An introduction to "introduction": a common but rarely-named systems concept

Introduction is simple to describe: It's when you (A) talk to some server B, which gives you a way to communicate directly to (introduces you to) some other server C.

A few things which do introduction:

Introduction is not always just a simple map from a name to a response containing addresses. In fact, it's more interesting when it's not simple.

Note that introduction can be iterated. Once I'm introduced to C, C might in turn introduce me to still more servers: D and E, which introduce F and G, and so on. For example, an NS record pointing to another DNS server, or a URL pointing to another webpage with still more URLs on it.

An example of something that is not introduction is a typical load-balancer. You (A) talk to a load-balancer (B), and then send and receive some data from another server, but you're typically not in direct communication with the other server: All your communication with that server is proxied through the load-balancer. Among other differences, this creates a performance overhead.

Once you know what introduction is, you begin to see it everywhere. Some more examples of introduction: