython or C. It's even possible to have external C code call back into shrapnel. This makes it easy to prototype your application in Python, and then push only the hot spots into Cython.
  • Timeouts: Shrapnel provides a general timeout mechanism that can be used to wrap any function call with a timeout.
  • Profiler: Thread-aware profiler generates HTML reports.



API Documentation