Memcached CAS Command

The Memcached CAS (Check-And-Set or Compare-And-Swap) command is used to perform a "check and set" operation. It only allows the value to be written if the value associated with the key has not been modified by another client since the last time the current client retrieved it.

The check is performed using the cas_token parameter, which is a unique 64-bit value assigned by Memcached to an existing element.

Syntax:

The basic syntax format for the CAS command is as follows:

cas key flags exptime bytes unique_cas_token [noreply]
value

Parameter descriptions are as follows:

• key: The key in the key-value structure used to find the cached value.
• flags: An integer parameter that the client can use to store additional information about the key-value pair.
• exptime: The duration for which the key-value pair is stored in the cache (in seconds, 0 means forever).
• bytes: The number of bytes stored in the cache.
• uniquecastoken: A unique 64-bit value obtained through the gets command.
• noreply (optional): This parameter informs the server not to return data.
• value: The stored value (always on the second line), which can be directly understood as the value in the key-value structure.

Example

To use the CAS command on Memcached, you need to obtain a token from the Memcached service provider via the gets command.

The gets command is similar to the basic get command, but it returns slightly more information: a 64-bit integer value that acts as a "version" identifier for the name/value pair.

Example steps are as follows:

• The CAS command will fail if no unique token is set.
• The operation will fail if the key does not exist.
• Add the key-value pair.
• Obtain the unique token via the gets command.
• Update the data using the cas command.
• Check if the data is updated using the get command.

  cas tp 0 900 9
  ERROR             <− Missing token
  
  cas tp 0 900 9 2
  memcached
  NOT_FOUND         <− Key tp does not exist
  
  set tp 0 900 9
  memcached
  STORED
  
  gets tp
  VALUE tp 0 9 1
  memcached
  END
  
  cas tp 0 900 5 1
  redis
  STORED
  
  get tp
  VALUE tp 0 5
  redis
  END
  

Output

If the data is added successfully, the output will be:

STORED

Output information descriptions are as follows:

• STORED: Output after successful save.
• ERROR: Indicates an error in saving or syntax.
• EXISTS: Another user has updated the data since the last retrieval.
• NOT_FOUND: The key-value does not exist on the Memcached service.