Appending Data in Memcached using Ruby (Detailed Guide w/ Code Examples)

Use Case(s)

In Ruby, appending data to an existing key-value pair in Memcached can be beneficial in various scenarios, such as:

  1. Session Management: Storing session information that constantly updates, like a user's activity on a website or app.
  2. Real-time Analytics: Tracking real-time actions, where you need to append new events to an existing list.
  3. Caching: When the cached data is updated frequently and needs to be appended rather than replaced.

Code Examples

Let's consider some examples using the popular Ruby client for Memcached, Dalli.

Example 1: Basic Append Operation

require 'dalli' dc ='localhost:11211') # Set an initial value dc.set('key', 'Hello') # Append to the existing value dc.append('key', ', World!') # Get the final value puts dc.get('key') # It will output: "Hello, World!"

In this example, we first set an initial value ('Hello') for the key 'key'. Then, we use the append method to add more data (', World!') at the end of our existing value. Finally, we retrieve the updated value.

Example 2: Append Operation with non-existing Key

require 'dalli' dc ='localhost:11211') # Try to append to a non-existent key dc.append('non_existent_key', ', Universe!') # Get the value puts dc.get('non_existent_key') # It will be nil

In this case, we tried to append data to a non-existent key. As the key was not previously set, the append operation fails and returns nil when we try to get the value.

Best Practices

  1. Check if Key Exists: Before attempting to append data, ensure that the key exists. Append operations on non-existing keys will fail.
  2. Exception Handling: Implement proper error handling for cache server connection issues to prevent application crashes.
  3. Avoid Large Values: Memcached has a limit on the size of values it can store (default: 1MB). Keep your values within this limit.

Common Mistakes

  1. Ignoring Return Values: The append operation in Dalli returns a boolean indicating success or failure. Neglecting to check this can lead to unseen failures.
  2. Appending Large Data: Trying to append data such that the total size exceeds Memcached's limit will cause the operation to fail.


Q: Can I prepend data in Memcached using Ruby? A: Yes, similar to append, Dalli provides a prepend function to add data at the start of an existing value.

Q: What happens if the server runs out of memory while appending data? A: If the server runs out of memory during an append operation, the operation will fail and no data will be appended.

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