Read Cursor isolation level

Question

Using Berkeley DB version 4.5.
DB environment is opened with following flags:
DB_CREATE, DB_RECOVER, DB_INIT_LOCK, DB_INIT_LOG, DB_INIT_TXN, DB_INIT_MPOOL, DB_PRIVATE and DB_THREAD.
My requirement: I need to use the cursor to iterate over all objects in a database which is only for ready only operation. However, it should allow other threads to update the same database in a transaction (w/ less wait time due to lock held by the read cursor) .
From the API referennce, Cursor isolation level can be controlled by setting the flag in DB->cursor() .
I can't use DB_READ_UNCOMMITTED as it returns dirty data. It looks like DB_TXN_SNAPSHOT is the perfect for cursor read as it doesn't take the read lock so other threads can continue with the write operation but then according to the manual this will affect peformance of write operation (as it has to update multiple version of the object in cache).
So, it looks like my next option is to use DB_READ_COMMITTED flag but I am not sure it affect with or without transactionId passed to DB->cursor().
Can you please suggest the appropriate flag to set in DB->cursor() for read-cursor (which satisfies my requirement) and what is the affect of passing txnId (and if txnId is not passed) ?
when Cursor is iterated at what point read lock is acquired and released?
Thanks in advance for your help,
Raj

Answer 1

Hi Raj,
So, it looks like my next option is to use DB_READ_COMMITTED flag but I am not sure it affect with or without transactionId passed to DB->cursor().
Can you please suggest the appropriate flag to set in DB->cursor() for read-cursor (which satisfies my requirement) and what is the affect of passing txnId (and if txnId is not passed) ?
when Cursor is iterated at what point read lock is acquired and released?Degree 2 isolation, committed reads, means that the cursor will only read data that is committed (never dirty data, non-committed data); data will never change so long as it is addressed by the cursor, but the data may change before the reading cursor is closed. The read lock on a page is acquired when the cursor needs to move onto that page to read from it a record; the read lock is released as soon as the cursor moves away from that page.
The cursor has the same behavior whether a transaction id (txnId) is specified or not.
Please review the following documentation sections for more information:
- Isolation and Committed Reads in Getting Started with Berkeley DB Transaction Processing
- Degrees of isolation in Berkeley DB Programmer's Reference Guide
Regards,
Andrei