in-memory cache design using OO concept

What is in-memory cache? I could not find much information on it from the Web.

In fact, I was asked to design a in-memory cache based on OO concept using C++, but j开发者_如何学运维ust do not know how to start. Any suggestions would be appreciated.

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This depends on the context but generally, an in-memory cache stores some value so that it can be retrieved later, instead of creating a new object. This is most often used in conjunction with databases – or really any application where the construction / retrieval of an object is expensive.

For a simple memory cache, imagine the following dummy class (which violates tons of best practices, so don’t copy it!):

class Integer {
    int value;

public:

    Integer(int value) : value(value) {
        sleep(1000); // Simulates an expensive constructor
    }
};

Now imagine that you need to create instances of this class:

Integer one(1);
Integer two(2);
// etc.

… but later (in another method) perhaps you need to create a new instance of 2:

Integer two(2);

This is expensive. What if you could recycle the old value? Using constructors, this isn’t possible but using factory methods we can do this easily:

class Integer {
    int value;

    static std::map<int, Integer> cache;

    Integer(int value) : value(value) {
        sleep(1000); // Simulates an expensive constructor
    }

    friend Integer make_int(int);
};

Integer make_int(int value) {
    std::map<int, Integer>::iterator i = Integer::cache.find(value);
    if (i != Integer::cache.end())
        return i->second;

    Integer ret = Integer(value);
    Integer::cache[value] = ret;
    return ret;
}

Now we can use make_int to create or retrieve an integer. Each value will only be created once:

Integer one = make_int(1);
Integer two = make_int(2);
Integer other = make_int(2); // Recycles instance from above.

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An in-memory cache is used to store things that are typically stored externally, like a database record, an HTTP request, or the contents of a file. Typically, you'll maintain an LRU list of objects. This is pretty easy to manage, as all you have to do is move an object to the head of the list when you get a request for it, and remove the last object in the list when you need to add a new one. Obviously, you can add a lot of complexity to tune this behavior, but that's the basic algorithm.

You'll probably want a "read-through" cache. That is, the program always asks the cache for the object it needs, and if it doesn't exist in the cache then it's up to the cache to fetch it. This way, it simplifies the programming, as the application doesn't have to go to two different places to get an object. It can also make it possible to invisibly add caching, if you can modify an existing module (e.g., a DAO) that is already responsible for reading the object.

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memcached has a C++ API and may be worth a look to see if it meets your needs:

Free & open source, high-performance, distributed memory object caching system, generic in nature, but intended for use in speeding up dynamic web applications by alleviating database load.

Memcached is an in-memory key-value store for small chunks of arbitrary data (strings, objects) from results of database calls, API calls, or page rendering.