Section 4.2. Finding Services, Not Devices

4.2. Finding Services, Not Devices

In the world of networked devices, it does you no good to locate a device with which you cannot communicate. We commonly anthropomorphize devices in ways that are not quite correct. We say that we "pinged a server," though, in fact, what we pinged was a piece of software on the server that answers ICMP echo request packets. If you take away that software, it stops answering ping requests, even though the server is still there and may still be performing other functions perfectly well.

When designing a service discovery system, it's important to remember that what network software clients need to discover are software entities with which they can communicate, not pieces of hardware. The difference between discovering services and discovering hardware may seem small and subtle, but it makes all the difference in actual use. In a print dialog, you want to see the list of things you can print to. In iTunes, you want to see the list of music sources you can play. In iPhoto, you want to see the list of photo albums you can view. In a web browser, you want to see a list of offered web pages you can view. Any given piece of hardware on the network may offer zero, one, or more of each of these kinds of resources. What you want to see is the list of resources you can use, not a list of the hardware where they may or may not reside.