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Tips for Running Scripts in Production

Molly Struve (she/her) on June 16, 2020

I know, how dare I suggest running a script in production. I am a Site Reliability Engineer, I should never condone such craziness. But truth is,...

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Ben Sinclair • Jun 17 '20

When I do this (which is more often than I'd admit in an interview) I'll usually do something like this:

# result = article.update(cached_tag_list: article.tags.pluck(:name).join(", "))
puts "result = article.update(cached_tag_list: #{article.tags.pluck(:name).join(", ")}"
continue

Excuse my butchered pseudo-ruby, I don't use it so am inferring from yours. What I mean, though, is I display what would be called as a kind of dry-run, and short-circuit the rest of the loop. Then when I'm ready, I uncomment the "real" line.

Molly Struve (she/her) • Jun 17 '20

That is definitely another great way to try it out first before actually doing it. When I "dry run" a script I usually run the script for a single object, check that it looks how I expect and then l let the script loose on the rest.

Rafi • Jun 17 '20

I recently came across this gem data-migrate which allows you to do data migrations like you do schema migrations.

Molly Struve (she/her) • Jun 17 '20

One reason I am wary about doing data migrations in schema migrations is bc sometimes they can take a large amount of time and if your schema migrations are executed inline with your deploy pipeline that can block a deploy. At DEV we use DataUpdateScripts which is pretty similar but it runs asynchronously in the background.

Set Up Framework For Running Data Update Scripts #6025

mstruve posted on Feb 11, 2020

What type of PR is this? (check all applicable)

[x] Feature

Description

While hooking up our first Elasticsearch model Tags, I realized that in order for search to work I would have to manually reindex all of the tags in our database before the search code went live. This is not a huge deal for us, but it means an extra undocumented step that others who are using this codebase might miss. This framework would give us the ability to run scripts like this the same way we run migrations.

module DataUpdateScripts
  class IndexTags
    def run
      Tag.find_each(&:index_to_elasticsearch)
    end
  end
end

A script like this would be deployed ahead of using the new data. When the app deploys or a local environment is updated the DataUpdateWorker would look at all the files in the data_update_scripts folder. Any file that has not been run, ie is not in our database, it will create a record for and then call run on that class.

We used something like this at my prior company because we had to keep 5+ VPCs data in sync and it worked out really well.

Why not use migrations? The reason we may want to separate this from migrations is so that it is not tied to our deploy process. In the past, I have had scripts that take hours to run bc they touch a lot of data and you don't want that holding up a deploy. This way a deploy goes out, kicks off a worker, and that worker does its thing in the background for however long it needs.

THOUGHTS?!

Added to documentation?

[x] readme

If people are on board with this approach I will add the necessary documentation to this branch as well

View on GitHub

Rafi • Jun 17 '20

Awesome !!!

Rafi • Jun 17 '20 • Edited

But how do you keep track of the order in which script ran? Do the script file names get timestamp attached to them similar to regular migrations?

Molly Struve (she/her) • Jun 17 '20

Yep! That is exactly how it works, same as migrations. Here you can see a list of our scripts github.com/thepracticaldev/dev.to/...

Alejandro AR • Jun 17 '20

If the task takes a good amount of time, you have risks of being disconnected from either the SSH session, your internet provider, etc.

To avoid this, I recommended running these scripts (although, I do not recommend running scripts like this at all) using screen. It's super easy to use:

SSH into the desired instance
Start a new screen session using screen
Run the long running script
Press Ctrl + a followed by d to detach the session
You can now close everything, even the SSH session
You can reattach to the screen session using screen -r

Screen has a lot of awesome things, make sure to check out the man page.

Juan A. Fuentest Torcat • Jun 18 '20

I actually learned about tmux one time I had to run a script that took hours to finish, total savior <3

Molly Struve (she/her) • Jun 17 '20

Oh man, YES! screen or tmux sessions are a must as well for long-running scripts!

Ian Pride • Jun 19 '20

Screen is a must for SSH... or even local if you want to be minimal in your shells.

Shaiju T • Jun 16 '20 • Edited

Hi, 😄, By the way are you talking about SQL Script or YML Script . Because Ruby is a programming language right ?

Juan A. Fuentest Torcat • Jun 17 '20

Programming languages can still be used to write scripts ;)

Shaiju T • Jun 17 '20

What kind of script is used in this post. Is there a documentation or tutorial. so I can learn?

Molly Struve (she/her) • Jun 17 '20

The script in this post is written in Ruby. I highly recommend Googling Ruby Tutorial and you will find MANY resources to help you learn Ruby in whatever learning style is best for you.

Molly Struve (she/her) • Jun 16 '20

Nope, I am talking about Ruby. In this sense, I am using "script" to define a small chunk of Ruby code that is run standalone in a console.

Julien Camblan • Jun 17 '20

I learned all these lessons the hard way by running scripts in production without applying them in the first place. I think I'd still have hair if I'd read this kind of article before. So thank you, it will save lives for sure!

Lee Noble • Jun 20 '20

A technique I used recently was to have my script write out two SQL files. One with all the fixes, and the other to revert everything back to how it was. The script took a long time to run but were completely non destructive. The SQL could actually then be examined and run on a copy of the database and the results checked, both the fixing SQL and the reversion (just check that both databases are the same again). Then just run the fix SQL on production.

Alex Bit • Jul 11 '23

great article. while this is about a script for changing data, the points are also applicable to writing scripts for transforming code / codemods. thanks for sharing molly.