I want to create an application that will run as a service that subscribes to a Google Cloud pub/sub topic, and delivers emails based on 'alerts' that it receives through the pub/sub subscription. It will read from Cloud DataStore to determine the email addresses to mail to, and it will probably use some type of 3rd party service and API to do the actual mailing. In future, there will be a requirement to send other types of notification, for example SMS alerts.
What types of Google Cloud application should I consider for this? I'm not sure what type of application would be appropriate for something that effectively acts as a constantly available 'service'. Could I just create a normal Java application that subscribes to the topic in its 'main' method, and deploy this as an AppEngine app? Or are there more complex considerations I need to bear in mind? I've previously used DataFlow, which handles parallel processing of data very well, when subscribed to a pub/sub topic, however it's really designed for processing Big Data and I don't think would be appropriate.
Apologies for the noob question - I'm still learning to think in terms of Cloud architectures as opposed to traditionally running services. I'd really appreciate some basic guidance on what type of application I should be looking at creating.