Cache invalidation

Cache invalidation is a process in a computer system whereby entries in a cache are deleted. It can be done explicitly, as part of a cache coherence protocol in a parallel computer. In such a case, a compute node changes a variable and then invalidates the cached values of that variable across the rest of the computer system.

Explicit invalidation

Cache invalidation can be used to push new content to a client. 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is carried out by changing the application data, which in turn marks the information received by the client as out-of-date.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.^[1]

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.^[1]

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 effects only one variant of the cached content.^[1]

Ban

A reference to the cached content is added to 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.^[1]

This method, unlike purge, does not immediately remove cached content from the caching proxy. Instead, the cached content is updated happens 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 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.^[1]

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.^[1]

Parallel computing

General	Distributed computing Cloud computing High-performance computing

Levels	Bit Instruction Task Data Memory

Multithreading	Temporal Simultaneous Preemptive Cooperative

Theory	PRAM model Analysis of parallel algorithms Amdahl's law Gustafson's law Cost efficiency Karp–Flatt metric Slowdown Speedup

Elements	Process Thread Fiber Instruction window

Coordination	Multiprocessing Memory coherency Cache coherency Cache invalidation Barrier Synchronization Application checkpointing

Programming	Models Implicit parallelism Explicit parallelism Concurrency Non-blocking algorithm

Hardware	Flynn's taxonomy SISD SIMD MISD MIMD Pipelined processor Superscalar processor Vector processor Multiprocessor symmetric asymmetric Memory shared distributed distributed shared UMA NUMA COMA Massively parallel computer Computer cluster Grid computer

APIs	POSIX Threads OpenMP OpenCL OpenHMPP OpenACC MPI PVM UPC TBB Boost.Thread Global Arrays Ateji PX Charm++ Cilk Coarray Fortran CUDA Dryad C++ AMP PLINQ TPL

Problems	Embarrassingly parallel Software lockout Scalability Race condition Deadlock Livelock Starvation Deterministic algorithm Parallel slowdown

Category: parallel computing Media related to parallel computing at Wikimedia Commons

References

1 2 3 4 5 6 "An Introduction to Cache Invalidation — FOSHttpCache Documentation". foshttpcache.readthedocs.org. Retrieved 2016-01-02.

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