[GitHub] [pulsar] candlerb opened a new issue #5520: Missing documentation for window functions

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candlerb opened a new issue #5520: Missing documentation for window functions
URL: https://github.com/apache/pulsar/issues/5520
 
 
   **Describe the bug**
   There is no documentation for window functions, and no python examples for window functions.
   
   **To Reproduce**
   Search for "window" in the docs (site2).
   
   It's mentioned in passing:
   
   > A Java function listens for the sanitized-sentences topic, counts the number of times each word appears **within a specified time window**, and publishes the results to a results topic
   
   (my emphasis)
   
   In some CLI flags:
   
   ```
   ./docs/functions-cli.md:sliding-interval-count | The number of messages after which the window slides. |  |
   ./docs/functions-cli.md:sliding-interval-duration-ms | The time duration after which the window slides. |  |
   ./docs/functions-cli.md:window-length-count | The number of messages per window. |  |
   ./docs/functions-cli.md:window-length-duration-ms | The time duration of the window in milliseconds. | |
   ```
   
   And in a reference to release notes:
   
   > * Add Windowfunction interface to functions api [#3324](https://github.com/apache/pulsar/pull/3324)
   
   **Expected behavior**
   N/A
   
   **Screenshots**
   N/A
   
   **Desktop (please complete the following information):**
   N/A
   
   **Additional context**
   This functionality is really interesting to me, and I'd like to know (a) what it can do, and (b) if I can use it from python.
   
   I'm particularly interested if it's possible to do e.g. a ten-minute rolling window, where I can add 1 to an internal counter for an event which enters the window, and remove 1 from a count when that event leaves the window, thus maintaining a running count.
   
   It could be done without external support, but would require in-RAM buffering for all events over than 10 minute window - but buffering is what pulsar is for :-)
   
   Externally I could maintain two readers: one at the current location and one at T-10 mins.  But that wouldn't work as a pulsar function AFAICS.

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