Getting All Redis Keys Starting With a Prefix in Ruby (Detailed Guide w/ Code Examples)

Use Case(s)

In a Ruby application using Redis as a data store, you might have stored data with keys following certain naming conventions or patterns. For example, you may have prefixed user-related data with 'user:', session-related data with 'session:', and so on. In such cases, you may need to fetch all keys starting with a particular prefix.

Code Examples

To get all keys starting with a specific string, Redis provides the KEYS command which accepts a pattern. Here's how you can use it with the redis-rb gem in Ruby:

require 'redis'

redis = Redis.new(host: 'localhost', port: 6379)

keys = redis.keys('user:*')
puts keys

In this example, we're connecting to a local Redis instance and using the keys method with a pattern 'user:' to get all keys that start with 'user:'. The '' is a wildcard character that matches any sequence of characters.

Best Practices

While the KEYS command can be incredibly useful, it’s important to note that it may affect performance when run against large databases because it's a blocking operation. Instead, consider using the SCAN command for production environments if you need to retrieve keys matching a pattern. The SCAN command iteratively scans the database and returns a cursor that you can pass back to iterate over the next batch of keys.

require 'redis'

redis = Redis.new(host: 'localhost', port: 6379)

cursor = '0'

loop do
  cursor, keys = redis.scan(cursor, match: 'user:*')
  puts keys
  break if cursor == '0'
end

This example uses the SCAN command to get all keys starting with 'user:' in a non-blocking manner.

Common Mistakes

One common mistake is to use the KEYS command in production code which can lead to performance issues. As mentioned above, it's better to use SCAN for such operations.

FAQs

Q: Can I use other patterns with the keys or scan method?

A: Yes, you can use any pattern supported by Redis. For example, 'user:?' matches 'user:x' but not 'user:xx'. '[abc]*' matches any key starting with 'a', 'b', or 'c'.