If you are working on a new version of a software that requires a very large number of users, there is usually one big bottleneck: the database.
For that reason, many large scale websites, such as Facebook and Twitter have moved away from SQL databases and moved on using systems that allow them to scale mostly horizontally. These are call NoSQL databases. This one I use is called Cassandra. The more users you get, the more Cassandra nodes you add and it can grow pretty much as big as you'd like. Netflix said around 2015 that they had over 2,000 such nodes.
Cassandra automatically mirrors data between nodes.
If you'd like, you can also look at Scyllab which is a Cassandra in C++ instead of Java. I would be using that if I did not have a problem with deletion from the database... right now, it is not an option for me, unfortunately. There are several other NoSQL systems, but these are the only two I tried.
Next... you write your application that runs on a separate computer (while doing development you can do it on the same computer, that's fine, but you need at least 4Gb of RAM for Cassandra...) Make 100% sure that ALL the data which is not read-only static data installed by your application is saved IN THE DATABASE. Otherwise you would have a copy on computer A and not on computer B. So someone accessing computer B would never have access to that data on computer A. However, if all the data (i.e. even files uploaded) is in the database, any front end application instance can access it. What does this mean? That you can have as many front ends as you need to sustain your load. You structure becomes something like this:
Internet Users
|
v
+--------------------+
| |
| Load Balancer |
| |
+--------------------+
|
+-----------------------+--------------------+
| | |
v v v
+----------------+ +----------------+ +----------------+
| | | | | |
| Apache/App | | Apache/App | | Apache/App |
| | | | | |
+----------------+ +----------------+ +----------------+
^ ^ ^
| | |
v v v
+----------------+ +----------------+ +----------------+
| | | | | |
| Cassandra |<-->| Cassandra |<-->| Cassandra |
| | | | | |
+----------------+ +----------------+ +----------------+
Note: Each box represents a computer, your minimum cluster will probably be around 7 computers. However, each can be a pretty cheap Cloud computer instead of a 16 processors dedicated server...
One important note: To run properly (as in safely for the data), Cassandra requires at least 4 nodes and a replication factor of 3. You also need a secure (private) LAN network. If such is not available, you will have to install something such as OpenVPN to make sure all data travels encrypted between nodes (this is true even with other products that offer such duplication.)
The Load Balancer is just a process that checks the load of the frontend computers and decides to which to send the next request. Apache2 has a module for that. Note that I know of Apache, I'd bet other HTTP servers also offer similar modules. That's up to you to decide which solution is best for your company. You may also scratch that completely and use the DNS round robins capability. It's not as good, but definitely much easier to setup and may be enough if each access to your app. is pretty symmetrical (uses about the same amount of time). If some requests take 3 seconds when others use 50 ms, you may run in problems with just a DNS RR setup.
The Application can be in any language. I use C++, but I could have used Java, PHP, plain C (really?!), perl, etc. Whatever works for you. Just remember, NO DATA on the frontends. All in the database and you'll be fine. The mirroring is done by Cassandra so your work in done at that point.
Note that the graph I showed above is not quite correct. The frontends should connect to any Cassandra node. Often the drivers will connect to 2 or 3 nodes to make sure you always have access to the data. Then, once you go from 1,000 users to 10,000, you just add a few frontends and a few Cassandra nodes, and voilà! It will all work as before (same speed for the clients).
As far as using ISPConfig, certainly. Will ISPConfig help you with all of those pieces? Probably not. At least not without adding your own module(s)... But you should be able to use it for what it offers: setup your DNS, setup your emails, probably do a large part of the Apache setup, etc.
