Struct Maglev 

pub struct Maglev { /* private fields */ }

Maglev consistent hashing (Google, NSDI’16).

Builds a precomputed lookup table of M (a prime, default 65537) slots from the stable, full backend set. Each backend derives a (offset, skip) permutation from two seeded hashes and claims slots in permutation order until the table is full; a backend’s share of slots is proportional to its weight.

Rebuild discipline (never on the hot path)

The table is rebuilt only when the full backend set changes (Maglev::rebuild, wired into BackendList::add_backend / remove_backend), never per packet. Building the table is O(M) = 65537 slot writes; doing it per datagram would be a DoS amplifier (one unhealthy backend would trigger a full rebuild on every selection) and would also destroy Maglev’s stability, since the table would track a shifting subset.

Selection (handles a shrunk healthy subset without rebuilding)

At selection time the caller passes the healthy subset of backends. Lookup is near-O(1): compute slot = key % M, then probe forward through the table (table[(slot + i) % M]) and return the first entry whose address is present in the healthy subset. The table maps to the full set it was built from; membership is checked against the subset, so an unhealthy backend is simply skipped — no rebuild, and healthy keys stay pinned. If no table entry resolves to a healthy backend, fall back to round-robin over the subset. With no key it falls back to round-robin.

Implementations

impl Maglev

pub const DEFAULT_TABLE_SIZE: usize = 65537

Default prime table size. 65537 is the smallest prime above 2^16; large enough to keep per-key disruption small on backend churn, small enough to rebuild cheaply off the datapath.

pub fn new() -> Self

pub fn with_seed(seed: u64) -> Self

pub fn with_seed_and_size(seed: u64, size: usize) -> Self

pub fn rebuild(&mut self, backends: &[Rc<RefCell<Backend>>])

Rebuild the lookup table from backends. Called on backend-set change, NOT per packet. Honors backend weight via proportional slot share.

Trait Implementations

impl Debug for Maglev

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for Maglev

fn default() -> Self

Returns the “default value” for a type.

impl LoadBalancingAlgorithm for Maglev

fn next_available_backend( &mut self, key: Option<u64>, backends: &mut Vec<Rc<RefCell<Backend>>>, ) -> Option<Rc<RefCell<Backend>>>

Select the next backend.

fn rebuild(&mut self, backends: &[Rc<RefCell<Backend>>])

Called by the control plane when the live backend set for a cluster changes, so table-based policies (Maglev) can recompute their lookup table off the datapath. The default is a no-op; only Maglev overrides it.