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javamongodbmapreducemongo-java

Error in db.eval() in MongoDB

I am trying to run a map-reduce script using db.eval() method. 

    MongoClient mongo = new MongoClient("localhost", 27017);
    DB mongodb = (DB) mongo.getDB("testDB");

    String script = "db.collection.mapReduce("
            + "function() {emit(this.class, this.marks);},"
            + "function(key, values) { return {\"sum\":Array.sum(values)};},"
               + "{ "
                   + " query : {_id:{$lt:50}},"
                   + "out:\"collectionMapReduce\""
               + "}"
           + ")";

    Object result = mongodb.eval(script);

I got the following error : 

Exception in thread "main" com.mongodb.MongoException$Network: Read operation to server localhost:27017 failed on database testDB
at com.mongodb.DBTCPConnector.innerCall(DBTCPConnector.java:298)
at com.mongodb.DBTCPConnector.call(DBTCPConnector.java:269)
at com.mongodb.DBCollectionImpl.find(DBCollectionImpl.java:84)
at com.mongodb.DB.command(DB.java:320)
at com.mongodb.DB.command(DB.java:299)
at com.mongodb.DB.command(DB.java:374)
at com.mongodb.DB.command(DB.java:246)
at com.mongodb.DB.doEval(DB.java:445)
at com.mongodb.DB.eval(DB.java:463)
at com.dev.TestMapReduce.main(TestMapReduce.java:25)

Caused by: java.io.EOFException
at org.bson.io.Bits.readFully(Bits.java:75)
at org.bson.io.Bits.readFully(Bits.java:50)
at org.bson.io.Bits.readFully(Bits.java:37)
at com.mongodb.Response.<init>(Response.java:42)
at com.mongodb.DBPort$1.execute(DBPort.java:164)
at com.mongodb.DBPort$1.execute(DBPort.java:158)
at com.mongodb.DBPort.doOperation(DBPort.java:187)
at com.mongodb.DBPort.call(DBPort.java:158)
at com.mongodb.DBTCPConnector.innerCall(DBTCPConnector.java:290)
... 9 more

I am not able to understand the reason for this error. How can I resolve this?

Note: I am able to perform this job using db.collection.mapReduce(). But I am just trying to achieve this using db.eval()

Solution

  • The correct way to run a map/reduce job is via the mapReduce command which is exposed in the Java driver via the MapReduceCommand helper.

    The eval command is not intended for this purpose (and has also been deprecated as at MongoDB 3.0).