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\it\noindent Distributed in-memory caching of application data has
recently become a popular solution for improving the scalability of
database-driven web sites. These systems, such as \memcached, provide a
simple and flexible interface, are easy to add to existing setups and
scale near-linearly with the number of servers. Unfortunately, such
caches do not guarantee transactional consistency, violating the
consistency guarantees of the underlying storage layer.  Moreover, the
application is responsible for updating the data in the cache, making
programming difficult because the developer must reason globally to
identify every cached value that an update that might affect.

We introduce a transactional cache, which provides transactional
consistency and a simple programming model, while retaining the
flexibility and scalability of key/value application data caches like
\memcached.  Our system, \emph{\txcache{}}, uses a novel technique to
improve performance and avoid explicit invalidations: it allows
read-only transactions to run slightly in the past, but ensures that
they see a consistent view of the database.  Our system achieves this
by retaining multiple versions of each cached value and automatically
tracking the range of times over which each value is valid.

% Our system, \emph{\txcache{}} uses a multi-version cache to allow
% read-only transaction to run slightly in the past while ensuring
% that they still see a consistent view of the database.  This novel
% technique ...


% Distributed in-memory caching of application data has recently become
% a popular approach for improving the scalability of database-driven
% web sites. These systems, such as \memcached, provide a simple and
% flexible interface: they store application-provided key/value
% mappings, like a distributed hash table. The cache can be used to
% cache the results of both database queries and application
% computations, reducing load on both the database and application
% servers. Because cache performance scales near-linearly with the
% number of servers, this technique is used extensively by a number of
% large websites, who find it to be the most cost-effective approach to
% improving performance.

% Unfortunately, such caches do not guarantee transactional consistency
% of the results they return, making them unsuitable for use in
% applications with strict consistency requirements. Moreover, the
% application is responsible for invalidating the cache, making
% programming difficult because the developer must reason globally to
% identify every change that might affect a cached value.

% We provide an alternative in a transactional cache, \emph{\txcache{}},
% which retains the flexibility and scalability of simple application
% data caches while providing transactional consistency and a simpler
% programming model.  \txcache uses a novel technique to improve
% performance and avoid explicit invalidations: it allows read-only
% transactions to run slightly in the past, but ensures that they see a
% consistent view of the database. This is achieved using a versioned
% cache, in which entries are automatically tagged with the range of
% times at which they are valid.


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% LocalWords:  invalidations versioned