## Enabled group

Chaos Monkey will only consider server groups eligible for termination if they are marked as enabled by Spinnaker. The Spinnaker API exposes an isDisabled boolean flag to indicate whether a group is disabled. Chaos Monkey filters on this to ensure that it only terminates from active groups.

## Probability

For each app, Chaos Monkey divides the instances into instance groups (the groupings depend on how the app is configured). Every weekday, for each instance group, Chaos Monkey flips a weighted coin to decide whether to terminate an instance from that group. If the coin comes up heads, Chaos Monkey schedules a termination at a random time between 9AM and 3PM that day.

Under this behavior, the number of work days between terminations for an instance group is a random variable that has a geometric distribution.

The equation below describes the probability distribution for the time between terminations. X is the random variable, n is the number of work days between terminations, and p is the probability that the coin comes up heads.

``````P(X=n) = (1-p)^(n-1) × p,   n>=1
``````

Taking expectation over X gives the mean:

``````E[X] = 1/p
``````

Each app defines two parameters that governs how often Chaos Monkey should instances for that app:

• mean time between terminations in work days (μ)
• min time between terminations in work days (ɛ)

Chaos Monkey uses μ to determine what p should be. If we ignore the effect of ɛ and solve for p:

``````μ = E[X] = 1/p
p = 1/μ
``````

As an example, for a given app, assume that μ=5. On each day, the probability of a termination is 1/5.

Note that if ɛ>1, Chaos Monkey termination behavior is no longer a geometric distribution:

``````P(X=n) = (1-p)^(n-1) × p,  n>=ɛ
``````

In particular, as ɛ grows larger, E[X]-μ gets larger. We don't apply a correction for this, because the additional complexity in the math isn't worth having E[X] exactly equal μ.

Also note that if μ=1, then p=1, which guarantees a termination each day.