# Thomas write rula andview serializability

Yesterday our Professor told in the class that Thomas Write rule ensures view serializability, but while surfing on this topic today on internet I am not able to find any information about that claim. So is it always $$TRUE?$$

What he told was this-->

Timestamp ordering ensures conflict serializability

Thomas write rule ensures view serializability

• Did you just try asking your professor?
– Juho
Jan 4 '20 at 8:32
• What to ask when he has already mentioned that in class... But i am not able to find a single line about it on the web Jan 4 '20 at 13:32
• For instance, couldn't you say "You said that X. Is it really always true? Why?". Anyway, I hope your question attracts attention here.
– Juho
Jan 4 '20 at 13:58
• Will do when college resumes on monday..Thanks anyway.. :) Jan 4 '20 at 14:07

## 1 Answer

Timestamp ordering ensures conflict serializability

proof :

Assume that in precedence graph of schedule, we have edge $$T_i \rightarrow T_j$$.

Now, When $$T_j$$ puts it's request for this conflicting operation it will be continue only if $$\text{timestamp}(T_i) < \text{timestamp}(T_j)$$.

Now, for schedule to be non conflict serializable there must exist a cycle in precedence graph of that schedule and let say that cycle is $$T_i, T_{i+1}, ...., T_i$$.

Now, note that that cycle can't exist because existence of cycle implies $$\text{timestamp}(T_i) < \text{timestamp}(T_i)$$. (As timestamp are unique.)

So, we conclude that there can't be any such cycle which in turn implies that timestamp ordering protocol allows only conflict serializable schedules.

• Ya..this I know brother but i want to confirm the claim about view serailizibility Jan 5 '20 at 13:47
• Yes, working on that will post if succeed. :) Jan 5 '20 at 13:49
• Thanks a lot.. :) Jan 5 '20 at 13:57
• @Turing101, here is a proof of second one Jan 5 '20 at 14:34
• Thanks a ton mate.. :) Jan 5 '20 at 18:00