Mercurial > lbo > hg > localmr
view src/mapreducer.rs @ 50:603c38d64cd5
Experiment: Pre-fetch larger buffers from shards
author | Lewin Bormann <lbo@spheniscida.de> |
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date | Wed, 03 Feb 2016 21:15:39 +0000 |
parents | ab7d19e012e0 |
children | 7a059fb6186a |
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//! The MapReducer trait and associated types. use record_types::{REmitter, MEmitter, Record, MultiRecord}; use std::clone::Clone; use std::hash::{Hasher, SipHasher}; /// Default sharding function. pub fn _std_shard(n: usize, key: &String) -> usize { let mut h = SipHasher::new(); h.write(key.as_bytes()); h.finish() as usize % n } /// Map() function type. The MEmitter argument is used to emit values from /// the map() function. pub type MapperF = fn(&mut MEmitter, Record); /// Reduce() function type. The REmitter argument is used to emit values /// from the reduce() function. pub type ReducerF = fn(&mut REmitter, MultiRecord); /// A function used to determine the shard a key belongs in. /// The first argument is the number of shards, the second one the key; /// the return value should be in [0; n). pub type SharderF = fn(usize, &String) -> usize; /// A type implementing map() and reduce() functions. /// The MapReducer is cloned once per mapper/reducer thread. pub trait MapReducer: Clone { /// Takes one <key,value> pair and an emitter. /// The emitter is used to yield results from the map phase. fn map(&self, em: &mut MEmitter, record: Record); /// Takes one key and one or more values and emits one or more /// values. fn reduce(&self, em: &mut REmitter, records: MultiRecord); /// Determines how to map keys to (reduce) shards. /// Returns a number in [0; n) determining the shard the key belongs in. /// The default implementation uses a simple hash (SipHasher) and modulo. fn shard(&self, n: usize, key: &String) -> usize { _std_shard(n, key) } }