Adds transaction with watched key example script.

[sailingwithsandeep]

My pull request is about creating an example of redis transactions which shows a sample code that runs in isolated connection and provides optimistic locking with check-and-set (CAS) behavior.

#2280

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Checklist

• Does npm test pass with this change (including linting)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?

[simonprickett]

Is there any reason to change anything in this README.md at all except for adding a new line entry in the table for your new script?

[SuzeShardlow]

@sailingwithsandeep please see @simonprickett's question above.

[sailingwithsandeep]

@simonprickett Hey so sorry i just realised i changed the order of readme file's coding guidelines list. it happened because i copied that readme file from previous pull request and somehow changed the order. i hope you understand my concern. Again extremely sorry for changing unwanted things. (P.S. I am so excited that i am contributing into redis community so, given that state i might have rushed it.)

[SuzeShardlow]

Please don't rush @sailingwithsandeep and only change the things that are necessary :) please change it back and we will re-review.

[sailingwithsandeep]

@simonprickett @SuzeShardlow Hey i committed the changes hopefully this time there won't be an issue. Again sorry for the trouble.

[codecov-commenter]

Codecov Report

Base: 95.85% // Head: 95.85% // No change to project coverage 👍

Coverage data is based on head (81738e8) compared to base (d0bfa77).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2297   +/-   ##
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  Coverage   95.85%   95.85%           
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  Files         433      433           
  Lines        4002     4002           
  Branches      451      451           
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  Hits         3836     3836           
  Misses        102      102           
  Partials       64       64           

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[simonprickett]

@sailingwithsandeep I was able to test the happy path successfully, however if there's concurrent modification of paymentId:1259 the code errors like this:

$ node transaction-with-watch.js
Transaction Failed Due To Concurrent Modification!
Error: Error: pool is draining and cannot accept work`

to reproduce this, add a delay between the watch and exec during which you change the value stored at paymentId:1259 in a redis-cli session.

I added the delay using some temporary code that allows me time to modify the key in redis-cli:

async function slowDown() {
  return new Promise(resolve => setTimeout(resolve, 5000));
}

and called it like this:

      const multi = isolatedClient
        .multi()
        .set('paymentId:1259', 'Payment Successfully Completed!')
        .set('paymentId:1260', 'Refund Processed Successfully!');

      await slowDown();

      await multi.exec();

[sailingwithsandeep]

Hello @simonprickett , i have added the delay which you have mentioned in previous review!

[SuzeShardlow]

@sailingwithsandeep you still need to look at your code. The delay which @simonprickett mentioned was purely to reproduce / demonstrate the error. It isn't a fix. You still need to fix the error.

Please have another read of Simon's comment: #2297 (review) and amend your PR accordingly so the error no longer occurs.

[sailingwithsandeep]

Hello, @SuzeShardlow @simonprickett i have handled that pooling error please have a look!

[simonprickett]

Thanks - both cases (concurrent modification and success) work for me, @SuzeShardlow this is good to merge.

[SuzeShardlow]

Thanks for your contribution @sailingwithsandeep :)

[sailingwithsandeep]

@SuzeShardlow @simonprickett Thank you! for supporting me! this was my first open source contribution.

[SuzeShardlow]

Welcome to the open source world 🙂