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Node Memcached Replace Operation (Detailed Guide w/ Code Examples)

Use Case(s)

The replace operation in Memcached is used when you want to change the value of an existing key. Unlike the set operation, replace will only succeed if the key already exists in the cache. This can be useful when your application needs to ensure that it's only updating previously cached values and not creating new keys.

Code Examples

Here's a basic example of using replace with memcached in Node.js:

var memcached = require('memcached'); var client = new memcached('localhost:11211'); var key = 'myKey'; var value = 'Hello World'; client.set(key, value, 10000, function(err) { //set value first if(err) console.log(err); }); var newValue = 'Hello Universe'; client.replace(key, newValue, 10000, function(err) { //replace value if(err) console.log(err); });

In this example, we first set the key "myKey" to have the value "Hello World". We then replace that value with "Hello Universe". If the key didn't exist, the replace operation would fail.

Best Practices

  1. Always handle errors from your memcached operations. Network issues or misconfigurations can lead to failures.
  2. Be aware of your TTL (Time To Live). If a key expires before you call replace, the operation will fail because the key no longer exists.

Common Mistakes

  1. A common mistake is to use replace without understanding its behavior. If the key doesn't exist, replace will fail whereas set would succeed.
  2. Not checking for errors after a replace operation. This can lead to silent failures and hard-to-debug problems.


  1. Question: What happens if I call replace on a key that doesn't exist? Answer: The replace operation will fail. It only works on keys that already exist in the cache.

  2. Question: How is replace different from set? Answer: The set operation will create a new key-value pair if the key doesn't exist, whereas replace only modifies existing key-value pairs.

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