Repeatedly invoking a Phoenix++ MapReduce job over a stream results in many redundant computations (at both Map and Reduce operations). C-MR allows data to be processed only once by Map and the inclusion of the Combine operator significantly decreases redundant work performed at the Reduce operator.
1. Data is often generated from a source that can potentially produce an unbounded stream.2. A stream’s contents can only be accessed sequentially.Traditional queries are comprised of relational operators that assume a finite data source that can be accessed randomly.
C-MR: Continuously Executing MapReduce Workflows on Multi-Core Processors
C-MR: Continuously ExecutingMapReduce Workflows on Multi- Core Processors Speaker: LIN Qianhttp://www.comp.nus.edu.sg/~linqian
Problem• Stream applications are often time-critical• Enabling stream support for MapReduce jobs – Simple for the Map operations – Hard for the Reduce operations• Continuously executing MapReduce workflows requires a great deal of coordination 1
C-MR Workflow• Windows: temporal subdivisions of a stream described by – size (the amount of the stream spanning) – slide (the interval between windows) 2
C-MR Programming Interface (cont.2)• Create workflows of continuous MapReduce jobs
C-MR vs. MapReduce• MapReduce computing nodes receive a set of Map or Reduce tasks and each node must wait for all other nodes to complete their tasks before being allocated additional tasks.• C-MR uses pull-based data acquisition allowing computing nodes to execute any Map or Reduce workload as they are able. Thus, straggling nodes will not hinder the progress of the other nodes if there is data available to process elsewhere in the workflow. 6
Two Properties of Streams• Unbounded• Accessed sequentially Hard to be handled using traditional DBMS 19
Query Operators• Unbounded stateful operators – maintain state with no upper bound in size run out of memory• Blocking operators – read an entire input before emitting a single output might never produce a result • Never use them, or • Use them under a refactoring 20
Punctuations• Mark the end of substreams – allowing us to view an infinite stream as a mixture of finite streams 21