Law

The latency floor of consistency

You may make reads local, or writes local, never both. You are not paying for the distance. You are paying for not knowing the distance.

r + w \geq d (sequential consistency), r \geq \frac{u}{4}, w \geq \frac{u}{2} (linearizability)

$d$: One-way delay; the one-way delay between the two nodes
$r, w$: Read and write time; the time a read takes and the time a write takes
$u$: Delay uncertainty; the spread between the smallest and largest possible delay, d_max − d_min

How it behaves

One input on the horizontal axis, the others held fixed.

Data table

Every number the mechanisms quote for this relationship is recomputed from it, so none of them disagrees with it.

Each mechanism states it in its own names. The general symbol each name stands for is shown with it.

Consistency & Isolation Models
When many users, many objects and many copies exist at once, which histories does the system promise never to show, and who writes the code for the histories it does show?
$r + w \geq d$
Read it in place →
Consistency & Isolation Models
When many users, many objects and many copies exist at once, which histories does the system promise never to show, and who writes the code for the histories it does show?
$read latency \geq u /4, write latency \geq u /2$
Read it in place →