Twisted (software)

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Twisted is an event-driven network programming framework written in Python and licensed under the MIT License.

Twisted projects variously support TCP, UDP, SSL/TLS, IP Multicast, Unix domain sockets, a large number of protocols (including HTTP, NNTP, IMAP, SSH, IRC, FTP, and others), and much more. Twisted is based on the event-driven programming paradigm, which means that users of Twisted write short callbacks which are called by the framework.

1 Separation of Protocols and Transports
2 Deferreds
3 Thread support
4 Foreign loop support
5 See also
6 External links

[edit] Separation of Protocols and Transports

One of Twisted's core ideas is complete separation between logical protocols (usually relying on stream-based connection semantics, such as HTTP or POP3) and physical transport layers supporting such stream-based semantics (such as files, sockets or SSL libraries). Connection between a logical protocol and a transport layer happens at the last possible moment—just before information is passed into the logical protocol instance. The logical protocol is informed of the transport layer instance, and can use it to send messages back and to check for the peer's identity. Note that it is still possible, in protocol code, to deeply query the transport layer on transport issues (such as checking a client-side SSL certificate). Naturally, such protocol code will fail if the transport layer does not support such semantics.

[edit] Deferreds

Another core idea in Twisted is the concept of "deferreds". A deferred is a value which has not been computed yet, for example because it needs data from a remote peer. Deferreds can be passed around, just like regular objects, but cannot be asked for their value. Each deferred supports a callback chain. When the deferred gets the value, it is transferred through the callback chain, with the result of each callback being the input for the next one. This allows operating on the values of a deferred without knowing what they are. For example, if a deferred returns a string from a remote peer containing an IP address in quad format, a callback can be attached to translate it into a 32-bit number. Any user of the deferred can now treat it is as a deferred returning a 32-bit number. This, and the related ability to define "errbacks" (callbacks which are called as error handlers), allows code which looks as though it is serial, while still maintaining the event-driven abstraction.

[edit] Thread support

Twisted supports an abstraction over raw threads—using a thread as a deferred source. Thus, a deferred is returned immediately, which will receive a value when the thread finishes. Callbacks can be attached which will run in the main thread, thus alleviating the need for complex locking solutions. A prime example of such usage, which comes from Twisted's support libraries, is using this model to call into databases. The database call itself happens on a foreign thread, but the analysis of the result happens in the main thread.

[edit] Foreign loop support

Twisted can integrate with foreign event loops, such as GTK+ or Qt's. This allows using Twisted as the networking support layer in GUI programs, utilizing all of its libraries without adding a thread-per-socket overhead, as using Python's native library would. A full-fledged web server can be integrated in-process with a GUI program using this model, for example.