Append Operation in Memcached using Java (Detailed Guide w/ Code Examples)

Use Case(s)

Appending data to existing values in a Memcached server is a common need when you want to extend data without reading and writing the entire blob. This is useful in scenarios such as logging, where new entries are appended to existing ones, or building large data structures over time.

Code Examples

Here's an example of how to append data to already stored data in Memcached using the spyMemcached client library for Java.

First, add the dependency to your Maven project:

<dependency> <groupId>net.spy</groupId> <artifactId>spymemcached</artifactId> <version>2.12.1</version> </dependency>

Now, let's connect to the memcached server, store a value, and then append to it:

import net.spy.memcached.MemcachedClient; import; import java.util.concurrent.Future; public class App { public static void main(String[] args) throws Exception { // Create a memcached client MemcachedClient client = new MemcachedClient(new InetSocketAddress("localhost", 11211)); // Set a key-value pair Future<Boolean> f = client.set("key", 0, "value"); System.out.println("set status: " + f.get()); // Append to the existing value Future<Boolean> fa = client.append("key", " appended"); System.out.println("append status: " + fa.get()); // Get the value to confirm the append operation was successful System.out.println("get: " + client.get("key")); // Shut down the client client.shutdown(); } }

When you run this, you should see:

set status: true
append status: true
get: value appended

Best Practices

  • Always check the result of append operation. It could fail if the key does not exist.
  • Consider network latency and the added complexity when deciding whether appending is the right solution or if a full set would be better.
  • Be aware that Memcached does not support multithreaded appends on the same key.

Common Mistakes

  • Trying to append to non-existent keys. The append operation does not create a new key; it only works on existing keys.
  • Overloading the server by appending large amounts of data at once. It's more efficient to write larger changes in one go rather than appending multiple times.


Q: What happens if I try to append to a key that doesn't exist? A: The append operation will fail. Memcached does not automatically create a key if it doesn't exist during an append operation.

Q: Is there a limit to how much data I can append? A: Yes, the maximum amount of data a single key can hold in Memcached is 1MB. If your append operations cause the total size to exceed this, the operation will fail.

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