Domain-Driven Design – How to Create UUIDs in DDD Entities/Aggregates

domain-driven-designevent-sourcinguuid

As I am learning DDD to help build an app idea the proper way 😉 I have come across a confusing aspect that I am trying to find a solution for.

I understand the need for Uuids in an app the size of which I am creating, but I am a little lost as to what method I should use to create Uuids for user-generated objects.

I have read all about the different variants of Uuid, and I have watched talks by Martin Fowler on the topic of Event Sourcing, so I am of the thinking I should be creating Uuids that are going to be the same every time the object is created.

How do I even tackle this with user-generated data? The reason is that if I want to mimic a system on testing to fix a live bug, shouldn't I create the exact same IDs?

Am I over thinking this? Will v4 Uuids be sufficient? This would mean that the system is not the same on either environment though, or does Event Sourcing not really care about Uids?

I am still pretty new to this, and I have tried thoroughly to answer my own questions with Google, but can't find anything.

Thanks in advance.

Best Answer

I have read all about the different variants of Uuid, and I have watched talks by Martin Fowler on the topic of Event Sourcing, so I am of the thinking I should be creating Uuids that are going to be the same every time the object is created.

Named UUIDs are great for this.

How do I even tackle this with user-generated data? The reason is that if I want to mimic a system on testing to fix a live bug, shouldn't I create the exact same IDs?

Same game as above - the point to using named uuid is that you are generating your identifiers deterministically, which means that you can reproduce them from the same inputs.

For the most part, you don't ever really care what the identifier is, just that it is determined. So if your user generated data includes an identifier, then that's fine -- when testing your system, you can reproduce the user's request with the same identifier that the client provided. That identifier can be fixed or random, as necessary.

If you get user requests without an identifier, then you'll want to add one in. You may be able to do so by simply hashing the data provided by the client; but if that can be duplicated then you'll need to introduce some additional element to ensure that the identifiers are unique.

Would you suggest that the client (written in the same language) would just use the same Uuid generating code?

Deterministic code is always much easier to test than not, so where it makes sense, sure. But from the point of view of the server, I don't really care where the ids generated by the client come from.

Also, how would you suggest using the client inputs? A sha1 of all the data for example?

Maybe, but not necessarily. Give some thought to when you want identifiers to collide.

For instance, if I'm a client trying to send a command message, then I'm probably going to give that message an identifier that is unique, so that the message will collide with a duplicate copy of itself.

If the message is intended to create a new entity, then it may be an advantage to have that identifier naturally collide with an entity created by a different client. In that case, there are two cases you may wish to handle -- the two clients try to create the entity with the same state, the two clients try to create the entity with different states.

If you and I both try to create the "same" customer, but one of us makes a mistake spelling the street address, then we probably don't want two copies of the customer in the model, but rather one customer and maybe some handling to get the address confirmed.

Related Solutions

How to generate “language-safe” UUIDs

A couple of tips that will lower the chances of inadvertently creating meaningful words:

Add some non-alpha, non-numerical characters to the mix, such as "-", "!" or "_".
Compose your UUIDs by accumulating sequences of characters (rather than single characters) that are unlikely to occur in real words, such as "zx" or "aa".

This is some C# sample code (using .NET 4):

private string MakeRandomString()  
{  
    var bits = new List<string>()  
    {  
            "a",  
            "b",  
            "c",  
            "d",  
            "e",  
            //keep going with letters.  
            "0",  
            "1",  
            "2",  
            "3",  
            //keep going with numbers.  
            "-",  
            "!",  
            "_",  
            //add some more non-alpha, non-numeric characters.  
            "zx",  
            "aa",  
            "kq",  
            "jr",  
            "yq",  
            //add some more odd combinations to the mix.  
    };  

    StringBuilder sb = new StringBuilder();  
    Random r = new Random();  
    for (int i = 0; i < 8; i++)  
    {  
        sb.Append(bits[r.Next(bits.Count)]);  
    }  

    return sb.ToString();  
}

This doesn't guarantee that you won't offend anyone, but I agree with @DeadMG that you cannot aim so high.

Unique Identifiers – Generating Secure IDs for Offline Web Apps

Your approach will work. A lot of document management systems use this type of approach.

One thing to consider is that you don't need to use both the user uuid and the random item id as part of the string. You can instead hash the concatination of both. This will give you a shorter identifier, and possibly some other benefits because the resultant id's will be more evenly distributed (better balanced for indexing, and file storage if you are storing files based on their uuid).

Another option you have is to generate just a temporary uuid for each item. Then when you do connect and post them to the server, the server generates (guaranteed) uuid's for each item and returns that to you. You then update your local copy.

Best Answer

Related Solutions

How to generate “language-safe” UUIDs

Unique Identifiers – Generating Secure IDs for Offline Web Apps

Related Topic