良好的数据库设计/规范化

Question

I was assigned a task, with which I am not sure how to approach it:

I have to build a Messaging-System which supports multiple devices and it should be as efficient as possible. Users can have up to 10 devices, which all need to receive a Message when the User receives it.

I have two Ideas:

Table Messages:
- ID (PK)
- SenderID
- ReceiverID
- Data

Table PendingTransmissions:
- MessageID (FK (PK of above table))
- DeviceID (FK)

The Problem here is that every time a message is sent, one entry per device is created and results in big overhead.

Table Messages:
- ID (PK)
- SenderID
- ReceiverID
- Data
- ReceivedDevice1
- ReceivedDevice2
- ReceivedDevice3
- ReceivedDevice4
- ReceivedDevice5
- ReceivedDevice6
- ReceivedDevice7
- ReceivedDevice8
- ReceivedDevice9
- ReceivedDevice10

The problem here is obviously the redundance of Devices, but the overhead would be lower.

What is the better solution or is there something I have missed completely?

Thanks in advance!

Answer 1

The first approach is totally valid and it should be the way to go. The maintainability of the second approach is a horror and on a later time when some code is added the code won't be very readable (i had to maintain multiple DBs already that where designed this way).

Answer 2

I would recommend against the 2nd approach for (at least) two reasons:

• Today, one User can have 10 devices. If that number changes to, say, 20 in future, that means changes in many layers in your application: database, application entity classes, DAO etc.
• In future, you might want to add functionality for groups. It'll be cumbersome to extend this design for that.

In 1st approach, if you are concerned about your PendingTransmissions becoming too big in size, you can take care of it like this:

• Add two columns: isDelivered and deliveredTimestamp . You can then periodically archive all rows which are both delivered and older than, say, 1 month.