Unique client-supplied ids in a multi-tenant application - sql

Question

We are building a multi-tenant application that supports multiple clients. Let's use an accounting application as an example - each organization has its own accounts, receipts, etc (with their own unique id). In our case, the numbers of clients is small.
There are two options to go about it
Unique ids (UUIDs) are created by our app, and the client is responsible for maintaining a mapping of their own ids, to our UUIDs. This is easier to support but adds complexity to the client (need to maintain an extra UUID, and potentially have to pass it between their own micro-services)
We let the client specify the ids of the object in the API call as if they are the only tenant, and somehow handle the uniqueness in the background.
If we go with the 2nd approach, then we need to combine the clientId with the objectId. We could think of 3 ways to do it
A DB table per client. We decide which table to use based on the client_id. Requires either manually, or automatically creating a full set of tables per client.
DB composite keys.(I am aware of the performance hit of using Strings as pkeys, )
CREATE TABLE User (
clientId String,
userId String,
PRIMARY KEY (clientId, userId)
)
Application level: Application maintains both, and is responsible for returning the client their id, while producing the internal UUID for internal use. The internal id can be optimized for storage type. For example (Scala)
trait UniqueId[T]{
val toClientId: String // The unique id we got from a client
val to InteralId: T // The unique id we use internally
}
case class Id(client: Client, userId: String) extends UniqueId[...]{
val toClientId = userId
// Could have any of...
val to InteralId = s"${client.name}_${userId}"
val to InteralId = MD5(s"${client.name}_${userId}")
}

Answer 1

I do not see any question here, but I supposed you want some advice on the way to go ;-)
If you have a database that is multitenant aware (oracle multitenant, or a db with partition) you could easily implement your solution 1.
I would personnaly use an internal guid or int surrogate to identify each objects and would store the client ID only once as an attribute...