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 IDentifier". You use it anywhere that you need an identifier that guaranteed to be different than every other.

Usually, you only need a value to be "locally unique" -- the Primary Key identity in a database table,for example, needs only be different from the other rows in that table, but can be the same as the ID in other tables. (no need for a GUID here)

GUIDs are generally used when you will be defining an ID that must be different from an ID that someone else (outside of your control) will be defining. One such place in the Interface identifier on ActiveX controls. Anyone can create an ActiveX, and not know with what other control someone will be using them with --- and there's nothing to stop everyone from giving their controls the same name. GUIDs keep them distinct.

GUIDs are a combination of the time (in very small fractions of a second) (so it assured to be different from any GUID defined before or later), and a number defining your location (sometimes taken from the MAC address of you network card) (so it's assured to be different from any other GUID defined right now by someone else).

They are also sometimes known as UUIDs (universally unique ID).

Answer 2

GUID technically stands for globally unique identifier. What it is, actually, is a 128 bit structure that is unlikely to ever repeat or create a collision. If you do the maths, the domain of values is in the undecillions.

Use guids when you have multiple independent systems or clients generating ID's that need to be unique.

For example, if I have 5 client apps creating and inserting transactional data into a table that has a unique constraint on the ID, then use guids. This prevents having to force a client to request an issued ID from the server first.

This is also great for object factories and systems that have numerous object types stored in different tables where you don't want any 2 objects to have the same ID. This makes caching and scavenging schemas much easier to implement.

Answer 3

GUID stands for "Globally Unique Identifier" and you use it when you want to have, erm, a Globally Unique Identifier.

In RSS feeds, for example, you should have a GUID for each item in the feed. That way, the feed reader software can keep track of whether you have read that item or not. Without a GUID, it would be impossible to tell.

A GUID differs from something like a database ID in that no matter who creates an object -- you, me, the guy down the street -- our GUIDs will always be different. There should be no collisions using a GUID.

You'll also see the term UUID, which stands for "Universally Unique Identifier." There is essentially no difference between the two. UUID is the more appropriate term. GUID is the term used by Microsoft.