What exactly is GUID? Why and where I should use it?

Question

What exactly is GUID? Why and where I should use it?
I\'ve seen references to GUID in a lot of places, and in wikipedia, but it is not very clear telling you where to

Answer 1

A GUID is a "Globally Unique ID". Also called a UUID (Universally Unique ID).

It's basically a 128 bit number that is generated in a way (see RFC 4112 http://www.ietf.org/rfc/rfc4122.txt) that makes it nearly impossible for duplicates to be generated. This way, I can generate GUIDs without some third party organization having to give them to me to ensure they are unique.

One widespread use of GUIDs is as identifiers for COM entities on Windows (classes, typelibs, interfaces, etc.). Using GUIDs, developers could build their COM components without going to Microsoft to get a unique identifier. Even though identifying COM entities is a major use of GUIDs, they are used for many things that need unique identifiers. Some developers will generate GUIDs for database records to provide them an ID that can be used even when they must be unique across many different databases.

Generally, you can think of a GUID as a serial number that can be generated by anyone at anytime and they'll know that the serial number will be unique.

Other ways to get unique identifiers include getting a domain name. To ensure the uniqueness of domain names, you have to get it from some organization (ultimately administered by ICANN).

Because GUIDs can be unwieldy (from a human readable point of view they are a string of hexadecimal numbers, usually grouped like so: aaaaaaaa-bbbb-cccc-ffffdd-ffffffffffff), some namespaces that need unique names across different organization use another scheme (often based on Internet domain names).

So, the namespace for Java packages by convention starts with the orgnaization's domain name (reversed) followed by names that are determined in some organization specfic way. For example, a Java package might be named:

com.example.jpackage

This means that dealing with name collisions becomes the responsibility of each organization.

XML namespaces are also made unique in a similar way - by convention, someone creating an XML namespace is supposed to make it 'underneath' a registered domain name under their control. For example:

xmlns="http://www.w3.org/1999/xhtml"

Another way that unique IDs have been managed is for Ethernet MAC addresses. A company that makes Ethernet cards has to get a block of addresses assigned to them by the IEEE (I think it's the IEEE). In this case the scheme has worked pretty well, and even if a manufacturer screws up and issues cards with duplicate MAC addresses, things will still work OK as long as those cards are not on the same subnet, since beyond a subnet, only the IP address is used to route packets. Although there are some other uses of MAC addresses that might be affected - one of the algorithms for generating GUIDs uses the MAC address as one parameter. This GUID generation method is not as widely used anymore because it is considered a privacy threat.

One example of a scheme to come up with unique identifiers that didn't work very well was the Microsoft provided ID's for 'VxD' drivers in Windows 9x. Developers of third party VxD drivers were supposed to ask Microsoft for a set of IDs to use for any drivers the third party wrote. This way, Microsoft could ensure there were not duplicate IDs. Unfortunately, many driver writers never bothered, and simply used whatever ID was in the example VxD they used as a starting point. I'm not sure how much trouble this caused - I don't think VxD ID uniqueness was absolutely necessary, but it probably affected some functionality in some APIs.