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Input: (lineNumber, line) records Output: line matching a given pattern Map: if(line matches pattern): output(line) Reduce: identify function -alternative: no reducer(map only job) |
2. Sort:
Input: (key, value) records Output: Same records, sorted by key map: identify function Reduce: Identify function Trick: Pick partitioning function h such that k1<k2 => h(k1) <(k2) |
3. Inverted Index
Input: (filename, text) records Output: list of files containing each word Map: foreach word in text.split(): output(word, filename) Combine: uniqify filenames for each word Reduce: def reduce(word, filenames): output(word, sort(filenames)) |
Hadoop Mapreduce | Hive | Pig |
Compiled language | SQL like query language | Scripting language |
Lower level of abstraction | Highter level of abstruction | Highter level of abstraction |
More lines of code | Comparatively less line of code than mapreduce and apache pig | Comparatively less lines of code than mapreduce |
Code efficiency is high when compared to pig and hive. | Code efficiency is relatively less | Code efficiency is relatively less |
1) You have a huge amount of data.
2) Hide system level details from the developers
- No more race conditions, lock contention, etc
3) Two strong merits for big data analytics
- Scalability
- Fault tolerance
4) Move computing to data
- Cluster have limited bandwidth
5) Hadoop is the most widely used implementation of MapReduce
2) Map function:
(Kin, Vin) -> list(Kinter, Vinter)
3) Reduce function:
(Kinter, list(Vinter)) -> list(Kout, Vout)
def mapper(line): foreach word in line.split(): output(word, 1) Def reducer(key, values): output(key, sum(values)) |
map() input <fileneme, file text> parses file and emits <word, count> pairs. eg. <"hello", 1> Reduce() sums values for the some key and emits <word, totalcount>. eg, <"hello", (3 5 2 7)> = <"hello", 17> |