IBM Informix is a relational database system developed and marketed by IBM. In Informix we can have more than one database under one Informix server instance. When you install Informix software and bring up an instance by default Informix gives us 3 to 4 databases, which are used for administration, Authentication and auditing purposes.
What is Informix instance?
An Informix instance is a single connection to shared memory. It is represented by one or more oninit processes.
The below diagram explains the Informix architecture at a very high level:
Note1: Server Instance in the above diagram is Informix database server instance. Please don’t get confused with UNIX server here.
Note2: A single Informix server product can have N number of Informix server instances and each instance can have N number of databases. All these instances can run on a single UNIX server or multiple UNIX servers. It all depends on the client, hardware requirements and the way DBA configures the instances.
Note3: Clients/applications/users only talks to Informix server instance, which is listening at a particular port, but never with databases directly.