Purpose of ckpt and how its realted to dbwr and lgwr

Hi ,
I have read lot about check point (ckpt) still i am not clear with it ,what i want to know is .
when the ckpt scenario occurs
How its realated to data buffer cache ,dbwr and lgwr
How is scn realted to ckpt
what role does ckpt play when there is shutdown immedeate and abort

I feel this can explained only the person who has worked practically ,i have'nt worked much practically since i am learning it now .

Thanks
lnr

Checkpoints will update the datafile headers and the controlfiles.when the database is opened the controlfile will read all the datafile headers and check whether all the checkpoint matches the datafile and controlfile headers.if it matches it will open the database otherwise it will be in recovery mode...

when you shutdown your database by abort mode the datafile headers will not match the controlfile readings that is the reason it does instance recovey...



checkpoint will happen in the following conditions

1.alter system checkpoint;
2.alter system switch logfile;
3.shutdown normal
4.shutdown immedate
5.whenever a logswitch happens
depending upon the value you set on
6.log_checkpoint_interval
7.log_checkpoint_timeout

CKPT now replaces the work (in 8 > ) that LGWR used to do.
A checkpoint ensures a consistent state of affairs as said before.

Steve