How to use Armada
The main concepts to understand in Armada are:
- Job Set
A Job in Armada is equivalent to a Pod in Kubernetes, with a few extra metadata fields.
It is the most basic unit of work in Armada and if you are familiar with Kubernetes it will be very familiar to you.
A simple example of a job in yaml form looks like:
queue: test priority: 0 jobSetId: set1 podSpec: terminationGracePeriodSeconds: 0 restartPolicy: Never containers: - name: sleep imagePullPolicy: IfNotPresent image: busybox:latest args: - sleep - 60s resources: limits: memory: 64Mi cpu: 150m requests: memory: 64Mi cpu: 150m
A Job belongs to a Queue and a Job Set, seen by the corresponding fields. Finally it has a priority, which is a relative priority to other Jobs in the same queue; the lower the number, the higher the priority.
If you submit 2 Jobs, A and B, and you want B to be ordered before A in the queue, B should be given a priority lower than A using the priority field.
All jobs of priority 0 will be taken from the queue before any with priority 1 and time of submission is not taken into account.
Note: Job resource request and limit should be equal. Armada does not support limit > request currently.
For more details on the options available for an Armada Job see here
Multi node jobs
Certain workloads requires multiple pods starting at the same time, job specification can contain multiple podSpecs this way:
queue: test priority: 0 jobSetId: multi-node-set1 podSpecs: - terminationGracePeriodSeconds: 0 restartPolicy: Never containers: - name: sleep imagePullPolicy: IfNotPresent image: busybox:latest args: - sleep - 60s resources: limits: memory: 64Mi cpu: 150m requests: memory: 64Mi cpu: 150m - terminationGracePeriodSeconds: 0 restartPolicy: Never containers: ...
Armada will schedule this type of job on one cluster, and the pods will be created at the same
time. If for any reason any of the pods can’t start, all pods will be eventually removed (using
stuckPodExpiry timeout). If the problem is deemed to be recoverable, the job will be retried.
All events related to multi node job pods have identifier
podNumber which corresponds with index of pod in the
A Job Set is a logical grouping of Jobs.
The reasons you may want to do this is because you can;
- Watch all events of a Job Set together
- Cancel all jobs in a Job Set together
The Job Sets are typically meant to be used to represent a single task, where many Jobs are involved.
You can then follow this Job Set as a single entity rather than having to track multiple associated Jobs.
A Job Set has no impact on the running of jobs a this moment and is purely an abstraction over a group of Jobs.
A queue is the likely most important aspect of Armada.
Conceptually is it quite simple, it is just a Queue of jobs waiting to be run on a Kubernetes cluster.
In addition to being a queue, it is also used:
- Maintaining fair share over time
- Security boundary (Not yet implemented) between jobs
- Resource limits
Queues are provided with a fair share of the resource available on all clusters over time.
That is to say if there are 5 queues and they were all to submit infinite jobs (well in excess of cluster capacity), then each queue should receive the same amount of compute resource over the course of time. No single queue should get a larger share of the available resource.
The reason we say over time, as at any single moment we cannot guarantee the queues will be exactly equal, however each job submitted has a cost. So if one queue gets a larger share early, they’ll receive a lesser share later to make up for it.
A Queue has a Priority Factor.
The lower the number, the greater the share that queue will receive.
So if you have 2 Queues:
- A - Priority 10
- B - Priority 1
We would expect B to receive substantially more resource on the clusters than A over time.
All priorities are relative, so you can make your own scheme of priority numbers. So for example you could have “normal” be 100, “high” be 50 and “max” be 10.
Priority allows great flexibility, as it means you can predictably give certain queues a bigger share of resource than others.
Armada allows to set user (and group) permissions for a specific Queue using owners (and groupOwners) options.
The job set events are available to all users with “watch_all_events” permissions.
kubernetes.impersonateUsers is turned on, Armada will create pods in kubernetes impersonating owner of the job. This will enforce Kubernetes permissions and limit access to namespaces.
Queues can also have a resource limit which is particularly useful if your installation has irregular usage, with busy and quiet periods.
It will prevent a user submitting lots of long running jobs in a quiet period and holding all the resource in your entire installation.
The limit will restrict them to a given percentage of all available resource over all clusters.
Using armadactl it’ll look like:
armadactl create queue test --resourceLimits cpu=0.3,memory=0.2
Which means the queue at maximum can only ever be using 30% of the total cpu and 20% of the memory available over all clusters.
Considerations when setting up Queues
So now you know what Queues are and what they can do. We’ll briefly cover what to consider when setting them up.
You should consider how your organisation should be split up so users’ work is provided with an appropriate amount of resource allocation.
There are many ways this could be done, here are some ideas:
- By user. All users will be equal (or scaled) relative to each other
- By team. All teams will be equal (or scaled) relative to each other
- By project. All projects will be equal (or scaled) relative to each other
- A mixture of all the above. So a given user may have their own queue, be part of a team queue and part of multiple project queues. They would then submit the appropriate jobs to the appropriate queues and expect them to all be given a fair amount of resource.
- This allows a lot of flexibility in how you slice up your organisation, and combined with differing priorities it is possible to achieve any mix of resource allocation you want.
For each of the above, if you went for “By user”. Simply make each user of the system their own Queue and tell them to submit to it.
They will then receive the same amount of resource as any other user in the system.