When to use one queue vs multiple?

Question

Say I want to send emails to users.

• When users sign up for an account they get an email.
• When they complete an order they get an email.
• When they receive a message they get an email.

My hunch is that a event Fanout is a good approach to use here. Say I have multiple SNS topics e.g. orders, users, messages. Each topic has their respective events e.g. order_created, user_created, message_created.

My question is which one to choose:

1. Create a single queue that subscribes to all of these topics with an attached worker that will send the appropriate email based on the event
2. Create a separate queue for each of these topics where the attached worker only sends a specific email

I am having a hard time identifying obvious pros / cons. Also more generally what are scenarios where you would have one queue subscribe to multiple topics vs multiple queues each subscribed to a single topic.

Thanks!

Answer 1

Here are my pros and cons for the two options that you have listed.

Single queue for multiple topics

Pros

• Less components that needs to be monitored
• Better resource utilization if the individual topics have not too many messages

Cons

• If one topic can have a lot more messages than others then it can cause delay for other topics' messages
• It is easier to reach the max messages / queue quota with a single queue
• If the queue becomes temporarily unreachable then none of the message types processing can advance

Dedicated queue per topic

Pros

• One queue could be setup as FIFO while others as standard
  • Different settings can be used for each queue (like: retention period, maximum message size, etc.)
• Easier to introduce different workers for different queues if the increased load requires that
  • Also you can scale up and down different workers based on the different queues' length

Cons

• If you introduce more and more message types then the number of topics, queues and workers can impose some maintenance burden on you
• It is harder to correlate two messages from different topics