Decrementing Values in Memcached with Java (Detailed Guide w/ Code Examples)

Use Case(s)

The decr function is used when you need to decrement a numeric value stored in Memcached. This is often utilized in scenarios like:

• Tracking available inventory of an item
• Counting down from a specific number, like during a countdown
• Rate limiting where you increment a counter each time a user accesses a service and decrement it after a certain period

Code Examples

Let's assume that we have a system that tracks the remaining seats for a concert. Here is how you could set up and use decr.

// Assuming you have already established a connection to Memcached named 'memcachedClient'
String key = "remaining_seats";
int initialValue = 100;
int expiryTime = 3600; // Setting the expiry time for 1 hour

// Set initial value in memcached
memcachedClient.set(key, expiryTime, initialValue);

// Now lets decrement the value by 1
long newValue = memcachedClient.decr(key, 1);

System.out.println("New Value: " + newValue); // Output: New Value: 99

This will reduce the value of 'remaining_seats' key in Memcached by 1.

Best Practices

• Always check if the key exists before trying to decrement its value. If the key doesn't exist, Memcached will return 0.
• Be aware of underflows. Memcached won't allow the value to go below 0, so if you decrement a value that's currently at 0, the operation will fail.
• When decrementing, make sure the stored value is a numeric string. Other types may lead to unpredictable results.

Common Mistakes

• Trying to decrement non-numeric values. Memcached decr function works only on numeric values. If the value is non-numeric, you'll get an error.
• Not handling missing keys. If a key does not exist, Memcached will return 0 and will not create this key.

FAQs

Q: What happens if the new value should be less than zero? A: Memcached will not let the value go below 0. If a decrement operation would result in a negative number, the new value will instead be 0.

Q: What will happen if I try to decrement a key that does not exist? A: If a key does not exist in Memcached and you try to decrement it, Memcached will simply return 0.