db_try_consume function

Try to consume a message from the queue

If there is a message that it READY, it returns that. Otherwise it checks for crashed workers.

db_try_consume(db, queue, crashed = TRUE, con = NULL)

Arguments

• db: DB file name.
• queue: Name of the queue.

Details of the implementation

The database must be locked for the whole operation, including checking on or creating the lock databases.

1. If there is a READY message, that one is taken.
2. Otherwise if there are WORKING messages, then we check them one by one. This might take a lot of time, and the DB must be locked for the whole search, so it is not ideal. But I don't have a better solution right now.

Taking a message means

1. Updating its row.status to WORKING.
2. Creating another database that serves as the lock for this message.