Log cursors

WiredTiger cursors provide access to data from a variety of sources.

One of these sources is the set of records in the transaction log files. WiredTiger log records can be made up of multiple operations from a single transaction. One can use a log cursor to access the operations comprising the transaction log record.

When using a log cursor the key is the LSN and also an operation counter within that log record. When a log record is a single entity (i.e. any record that is not a transaction commit made up of operations), the operation counter returned in the key will be zero.

The value returned contains the following:

• transaction id (commit records only, otherwise 0)
• record type
• operation type (commit records only, otherwise 0)
• file id (if applicable, otherwise 0)
• operation key (commit records only, otherwise empty)
• operation value

The following example opens a log cursor on the database:

        ret = session->open_cursor(session, "log:", NULL, NULL, &cursor);

and using the key:

                ret = cursor->get_key(cursor, &lsn.file, &lsn.offset, &opcount);

and using the value:

                ret = cursor->get_value(cursor, &txnid, &rectype,
                    &optype, &fileid, &logrec_key, &logrec_value);

Notes:

• Although the key format includes an operation counter, that field is ignored when setting the key for a WT_CURSOR::search. The following is an example of setting the key:

        cursor->set_key(cursor, lsnsave.file, lsnsave.offset, 0);

• Performing a WT_CURSOR::next call on a log cursor moves to the next operation within a commit record or the first operation in the next log record (which may be the entire log record).
• Log cursors are always read-only.