Spark: All RDD data not getting saved to Cassandra table - scala

Question

Hi, I am trying to load RDD data to a Cassandra Column family using Scala. Out of a total 50 rows , only 28 are getting stored into cassandra table.
Below is the Code snippet:
val states = sc.textFile("state.txt")
//list o fall the 50 states of the USA
var n =0 // corrected to var
val statesRDD = states.map{a =>
n=n+1
(n, a)
}
scala> statesRDD.count
res2: Long = 50
cqlsh:brs> CREATE TABLE BRS.state(state_id int PRIMARY KEY, state_name text);
statesRDD.saveToCassandra("brs","state", SomeColumns("state_id","state_name"))
// this statement saves only 28 rows out of 50, not sure why!!!!
cqlsh:brs> select * from state;
state_id | state_name
----------+-------------
23 | Minnesota
5 | California
28 | Nevada
10 | Georgia
16 | Kansas
13 | Illinois
11 | Hawaii
1 | Alabama
19 | Maine
8 | Oklahoma
2 | Alaska
4 | New York
18 | Virginia
15 | Iowa
22 | Wyoming
27 | Nebraska
20 | Maryland
7 | Ohio
6 | Colorado
9 | Florida
14 | Indiana
26 | Montana
21 | Wisconsin
17 | Vermont
24 | Mississippi
25 | Missouri
12 | Idaho
3 | Arizona
(28 rows)
Can anyone please help me in finding where the issue is?
Edit:
I understood why only 28 rows are getting stored in Cassandra, it's because I have made the first column a PRIMARY KEY and It looks like in my code, n is incremented maximum to 28 and then it starts again with 1 till 22 (total 50).
val states = sc.textFile("states.txt")
var n =0
var statesRDD = states.map{a =>
n+=1
(n, a)
}
I tried making n an accumulator variable as well(viz. val n = sc.accumulator(0,"Counter")), but I don't see any differnce in the output.
scala> statesRDD.foreach(println)
[Stage 2:> (0 + 0) / 2]
(1,New Hampshire)
(2,New Jersey)
(3,New Mexico)
(4,New York)
(5,North Carolina)
(6,North Dakota)
(7,Ohio)
(8,Oklahoma)
(9,Oregon)
(10,Pennsylvania)
(11,Rhode Island)
(12,South Carolina)
(13,South Dakota)
(14,Tennessee)
(15,Texas)
(16,Utah)
(17,Vermont)
(18,Virginia)
(19,Washington)
(20,West Virginia)
(21,Wisconsin)
(22,Wyoming)
(1,Alabama)
(2,Alaska)
(3,Arizona)
(4,Arkansas)
(5,California)
(6,Colorado)
(7,Connecticut)
(8,Delaware)
(9,Florida)
(10,Georgia)
(11,Hawaii)
(12,Idaho)
(13,Illinois)
(14,Indiana)
(15,Iowa)
(16,Kansas)
(17,Kentucky)
(18,Louisiana)
(19,Maine)
(20,Maryland)
(21,Massachusetts)
(22,Michigan)
(23,Minnesota)
(24,Mississippi)
(25,Missouri)
(26,Montana)
(27,Nebraska)
(28,Nevada)
I am curious to know what is causing n to not getting updated after value 28? Also, what are the ways in which I can create a counter which I can use for creating RDD?

Answer 1

There are some misconceptions about distributed systems embedded inside your question. The real heart of this is "How do I have a counter in a distributed system?"
The short answer is you don't. For example what you've done in your code example originally is something like this.
Task One {
var x = 0
record 1: x = 1
record 2: x = 2
}
Task Two {
var x = 0
record 20: x = 1
record 21: x = 2
}
Each machine is independently creating a new x variable set at 0 which gets incremented within it's own context, independently over the other nodes.
For most use cases the "counter" question can be replaced with "How can I get a Unique Identifier per Record in a distributed system?"
For this most users end up using a UUID which can be generated on independent machines with infinitesimal chances of conflicts.
If the question can be "How can I get a monotonic increasing unique indentifier?"
Then you can use zipWithUniqueIndex which will not count but will generate monotonically increasing ids.
If you just want them number to start with it's best to do it on the local system.
Edit; Why can't I use an accumulator?
Accumulators store their state (surprise) per task. You can see this with a little example:
val x = sc.accumulator(0, "x")
sc.parallelize(1 to 50).foreachPartition{ it => it.foreach(y => x+= 1); println(x)}
/*
6
7
6
6
6
6
6
7
*/
x.value
// res38: Int = 50
The accumulators combine their state after finishing their tasks, which means you can't use them as a global distributed counter.