improve setup:static-content:deploy downtime period

Summary (*)

setup:static-content deploy is very annoying to use in production because it produces huge downtime for websites.

Examples (*)

Currently in order to force any changes in CSS no matter how small, change red to yellow for example requires the following process.

rm -rf var/view/preprocessed/* pub/static/frontend/*

Any other procedure will force obsolete files to be used.
While everything is deleted there is downtime.
While everything is recreated there is downtime.
Depending on the server this can range from a few seconds to 10minutes+ and it's unacceptable for all users, customers and developers.

Proposed solution

Is it possible to change the way deployment is handled by using a basic way of temporary folders?
For example everytime we issue the command setup:static-content:deploy a temporary folder is created to execute the procedure in and then the final contents are moved back to the locations we need.

This would mean that cached files and files that are currently being used in the website will be accessible during the whole deployment process without causing downtime. Afterwards, only renaming the top level directories and changing the version would be enough.

To depict better

Currently the pub/static folder structure in production mode looks like this

adminhtml
_cache
deployed_version.txt
frontend

and in var/view/preprocessed we have the folder
pub

So my recommendation would be,
create the folder var/view/preprocessed/pub_version
create the folders in pub/static folder

adminhtml_version
frontend_version

Do all the work in the temp folders, and then rename them accordingly.

Although it will not reduce the time for compilation it will still reduce the downtime.

Start by tracing the setup:static-content:deploy entry point and the handling of pub/static and var/view/preprocessed. Review how deployed content and deployed_version.txt are created; done means new content can be prepared without removing files currently serving requests, then switched with no deployment downtime.