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//! Module containing types related to computing nodes which should receive a copy of data blocks
//! and metadata
use std::convert::TryInto;
use garage_util::data::*;
use crate::layout::ClusterLayout;
/// A partition id, which is stored on 16 bits
/// i.e. we have up to 2**16 partitions.
/// (in practice we have exactly 2**PARTITION_BITS partitions)
pub type Partition = u16;
// TODO: make this constant parametrizable in the config file
// For deployments with many nodes it might make sense to bump
// it up to 10.
// Maximum value : 16
/// How many bits from the hash are used to make partitions. Higher numbers means more fairness in
/// presence of numerous nodes, but exponentially bigger ring. Max 16
pub const PARTITION_BITS: usize = 8;
const PARTITION_MASK_U16: u16 = ((1 << PARTITION_BITS) - 1) << (16 - PARTITION_BITS);
/// A ring distributing fairly objects to nodes
#[derive(Clone)]
pub struct Ring {
/// The replication factor for this ring
pub replication_factor: usize,
/// The network configuration used to generate this ring
pub layout: ClusterLayout,
// Internal order of nodes used to make a more compact representation of the ring
nodes: Vec<Uuid>,
// The list of entries in the ring
ring: Vec<RingEntry>,
}
// Type to store compactly the id of a node in the system
// Change this to u16 the day we want to have more than 256 nodes in a cluster
pub type CompactNodeType = u8;
pub const MAX_NODE_NUMBER: usize = 256;
// The maximum number of times an object might get replicated
// This must be at least 3 because Garage supports 3-way replication
// Here we use 6 so that the size of a ring entry is 8 bytes
// (2 bytes partition id, 6 bytes node numbers as u8s)
const MAX_REPLICATION: usize = 6;
/// An entry in the ring
#[derive(Clone, Debug)]
struct RingEntry {
// The two first bytes of the first hash that goes in this partition
// (the next bytes are zeroes)
hash_prefix: u16,
// The nodes that store this partition, stored as a list of positions in the `nodes`
// field of the Ring structure
// Only items 0 up to ring.replication_factor - 1 are used, others are zeros
nodes_buf: [CompactNodeType; MAX_REPLICATION],
}
impl Ring {
pub(crate) fn new(layout: ClusterLayout, replication_factor: usize) -> Self {
if replication_factor != layout.replication_factor {
warn!("Could not build ring: replication factor does not match between local configuration and network role assignation.");
return Self::empty(layout, replication_factor);
}
if layout.ring_assignation_data.len() != replication_factor * (1 << PARTITION_BITS) {
warn!("Could not build ring: network role assignation data has invalid length");
return Self::empty(layout, replication_factor);
}
let nodes = layout.node_id_vec.clone();
let ring = (0..(1 << PARTITION_BITS))
.map(|i| {
let top = (i as u16) << (16 - PARTITION_BITS);
let mut nodes_buf = [0u8; MAX_REPLICATION];
nodes_buf[..replication_factor].copy_from_slice(
&layout.ring_assignation_data
[replication_factor * i..replication_factor * (i + 1)],
);
RingEntry {
hash_prefix: top,
nodes_buf,
}
})
.collect::<Vec<_>>();
Self {
replication_factor,
layout,
nodes,
ring,
}
}
fn empty(layout: ClusterLayout, replication_factor: usize) -> Self {
Self {
replication_factor,
layout,
nodes: vec![],
ring: vec![],
}
}
/// Get the partition in which data would fall on
pub fn partition_of(&self, position: &Hash) -> Partition {
let top = u16::from_be_bytes(position.as_slice()[0..2].try_into().unwrap());
top >> (16 - PARTITION_BITS)
}
/// Get the list of partitions and the first hash of a partition key that would fall in it
pub fn partitions(&self) -> Vec<(Partition, Hash)> {
let mut ret = vec![];
for (i, entry) in self.ring.iter().enumerate() {
let mut location = [0u8; 32];
location[..2].copy_from_slice(&u16::to_be_bytes(entry.hash_prefix)[..]);
ret.push((i as u16, location.into()));
}
if !ret.is_empty() {
assert_eq!(ret[0].1, [0u8; 32].into());
}
ret
}
/// Walk the ring to find the n servers in which data should be replicated
pub fn get_nodes(&self, position: &Hash, n: usize) -> Vec<Uuid> {
if self.ring.len() != 1 << PARTITION_BITS {
warn!("Ring not yet ready, read/writes will be lost!");
return vec![];
}
let partition_idx = self.partition_of(position) as usize;
let partition = &self.ring[partition_idx];
let top = u16::from_be_bytes(position.as_slice()[0..2].try_into().unwrap());
// Check that we haven't messed up our partition table, i.e. that this partition
// table entrey indeed corresponds to the item we are storing
assert_eq!(
partition.hash_prefix & PARTITION_MASK_U16,
top & PARTITION_MASK_U16
);
assert!(n <= self.replication_factor);
partition.nodes_buf[..n]
.iter()
.map(|i| self.nodes[*i as usize])
.collect::<Vec<_>>()
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_ring_entry_size() {
assert_eq!(std::mem::size_of::<RingEntry>(), 8);
}
}
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