Node.js avoid db race condition with cluster/pm2

Question

I have a Node application which runs in cluster mode with pm2.

I also have a function which checks if a specific row is in a db table. If the row is missing it creates the row otherwise a value is set and saved.

I only need one row for each combination of userId and groupId.

function someFunction()={
  return Activation.findOne({ where: { userId: userId, groupId: groupId } })
    .then(activationObject => {
        if (!activationObject) {
          return Activation.create({ userId: userId, groupId: groupId, activationTime: sequelize.fn('NOW') })
        } else {
          activationObject.activationTime = sequelize.fn('NOW');
          return activationObject.save()
        }
    })      
}

How can I avoid race conditions when running node in cluster mode? Currently if first worker checks the row is available and the second checks at the same time both get no result and in the end we have two newly created rows instead of one.

I know that Sequelize provides a findOrCreate() method but I wanted an easy understandable example.

Answer 1

The easiest way would be to add a UNIQUE constraint for the combination of userId and groupId with an ON CONFLICT REPLACE clause, and always create a new row instead of updating. This will cause a newly inserted row with the new activationTime to replace the old row.

You can additionally check the number of rows inserted to tell whether the insert succeeded or not.

Example: UNIQUE (userId, groupId) ON CONFLICT REPLACE