Is checksum a good way to see if table has been modified in MySQL?

Question

I'm currently developing an application in Java that connects to a MySQL database using JDBC, and displays records in jTable. The application is going to be run by more than one user at a time and I'm trying to implement a way to see if the table has been modified. EG if user one modifies a column such as stock level, and then user two tries to access the same record tries to change it based on level before user one interacts.

At the moment I'm storing the checksum of the table that's being displayed as a variable and when a user tries to modify a record it will do a check whether the stored checksum is the same as the one generated before the edit.

As I'm new to this I'm not sure if this a correct way to do it or not; as I have no experience in this matter.

Answer 1

Calculating the checksum of an entire table seems like a very heavy-handed solution and definitely something that wouldn't scale in the long term. There are multiple ways of handling this but the core theme is to do as little work as possible to ensure that you can scale as the number of users increase. Imagine implementing the checksum based solution on table with million rows continuously updated by hundreds of users!

One of the solutions (which requires minimal re-work) would be to "check" the stock name against which the value is updated. In the background, you'll fire across a query to the table to see if the data for "that particular stock" has been updated after the table was populated. If yes, you can warn the user or mark the updated cell as dirty to indicate that that value has changed. The problem here is that the query won't be fired off till the user tries to save the updated value. Or you could poll the database to avoid that but again hardly an efficient solution.

As a more robust solution, I would recommend using a database which implements native "push notifications" to all the connected clients. Redis is a NoSQL database which comes to mind for this.

Another tried and tested technique would be to forgo direct database connection and use a middleware layer like a messaging queue (eg RabbitMQ). Message queues enable design of systems which communicate using message. So for eg every update the stock value in the JTable would be sent across as a message to an "update database queue". Once the update is done, a message would be sent across to a "update notification queue" to which all clients would be connected. This will enable all of them to know that the value of a given stock has been updated and act accordingly. The advantage to this solution is that you get to keep your existing stack (Java, MySQL) and can implement notifications without polling the DB and killing it.

Answer 2

Checksum is a way to see if data has changed. Anyway I would suggest you store a column "last_update_date", this column is supposed to be always updated at every update of the record. So you juste have to store this date (precision date time) and do the check with that. You can also add a column version number : a simple counter incremented by 1 at each update.

Note: You can add a trigger on update for updating last_update_date, it should be 100% reliable, maybe you don't need a trigger if you control all updates.

Answer 3

When using in network communication:

A checksum is a count of the number of bits in a transmission unit that is included with the unit so that the receiver can check to see whether the same number of bits arrived. If the counts match, it's assumed that the complete transmission was received.

So it can be translated to check 2 objects are different, your approach is correct.