You signed in with another tab or window. Reload to refresh your session.You signed out in another tab or window. Reload to refresh your session.You switched accounts on another tab or window. Reload to refresh your session.Dismiss alert
Copy file name to clipboardExpand all lines: crates/consistent-hashing/README.md
+3-3Lines changed: 3 additions & 3 deletions
Display the source diff
Display the rich diff
Original file line number
Diff line number
Diff line change
@@ -128,16 +128,16 @@ Properties 2, 3, and 4 can be proven via induction as follows.
128
128
129
129
### Property 4
130
130
131
-
`k = 1`: We expect that `consistent_hash` returns a single uniformly distributed node index which is consistent in `n`, i.e. changes the hash value with probability `1/(n+1)`, when `n` increments by one. In our implementation, we use an `O(1)` implementation of the jump-hash algorithm. For `k=1`, `consistent_choose_k(key, 1, n)` becomes a single function call to `consistent_choose_max(key, 1, n)` which in turn calls `consistent_hash(key, 0, n)`. I.e. `consistent_choose_k` inherits the all the desired properties from `consistent_hash` for `k=1` and all `n>=1`.
131
+
`k = 1`: We expect that `consistent_hash` returns a single uniformly distributed node index which is consistent in `n`, i.e. changes the hash value with probability `1/(n+1)`, when `n` increments by one. In our implementation, we use an `O(1)` implementation of the jump-hash algorithm. For `k=1`, `consistent_choose_k(key, 1, n)` becomes a single function call to `consistent_choose_max(key, 1, n)` which in turn calls `consistent_hash(key, 0, n)`. I.e. `consistent_choose_k` inherits all the desired properties from `consistent_hash` for `k=1` and all `n>=1`.
132
132
133
-
`k → k+1`: `M(k+1, n+1) = M(k+1, n)` iff `M(k, n+1) < n` and `consistent_hash(_, k, n+1-k) < n - k`. The probability for this is `(n+1-k)/(n+1)`for the former by induction and `(n-k)/(n+1-k)` by the assumption that `consistent_hash` is a proper consistent hash function. Since both these probabilities are assumed to be independent, the probability that our initial value changes is `1 - (n+1-k)/(n+1) * (n-k)/(n+1-k) = 1 - (n-k)/(n+1) = (k+1)/(n+1)` proving property 4.
133
+
`k → k+1`: `M(k+1, n+1) = M(k+1, n)` iff `M(k, n+1) < n` and `consistent_hash(_, k, n+1-k) < n - k`. The probability for the former is `(n+1-k)/(n+1)` by induction and `(n-k)/(n+1-k)` for the latter by the assumption that `consistent_hash` is a proper consistent hash function. Since both these probabilities are assumed to be independent, the probability that our initial value changes is `1 - (n+1-k)/(n+1) * (n-k)/(n+1-k) = 1 - (n-k)/(n+1) = (k+1)/(n+1)` proving property 4.
134
134
135
135
### Property 3
136
136
137
137
Property 3 is trivially satisfied if `S(k+1, n+1) = S(k+1, n)`. So, we focus on the case where `S(k+1, n+1) != S(k+1, n)`, which implies that `n ∈ S(k+1, n+1)` as largest element.
138
138
We know that `S(k+1, n) = {m} ∪ S(k, m)` for some `m` by definition and `S(k, n) = S(k, u) ∖ {v} ∪ {w}` by induction for some `u`, `v`, and `w`. Thus far we have `S(k+1, n+1) = {n} ∪ S(k, n) = {n} ∪ S(k, u) ∖ {v} ∪ {w}`.
139
139
140
-
If `u = m`, then `S(k+1, n) = {m} ∪ S(k, m) ∖ {v} ∪ {w}` and `S(k+1, n+1) = {n} ∪ S(k, n) = {n} ∪ S(k, m) ∖ {v} ∪ {w}` and the two differ exactly in the elemetns`m` and `n` proving property 3.
140
+
If `u = m`, then `S(k+1, n) = {m} ∪ S(k, m) ∖ {v} ∪ {w}` and `S(k+1, n+1) = {n} ∪ S(k, n) = {n} ∪ S(k, m) ∖ {v} ∪ {w}` and the two differ exactly in the elements`m` and `n` proving property 3.
141
141
142
142
If `u ≠ m`, then `consistent_hash(_, k, n) = m`, since that's the only way how the largest values in `S(k+1, n)` and `S(k, n)` can differ. In this case, `m ∉ S(k+1, n+1)`, since `n` (and not `m`) is the largest element of `S(k+1, n+1)`. Furthermore, `S(k, n) = S(k, m)`, since `consistent_hash(_, i, n) < m` for all `i < k` (otherwise there is a contradiction).
143
143
Putting it together leads to `S(k+1, n+1) = {n} ∪ S(k, m)` and `S(k+1, n) = {m} ∪ S(k, m)` which differ exactly in the elements `n` and `m` which concludes the proof.
0 commit comments