feat: add autoscaling configuration for prebuilds #408

evgeniy-scherbina · 2025-05-29T14:45:53Z

Relates to coder/internal#312
Needed for coder/coder#18126

Exposes an autoscaling field under coder_workspace_preset:

data "coder_workspace_preset" "us-nix" {
  ...
  
  prebuilds = {
	  instances = 0                  # default to 0 instances
	  
	  autoscaling {
		  timezone = "UTC"             # only a single timezone may be used for simplicity
		  
		  # scale to 3 instances during the work week
		  schedule {
		    cron = "* 8-18 * * 1-5"    # from 8AM-6:59PM, Mon-Fri, UTC
		    instances = 3              # scale to 3 instances
		  }
		  
		  # scale to 1 instance on Saturdays for urgent support queries
		  schedule {
		    cron = "* 8-14 * * 6"      # from 8AM-2:59PM, Sat, UTC
		    instances = 1              # scale to 1 instance
		  }
	  }
  }
}

dannykopping · 2025-06-12T20:10:05Z

provider/workspace_preset.go

+								Schema: map[string]*schema.Schema{
+									"timezone": {
+										Type:        schema.TypeString,
+										Description: "The timezone to use for the autoscaling schedule (e.g., \"UTC\", \"America/New_York\").",


Nit: can you include a link to an acceptable list of TZs here? I think https://en.wikipedia.org/wiki/List_of_tz_database_time_zones suffices but I'm not sure.

https://pkg.go.dev/time#LoadLocation says it references the "IANA Time Zone database".

See the help output of coder schedule {start|stop} for some sample wording. The phrase "timezone" is slightly misleading here as it's the location that influences the timezone depending on wibbly wobbly timey wimey stuff.

dannykopping · 2025-06-12T20:13:42Z

provider/workspace_preset.go

+
+											_, err := time.LoadLocation(timezone)
+											if err != nil {
+												return nil, []error{fmt.Errorf("failed to load location: %w", err)}


Suggested change

return nil, []error{fmt.Errorf("failed to load location: %w", err)}

return nil, []error{fmt.Errorf("failed to load timezone %q: %w", timezone, err)}

dannykopping · 2025-06-12T20:14:17Z

provider/workspace_preset.go

+							MaxItems:    1,
+							Elem: &schema.Resource{
+								Schema: map[string]*schema.Schema{
+									"timezone": {


I love that we explicitly don't have a default value here 👍 nice.

~~I recommend defaulting to TZ or /etc/localtime, and falling back to UTC. There's a helper function tz.TimezoneIANA for this.~~

EDIT: given that this is for the provider, you might actually be correct in not specifying any default here as we can't assume that the timezone in which the provider is running is the same timezone as coderd.

dannykopping · 2025-06-12T20:25:10Z

provider/workspace_preset.go

+											Schema: map[string]*schema.Schema{
+												"cron": {
+													Type:        schema.TypeString,
+													Description: "A cron expression that defines when this schedule should be active. The cron expression must be in the format \"* HOUR * * DAY-OF-WEEK\" where HOUR is 0-23 and DAY-OF-WEEK is 0-6 (Sunday-Saturday). The minute, day-of-month, and month fields must be \"*\".",


Why must day-of-month and month be *?

I'm assuming this relates to the "weekly cron schedule" concept we already use for schedules?

Seems so, but I think that's a strange requirement since nothing in this implementation is on a weekly cadence.

dannykopping · 2025-06-12T20:25:33Z

provider/workspace_preset.go

 					},
 				},
 			},
 		},
 	}
 }
+
+// validatePrebuildsCronSpec ensures that the minute, day-of-month and month options of spec are all set to *


It'd be good to add the why here, not just the what.

~~This should be moved to coderd/schedule/cron~~ EDIT: sorry, thought this was a different project :P

dannykopping · 2025-06-12T20:27:02Z

provider/workspace_preset_test.go

+					autoscaling {}
+				}
+			}`,
+			ExpectError: regexp.MustCompile(`The argument "[^"]+" is required, but no definition was found.`),


Nit: let's check for a precise name to ensure we don't get false positives.
Or is it like this because we can't know which argument will show up first?

dannykopping · 2025-06-12T20:34:20Z

provider/workspace_preset_test.go

+			},
+		},
+		{
+			Name: "Prebuilds is set with an autoscaling field with 2 schedules",


Where do we check that these schedules are not conflicting? I didn't find any checks in coder/coder#18126 from a quick scan. I'll review that PR in more detail tomorrow.

If the current time matches any defined autoscaling schedule, the corresponding number of instances is used.

This makes me nervous because it introduces some non-determinism in the case of conflicting/overlapping schedules. If we don't evaluate cron schedules in the same order, we could flip back and forth between conflicting schedules, which could cause a lot of churn and confusion.

dannykopping · 2025-06-12T20:36:13Z

provider/workspace_preset.go

+
+// validatePrebuildsCronSpec ensures that the minute, day-of-month and month options of spec are all set to *
+func validatePrebuildsCronSpec(spec string) error {
+	parts := strings.Fields(spec)


Howcome we don't use PrebuildsCRONParser here?

spikecurtis · 2025-06-13T11:27:24Z

I really don't think we should call this "autoscaling" as that already has a specific meaning in Cloud circles which this is not.

Autoscaling is adjusting instance counts dynamically based on measured load.

This is adjusting instance counts based on a schedule. Some possible names:

Scale sceduling
Schedule-scaling
Pool scheduling
Prebuild scheduling

evgeniy-scherbina force-pushed the prebuilds-autoscaling-mechanism branch from 15cae29 to 543a735 Compare June 11, 2025 17:35

feat: add autoscaling configuration for prebuilds

d61894d

evgeniy-scherbina force-pushed the prebuilds-autoscaling-mechanism branch from 2f82387 to d61894d Compare June 11, 2025 18:01

evgeniy-scherbina marked this pull request as ready for review June 12, 2025 19:28

evgeniy-scherbina mentioned this pull request Jun 12, 2025

feat: implement autoscaling mechanism for prebuilds coder/coder#18126

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spikecurtis commented Jun 13, 2025

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	return nil, []error{fmt.Errorf("failed to load location: %w", err)}
	return nil, []error{fmt.Errorf("failed to load timezone %q: %w", timezone, err)}

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