Beyond the Twelve-Factor App

can create and consume transient state in the middle of handling a
request or processing a transaction, but that data should all be gone
by the time the client has been given a response.
To put it as simply as possible, all long-lasting state must be external
to the application, provided by backing services. So the concept isn’t
that state cannot exist; it is that it cannot be maintained within your
application.
As an example, a microservice that exposes functionality for user
management must be stateless, so the list of all users is maintained
in a backing service (an Oracle or MongoDB database, for instance).
For obvious reasons, it would make no sense for a database to be
stateless.
The Share-Nothing Pattern
Processes often communicate with each other by sharing common
resources. Even without considering the move to the cloud, there
are a number of benefits to be gained from adopting the sharenothing
pattern.
Firstly, anything shared among processes is a liability that makes all
of those processes more brittle. In many high-availability patterns,
processes will share data through a wide variety of techniques to
elect cluster leaders, to decide on whether a process is a primary or
backup, and so on.
All of these options need to be avoided when running in the cloud.
Your processes can vanish at a moment’s notice with no warning,
and that’s a good thing. Processes come and go, scale horizontally
and vertically, and are highly disposable. This means that anything
shared among processes could also vanish, potentially causing a cascading
failure.
It should go without saying, but the filesystem is not a backing service.
This means that you cannot consider files a means by which
applications can share data. Disks in the cloud are ephemeral and, in
some cases, even read-only.
If processes need to share data, like session state for a group of processes
forming a web farm, then that session state should be externalized
and made available through a true backing service.
40 | Chapter 12: Stateless Processes