The numReplicas field in the serviceInstanceUpdate mutation has a default value of 1. When you call the mutation without explicitly including numReplicas in your input, it resets to that default. To preserve your replica count, include numReplicas: 2 (or your desired count) in the input alongside the source field in your serviceInstanceUpdate call.

I would say that this behavior is a little unexpected, but okay.

I can't set a specific number of replicas because it can change between deployments via the dashboard. I need the same number of replicas to be retained after calling serviceInstanceUpdate() as there were before.

Also, as far as I know, the numReplicas field is deprecated (https://station.railway.com/questions/update-number-of-instances-via-api-2499832f), so I need to use multiRegionConfig instead.

I tried to get the current number of replicas by calling serviceInstance(). In response, I received json, which says that I have one replica here .data.serviceInstance.latestDeployment.meta.serviceManifest.deploy.multiRegionConfig, although right now there are two when I look at the UI. Am I getting the number of replicas in some incorrect way, or is something wrong with the API?

I used railway-cli as a reference to get current number of replicas (https://github.com/railwayapp/cli/blob/427c325f220ba2ecdd64828a4bace5426165e86e/src/commands/scale.rs#L161-L168). When I run railway service scale ... I see 1 replica instead of 2 there too.

Is your use case just to update the image tag and redeploy?

If so, you can do that via the CLI:

railway link --workspace $WORKSPACE_ID --environment $ENVIRONMENT_ID --project $PROJECT_ID
railway env edit —-service-config $SERVICE_ID "source.image" “user/repo:new-tag-vXXX”

It worked, thank you!