Separate Database and Application on different Servers or run both on same Server?

- A discussion of several aspects

There exists no general valid answer to the question if database and application should be separated on different physical servers or share the same physical server.

Summary

Trends as "Server Consolidation" or "Multi-Tier-Architecture" provide arguments for each side – the best solution depends on the individual combination of several factors which are listed in tabular form.

In case that the per-CPU software license costs are magnitudes higher than the hardware costs, the the decision is very likely driven by costs rather than based on the companies standard / reference architecture.

In case that an old or not popular application software requires a specific operating system or an old version, the possibility to run the database on a separated server on your strategic operating system can be a strong argument for the separation.

The "virtualized architecture" will be somewhere between the two base architectures.

This table provides a high level overview, more specific tables in the main section drill down to a more detailed level.

Scenario / Topic	on same server	separated servers
License costs for database, application server / middle ware and application in case of CPU-based licensing	In worst case you pay for all CPU's in the server the database license and the application (server) license.	Total range from "no difference" to "significant savings"
Different operating systems	Not possible	Possible
Number of Servers (installation, maintenance, rack-space, energy, operating system licenses, backup licenses)	1	2
Connectivity between database and application	local interprocess communication or local TCP/IP communication	usually TCP/IP + network + switch
Total number of CPU's		Equal or Higher
Server Class		more likelihood to avoid the higher server class
General	Simple	Popular Architecture

The whitepaper contains a detailed evaluation of those and more arguments:

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