ScheduledScaling.md

Scheduled Scaling

Overview

• Definition of scheduled scaling
• How does sokar behave when scheduled scaling is active
• How to configure scheduled scaling
• Example configurations

What is Scheduled Scaling?

Background

Scheduled scaling is intended for scenarios where the load is roughly known beforehand and where this load is present in a repeating manner (e.g. each day). If this a-priori knowledge is incorporated into the automatic scaling one can imagine that it would improve the scaling reliability of the system. Furthermore, instead of just reacting on the scaling alerts caused by the current load it is possible to prepare the application for the incoming requests by prescaling the system.

Behavior and Implementation

With scheduled scaling sokar introduces a way to define at which time span what scale range is allowed. A scale range is a pair of limit values, a min- and a max scale. The time span (within one day) is defined by a start, an end. The day(s) for which the values shall be applied can also be specified. These three components (day, time and scale range) form a so called scheduling entry. Then multiple schedule entries define a complete schedule.

In general, for the time one schedule entry is active, sokar ensures that the scale is at least min and at most max. Between those two limits sokar will scale according to the incoming scale alerts. This fact is depicted in the image above. As shown the defined min (blue dashed line) and max (red dashed line) for the active schedules keep the scale (actual scale, black line) in the specified scale range. Even though the planned scale (yellow dashed line) differs at some points. As shown, sokar will scale only in between the grey marked area.

For example in the active schedule (a) sokar would like to scale down since there is currently no load on the system, but the defined min scale for that schedule permits to do so. The same can happen for scaling up. In active schedule (b) sokar want's to scale up to satisfy to the currently high demands on the application but the max scale defined for that schedule permits this too. To summarize one can prescale the system by defining an appropriate min scale and can limit the costs based on daytime by selecting the according value for max.

If the min or max is not known or the according limitation in automatic scaling is not wanted, it is possible to just not define the min/ max bounds.

The picture above shows also another important fact to keep in mind - there is also a global min (solid blue line) and max limit (solid red line). Global means that it has higher priority than the limits defined for a schedule. This means even if the min/ max of the schedule is lower/ higher than the globally defined min/ max, just the global one is finally regarded. Hint: The global limits are specified on the scale-object level (see: --scale-object.min and --scale-object.max in Config.md). For example in active schedule (c) sokar planned to scale up but the global max limit permitted to do so although the max for the schedule is high enough.

No Replacement of Auto Scaling

It is possible to specify a scale range where min and max are identical. Thus it is possible to tie the scale sokar shall ensure to a fixed and exact value at a certain time. But the feature of scheduled scaling is not meant to be used as a replacement for the alert based auto scaling. Furthermore the schedule is not evaluated as frequent enough (just every 30s) to have fast reaction times for scaling. The time accuracy for scheduled scaling is 1min +-30s.

Configuration

The scheduled scaling is active as soon as a scaling schedule is configured. As usual this can be done via environment variable, via command line parameter or configuration file. For details see the description of scale-schedule in Config.md.

Examples

ENV vs CMD-Line vs Cfg-File

ENV

export SK_CAP_SCALE_SCHEDULE="MON-FRI 7 9 10-30|MON-FRI 14 18 10-30"

CMD-Line

--cap.scale-schedule="MON-FRI 7 9 10-30|MON-FRI 14 18 10-30"

Cfg-File

cap:
  scale-schedule:
    - days: "MON-FRI"
      start-time: 7:00
      end-time: 9:00
      min: 10
      max: 30
    - days: "MON-FRI"
      start-time: 14:00
      end-time: 18:00
      min: 10
      max: 30

More Examples

Rush hour peaks

• Scale up at weekdays during 7:30-9:30 and 15:30-17:30.
• No schedule in between and on weekends.

ENV/ CMD-Line

MON-FRI 7:30 9:30 10-30|MON-FRI 15:30 17:30 10-30

Cfg-File

cap:
  scale-schedule:
    - days: "MON-FRI"
      start-time: 7:30
      end-time: 9:30
      min: 10
      max: 30
    - days: "MON-FRI"
      start-time: 15:30
      end-time: 17:30
      min: 10
      max: 30

Low Scale at Weekend

• Scale down at weekends during the whole day.
• No schedule for weekdays

ENV/ CMD-Line

SAT-SUN * * 2-10

Cfg-File

cap:
  scale-schedule:
    - days: "SAT-SUN"
      start-time: "*"
      end-time: "*"
      min: 2
      max: 10

Prescale, but don't limit the maximum scale

• To be prepared for 'Black Friday' ensure that the system can handle the incoming spike of requests.
• Not knowing the peak amount of requests for that time, don't limit the system from up scaling.
• Starting at 7 and keep it until midnight.

ENV/ CMD-Line

FRI 7 * 20-*

Cfg-File

cap:
  scale-schedule:
    - days: "FRI"
      start-time: 7
      end-time: "*"
      min: 20
      max: *

Full week

• Scale up at weekdays at lunch for 2h.
• Modest scale at remaining day time at weekdays.
• Scale down at weekend.

ENV/ CMD-Line

MON-FRI * 11:59 5-10|MON-FRI 12 14 10-30|MON-FRI 14:01 * 5-10|SAT-SUN * * 2-5

Cfg-File

cap:
  scale-schedule:
    - days: "MON-FRI"
      start-time: "*"
      end-time: 11:59
      min: 5
      max: 10
    - days: "MON-FRI"
      start-time: 12:00
      end-time: 14:00
      min: 10
      max: 30
    - days: "MON-FRI"
      start-time: 14:01
      end-time: "*"
      min: 5
      max: 10
    - days: "SAT-SUN"
      start-time: "*"
      end-time: "*"
      min: 2
      max: 5