We have all services running inside a VPN (see one of our older posts: https://blog.yourkarma.com/building-private-clouds-with-amaz...) and we also use HTTP Basic token auth that are configured upon deployment. Every app gets its own token, so we can trace which app does what.

I'd figured the services were on a private network (or at least the app is listening on an interface on a private network), but it is good someone confirmed it. Thanks for your insight on security.

This. Also SOA means STD in Dutch, so we don't want to use it that often ;)

We use SQS for basically everything that happens asynchronously to the main flow. So when a user signs up there are a bunch of things that need to happen straight away (create an account, give 100MB to every new user), these go via HTTP. The others (email, give the owner of a Karma an additional 100MB and a push notification) goes through SNS and SQS.

All these async things don't impact the main flow of the user, so it's no problem that it isn't instantaneous. We are running background processes that do this.

We only use Rails for our frontend applications, they don't contain any business logic. Our backend processes are usually Sinatra for HTTP requests, or custom daemons.

Each app gets their own queue. So there is a queue for the shipment app and for the mailer app. When an event is published to SNS, it gets added to both queues.

I see. So you never add a listener, without also modifying the emitter to send to an additional queue.

We don't change the emitter, the listener changes the SNS config. The emitter publishes to SQS under a certain topic. Listeners that interested in that topic subscribe to it by changing the SNS config, basically saying "give me a copy of those events too and put it onto my own queue". SNS copies the messages to every queue.

Yup we are. There might be multiple applications interested in the event and they each get their own queue.

Have you looked into a "real" messaging queue for this, like RabbitMQ or another AMQP based system for this? Or are you bound to SNS/SQS for a particular reason?

"Real messaging" aside, this is an interesting question. It'd be great to hear from iain_hecker if they considered using RabbitMQ (or similar) and if yes, why SQS/SNS were chosen.

Also, I'd be interested to read more about how you handle authentication across web/mobile. Thank you for the blog post and for taking the time to answer questions here. :-)

We have considered stuff like RabbitMQ. They would be excellent solutions too and we are working on a part of our architecture with MQTT/protobuf (more on that later, I'm sure). We went with SNS/SQS because we don't have to build a cluster with that. Building clusters is hard and Amazon takes care of that. Since we were already using a bunch of AWS products, this was a nice fit and we're quite happy with it.

Authentication is a good idea for an article too. Thanks :)

Why isn't SNS/SQS a "real" messaging queue ?

Sorry, my question was a bit vague. Generally SQS works with polling, and once a message has been pulled from a queue it is not available to other viewers any longer. This makes it hard to create a system where you have an unknown number of clients that may or may not be interested in certain queues, or even in messages of certain types.

RabbitMQ (and other AMQP based queueing systems) have built-in solutions for this that SQS doesn't offer. Karma solved it by sending the same message to a number of different queues, but this makes scaling to more clients harder, and reduces flexibility in subscribing clients to certain message types.

Hope this makes it clearer :-).

That's where SNS comes in. We publish to SNS which will fan out to multiple SQS queues. When a new service is built, it says to SNS "subscribe my queue to these topics". The publisher of the event doesn't know about the subscribers.

Ah, nice, I didn't know the integration of SNS and SQS worked that way. Cool!

We will! There are however a couple of really nasty issues that we don't feel comfortable to release to the public yet.

It's easier to test individual items, for sure. Testing small code bases is the best thing ever. However, some actions require stuff to happen in multiple services.

There are "contracts" on how to talk to other services. You can test if you follow that contract, but it's hard to verify automatically that these contracts are in sync between apps.