Going Serverless with Azure
Modern SharePoint Online gives developers many architecture choices.
For full blown SharePoint Point apps use SharePoint Framework and build server components as Azure Web Apps (http://1jj.uk/azurewebapps). Store your transactional data in Azure SQL (http://1jj.uk/azuresql), use CosmosDB (http://1jj.uk/cosmosdb) for huge scale, or Azure Table Storage (http://1jj.uk/azuretables) for ultimate low cost.
Automating Business Processes?
Add your logic on top of existing SharePoint artefacts.
Reacting to events?
Logic Apps (http://1jj.uk/azurelogicapps) or Flow can connect to SharePoint and be triggered by events on Lists and Libraries.
Logic Apps can easily create an Azure Function (http://1jj.uk/azurefunction) in C# or node.js, giving you full CSOM.
Logic Apps and Azure Function can both be triggered by time-based events.
Gluing things together?
Logic Apps with HTTP endpoints, or webhook-style Azure Functions for when you need synchronous behaviour.
For asynchrony, consider Azure Storage Queues (http://1jj.uk/azurequeues). Logic Apps and Azure Functions can send and receive messages to and from these.