DISTCACHE(8) distcache DISTCACHE(8)
distcache - Distributed session caching
The distcache architecture provides a protocol and set of accompanying
tools to allow applications, and indeed machines, to share session
state between them by way of a network service.
The primary use of distcache right now is SSL/TLS session caching. This
allows SSL/TLS servers (eg. a secure Apache web server providing HTTPS
support) to use a centralised session cache, i.e any server may resume
SSL/TLS sessions negotiated by any other server on the network. The
advantages to this approach include increased freedom of mechanisms for
Existing SSL/TLS load-balancing solutions
Many load-balancers attempt to route incoming connections to servers
based on remembering the last mapping from the same source network
address. Others, called "SSL sticky" load-balancers, attempt to parse
SSL/TLS session ids from handshake messages and so map future session-
resume attempts. Both methods have serious weaknesses - the former is
generally confused by any form of network address translation (eg. when
clients are behind masquerading gateways), and the latter is confused
by any SSL/TLS renegotiations. Moreover both are stateful and a poten-
tial bottleneck, because there is no obvious way to scale the architec-
ture to multiple load-balancers.
Arbitrary SSL/TLS load-balancing with distcache
There is no need nor motivation to route incoming connections to "the
same server" to improve the chances for SSL/TLS session resumption, and
indeed doing so defeats the point of load-balancing (which is to bal-
ance according load or availability). The use of distcache is to ensure
that all servers share the same "cache" and so can respond to SSL/TLS
session resume requests irrespective of where the previous SSL/TLS con-
nection from the same client was mapped to.
It ainâ€â€™t just for SSL/TLS ...
Future versions of distcache will expand on the protocol and should
provide for a variety of "shared-state" uses besides SSL/TLS session
caching. The possibilities include application state caching, network-
based shared virtual memory, etc.
Runs a cache server listening on a configurable network address.
Runs a local client proxy. From the point of view of applications
this behaves like dc_server, but manages multiplexing application
requests to/from a cache server over a single persistent connec-
tion. See dc_client(1).
Sends a (configurable) barrage of session caching requests to a
given network address using the distcache protocol. Useful for
testing correctness of an installation as well as benchmarking. Can
be used directly against an instance of dc_server or against a
dc_client proxy. See dc_test(1).
A transparent proxy tool supporting the distcache protocol that can
be used to monitor cache operation requests and responses between
any two end-points (eg. between an application and dc_client, or
between dc_client and dc_server). See dc_snoop(1).
The comments below provide a short summary of the APIs available in
distcache. To view more details, consult the section 2 man pages these
summaries refer to. If you are using a packaged version of distcache,
you may need to ensure that a corresponding "devel" package is
installed as the libraries, headers, and API documentation is often
packaged independantly of the user tools.
This is the underlying Network Abstraction Library (hence "NAL") used
by the distcache libraries and tools. libnal uses non-blocking sockets,
with an addressing abstraction that allows tools to transparently work
over unix domain sockets or TCP/IPv4 sockets by a change of address
text. For this reason, all the distcache tools can have their "-listen"
and "-connect" switches set to work over either kind of transport.
libnal defines various object types;
NAL_ADDRESS (see NAL_ADDRESS_new(2))
The addressing abstraction converts to and from text representa-
tions, indicates whether given addresses are valid for listening
on, connecting to, or both.
NAL_CONNECTION (see NAL_CONNECTION_new(2))
This encapsulates a network connection that can be used for sending
and receiving arbitrary binary data.
NAL_LISTENER (see NAL_LISTENER_new(2))
This encapsulates a listening socket that can be used to accept
incoming connection requests on a configured address, creating a
NAL_CONNECTION wrapper for each accepted connection.
NAL_SELECTOR (see NAL_SELECTOR_new(2))
This provides an object that can be prepared with various NAL_LIS-
TENER and NAL_CONNECTION objects, and then can block waiting for
network activity up to some configurable limit of time. This is the
basis of non-blocking I/O and is an encapsulation of the tradi-
tional select(2) function.
NAL_BUFFER (see NAL_BUFFER_new(2))
This abstraction implements a FIFO data array and is used primarily
for representing the read and send parts of a NAL_CONNECTION
There are also some helper functions to assist in serialising data,
particularly with respect to putting integral data into network byte
order (allowing interoperability between platforms with differing
byte-order). These functions are documented in NAL_decode_uint32(2).
There are two APIs implemented by the libdistcache library;
This header provides the DC_PLUG abstraction. This encapsulates a
connection and implements the distcache protocol and various func-
tions for manipulating the reading and writing of distcache mes-
sages (requests or responses). This abstraction can support client
and server implementations of the distcache protocol and supports
asynchronous behaviour by interacting with libnalâ€™s NAL_SELECTOR
type. For more information, see DC_PLUG_new(2).
This header declares a higher-level (and much simpler) API than
dc_plug.h, and is useful in applications that want API functions
that â€˜â€˜do cache operationsâ€™â€™. The API is blocking, and provides
simplistic â€˜â€˜addâ€™â€™, â€˜â€˜removeâ€™â€™, and â€˜â€˜getâ€™â€™ functions that only
return once the full request/response cycle is complete or an error
has occured. This is the API used to add distcache support to
applications like Apache, stunnel, etc. For more information, see
This header declares an API for implementing a session cache supporting
the distcache protocol. It is primarily intended for environments that
wish to implement an alternative method for session storage. As with
elements of libdistcache, this API is likely to be undergoing some
important restructuring and enhancements. Please consider subscribing
to the distcache mail list and/or monitoring CVS, this gives you an
opportunity to influence ongoing development and be less surprised at
changes the turn up in future versions. For more information, see
The distcache toolkit, including the libnal network abstraction library
that comes bundled with it, is distributed under the LGPL license
("Library GNU Public License") and you should have received a copy of
this license with this software and its documents.
Quite possibly. In particular, portability has not been tested under
many platforms as the current developers have limited OS resources.
Feedback, access to alternative platforms, bug-reports, and questions
are all welcome - please go to the distcache website and subscribe to
the distcache-users mail list.
Distributed caching server.
Distributed caching client proxy
Distcache protocol analyser and debugging tool.
Distcache home page.
This toolkit was designed and implemented by Geoff Thorpe for Crypto-
graphic Appliances Incorporated. Since the project was released into
open source, it has a home page and a project environment where devel-
opment, mailing lists, and releases are organised. For problems with
the software or this man page please check for new releases at the
project web-site below, mail the users mailing list described there, or
contact the author at email@example.com.
Home Page: http://www.distcache.org
1.4.5 2004.03.23 DISTCACHE(8)
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