Goal
=========
Devise a mongo database replication topology where there is one
primary location (master) and up to 50 secondary locations (slaves).
Configuration information will flow from primary to secondary machines
only. Local data like registrations will flow from any location to
any, all or multiple other locations. Admins will want to bring
locations on and off line w/minimal disruption to the services.
Assertions
========
- in 4.6, for security reasons each mongod service will be configured
to disallow connections from remote machines. Instead connections to
each mongod instance will go thru stunnel to authorize, authenticate,
encrypt.
Plan
========
So i'm going to be creating an stunnel config that could potentially
open a connection to the mongod service on any other location. The
port to connect to the primary location will be 27019, The port range
to connect to secondary locations will start at 27020 and increment by
one until the last location is mapped.
Example:
to connect to mongod on primary location use localhost:27019
to connect to mongod on location 1 use localhost:27020
to connect to mongod on location 2 use localhost:27021
...
to connect to mongod on location N use localhost:${27020 + N}
So this means every time a new location is added or removed, a new
stunnel config will need to be replicated and the stunnel service on
each server will need to be restarted. Consequently all open
replication connections will be interrupted. However, we will be
introducing the concepts of locations that are enabled and disabled.
This will be important to take servers out of rotation in case they
are unavailable for whatever reason. When locations are enable or
disabled, the stunnel configuration will not change, where to
replicate based on enabled state will be handled in the application
layer.
Implementation Notes
==================
* It will be up to the local replication modules like the registration
replicator to honor enabled or disabled locations and act
appropriately. This replicator will also have to be able to map a
location with it's stunnel local port mapping. E.g. If location
number 6 on host foo.example.com is disabled, then all other locations
should not attempt to replicate to localhost:27025. This mapping
could easily be written to a config file as part of stunnel config
file replication to abstract out the mapping.
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