12 Principles for Building Software as a Service Apps

The Twelve Factors:

ICodebaseThere should be exactly one codebase for a deployed service with the codebase being used for many deployments.
IIDependenciesAll dependencies should be declared, with no implicit reliance on system tools or libraries.
IIIConfigThe configuration that varies between deployments should be stored in the environment.
IVBacking servicesAll backing services are treated as attached resources and attached and detached by the execution environment.
VBuild, release, runThe delivery pipeline should strictly consist of build, release, run.
VIProcessesApplications should be deployed as one or more stateless processes with persisted data stored on a backing service.
VIIPort bindingSelf-contained services should make themselves available to other services by specified ports.
VIIIConcurrencyConcurrency is advocated by scaling individual processes.
IXDisposabilityFast startup and shutdown are advocated for a more robust and resilient system.
XDev/Prod parityAll environments should be as similar as possible.
XILogsApplications should produce logs as event streams and leave the execution environment to aggregate.
XIIAdmin ProcessesAll admin tasks should be kept in source control and packaged with the application.

See https://12factor.net/ for more details and this page for nice illustrations: https://www.redhat.com/architect/12-factor-app.