Generate a sequential guid by myself

Question

Possible Duplicate:
Sequential Guid Generator C#

I got documents which might be stored in raven. I'll like to generate a sequential GUID for them (I saw at RavenDB.net that raven uses them).

Are there a ravendb API that I can use to generate the id?

Notice that the question is RavenDB specific

@ChrisGessler: Not duplicate. I asked about ***RavenDb:s*** implementation — jgauffin, Sep 03 '12 at 18:31
@jgauffin: do you want to generate on DB side, cause according to the [Ayende](https://groups.google.com/forum/?fromgroups#!topic/ravendb/5awuv842u38) post, it's enough to set `idColumn=null` ? — Tigran, Sep 03 '12 at 18:36
@Tigran: The documents might be stored. I want to generate the keys up front — jgauffin, Sep 03 '12 at 18:38
@Dennis: http://www.codeproject.com/Articles/388157/GUIDs-as-fast-primary-keys-under-multiple-database — jgauffin, Sep 03 '12 at 18:38
@jgauffin: don't believe, at this point, that you have more choices then: 1. Simply look in the code 2. Write to Ayende and ask about this. — Tigran, Sep 03 '12 at 18:41
@Tigran: He or Matt Warren usually answers questions here too.. — jgauffin, Sep 03 '12 at 18:45
@jgauffin: entities, described in that article, are not **GUIDs**. They are looking **like** GUIDS, that's all. — Dennis, Sep 03 '12 at 19:06

Matt Warren · Accepted Answer · 2012-09-04T11:15:12.547

The code below shows how RavenDB achieves this on the server for eTags, you can see it in context here. The other piece of the code is here

public Guid CreateSequentialUuid()
{
    var ticksAsBytes = BitConverter.GetBytes(currentEtagBase);
    Array.Reverse(ticksAsBytes);
    var increment = Interlocked.Increment(ref sequentialUuidCounter);
    var currentAsBytes = BitConverter.GetBytes(increment);
    Array.Reverse(currentAsBytes);
    var bytes = new byte[16];
    Array.Copy(ticksAsBytes, 0, bytes, 0, ticksAsBytes.Length);
    Array.Copy(currentAsBytes, 0, bytes, 8, currentAsBytes.Length);
    return bytes.TransfromToGuidWithProperSorting();
}

I don't think this is accessible via the API as it's an internal detail, but you could implement something similar yourself. Basically it relies on having a global counter (currentEtagBase and sequentialUuidCounter).

score 0 · Answer 2 · answered Sep 03 '12 at 18:28

0

Typically, if you're using GUID values, you wouldn't worry about them being sequential.

Guid.NewGuid will provide you with unique values you can use directly.

If you truly need sequential values, you can use the Guid constructor which accepts integral types, and build sequentially increasing values. This would be highly unusual, however.

answered Sep 03 '12 at 18:28

Reed Copsey

554,122
78
1,158
1,373

2

Globally unique identifiers with a sequential part is nothing new. They exists for instance in SqlServer and speedup the lookup siginifically – jgauffin Sep 03 '12 at 18:31
@jgauffin Yes, but then they're typically auto-generated at insert. Making new, sequential values manually isn't that common. – Reed Copsey Sep 03 '12 at 18:31
That's why I asked. I want to be able to use ravendbs implementation – jgauffin Sep 03 '12 at 18:32
Then check the code at github. Isn't it open source? – Piotr Perak Sep 03 '12 at 18:37
@jgauffin My understanding with Raven is that you just leave it as `Guid.Empty`, and when you call `Store`, Raven will give you a proper Guid. You don't typically provide your own... – Reed Copsey Sep 03 '12 at 18:39
@ReedCopsey: Depends on what you want to do with the id.. – jgauffin Sep 03 '12 at 18:40
@Peri: Do you always provide useful comments like that? gee, thanks. – jgauffin Sep 03 '12 at 18:40

Tigran · Answer 3 · 2012-09-03T18:48:15.607

It's not usual request: create a sequential GUID, as by definition, GUID is unique (as much as it possible) and presence of some mathematical, predictable sequence will introduce additional problems in its uniqueness support.

But, by the way, as usual, there is always someone that think about this already.

Seems that guy on this link had the same problem:

Generate Sequential GUID

Note: the code provided is for C# 2005, but not much changed since that, in regard of this question at least.

EDIT

According to documentation, in regard of the sequential GUID generation:

Numeric or Guid Id properties are supported and will work seamlessly. In this case, RavenDB will automatically make the translation between the inner string ID to the numeric or Guid value shown in the entity and back.

Using this approach, IDs are available immediately after calling Store on the object - the RavenDB session will set a unique ID in memory without asking one from the serve

That means even if it's generated by "server", its also stored in the memory, so ask for it will cost almost nothing, if this is your warry...

Generate a sequential guid by myself

3 Answers3