What is the best approach for caching database queries

Question

I'm writing a management intelligence application which requires quite a lot of complex database querying with some queries being quite expensive. To aid performance I'm using Memcached quite heavily to store as much as I can in memory.

This has led to quite a lot of duplication in my code which I'm eager to get rid of and build a cleaner data access solution. Quite a lot of my data access functions have ended up looking like this..

public int NumberOfTimeouts(DateTime date, int? applicationId)
{
    var functionCacheKey = "NumberOfTimeouts";
    var cacheKey = string.Format("{0}-{1}-{2}-{3}", RepositoryCacheKey, functionCacheKey, date, applicationId);
    var cachedNumberTimeouts = _cache.Retrieve(cacheKey);
    if (cachedNumberTimeouts != null)
    {
        return (int)cachedNumberTimeouts;
    }

    //query logic here, calculates numberOfTimeouts

    UpdateCache(date, cacheKey, numberOfTimeouts);
    return numberOfTimeouts;
}

I'm just not too sure what the standard approach is to this, could it involve using a custom attribute class or something similar?

Answer 1

This is a cross-cutting concern. The Decorator pattern may be applicable here. I may be inexperienced in this pattern, however I will give it a shot

// model
public class CustomObject
{
    public int Id { get; set; }
}
// interface
public interface IRepository<T>
{
    IEnumerable<T> Find(Expression<Func<T, bool>> expression);
}
public interface ICacheableRepository<T>
{
    IEnumerable<T> Find(Expression<Func<T, bool>> expression, Func<int> cacheKey);
}
public interface IRepositoryCacheManager<T>
{
    IEnumerable<T> Get(int key);
    bool Any(int key);
    void Add(int key, IEnumerable<T> result);
}
// cache manager
public class RepositoryCacheManager<T> : IRepositoryCacheManager<T>
{
    private Dictionary<int, IEnumerable<T>> cache = new Dictionary<int,IEnumerable<T>>();
    #region IRepositoryCache<T> Members

    public IEnumerable<T> Get(int key)
    {
        return cache[key];
    }

    public bool Any(int key)
    {
        IEnumerable<T> result = null;
        return cache.TryGetValue(key, out result);
    }

    public void Add(int key, IEnumerable<T> result)
    {
        cache.Add(key, result);
    }

    #endregion
}

// cache repository decorator
public class CachedRepositoryDecorator<T> : IRepository<T>, ICacheableRepository<T>
{
    public CachedRepositoryDecorator(IRepositoryCacheManager<T> cache
        , IRepository<T> member)
    {
        this.member = member;
        this.cache = cache;
    }

    private IRepository<T> member;
    private IRepositoryCacheManager<T> cache;

    #region IRepository<T> Members

    // this is not caching
    public IEnumerable<T> Find(Expression<Func<T, bool>> expression)
    {
        return member.Find(expression);
    }

    #endregion

    #region ICacheableRepository<T> Members

    public IEnumerable<T> Find(Expression<Func<T, bool>> expression, Func<int> cacheKey)
    {
        if (cache.Any(cacheKey()))
        {
            return cache.Get(cacheKey());
        }
        else
        {
            IEnumerable<T> result = member.Find(expression);
            cache.Add(cacheKey(), result);
            return result;
        }
    }

    #endregion
}
// object repository
public class CustomObjectRepository : IRepository<CustomObject>
{
    #region IRepository<CustomObject> Members

    public IEnumerable<CustomObject> Find(Expression<Func<CustomObject, bool>> expression)
    {
        List<CustomObject> cust = new List<CustomObject>();
        // retrieve data here
        return cust;
    }

    #endregion
}
// example
public class Consumer
{
    // this cache manager should be persistent, maybe can be used in static, etc
    IRepositoryCacheManager<CustomObject> cache = new RepositoryCacheManager<CustomObject>();
    public Consumer()
    {
        int id = 25;

        ICacheableRepository<CustomObject> customObjectRepository =
            new CachedRepositoryDecorator<CustomObject>(
                cache
                , new CustomObjectRepository()
                );
        customObjectRepository.Find(k => k.Id == id, () => { return id; });
    }
}

Please note:

• I haven't tested this code, don't know whether it is fully functional or not. I just describe the illustration
• Yes, this has code smell by having the ICacheableRepository overloading for Find , however I am incapable in using Expression as Key in Dictionary

The pros:

• This CachedRepositoryDecorator can be used to ANY generic repository (reusable)
• No caching logic inside the select process, emphasize SRP

The cons:

• Hard to implement without ORM, maybe you will need some tweaks with reflection to make it works without ORM
• Hard to understand at beginning
• Hard to wire without DI Container

Credit to this article :)

Answer 2

I think the Unit of Work pattern is what you need. More info:

• http://martinfowler.com/eaaCatalog/unitOfWork.html
• http://www.codeproject.com/Articles/581487/Unit-of-Work-Design-Pattern
• http://msdn.microsoft.com/en-us/magazine/dd882510.aspx

Or a framework that contains the UoW pattern: https://github.com/riteshrao/ncommon