Cache invalidation is a process in a
computer system
A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations ( computation) automatically. Modern digital electronic computers can perform generic sets of operations known as programs. These prog ...
whereby entries in a
cache
Cache, caching, or caché may refer to:
Places United States
* Cache, Idaho, an unincorporated community
* Cache, Illinois, an unincorporated community
* Cache, Oklahoma, a city in Comanche County
* Cache, Utah, Cache County, Utah
* Cache County ...
are replaced or removed.
It can be done explicitly, as part of a
cache coherence
In computer architecture, cache coherence is the uniformity of shared resource data that ends up stored in multiple local caches. When clients in a system maintain caches of a common memory resource, problems may arise with incoherent data, wh ...
protocol. In such a case, a processor changes a memory location and then invalidates the cached
values
In ethics and social sciences, value denotes the degree of importance of something or action, with the aim of determining which actions are best to do or what way is best to live (normative ethics in ethics), or to describe the significance of di ...
of that memory location across the rest of the computer system.
Explicit invalidation
''Cache invalidation'' can be used to
push
Push may refer to:
Music
* Mike Dierickx (born 1973), a Belgian producer also known as Push
Albums
* ''Push'' (Bros album), 1988
* ''Push'' (Gruntruck album), 1992
* ''Push'' (Jacky Terrasson album), 2010
Songs
* "Push" (Enrique Iglesias s ...
new content to a
client
Client(s) or The Client may refer to:
* Client (business)
* Client (computing), hardware or software that accesses a remote service on another computer
* Customer or client, a recipient of goods or services in return for monetary or other valuabl ...
. This method functions as an alternative to other methods of displaying new content to connected clients. ''Invalidation'' is carried out by changing the application data, which in turn marks the information received by the client as out-of-date. After the cache is invalidated, if the client requests the cache, they are delivered a new version.
Methods
There are three specific methods to invalidate a cache, but not all
caching proxies support these methods.
Purge
Removes content from caching proxy immediately. When the client requests the data again, it is
fetched from the application and stored in the caching proxy. This method removes all variants of the cached content.
Refresh
Fetches requested content from the application, even if cached content is available. The content previously stored in the cache is replaced with a new version from the application. This method affects only one variant of the cached content.
Ban
A reference to the cached content is added to a
blacklist
Blacklisting is the action of a group or authority compiling a blacklist (or black list) of people, countries or other entities to be avoided or distrusted as being deemed unacceptable to those making the list. If someone is on a blacklist, ...
(or ban list). Client requests are then checked against this blacklist, and if a request matches, new content is fetched from the application, returned to the client, and added to the cache.
This method, unlike purge, does not immediately remove cached content from the caching proxy. Instead, the cached content is updated after a client requests that specific information.
Alternatives
There are a few alternatives to cache invalidation that still deliver updated content to the client. One alternative is to expire the cached content quickly by reducing the
time-to-live (TTL) to a very low value. Another alternative is to validate the cached content at each request. A third option is to not cache volatile content requested by the client. These alternatives can cause issues, as they create high load on the application due to more frequent requests for information.
Disadvantages
Using invalidation to transfer new content can be difficult when invalidating multiple objects. Invalidating multiple representations adds a level of complexity to the application. Cache invalidation must be carried out through a caching proxy; these requests can impact performance of the caching proxy, causing information to be transferred at a slower rate to clients.
References
{{Parallel computing
Cache coherency