If you're using something like MySQL or Postgres, you can do locking with the built in transaction primitives (see, for instance, SELECT FOR UPDATE). Mongo has tools like findAndModify which can help.
If you're using SQLite you can use exclusive transactions to perform the read+write but I'm sure there's probably a more efficient way to go about it. You can craft an UPDATE that selects on the primary key and the condition and then use sqlite3_changes() to get back the number of records modified (and fail if it's zero), but that may not be possible with your setup.
If you're using SQLite you can use exclusive transactions to perform the read+write but I'm sure there's probably a more efficient way to go about it. You can craft an UPDATE that selects on the primary key and the condition and then use sqlite3_changes() to get back the number of records modified (and fail if it's zero), but that may not be possible with your setup.