How to Make Count thread-safe?

Option 1: Use update_counters

Rails provides a built-in method:
Foobar.increment_counter(:foo_count, foobar.id)
This translates into a SQL query like:
UPDATE foobars SET foo_count = foo_count + 1 WHERE id = X
Because this happens directly in the database in one atomic operation, it’s thread-safe and web-instance-safe.

Option 2: Raw SQL

If you prefer, or need something custom:
Foobar.where(id: foobar.id).update_all("foo_count = foo_count + 1")

Option 3: Database Locking (Pessimistic Lock)

If you absolutely need to perform other operations with the updated object:

Foobar.transaction do
  foobar = Foobar.lock(true).find(foobar.id)
  foobar.foo_count += 1
  foobar.save!
end

This uses a SELECT … FOR UPDATE query, locking the row until the transaction completes. But be cautious: this can reduce performance under high concurrency.

Option 4: PostgreSQL Triggers (Database-level)

Move the increment logic into the database using a trigger:

CREATE FUNCTION increment_foo_count()
RETURNS TRIGGER AS $$
BEGIN
  UPDATE foobars SET foo_count = foo_count + 1 WHERE id = NEW.foobar_id;
  RETURN NEW;
END;

CREATE TRIGGER trigger_increment_foo_count
AFTER INSERT ON foo_count_events
FOR EACH ROW
EXECUTE FUNCTION increment_foo_count();

Then in Rails, you insert a row into foo_count_events, and the database handles the rest.

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