What are the Advantage and Disadvantage of Caching in Web Development In PHP, how does it affect Database?_问答_开发者

In PHP, What are the Advantage and Disadvantage of 开发者_开发问答Caching in Web Development In PHP, how does it affect Database?

Caching works in many different ways, but for PHP specifically I can think of a few ways;

Database calls; they are slow, require computation, and can be quite intensive. If you've got repeated calls, caching the query is golden. There's two levels; at the PHP side where you control the cache, and at the database side where they do.
Running PHP code means the webserver calls the PHP interpreter, it parses the code, and the run it. A PHP cacher can cache the parsing part, and go straight for the running part. THen there's the next generation of directly compiling PHP code to C, and run it from there (like Facebook does).
Computations; if you're doing math or heavy lifting of repeated operation, you can cache the result instead of calculate it every time.

Advantages;

speed
less resources used
reuse
being smart

Disadvantages;

stale data
overhead
complexity

I'll only deal with the disadvantages here;

First, stale data; this means that when you use cached content/data you are at risk of presenting old data that's no longer relevant to the new situation. If you've cached a query of products, but in the mean time the product manager has delete four products, the users will get listings to products that don't exists. There's a great deal of complexity in figuring out how to deal with this, but mostly it's about creating hashes/identifiers for caches that mean something to the state of the data in the cache, or business logic that resets the cache (or updates, or appends) with the new data bits. This is a complicated field, and depends very much on your requirements.

Then overhead is all the business logic you use to make sure your data is somewhere between being fast and being stale, which lead to complexity, and complexity leads to more code that you need to maintain and understand. You'll easily lose oversight of where data exists in the caching complex, at what level, and how to fix the stale data if you get it. It can easily get out of hand, so instead of doing caching on complex logic you revert to simple timestamps, and just say that a query is cached for a minute or so, and hope for the best (which, admittedly, can be quite effective and not too crazy). You could give your cache life-times (say, it will live X minutes in the cache) vs. access (it will live for 10 requests) vs. timed (it will live until 10pm) and variations thereof. The more variation, the more complexity, of course.

However, having said that, caching can turn a bog of a system into quite a snappy little vixen without too much effort or complexity. A little can get you a long way, and writing systems that use caching as a core component is something I'd recommend.

The main advantage, and also the goal, of caching is speeding up loading and minimizing system resources needed to load a page.

The main disadvantage is how it's implemented by the developers, and then maintaining proper caching system for the website, making it properly manageable by the Admin.

The above statements are purely said in general terms.

Caching is used to reduce hefty/slow operations (heavy calculations/parsing/database operations) which will consistently product the same result. Caching this result will reduce the server load and speed up the application (because the hefty/slow operation does not need executing)

The disadvantage is that it'll often increase complexity of the application, because the cache should be purged/altered when the result of the operation will no longer be the result cached.

Simple example: a website whose navigation is stored in the database could cache the navigation once the navigation has been fetched from the database, thus reducing the total amount of db-calls, because we no longer need to execute a query to retrieve the navigation. When the navigation changes (e.g. a page had been added), the cached value for the navigation should be rebuilt, because the navigation that has been cached does not yet reflect the latest change: the new page is not present there.

When a page is Cached, instead of regenerating the page every time, they store a copy of what they send to your browser. The next time a visitor requests the same page, the script will know it'd already generated one recently, and simply send that to the browser without all the hassle of re-running database queries or searches.

Advantage of Caching: