Thanks Alexander.
So now this is my guess about the rules of the datastore's timestamp-
based scheduler. If someone could, pls correct where I'm wrong in here
(basically I consolidated the theoretical timestamp-based scheduler
with what I understand from Ryan's presentation):
Each transaction T is assigned a timestamp TS(T) at startup. Each root
entity R is given a read timestamp RTS(R) and a committed timestamp
CTS(R) (this is the timestamp mentioned in Ryan's presentation).
1. Transaction T wants to read some entity X whose root is R (X and R
may or may not be the same).
a). If TS(T) > CTS(R): T goes ahead and performs the read
(retrieved data from X is respective to CTS(R)). If TS(T) > RTS(R),
set RTS(R) := TS(T); otherwise leave RTS(R) intact.
b). If TS(T) < CTS(R): invalid read. Rollback T (abort T and
restart it with a new, larger timestamp).
2. T wants to write X.
a). If TS(T) >= RTS(R) and TS(T) > CTS(R): T performs the write to
X including writing the journal in R and applying that journal to X,
excluding setting the new value for CTS(R).
b). If TS(T) >= RTS(R), but TS(T) < CTS(R): the Thomas write rule
applies here. The write by T is ignored. T is allowed to proceed
without making change to R and X.
c). if TS(T) < RTS(R): invalid write. T must be rolled back.
3. T completes all its actions and now wants to commit. If CTS(R) at
this point is still the same as when T started, it is set to TS(T); T
effectively gets committed. If CTS(R) has changed, T must be rolled
back.
Note that in rolling back:
- If T has changed RTS(R), the old value must be restored.
- T may be retried at most n number of times (currently n is set to
4?).
I'd appreciate if someone could give comments asap on what I outlined
above.
Thanks a lot,
David.
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