Parallelism

--parallelism is a flag available on rolling-update. This flag is better thought of as partitioning a deployment instead of true parallelism.

Let's say we have hosts A, B, and C, each in the running state:

A	B	C
R1	R1	R1

We issue a rolling-update with --parallelism set to 2. The first thing that happens is that A and B undeploy their currently running job, entering the undeployed state. Note that undeploying is an asynchronous operation for the Helios master as well as the Helios agents on hosts A and B. The host C is unaffected:

A	B	C
U	U	R1

The next thing that happens is that A and B are put into the deploy state [in that order]. Again, this operation is asynchronous. Host C is still unaffected:

A	B	C
D	D	R1

Next, A and B go to the wait state. The rolling update will block until A converges to the running state. Host C is still unaffected:

A	B	C
W	W	R1

Now that A is running again, the rolling update will continue with waiting for B to complete deploying successfully. Since B has been deploying synchronously while the rolling update waited for A to finish, the time to wait is typically very short. This is the mechanism that makes --parallelism decrease the time it takes for a deployment to finish.

A	B	C
R2	W	R1

Host B has entered the running state, which means this partition has finished deploying.

A	B	C
R2	R2	R1

Host C is is the only machine in its partition, so it deploys in the same way a single job deployment would:

A	B	C
R2	R2	U

A	B	C
R2	R2	D

A	B	C
R2	R2	W

A	B	C
R2	R2	R2