* vs # in RabbitMQ Topic Exchange: Key Differences and Usage

In RabbitMQ topic exchanges, * matches exactly one word in a routing key, while # matches zero or more words. Use * for precise single-word matching and # for flexible multi-word matching in routing keys.

Quick Comparison

This table summarizes the main differences between * and # wildcards in RabbitMQ topic exchanges.

Feature	* Wildcard	# Wildcard
Match Type	Exactly one word	Zero or more words
Position in Routing Key	Matches one word at a specific position	Matches multiple words or none at any position
Example Match	stock.*.ny matches stock.us.ny	stock.# matches stock.us.ny and stock
Use Case	Precise routing with fixed word count	Flexible routing with variable word count
Wildcard Symbol	*	#

Key Differences

The * wildcard in RabbitMQ topic exchanges matches exactly one word in the routing key. Words are separated by dots (.), so * replaces a single word at a specific position. For example, a binding key log.*.error matches log.app.error but not log.app.db.error because the latter has two words between log and error.

In contrast, the # wildcard matches zero or more words. It can replace any number of words, including none. For example, log.# matches log, log.app, and log.app.db.error. This makes # more flexible for routing messages with variable word counts in routing keys.

In summary, use * when you want to match exactly one word at a certain position, and use # when you want to match any number of words, including none, in that part of the routing key.

Code Comparison

Here is an example of using the * wildcard in a topic exchange binding key to route messages with exactly one word in the middle position.

python

import pika

connection = pika.BlockingConnection(pika.ConnectionParameters('localhost'))
channel = connection.channel()

channel.exchange_declare(exchange='topic_logs', exchange_type='topic')

# Binding key with * wildcard
binding_key = 'stock.*.ny'

channel.queue_declare(queue='queue_star')
channel.queue_bind(exchange='topic_logs', queue='queue_star', routing_key=binding_key)

# Publish messages
channel.basic_publish(exchange='topic_logs', routing_key='stock.us.ny', body='Message for stock.us.ny')
channel.basic_publish(exchange='topic_logs', routing_key='stock.eu.ny', body='Message for stock.eu.ny')
channel.basic_publish(exchange='topic_logs', routing_key='stock.us.ca', body='Message for stock.us.ca')

print('Messages sent with * wildcard binding')
connection.close()

Output

Messages sent with * wildcard binding

# Equivalent

This example uses the # wildcard in a topic exchange binding key to route messages with zero or more words after stock.

python

import pika

connection = pika.BlockingConnection(pika.ConnectionParameters('localhost'))
channel = connection.channel()

channel.exchange_declare(exchange='topic_logs', exchange_type='topic')

# Binding key with # wildcard
binding_key = 'stock.#'

channel.queue_declare(queue='queue_hash')
channel.queue_bind(exchange='topic_logs', queue='queue_hash', routing_key=binding_key)

# Publish messages
channel.basic_publish(exchange='topic_logs', routing_key='stock', body='Message for stock')
channel.basic_publish(exchange='topic_logs', routing_key='stock.us.ny', body='Message for stock.us.ny')
channel.basic_publish(exchange='topic_logs', routing_key='stock.eu.ca.ny', body='Message for stock.eu.ca.ny')

print('Messages sent with # wildcard binding')
connection.close()

Output

Messages sent with # wildcard binding

When to Use Which

Choose * when you need to match routing keys with a fixed number of words and want precise control over which word is matched. For example, if you want to route messages only when the second word is specific, use *.

Choose # when you want to match routing keys with variable length or when you want to catch all messages under a certain prefix regardless of how many words follow. This is useful for broad routing rules.

In short, use * for exact single-word matching and # for flexible multi-word matching.

Key Takeaways

* matches exactly one word in a routing key segment.

# matches zero or more words, allowing flexible routing.

Use * for precise, fixed-length routing keys.

Use # for broad, variable-length routing keys.

Both wildcards help control message routing in RabbitMQ topic exchanges.