Need a way of sending a signal to an LP app server to switch it to (and from) read only mode

[Tom Haddon]

Currently to switch between read-only and read-write mode we need to stop an application server, change the configuration file it's using, and then start it up again. This is a problem because it's time consuming, as well as requiring more config files than is strictly necessary.

Here's how we'd like it to work:

- The configuration file for a given app server includes entries for read-write-master-main, read-write-slave-main, read-only-master-main, read-only-slave-main, etc.
- There is some way of specifying at start up whether an app server should be in read-write or read-only mode.
- There is some way of switching a server from read-write to read-only without restarting the app server - ideally some kind of signal you send it (similar to how we send a SIGUSR2 for a reload as part of logrotate)
- There is some way of confirming which state the app server is in

[Tom Haddon (mthaddon)]

This is somewhat related to bug 317697, fwiw - if it's appropriate to mark it as a dupe, that's fine by me.