Well, first I have to say, that this is not a strictly Joomla question, and most likely it's something should be asked at other SE sites (server fault or similar).
Here some things to consider for your project and also for anyone that might end up here looking for an answer for his business or other large project. (servers-wise speaking, I 'm not an expert, but have done my homework):
In terms of choosing the hosting provider, there can be many recommendations (see below in the answer). But considering the given number of 30000 users/second, the question here should be more of what resources you are going to need and how efficient will be your application.
Of course there can't be a direct answer to this, as you need more metrics, like max page views per second/minute/hour, page load time, and of course a lot of testing, benchmarking and optimizations while developing and after delivering the application.
Is Joomla Capable for such project?
Joomla is a web application that runs on top of LAMP, so what is Joomla capable to do is actually everything you can do using this software bundle. You realize that a lot of things are involved here and delivering performance is a combination of things.
So assuming the Server & Network proper configuration will be the responsibility of the Hosting Provider / System Administrators, your responsibility is to deliver an optimized web application.
Things to consider are database queries, long script execution time, memory usage for scripts etc. Also, ongoing monitoring and maintenance by testing, benchmarking and optimizing should be part of your tasks.
This is not because it will be a Joomla based site, but it is because how things work. The same would be true for any application disregarding the platform or (if you better like) the CMS.
Looking at Joomla, you should carefully select what extensions will be used and maybe be ready to customize or fix code that won't perform well - working in conjunction with the developers of the extensions if needed. That's because although Joomla is wonderful and powerful, this doesn't mean everything is perfect.
Keep in mind that a single line of code (memory consuming script), or a bad structured db query (with high execution times), or a not-efficiently designed database (absence of proper normalization) can make your web application a resource-heavy beast, not able to run even in the bigger and best optimized servers/networks.
Apparently an optimized application is also very important to keep hosting costs as down as possible.
An example could be a module that contains a non-optimized long-executable query that needs seconds to run. Can you think of what will happen if the module is being called some thousands times per second? In such scenario that db server will be exhausted and most likely will not be able to handle this.
When speaking for this big number of requests, each mili-second counts. So that's why the optimization is that important and you should be able to locate such flaws in your application and fix them.
In any case, if the 30000/second number is going to be true, then I don't think you can fit in a shared hosting. I don't know of any hosting provider that would accept a such project to his sharing packages.
You definitely are going to need dedicated resources. Depending on your demands for high availability and scalability, you might have to end-up with multi-tier server configuration. A common setup is the 3-tier architecture which includes:
- 1 (or more) servers dedicated for the databases,
- 1 (or more) application server for running the code,
- 1 (or more) front-end server delivering static content
This could be achieved with dedicated servers or cloud servers / VPS.
Of course there will be costs and if you are not qualified to set this up and maintain it, you will have extra costs for the administration.
This is a short list of hosting providers (better for dedicated or VPS solutions) that is in my top list after long personal research, including cloud hosting providers. Of course further research from your side is required:
(*Providers appear in no particular order.)
And the last few things to keep in mind, is that if you are going to end with dedicated servers, you can be the one that will define the setup, so it comes out that you can create the right environment for Joomla.
In such case, what you need is a knowledgable provider with high level and quick support, that will guarantee a secure, reliable and optimized environment, which is the important part, rather than the package features that come with.