C# Memcached CAS (Detailed Guide w/ Code Examples)

Use Case(s)

Check-and-set (CAS) operation is used in Memcached to solve the problem of lost updates due to overwrites. A common use case for the 'csharp memcached cas' query would be when you need to perform a read-modify-write operation on data stored in Memcached using C#, ensuring that the data has not been modified by another client between the read and write operations.

Code Examples

Here is a simple example using the EnyimMemcached client for C#:

using Enyim.Caching;
using Enyim.Caching.Memcached;

// Create client
MemcachedClient client = new MemcachedClient();

// Assume "Key1" already exists in cache
CasResult<string> result = client.GetWithCas<string>("Key1");

// Check if we got the value successfully
if (result.Result != null)
{
    // Modify the value
    string newValue = result.Result + "_modified";

    // Try to store it back using CAS
    CasResult<bool> setResult = client.Cas(StoreMode.Set, "Key1", newValue, result.Cas);

    if (!setResult.Result)
    {
        Console.WriteLine("The key was updated by someone else.");
    }
}

In this code snippet, initially, we're getting a key-value pair with its CAS value using GetWithCas(). If the key does exist, we modify the value, then try to update the key-value pair using Cas(). If the update fails because another client updated the data, we output a message indicating so.

Best Practices

• Be sure to handle the scenario where the CAS operation fails, usually by retrying the operation or by providing user feedback.
• Don't use CAS as a locking mechanism. It is designed to avoid overwriting data, not to control access to it.
• Always verify that the server supports CAS operations before using them.

Common Mistakes

• Not handling the case where the item was modified between the GetWithCas() and Cas() calls. In such cases, an exception won't be thrown; instead, the Result property of the CasResult will be false.
• Using CAS in high contention scenarios. If many clients are trying to update the same key concurrently, they could continuously invalidate each other's CAS tokens, leading to a situation known as "CAS thrashing".

FAQs

Q: What happens when a key is evicted from Memcached? Does its CAS value reset? A: Yes. When a key is evicted and then added again, it will have a new CAS value.

Q: Can I use CAS for transactions in Memcached? A: No. Memcached doesn't support transactions. CAS can help avoid overwrite conflicts, but it does not provide transactional integrity across multiple keys.