Decrementing Values in Memcached using Node.js (Detailed Guide w/ Code Examples)

Use Case(s)

The Memcached 'decr' function is commonly used when you need to decrease a numeric value stored in Memcached. Some typical use cases are:

• Decreasing a counter, such as the number of available items in inventory.
• Rate limiting, where you decrement a counter each time a user performs an action.

Code Examples

Here's a basic example to use decr in Node.js with memcached:

var memcached = require('memcached');

var client = new memcached('localhost:11211');

// Setting a key-value pair
client.set('counter', 100, 10000, function(err) {   
    if(err) console.error(err);
});

// Decrementing the value
client.decr('counter', 10, function(err, data) {
    if(err) console.error(err);
    console.log(data);  // Output: 90
});

In this example, we first set a key ('counter') with a value of 100. Then we use decr to decrease this value by 10. The resultant value (90) is logged in the console.

Best Practices

1. Confirm that the value you're trying to decrement is actually a number. Memcached will throw an error if you try to decrement a non-numeric value.
2. Don't assume the decrement operation will always succeed. Always check for errors in your callback function.
3. Avoid decrementing a value below 0. Memcached's decrement operation will not go below 0 and it will leave the value at 0.

Common Mistakes

1. Trying to decrement a key that doesn't exist: Memcached doesn't treat this as an error. Instead, it just does nothing. Always ensure the key exists before trying to decrement its value.
2. Not realizing that decr won't go below 0: If you're using decrement to manage resources and you need to know when you've run out, make sure to check if the result is 0.

FAQs

1. What happens if I try to decrement a non-numeric value? Memcached will return an error if you try to decrement a non-numeric value.

2. What if the value after decrementing becomes negative? Memcached's decrement operation will not let a value go below 0. It will set and return 0.

3. Does 'decr' create a key if it doesn't already exist? No, unlike incr, decr doesn't create a key if it doesn't already exist. The operation just fails without an error.