1. Examples
Undo, Redo, Undo/Redo

2. Undo Log <START S> <S,A,60> <COMMIT S>
<START T>
<T,A,10>
<START CKPT (T)>
<START U>
<U,B,20>
<T,C,30>
<START V>
<U,D,40>
<V,F,70>
<COMMIT U>
<T,E,50>
<COMMIT T>
<V,B,80>
<COMMIT V>
Suppose that we begin a non quiescent checkpoint immediately after <T,A,10>
Tell,
When the <END CKPT> is written?
For each possible point at which a crash could occur, how far back in the log we must look to find all the possible incomplete transactions?

3. Undo Log <START S> <S,A,60> <COMMIT S>
<START T>
<T,A,10>
<START CKPT (T)>
<START U>
<U,B,20>
<T,C,30>
<START V>
<U,D,40>
<V,F,70>
<COMMIT U>
<T,E,50>
<COMMIT T>
<END CKPT>
<V,B,80>
<COMMIT V>
Suppose that we begin a non quiescent checkpoint immediately after <T,A,10>
Tell,
When the <END CKPT> is written?
For each possible point at which a crash could occur, how far back in the log we must look to find all the possible incomplete transactions?
If the crash occurs after <END CKPT>, we have to search back only to the <START CKPT(T)>.
However, for crashes prior to the <COMMIT T> record, the search must continue back as far as the <START T> record, since that is the (lone) transaction that was active at the start of the checkpoint.

4. Redo Log <START S> <S,A,60> <COMMIT S>
<START T>
<T,A,10>
<START CKPT (T)>
<START U>
<U,B,20>
<T,C,30>
<START V>
<U,D,40>
<V,F,70>
<COMMIT U>
<T,E,50>
<COMMIT T>
<V,B,80>
<COMMIT V>
Suppose that we begin a non quiescent checkpoint immediately after <T,A,10>
Tell,
When the <END CKPT> is written?
For each possible point at which a crash could occur, how far back in the log we must look to find all the possible incomplete transactions?

5. Redo Log <START S> <S,A,60> <COMMIT S>
<START T>
<T,A,10>
<START CKPT (T)>
<START U>
<U,B,20>
<T,C,30>
<START V>
<U,D,40>
<V,F,70>
<COMMIT U>
<T,E,50>
<COMMIT T>
<V,B,80>
<COMMIT V>
Suppose that we begin a non quiescent checkpoint immediately after <T,A,10>
Tell,
When the <END CKPT> could be written?
For each possible point at which a crash could occur, how far back in the log we must look to find all the possible incomplete transactions? Consider both the case that the <END CKPT> record was or was not written prior to the crash.
<END CKPT> can occur at any point after the <START CKPT(T)>.
If the crash occurs after <END CKPT>, we restrict ourselves only to committed transactions that were listed in <START CKPT(..)>, i.e. T, and those that started after this point.
If the crash occurs in between, then, for this example, we have to consider all the log.

6. Undo/Redo Log <START S> <S,A,60,61> <COMMIT S>
<START T>
<T,A,61,62>
<START CKPT (T)>
<START U>
<U,B,20,21>
<T,C,30,31>
<START V>
<U,D,40,41>
<V,F,70,71>
<COMMIT U>
<T,E,50,51>
<COMMIT T>
<V,B,21,22>
<COMMIT V>
Suppose that we begin a non quiescent checkpoint immediately after <T,A,61,62>
Tell,
When the <END CKPT> could be written?
For each possible point at which a crash could occur, how far back in the log we must look at?

7. Undo/Redo Log <START S> <S,A,60,61> <COMMIT S>
<START T>
<T,A,61,62>
<START CKPT (T)>
<START U>
<U,B,20,21>
<T,C,30,31>
<START V>
<U,D,40,41>
<V,F,70,71>
<COMMIT U>
<T,E,50,51>
<COMMIT T>
<V,B,21,22>
<COMMIT V>
Suppose that we begin a non quiescent checkpoint immediately after <T,A,61,62>
Tell,
When the <END CKPT> could be written?
For each possible point at which a crash could occur, how far back in the log we must look at?
<END CKPT> can occur at any point after the <START CKPT(T)>.
If the crash occurs after <END CKPT>, we restrict ourselves only to transactions that were listed in <START CKPT(..)>, i.e. T, and those that started after this point.
If the crash occurs in between, then, for this example, we have to consider all the log.