Topic exchange (pattern matching) in RabbitMQ - Mini Project: Build & Apply

Topic Exchange with Pattern Matching in RabbitMQ

📖 Scenario: You are setting up a messaging system for a news service. Different news categories like sports, weather, and finance send messages. You want to route messages to queues based on topics using pattern matching.

🎯 Goal: Build a RabbitMQ topic exchange setup where messages are routed to queues based on matching routing keys with patterns.

📋 What You'll Learn

Create a topic exchange named news_topic_exchange

Create two queues named sports_queue and weather_queue

Bind sports_queue with the routing key pattern sports.*

Bind weather_queue with the routing key pattern weather.#

Publish messages with routing keys sports.football and weather.rain

Consume and print messages from both queues

💡 Why This Matters

🌍 Real World

Topic exchanges are used in messaging systems to route messages based on flexible patterns, like categorizing news or logs.

💼 Career

Understanding topic exchanges is important for configuring message brokers in microservices, event-driven architectures, and distributed systems.

1

Create the topic exchange and queues

Create a topic exchange called news_topic_exchange. Then create two queues named sports_queue and weather_queue.

RabbitMQ

# Your code here to declare exchange and queues

Hint

Use exchange_declare with exchange_type='topic' and queue_declare for queues.

2

Bind queues with routing key patterns

Bind the queue sports_queue to news_topic_exchange with the routing key pattern sports.*. Bind the queue weather_queue with the routing key pattern weather.#.

RabbitMQ

import pika

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

channel.exchange_declare(exchange='news_topic_exchange', exchange_type='topic')
channel.queue_declare(queue='sports_queue')
channel.queue_declare(queue='weather_queue')

# Bind sports_queue with 'sports.*'
# Bind weather_queue with 'weather.#'
# Your code here

Hint

Use queue_bind with the correct routing_key patterns.

3

Publish messages with routing keys

Publish a message with the routing key sports.football and body 'Football news'. Then publish a message with the routing key weather.rain and body 'Rain forecast' to the news_topic_exchange.

RabbitMQ

import pika

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

channel.exchange_declare(exchange='news_topic_exchange', exchange_type='topic')
channel.queue_declare(queue='sports_queue')
channel.queue_declare(queue='weather_queue')

channel.queue_bind(exchange='news_topic_exchange', queue='sports_queue', routing_key='sports.*')
channel.queue_bind(exchange='news_topic_exchange', queue='weather_queue', routing_key='weather.#')

# Publish message with routing key 'sports.football' and body 'Football news'
# Publish message with routing key 'weather.rain' and body 'Rain forecast'
# Your code here

Hint

Use basic_publish with the correct exchange, routing_key, and body.

4

Consume and print messages from queues

Consume one message from sports_queue and print its body. Then consume one message from weather_queue and print its body.

RabbitMQ

import pika

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

channel.exchange_declare(exchange='news_topic_exchange', exchange_type='topic')
channel.queue_declare(queue='sports_queue')
channel.queue_declare(queue='weather_queue')

channel.queue_bind(exchange='news_topic_exchange', queue='sports_queue', routing_key='sports.*')
channel.queue_bind(exchange='news_topic_exchange', queue='weather_queue', routing_key='weather.#')

channel.basic_publish(exchange='news_topic_exchange', routing_key='sports.football', body='Football news')
channel.basic_publish(exchange='news_topic_exchange', routing_key='weather.rain', body='Rain forecast')

# Consume one message from sports_queue and print its body
# Consume one message from weather_queue and print its body
# Your code here

Hint

Use basic_get with auto_ack=True to get messages and print their decoded bodies.

Practice

1. What is the main purpose of a topic exchange in RabbitMQ?

easy

A. To broadcast messages to all queues regardless of routing keys

B. To store messages until consumers are ready

C. To route messages based on pattern matching of routing keys

D. To encrypt messages before sending

Solution

1. Step 1: Understand the role of exchanges in RabbitMQ

   Exchanges route messages to queues based on rules. Topic exchanges use patterns in routing keys.

2. Step 2: Identify the unique feature of topic exchanges

   Topic exchanges route messages by matching routing keys with patterns containing * and # wildcards.

3. Final Answer:

   To route messages based on pattern matching of routing keys -> Option C

4. Quick Check:

   Topic exchange = pattern matching routing [OK]

Hint: Topic exchange uses wildcards to match routing keys [OK]

Common Mistakes:

• Confusing topic exchange with fanout exchange
• Thinking topic exchange stores messages
• Assuming topic exchange encrypts messages

2. Which of the following is the correct syntax to bind a queue named logs to a topic exchange named topic_logs with routing key pattern kern.* using RabbitMQ CLI?

easy

A. rabbitmqadmin declare binding source=topic_logs destination=logs routing_key=kern.*

B. rabbitmqctl bind queue=logs exchange=topic_logs routing_key=kern.*

C. rabbitmqadmin bind queue=logs exchange=topic_logs routing_key=kern.*

D. rabbitmqctl declare binding source=topic_logs destination=logs routing_key=kern.*

Solution

1. Step 1: Identify the correct RabbitMQ CLI tool for binding

   Binding queues to exchanges is done with rabbitmqadmin declare binding, not rabbitmqctl.

2. Step 2: Check the correct parameters for binding

   The correct parameters are source for exchange, destination for queue, and routing_key for pattern.

3. Final Answer:

   rabbitmqadmin declare binding source=topic_logs destination=logs routing_key=kern.* -> Option A

4. Quick Check:

   Use rabbitmqadmin declare binding with source, destination, routing_key [OK]

Hint: Use rabbitmqadmin declare binding with source and destination [OK]

Common Mistakes:

• Using rabbitmqctl instead of rabbitmqadmin for binding
• Swapping source and destination parameters
• Omitting routing_key parameter

3. Given a topic exchange with a queue bound using the routing key pattern *.orange.*, which routing keys will deliver messages to this queue?

medium

A. quick.orange.rabbit, orange

B. orange.rabbit, quick.orange

C. quick.orange, orange.lazy.rabbit

D. quick.orange.rabbit, lazy.orange.elephant

Solution

1. Step 1: Understand the pattern *.orange.*

   The pattern means exactly three words separated by dots, with 'orange' as the middle word. The * matches exactly one word.

2. Step 2: Check each routing key against the pattern

   quick.orange.rabbit, lazy.orange.elephant keys: 'quick.orange.rabbit' and 'lazy.orange.elephant' both have three words with 'orange' in the middle, so they match.

3. Final Answer:

   quick.orange.rabbit, lazy.orange.elephant -> Option D

4. Quick Check:

   Pattern *.orange.* matches three-word keys with orange in middle [OK]

Hint: Count words; * matches one word, # matches zero or more [OK]

Common Mistakes:

• Assuming * matches zero or multiple words
• Ignoring word count in routing key
• Confusing * with # wildcard

4. You have a queue bound to a topic exchange with the routing key pattern stock.#. Messages with routing key stock.us.nyse are not delivered to the queue. What is the most likely cause?

medium

A. The exchange type is not set to topic

B. The routing key pattern stock.# does not match stock.us.nyse

C. The queue is not bound correctly to the exchange

D. The message was published with an empty routing key

Solution

1. Step 1: Verify the exchange type

   The pattern stock.# matches routing keys starting with 'stock' followed by zero or more words. If messages are not delivered, the exchange might not be a topic exchange.

2. Step 2: Check other possible causes

   Binding and routing key pattern are correct. If the exchange is not topic type, pattern matching won't work.

3. Final Answer:

   The exchange type is not set to topic -> Option A

4. Quick Check:

   Topic exchange required for pattern matching [OK]

Hint: Ensure exchange type is topic for pattern matching [OK]

Common Mistakes:

• Assuming binding is always correct without verification
• Thinking # wildcard doesn't match multiple words
• Ignoring exchange type setting

5. You want to receive all messages related to any fruit except those about apples or bananas using a topic exchange. Which binding key pattern should you use?

hard

A. *.fruit.*

B. #.fruit.# and bind separate queues for apple and banana to discard

C. #.fruit.# but exclude apple and banana keys manually

D. #.fruit.#

Solution

1. Step 1: Understand topic exchange limitations

   Topic exchanges route messages by matching patterns but cannot exclude specific keys directly in binding patterns.

2. Step 2: Plan a solution using multiple queues

   Bind one queue with #.fruit.# to get all fruit messages. Bind separate queues for apple and banana keys to handle or discard them separately.

3. Step 3: Implement exclusion by filtering in consumers or separate queues

   Since exclusion is not supported in binding keys, use separate queues or consumer logic to exclude apples and bananas.

4. Final Answer:

   #.fruit.# and bind separate queues for apple and banana to discard -> Option B

5. Quick Check:

   Exclusion requires separate queues or consumer filtering [OK]

Hint: Topic exchange can't exclude keys; use separate queues [OK]

Common Mistakes:

• Trying to exclude keys using binding patterns
• Assuming * or # can exclude specific words
• Not using multiple queues or consumer filtering