Just thought I'd risk proposing some possible architectures to confirm I see how the licenses (and datomic) applies.
Standard web application using free edition. Three servers. Architecture is DB storing data locally, a web server and another server (say for slow report queries, an API or web worker). Could use beefy servers to scale up.
Business application using pro license allows better redundancy and scalability. Can use different data storage options. Can break beyond limits of 2 servers for processing and request handling.
The pro's peer license cost per process (which I think means cost per grunty server cost) is at worst $800 up front plus $400 per year.
The free topology is 3 servers, one of which is the transactor (the one hosting the db in free edition). You get 2 peers _in addition to_ the transactor (connecting to it). So you could have one additional server beyond the web server, or 2 API servers serving a non-peer web tier etc.
Standard web application using free edition. Three servers. Architecture is DB storing data locally, a web server and another server (say for slow report queries, an API or web worker). Could use beefy servers to scale up.
Business application using pro license allows better redundancy and scalability. Can use different data storage options. Can break beyond limits of 2 servers for processing and request handling.
The pro's peer license cost per process (which I think means cost per grunty server cost) is at worst $800 up front plus $400 per year.
Note: This is unlikely to be quite right.
Updated: thanks to RichHickey.