Strictness
It’s convenient to undo a write, w[x], by restoring its before image (=the value of x before w[x] executed)
Example - w1[x,1] writes the value “1” into x.
- w1[x,1] w1[y,3] c1 w2[y,1] r2[x] a2
- abort T2 by restoring the before image of w2[y,1], = 3
But this isn’t always possible.
- For example, consider w1[x,2] w2[x,3] a1 a2
- a1 & a2 can’t be implemented by restoring before images
- notice that w1[x,2] w2[x,3] a2 a1 would be OK
A system is strict if it only reads or overwrites committed data.