MapReduce

问题 A Combiner runs after the Mapper and before the Reducer,it will receive as input all data emitted by the Mapper instances on a given node. then emits output to the Reducers. And also,If a reduce function is both commutative and associative , then it can be used as a Combiner. My Question is what does the phrase " commutative and associative " mean in this situation? 回答1: Assume you have a list of numbers, 1 2 3 4 5 6. Associative here means you can take your operation and apply it to any

问题 I'm trying to learn to use Yelp's Python API for MapReduce, MRJob. Their simple word counter example makes sense, but I'm curious how one would handle an application involving multiple inputs. For instance, rather than simply counting the words in a document, multiplying a vector by a matrix. I came up with this solution, which functions, but feels silly: class MatrixVectMultiplyTast(MRJob): def multiply(self,key,line): line = map(float,line.split(" ")) v,col = line[-1],line[:-1] for i in

问题 I've been trying to use Hadoop to send N amount of lines to a single mapping. I don't require for the lines to be split already. I've tried to use NLineInputFormat, however that sends N lines of text from the data to each mapper one line at a time [giving up after the Nth line]. I have tried to set the option and it only takes N lines of input sending it at 1 line at a time to each map: job.setInt("mapred.line.input.format.linespermap", 10); I've found a mailing list recommending me to

问题 Using only a mapper (a Python script) and no reducer, how can I output a separate file with the key as the filename, for each line of output, rather than having long files of output? 回答1: You can either write to a text file on the local filesystem using python file functions or if you want to use HDFS use the Thrift API. 回答2: The input and outputformat classes can be replaced by use of the -inputformat and -outputformat commandline parameters. One example of how to do this can be found in the

问题 I am using rails 3 with mongoid. I have a collection of Stocks with an embedded collection of Prices : class Stock include Mongoid::Document field :name, :type => String field :code, :type => Integer embeds_many :prices class Price include Mongoid::Document field :date, :type => DateTime field :value, :type => Float embedded_in :stock, :inverse_of => :prices I would like to get the stocks whose the minimum price since a given date is lower than a given price p, and then be able to sort the

问题 I have a tiny cluster composed of 1 master (namenode, secondarynamenode, resourcemanager) and 2 slaves (datanode, nodemanager). I have set in the yarn-site.xml of the master : yarn.scheduler.minimum-allocation-mb : 512 yarn.scheduler.maximum-allocation-mb : 1024 yarn.scheduler.minimum-allocation-vcores : 1 yarn.scheduler.maximum-allocation-vcores : 2 I have set in the yarn-site.xml of the slaves : yarn.nodemanager.resource.memory-mb : 2048 yarn.nodemanager.resource.cpu-vcores : 4 Then in the

问题 I have my own MapReduce code that I'm trying to run, but it just stays at Accepted state. I tried running another sample MR job that I'd run previously and which was successful. But now, both the jobs stay in Accepted state. I tried changing various properties in the mapred-site.xml and yarn-site.xml as mentioned here and here but that didn't help either. Can someone please point out what could possibly be going wrong. I'm using hadoop-2.2.0 I've tried many values for the various properties,

问题 First, the background. I used to have a collection logs and used map/reduce to generate various reports. Most of these reports were based on data from within a single day, so I always had a condition d: SOME_DATE . When the logs collection grew extremely big, inserting became extremely slow (slower than the app we were monitoring was generating logs), even after dropping lots of indexes. So we decided to have each day's data in a separate collection - logs_YYYY-mm-dd - that way indexes are

问题 I have a system that accepts status updates from a variety of unique sources, and each status update creates a new document in the following structure: { "type": "status_update", "source_id": "truck1231", "timestamp": 13023123123, "location": "Boise, ID" } Data purely example, but gets the idea across. Now, these documents are generated at interval, once an hour or so. An hour later, we might the insert: { "type": "status_update", "source_id": "truck1231", "timestamp": 13023126723, "location"

问题 I am using hadoop 2.7.0 and java oracle jdk1.7.0_79 with NetBeans IDE 8.0.2. When I try to communicate with Hadoop using the Java file, then I get the following error. Is there any dependency issues involved? Or how can I resolve this error? I have seen posts with related issue, but none of them were helpful to convey the answer clearly. So, please help me out here. Thanks! Exception in thread "main" org.apache.hadoop.ipc.RemoteException: Server IPC version 9 cannot communicate with client