In a scenario, Bob and David clicked the like button on Instagram at the same time.
they all got the same value before it changed, like 1000.
So, after Bob clicked, and Added the like number, the result was 1001,
but same as David, got the previous value from the data base as 1000, so the result would not be 1002, but 1001.
How to resolve this problem?
We lock this process with transaction, the new command will not execute untill the last one complete (commit the transaction).
db.execute("BEGIN TRANSACTION"); rows = db.execute("SELECT likes FROM posts WHERE id = ?", id); like = row["likes"]; db.execute("UPDATE POSTS SET likes = ? WHERE id = ?", likes + 1, id); db.execute("COMMIT");