Memcached - Prepend Data

Memcached - Prepending Data to an Existing Item

The prepend command is an atomic operation that adds data to the beginning of an existing value for a given key. This is useful for tasks like adding metadata or updating a list without needing to first read the entire value.

Syntax

The basic syntax for the prepend command is as follows:

prepend key flags exptime bytes [noreply]
<value>

• key: The key of the item to modify.
• flags: A 32-bit integer stored with the item, retrievable on get calls.
• exptime: The new expiration time for the item in seconds. A value of 0 means it never expires.
• bytes: The size in bytes of the data you are prepending.
• [noreply] (optional): Instructs the server not to send a response, which can improve performance in write-heavy scenarios.
• value: The data to be prepended.

Command Line Example

Let’s demonstrate using telnet. First, we set a key, then we prepend data to it.

# Connect to Memcached
$ telnet localhost 11211

# 1. Set an initial key-value pair
set site 0 900 11
example.com
STORED

# 2. Prepend a prefix to the existing value
prepend site 0 900 8
https://
STORED

# 3. Retrieve the final value
get site
VALUE site 0 19
https://example.com
END

The server responds with STORED on success. If the key does not exist, it will respond with NOT_STORED.

Modern Development: Prepending Data with a Java Client

In a real application, you will use a client library to interact with Memcached. The following example uses the popular spymemcached library in Java.

1. Project Setup (Maven)

Add the spymemcached dependency to your project’s pom.xml:

<dependency>
    <groupId>net.spy</groupId>
    <artifactId>spymemcached</artifactId>
    <version>2.12.3</version> <!-- Use the latest stable version -->
</dependency>

2. Java Code Example

import net.spy.memcached.MemcachedClient;
import net.spy.memcached.internal.OperationFuture;
import java.net.InetSocketAddress;

public class PrependExample {
    public static void main(String[] args) {
        // Use try-with-resources for automatic connection management and cleanup
        try (MemcachedClient mcc = new MemcachedClient(new InetSocketAddress("127.0.0.1", 11211))) {
            System.out.println("Connection to Memcached server successful.");

            String key = "user:profile:activity";
            String initialActivity = "viewed_homepage";
            String newActivity = "clicked_login,";

            // 1. Set an initial value for the key (15-minute expiration)
            OperationFuture<Boolean> setFuture = mcc.set(key, 900, initialActivity);
            if (setFuture.get()) { // .get() blocks until the operation completes
                System.out.println("Successfully set initial value for key '" + key + "'.");
            }

            // 2. Retrieve the current value to verify
            System.out.println("Current value: " + mcc.get(key));

            // 3. Prepend new activity data to the existing string
            OperationFuture<Boolean> prependFuture = mcc.prepend(key, newActivity);
            if (prependFuture.get()) {
                System.out.println("Successfully prepended '" + newActivity + "'.");
            }

            // 4. Retrieve the updated value
            System.out.println("Updated value: " + mcc.get(key));

        } catch (Exception e) {
            System.err.println("An error occurred: " + e.getMessage());
            // In a production application, use a proper logging framework (e.g., SLF4J)
            e.printStackTrace();
        }
    }
}

3. Expected Output

Connection to Memcached server successful.
Successfully set initial value for key 'user:profile:activity'.
Current value: viewed_homepage
Successfully prepended 'clicked_login,'.
Updated value: clicked_login,viewed_homepage

Common Pitfalls and Best Practices

• Atomicity: prepend is an atomic operation. This makes it safe to use in concurrent environments without needing application-level locks for simple prefixing tasks.
• Key Must Exist: The operation will fail (NOT_STORED) if the key does not already exist in the cache. Your application logic must handle this case, perhaps by setting an initial value first.
• Item Size Limit: Be mindful of Memcached’s default item size limit (typically 1MB). Repeatedly prepending data to a key without it expiring can cause the item to exceed this limit, resulting in an error.
• Use Cases: prepend is ideal for maintaining time-ordered lists (where newest items come first), adding prefixes like version numbers or metadata, or building up a log string in cache.