title: Multi-tenant PHP SaaS - Separate DB's for each client, or group them?
body: <p>You'll have to bear with me here for possibly getting some of the terminology slightly wrong as I wasn't even aware that this fell into the whole 'multi-tenant' 'software as a service' category, but here it does.</p>
  
  <p>I've developed a membership system (in PHP) for a client. We're now looking at offering it as a completely hosted solution for our other clients, providing a subdomain (or even their own domain).</p>
  
  <p>The options I seem to have on the table, as far as data storage goes are:</p>
  
  <p><strong>Option 1</strong> - Store everything in 1 big database, and have a 'client_id' field on the tables that need it (there would be around 30 tables that it would apply to), and have a 'clients' table storing their main settings, details, etc and the domain to map to them. This then just sets a globally accessible variable containing their individual client id - I'd obviously have to modify every single query to check for the client_id column.</p>
  
  <p><strong>Option 2</strong> - Have a master table with the 'shared reference' tables, and the 'clients' table. Then have 'blocks' of other databases, which each contain, say 10 clients. The client would get their own database tables, prefixed with their client ID. This adds a little bit of security to protect against seeing other client data if something went really wrong.</p>
  
  <p><strong>Option 3</strong> - Exactly the same as option 2, except you have 1 database for each and every client, completely isolating them from other clients, and theoretically providing a bit more protection that if 1 client's tables were hacked or otherwise damaged, it wouldn't affect anyone else. The biggest downside is that when deploying a new client, an entire database, user and password need setting up, etc. Could this possibly also cause a fair amount of overhead, or would it be pretty much the same as if you had everyone in one database?</p>
  
  <p>A few points as well - some of these clients will have 5000+ 'customers' along with all the details for those customers - this is why option 1 may be a bit of an issue - if I've got 100  clients, that could equal over half a million rows in 1 table. </p>
  
  <p>Am I correct in thinking Option 3 would be the best way to go in a situation where security of customer data (and payment information) is key. From recommendations I've had, a few people have said to go with option 1 because 'its easier' however I really don't see it that way. I see it as a potential bottleneck down the line, as surely I can move clients around much easier if they have their own database.</p>
  
  <p><em>(FYI The system is PHP based with MySQL)</em></p>