Scalability Problem of Two-Tier
2-tier is feasible, but does not
- match OO application designs, which are inherently 3-tier
- scale as well as 3-tier due to session management
Session - shared state between communicating parties
- Entails a memory cost and a setup (processing) cost
Sessions reduce amount of per-request context passing (comm addr’s, authenticated user/device)
- Standard DB APIs (ODBC and OLE DB) work this way
- Hence, in 2-tier, N clients and M servers ? N? M sessions
Example -105 presentation servers and 100 servers
- 105 presentation sessions per server ? 107 sessions overall
Now guess why http has no sessions.