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 use it. If someone could answer this, it would be nice. Thanks

Answer 1

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 2

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 3

As addition to all the other answers, here is an online GUID generator:

http://www.guidgenerator.com/

What is a GUID?

GUID (or UUID) is an acronym for 'Globally Unique Identifier' (or 'Universally Unique Identifier'). It is a 128-bit integer number used to identify resources. The term GUID is generally used by developers working with Microsoft technologies, while UUID is used everywhere else.

How unique is a GUID?

128-bits is big enough and the generation algorithm is unique enough that if 1,0000,000,000 GUIDs per second were generated for 1 year the probability of a duplicate would be only 50%. Or if every human on Earth generated 600,000,000 GUIDs there would only be a 50% probability of a duplicate.

How are GUIDs used?

GUIDs are used in software development as database keys, component identifiers, or just about anywhere else a truly unique identifier is required. GUIDs are also used to identify all interfaces and objects in COM programming.

Answer 4

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-dddd-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.