Posted: 13/01/2023

Docker Compose Scaling Gotcha

Today I ran this command:

            docker-compose up -d --scale service1=10

Then ran this command:

            docker-compose up -d --scale anotherservice2=10

What do you think happens?

Well not what I was expecting.

I was expecting service1 to have 10 scaled versions, and anotherservice2 to have 10 scaled versions.

What I got was service1 was scaled back down to the default in the compose file (in this case 5), and anotherservice2 was bought up to 10.

Why does this happen? Would this have worked?

            docker-compose up -d --scale anotherservice2=10 anotherservice2

Turns out YES!

So what is going on?

Lets break the command down...

docker-compose up -d says bring the containers in the compose file up and detach from them.
--scale anotherservice2=10 says we want to ignore the scale options in the compose file for anotherservice2 and have 10 of them instead.
anotherservice2 says only apply the previous options to the anotherservice2

I can understand why, now I look at the command, but I would have preferred an indication that it would take service1 replicas down too.

Oh well another reset of the number of days since I took out PRODUCTION!

Thanks for reading this post.

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