2015 twenty four merry days of Perl Feed

Extracting more value out of your script's comments

Smart::Comments - 2015-12-11

'Twas the day before Christmas Eve and Blink the elf had just had a call telling him that he'd been reassigned to the NOC (North-Pole Operations Centre). "It's an emergency, get over there pronto!"

He quickly learned about the realities of the systems running Santa's operation. Many of them created by different teams who rarely talked to each other, and who used whatever language and frameworks were hot at the time their project started. A mixture of command-line tools, SOAP-based web services, RESTful APIS, and so on.

"It's all glued together by Perl scripts that Jack wrote; when things go wrong he always knows how to get things running again. But Jack's sick, so we need a Perl hacker to help us through tomorrow! Someone told me that you're the elf who wrote the Prancer web framework, so I asked the Boss for you personally".

It turned out there was a collection of scripts, but the biggest complaint in the NOC was that no-one knew what they were doing as they were running ("when something goes wrong it's always been Jack who works out what needs restarting, and at what step"). When Blink started looking at the scripts, his heart initially sank: they were enormous, and he only had a few hours before they had to be in use! But at least there were some comments, describing what was going on:


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# get list of children
... pile of code

# look them up in the Good Register
... pile of code

# get scanned images of letters to the Boss
... even more code

 

Blink remembered Smart::Comments, one of Damian Conway's modules, and realised it might help him out. Blink added use Smart::Comments and modified the highest level comments:


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use Smart::Comments;

### get list of children
... code
### looking children up in the Good Register
...

 

Any comments that started with three or more hash characters ("#") were printed out as the script ran.

The NOC elves thought this was great, but there were some parts of the scripts that took quite a while to run, so the next thing for Blink to do was to give a bit more detail. There were a lot of places where code looped over countries, so first he wanted to print out the country being processed:


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foreach my $country (@countries) {
#### Processing country: $country
}

 

Any perl expession can be put after the colon, so Blink was quickly able to display a country name, which Santa was assigned, as so on. On this next level of comments Blink used four hashes. When using Smart::Comments he could specify what level of comments should be handled:


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use Smart::Comments '###', '####';
 

This let the NOC decide how verbose the various scripts were when running. Blink then added the -ENV option:


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use Smart::Comments -ENV, '###', '####';
 

This let the NOC specify the what level of diagnostics they wanted, just by setting the Smart_Comments environment variable:

    export Smart_Comments='###:####'

The NOC can also set this environment variable to 0 to turn off the diagnostics completely.

There were still a lot of places where the scripts would iterate over large numbers of things, like children. Here it wasn't feasible to display each child's name, but they wanted some indication of progress. Blink used Smart::Comments's progress meters for these situations:


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for my $child (@children) { ### OCR [===| ] % done
    ocr_letter($child);
}

 

This displays a progress bar and a completion percentage. It will also track how long each step takes and estimate the time remaining, telling the NOC when a loop was close to finishing:

    OCR [=================|  ] 91% done  (about 20 seconds remaining)

After adding these three types of smart comments to all of the scripts, the NOC elves were very happy. Together they were able to get through Christmas Eve with no major mishaps.

The chief NOC elf asked Blink if he'd like to join the NOC full time, to work on a new microservices architecture. "Don't do it", whispered one of the junior NOC elves; "he says something like that every year. Why do you think Jack pulled a sickie?"

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Gravatar Image This article contributed by: Neil Bowers <neil@bowers.com>