How is uuid computed

As a developer or admin, there are certain situations when you need to generate a unique identifier for an object. Of course, you could just create a random string to do this, but the better and more popular way is to assign a universally unique identifier UUID to the object.

Per RFC section 4. In a random string such as this, we would have 32 hex characters, each with four bits of possibilities, for a total of bits of random data. However, RFC specifies fixed values of six bits: Specifically, the first four bits of the third segment and the first two bits of the fourth segment, so we are left with only random bits.

Okay, here we go. So that will generate characters for Segments 1, 2, and 5. But what about Segments 3 and 4? The answer is to simply create the necessary random data for the individual non-fixed segments i.

If we look at Segment 3 in binary i. What we would do next is generate the green parts as random bits, then just add the red number. The result is that our table acquires a new computed field, called U , which in the act of creation was also populated with unique, non- NULL values for each record.

The index we created on that field makes the table editable and selectable, as the white backgrounds of the DARMC and F3 columns indicate. The U column retains a light gray background indicating it is not editable since the values in that column are automatically created by a computed expression using the UuidMakeNew function.

That those values are long and meaningless UUID values does not matter: that field's sole purpose is to provide a convenient, guaranteed unique, identity field for an index so the table is editable and interactive selections can be made. For that purpose a btree index using the U computed field is fine. If we do not want to look at the U field in our table we can hide it using the Layers pane. Manifold is often used with spatial data that originated in GIS or other systems which do not take as rigorous an approach to database matters as do enterprise class DBMS applications.

It cannot be taken for granted that such data has indexes. Much data used in the GIS world does not. This makes it convenient always to have an automatically-created unique identifier for records and an index on that identifier. UUID values are guaranteed to be unique always so that records can be transferred between projects and tables even on different machines.

How to find the UuidMakeNew function? If we do not remember the name of the function we can always look it up in the Query Builder tab of the Command Window. Enter uuid into the filter box for the list of functions and right away the list will be reduced to make the function obvious.

Is a UUID really unique? Close enough. If we generated a UUID value every second for one billion years, we would have a If we plan on living for a billion years, our odds of getting wiped out by an extinction event asteroid impact are greater. Billion year old people should worry more about losing life the dinosaur way than about experiencing a UUID collision. Data Types. Example: Add a Computed Field to a Table - In this example we add a computed field to a table, illustrating how the computed field automatically changes when changes are made in the fields it uses for computation.

We also show how computed fields can use geometry, automatically updating centroids when areas are changed. Last, we show how geometry can be created using computed fields, to create effective radius circles for antennas based on the power of the antenna. Example: Add a Spatial Index to a Table - A typical use of an index is to provide a spatial index on a geom field in a table, so the geom data can be visualized in a drawing.

Appendix A - Sample Implementation. Appendix B - Sample Output of utest. A UUID is bits long, and requires no central registration process. Both sets of specifications have been aligned, and are fully technically compatible.

Motivation One of the main reasons for using UUIDs is that no centralized authority is required to administer them although one format uses IEEE node identifiers, others do not. As a result, generation on demand can be completely automated, and used for a variety of purposes. The UUID generation algorithm described here supports very high allocation rates of up to 10 million per second per machine if necessary, so that they could even be used as transaction IDs.

UUIDs are of a fixed size bits which is reasonably small compared to other alternatives. This lends itself well to sorting, ordering, and hashing of all sorts, storing in databases, simple allocation, and ease of programming in general. Since a UUID is a fixed size and contains a time field, it is possible for values to rollover around A. A UUID can be used for multiple purposes, from tagging objects with an extremely short lifetime, to reliably identifying very persistent objects across a network.

The internal representation of a UUID is a specific sequence of bits in memory, as described in Section 4. Each field is treated as an integer and has its value printed as a zero-filled hexadecimal digit string with the most significant digit first. The hexadecimal values "a" through "f" are output as lower case characters and are case insensitive on input.

Identifier persistence considerations: UUIDs are inherently very difficult to resolve in a global sense. Process of identifier assignment: Generating a UUID does not require that a registration authority be contacted. One algorithm requires a unique value over space for each generator. The address can be assigned from an address block obtained from the IEEE registration authority. If no such address is available, Leach, et al.

Another approach is to use version 3 or version 4 UUIDs as defined below. Then, to compare a pair of UUIDs, arithmetically compare the corresponding fields from each UUID in order of significance and according to their data type.

Two UUIDs are equal if and only if all the corresponding fields are equal. As an implementation note, equality comparison can be performed on many systems by doing the appropriate byte-order canonicalization, and then treating the two UUIDs as bit unsigned integers. UUIDs, as defined in this document, can also be ordered lexicographically.

When converting from a bit-oriented, in-memory representation of a UUID into a URN, care must be taken to strictly adhere to the byte order issues mentioned in the string representation section.

Validation mechanism: Apart from determining whether the timestamp portion of the UUID is in the future and therefore not yet assignable, there is no mechanism for determining whether a UUID is 'valid'. Scope: UUIDs are global in scope. Specification 4. Format The UUID format is 16 octets; some bits of the eight octet variant field specified below determine finer structure. That is, the interpretation of all other bits in the UUID depends on the setting of the bits in the variant field.

As such, it could more accurately be called a type field; we retain the original term for compatibility. The variant field consists of a variable number of the most significant bits of octet 8 of the UUID. The following table lists the contents of the variant field, where the letter "x" indicates a "don't-care" value. Interoperability, in any form, with variants other than the one defined here is not guaranteed, and is not likely to be an issue in practice.

Layout and Byte Order To minimize confusion about bit assignments within octets, the UUID record definition is defined only in terms of fields that are integral numbers of octets.

The fields are presented with the most significant one first. Note that the field names, particularly for multiplexed fields, follow historical practice. The following table lists the currently-defined versions for this UUID variant. The version is more accurately a sub-type; again, we retain the term for compatibility. Timestamp The timestamp is a bit value.

For systems that do not have UTC available, but do have the local time, they may use that instead of UTC, as long as they do so consistently throughout the system. However, this is not recommended since generating the UTC from local time only needs a time zone offset.