Upgrading Live Services - The Practice of Cloud System Administration - page 233

is responsible for upgrading or converting any data structures. All subsequent events will
trigger the new versions of the functions. No processes survive a restart of the Erlang en-
gine itself, but carefully planned upgrades can be arranged with zero downtime.
A very readable introduction to Erlang's basic features and structure is available online
at http://learnyousomeerlang.com/what-is-otp .
We expect to see more systems like this in the future. Geordi edited live code on Star
Trek: The Next Generation . We are optimistic that someday we will be able to do so, too.
11.10 Continuous Deployment
Continuous deployment means every release that passes tests is deployed to production
automatically. Continuous deployment should be the goal of most companies unless con-
strained by external regulatory or other factors.
Asdepictedin Figure11.1 , thisrequirescontinuousintegrationandcontinuousdelivery,
plus automating any other testing, approval, and code push processes.
Figure 11.1: Continuous integration, delivery, and deployment build on each other.
Recallthatcontinuousdeliveryresultsinpackagesthatareproductionready,butwheth-
er to actually deploy each package into production is a business decision. Continuous de-
ployment is a business decision to automate this approval and always deploy approved re-
leases.
It may sound risky to always be doing pushes, and it is. Therefore many precautions are
taken. One precaution is to use continuous deployment only to push to noncritical environ-
ments such as beta and UAT environments.
Anotheristousethisapproachinproduction,butonlyforspecificsubsystemsthathave
achieved high confidence in their releases. For example, it is common to see continuous
deploymentforsubsystemsthatprovideAPIsandbackendservicesbutnotwebUIsubsys-
tems, which may still be tested manually. This is another benefit of service-oriented archi-
tectures.
Even then, some people may be uncomfortable with continuous deployment. As an ex-
perienced system administrator, you may pay attention to various “hunches” that help you