Github user corneadoug commented on the pull request:
https://github.com/apache/incubator-zeppelin/pull/804#issuecomment-206663670
Maybe the GIFs are not obvious enough :)
The PR is mainly trying to improve the rendering of the streaming
capability introduced in https://github.com/apache/incubator-zeppelin/pull/611.
So I will try to explain more in detail what is happening and how things
work on the front-end side.
### How It Works
First of all, its good to know that this is only a case for TEXT type of
return values.
The way the back-end communicates data to the front-end end is through 2
different websocket events:
* PARAGRAPH_UPDATE_OUTPUT: Which remove previous result and replace it by
the event content
* PARAGRAPH_APPEND_OUTPUT: Which add to the end of previous result
So for example, if we take this code:
```
(1 to 40).foreach{ i=>
Thread.sleep(1000)
println(i)
}
```
It would send numbers from 1 to 40 with 1000ms interval, and this is what
would happen:
```
- PARAGRAPH_UPDATE_OUTPUT (sent: 1)
-- PARAGRAPH_APPEND_OUTPUT (sent: 2)
-- PARAGRAPH_APPEND_OUTPUT (sent: 3)
-- PARAGRAPH_APPEND_OUTPUT (sent: 4)
...
- PARAGRAPH_UPDATE_OUTPUT (sent: 1/n2/n3/n......40)
```
So the way it works is that the job results are never persisted inside the
Notebook before the end of the job.
### The Current Problem
Since the temporary results are not persisted, whenever you navigate in and
out of the Running Notebook, it will first reload the previous saved result in
that paragraph.
So if you did run a loop through the ABC before, The paragraph will render
that result.
Then you will receive some PARAGRAPH_APPEND_OUTPUT events from your running
job, which will transform the result you see in your paragraph into something
like:
```
A
B
...
Z
21 (PARAGRAPH_APPEND_OUTPUT)
22 (PARAGRAPH_APPEND_OUTPUT)
23 (PARAGRAPH_APPEND_OUTPUT)
```
Mainly because the flush of previous result (visually) was done in the
PARAGRAPH_UPDATE_OUTPUT event at the beginning of the job.
### What the PR achieve
In this PR, we try to clear the previous unrelated result of the paragraph
when the paragraph receives an PARAGRAPH_APPEND_OUTPUT event for the first time
in the Notebook, in order to keep result of that job only.
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