does sql statement ensure atomicity in postgres

Question

I have a simple bug in my program that uses multi user support. I'm using knex to build sql queries, and I have a pseudocode that depicts the scenerio:

const value = queryBuilder().readDataFromTheDatabase();//executes this
//do some other work and get value
queryBuilder.writeValueToTheDatabase(updateValue(value));

This piece of code is being use in sort of a middleware function. And as you can see, this is a possible race condition i.e. when multiple users access the thing, one of them gets a stale value when they try to execute this at roughly the same amount of time.

My solution

So, I was think a possible solution would be create a single queryBuilder statement:

queryBuilder().readAndUpdateValueInTheDatabase();  

So, I'll probably have to use a little bit of plpgsql. I was wondering if this solution will be sufficient. Will the statement be executed atomically? i.e. When one request reads and doesn't finish his write, does another request wait around to both read and write or just waits to write but, reads the stale value?

Answer 1

To avoid a race condition with such a setup, you must run both the read and the write in the same database transaction.

There are two ways to do that:

1. Use the default READ COMMITTED isolation level and lock the rows when you read them:

   SELECT ... FROM ... FOR NO KEY UPDATE;
   

   That locks the rows against concurrent modifications, and the lock is held until the end of the transaction.

2. Use the REPEATABLE READ isolation level and don't lock anything. Then your UPDATE will receive a serialization error (SQLSTATE 40001) if somebody modified the row concurrently. In that case, you roll the transaction back and try again in a new REPEATABLE READ transaction.

The first solution is usually better if you expect conflicts frequently, while the second is better if conflicts are rare.

Note that you should keep the database transaction as short as possible in both cases to keep the risk of conflicts low.