为实体生成唯一ID

Question

I'm working on Lagom project and one of the requirement is to uniquely identify each user's document.

A document has an id that I should generate each time a user adds a document.

My idea is simple to guarantee the unicity of the ID:

• generate a random string ID (NOT UUID)
• query the database for the generated ID
• IF query returns 0 hits THEN the ID is unique and I should return it to the caller
• IF query returns 1 hit THEN repeat all steps.

I've implemented the DocumentEventProcessor extends ReadSideProcessor<DocumentEvent> and DocumentRepository to be able to query the database.

My problem is how to write the function that iteratively (recursively maybe) query the database until new random ID returns.

Answer 1

Why not use a while loop?

SaveDocumentWithNewId(){
//what pseudocode. 
String newRandomId = null;
while(result.size() != 0 )
newRandomId = GenerateRandomID();
ArrayList result= queryToDatabase(newRandomId);
}

document.setId(newRandomId );
}

Answer 2

You should not use the read side (ie your database) to guarantee the uniqueness of ids. In Lagom (and any CQRS based system, for that matter), the read side will always be only eventually consistent. So you could end up in a situation where two nodes generate the same id and the database will tell both that it doesn't exist, because it hasn't caught up yet.

What you need to do instead is make sure you can distinguish between a newly created, unused PersistentEntity , and one that already has a document associated with it. Then you use the PersistentEntityRegistry to get the PersistentEntity for your newly generated key. If the entity already existed, generate a new key. If not, you have your key.

The algorithm - generate new IDs until you found an unused one - remains the same, just make sure you use the PersistentEntityRegistry to check for uniqueness, not the read side.

Answer 3

It's a costly process to check whether an ID generated is unique by searching the database. I will suggest you create your own UUID(Like a GUID) which should be unique across systems and which is also based on the requirement(ie 7 characters long).

You may create your own ID by taking the inetAddress of the system and attaching a random number to it and then generating a message digest out of it and then according to your requirement you can trim out 7 characters from it.

By doing so you will have unique ids across the system as well as fulfill your requirement.

For reference click here

Answer 4

Maybe you may use the following approach:

assumptions:
- distributed system,
- 7 characters long ID,
- you can assign each node a unique value - hostId (maybe last digits from your host IP?)

what to do:
- have a per node Singleton which will generate the ID,
- calculate uniqueNumericID by concatenating the hostId and the current time in millis (you should store last used timestamp, in case you need more than one ID per millisecond, just add one to the last value until last timestamp >= current timestamp).
- convert this numeric value (decimal) to the number with base say 62 (use characters '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ' and recurrently calculate modulo of the (rest of the) value, use the modulo as index of the character).

There should be more than enough space in your 7 characters long ID.