[SPARK-733] Add documentation on use of accumulators in lazy transfor…

…mation

I've added documentation clarifying the particular lack of clarity highlighted in the relevant JIRA. I've also added code examples for this issue to clarify the explanation.

Author: Ilya Ganelin <ilya.ganelin@capitalone.com>

Closes #4022 from ilganeli/SPARK-733 and squashes the following commits:

587def5 [Ilya Ganelin] Updated to clarify verbage
df3afd7 [Ilya Ganelin] Revert "Partially updated task metrics to make some vars private"
3f6c512 [Ilya Ganelin] Revert "Completed refactoring to make vars in TaskMetrics class private"
58034fb [Ilya Ganelin] Merge remote-tracking branch 'upstream/master' into SPARK-733
4dc2cdb [Ilya Ganelin] Merge remote-tracking branch 'upstream/master' into SPARK-733
3a38db1 [Ilya Ganelin] Verified documentation update by building via jekyll
33b5a2d [Ilya Ganelin] Added code examples for java and python
1fd59b2 [Ilya Ganelin] Updated documentation for accumulators to highlight lazy evaluation issue
5525c20 [Ilya Ganelin] Completed refactoring to make vars in TaskMetrics class private
c64da4f [Ilya Ganelin] Partially updated task metrics to make some vars private

docs/programming-guide.md

@@ -1316,7 +1316,35 @@ For accumulator updates performed inside <b>actions only</b>, Spark guarantees t
 will only be applied once, i.e. restarted tasks will not update the value. In transformations, users should be aware 
 of that each task's update may be applied more than once if tasks or job stages are re-executed.
 
+Accumulators do not change the lazy evaluation model of Spark. If they are being updated within an operation on an RDD, their value is only updated once that RDD is computed as part of an action. Consequently, accumulator updates are not guaranteed to be executed when made within a lazy transformation like `map()`. The below code fragment demonstrates this property:
 
+<div class="codetabs">
+
+<div data-lang="scala"  markdown="1">
+{% highlight scala %}
+val acc = sc.accumulator(0)
+data.map(x => acc += x; f(x))
+// Here, acc is still 0 because no actions have cause the `map` to be computed.
+{% endhighlight %}
+</div>
+
+<div data-lang="java"  markdown="1">
+{% highlight java %}
+Accumulator<Integer> accum = sc.accumulator(0);
+data.map(x -> accum.add(x); f(x););
+// Here, accum is still 0 because no actions have cause the `map` to be computed.
+{% endhighlight %}
+</div>
+
+<div data-lang="python"  markdown="1">
+{% highlight python %}
+accum = sc.accumulator(0)
+data.map(lambda x => acc.add(x); f(x))
+# Here, acc is still 0 because no actions have cause the `map` to be computed.
+{% endhighlight %}
+</div>
+
+</div>
 
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