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Network Access - Terminals

Communication between appliances in CA 3Tera AppLogic is over peer-to-peer IP connections. The endpoints of these connections are referred to as terminals. This is somewhat different from the usual setup of a multi-device network application, where most devices may be connected to a common bus (for example, an Ethernet switch) and each appliance may connect (or mis-connect) to any other appliance on the same bus.

In contrast, CA 3Tera AppLogic uses a separate virtual wire for each connection (equivalent to a port-to-port Ethernet cable between two network devices) and verifies that traffic goes only along those wires. This is made possible by the fact that the CA 3Tera AppLogic virtual appliances are not limited by a fixed number of physical connection ports and can therefore dedicate a separate virtual terminal for each connection.

There are two logical types of terminals:

Input terminal

A terminal on which the appliance provides a specific service (that is, acts as a server).

Output terminal

A terminal that an appliance uses to request a service from another appliance (that is, acts as a client).

An input terminal can have any number of output terminals connected to it. An output terminal can be connected to a single input terminal. Either kind of terminal may remain unconnected. An application designer may specify that an output terminal is mandatory, that is, the appliance requires the service provided on it and will not work without it being connected.

APK automatically configures the network settings for the virtual network interfaces used as terminals. It makes the terminals visible to the appliance code by name (providing the name resolution and the necessary IP routes), as follows:

• A connected output's name resolves to the address of the appliance that is connected on the other side, that is, the address to which the terminal name resolves can be used directly to request a connection. For example, if the appliance has an output terminal named web, connectible to an http server, one might do the equivalent of a command like this:
  wget http://web/some-page.html
• An input terminal's name resolves to the IP address of the terminal itself, thus allowing one to use the name when configuring a server, for example, if the appliance has an input terminal named in that is supposed to respond to HTTP requests, one might add this directive to the Apache configuration file:
  Listen in:80

An output terminal that is not connected will have its name resolve to an invalid IP address: 0.255.255.255. An appliance that is designed to have optional outputs (which may be left unconnected) should recognize this, or be prepared to handle gracefully the invalid address error on an attempt to connect.