Prepending in Memcached with Java (Detailed Guide w/ Code Examples)

Use Case(s)

A common use case for prepending in Memcached with Java is when you want to add data to the beginning of an existing cached value without having to read it first, then edit it, and finally write it back. This can be particularly useful for applications where the record grows dynamically, like log gathering or tracking user activity history.

Code Examples

The following code examples depict how to prepend data into existing keys in Memcached using Java. We'll be using the spyMemcached client.

import net.spy.memcached.MemcachedClient; public class Main { public static void main(String[] args) { try { // Create a memcached client MemcachedClient client = new MemcachedClient(new InetSocketAddress("localhost", 11211)); // Set a key client.set("key", 3600, "Hello"); // Prepend to a key client.prepend(0, "key", "World "); System.out.println(client.get("key")); // outputs: 'World Hello' // Close connection client.shutdown(); } catch (IOException e) { e.printStackTrace(); } } }

In this example, "Hello" is set as the value for the specified key. We then prepend "World " to the same key resulting in the final value: 'World Hello'. The first parameter in the prepend function is used as the CAS value, setting it to 0 means we're not using it.

Best Practices

  • Always check if the key exists before attempting to prepend. If the key doesn't exist, an error might occur.
  • Be mindful of the size of the data being stored. Memcached has a limit of 1MB per key/value pair.
  • Be aware that prepending operations are not atomic. If multiple clients prepend to the same key at the same time, the final value could be unpredictable.

Common Mistakes

  • Attempting to prepend to a non-existent key. Always ensure the key exists before using prepend.
  • Not considering thread safety. If multiple threads are manipulating the same key, consider implementing synchronization to avoid race conditions.


Q: Is it possible to prepend data to a memcached key with a specific expiry? A: No, the prepend operation doesn't affect the expiration time of the key. The expiration time is only set when you initially set or add the key.

Q: What happens if I try to prepend to a key that stores non-string data? A: The results are undefined. Prepend operation treats values as strings, so trying to prepend to a non-string value can lead to unexpected results.

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