Structuring database configuration settings

Question

I am structuring the config settings of a database, abstracting as far as possible the config data.
In the most common LAMP environments, connecting to a database requires the following four information:

• db_host
• db_name
• db_user
• db_password

I would like to understand two things:

1 - Are credentials related to host or db?

If user and password are related to the host, they could access all the databases available on that host, so I could represent the config data this way (no matter it is an array, an object or other):

db_config
 └─db_host
 ├─db_user
 ├─db_password
 └─databases
 ├─db_name
 └─db_engine

If user and password are instead related to a specific database, they could access the only db_name specified; in this case I could schematize this way the config data:

db_config
 └─db_host
 └─databases
 └─db_name
 ├─db_engine
 ├─db_user
 └─db_password

2 - May databases with a different engine live on the same host?

Databases may be powered by different engines (e.g. PostgreSQL, MySQL, MSSQL, Oracle, Firebird, etc).
If credentials are related to the host, and if database with different engines may cohexist on the same host, the config data could be represented this way:

db_config
 └─db_host
 ├─db_user
 ├─db_password
 └─databases
 └─db_name
 └─db_engine

But this structure does not allow databases with different engines to have the same name.
Can databases with different engines cohexist on the same host and use the same name?
If yes, the config data could be structured so:

db_config
 └─db_host
 └─db_engine
 └─databases
 ├─db_name
 ├─db_user
 └─db_password

What data structure fits most possible real situations?

• mmmmmm: Database schemas are not trees - define them as tables and in some cases some tables will have two foreign keys as a primary key - thus fixing the "tructure does not allow databases with different engines to have the same name"
• yodabar: So you are telling me that 1. credentials are related to a database and not to host, 2. one host may have databases with different engines and the same name, am I right?
• mmmmmm: Yes and I suspect more differences -0 what do you mean by name a data server of which they can be several of one type can have several databases and the credentials can differ between servers in all dbs and perhaps per database for some systems. This question is too brad as there are many combinations

Answer 1

It's kinda complicated, as different vendors have different features.

First, User/Password credentials should stand on their own. They might apply to a particular database server, but could be used for multiple databases too, or some other software. They are basically orthogonal.

Some databases support Active Directory/LDAP/Kerberos, SSL Certificates, PAM etc for login, not just username/passwords.

Some other points:

• A database server might live across multiple hosts due to replication
• A single computer can host multiple database servers on the same IP address (via different ports), or on different IP addresses. See PostgreSQL clusters
• A database cluster can have multiple instances. See SQL Server instances
• A database cluster can have many databases
• A database can have many schemas
• MySQL calls their schemas "databases"

In PostgreSQL at least, the users are children of the cluster, and not of a particular database, except I recall an obscure setting that lets you change this...