container.training/slides/k8s/batch-jobs.md

Executing batch jobs

• Deployments are great for stateless web apps

  (as well as workers that keep running forever)

• Pods are great for one-off execution that we don't care about

  (because they don't get automatically restarted if something goes wrong)

• Jobs are great for "long" background work

  ("long" being at least minutes or hours)

• CronJobs are great to schedule Jobs at regular intervals

  (just like the classic UNIX cron daemon with its crontab files)

---

Creating a Job

• A Job will create a Pod

• If the Pod fails, the Job will create another one

• The Job will keep trying until:

  • either a Pod succeeds,

  • or we hit the backoff limit of the Job (default=6)

.lab[

• Create a Job that has a 50% chance of success:

    kubectl create job flipcoin --image=alpine -- sh -c 'exit $(($RANDOM%2))' 
  

]

---

Our Job in action

• Our Job will create a Pod named flipcoin-xxxxx

• If the Pod succeeds, the Job stops

• If the Pod fails, the Job creates another Pod

.lab[

• Check the status of the Pod(s) created by the Job:

  kubectl get pods --selector=job-name=flipcoin
  

]

---

class: extra-details

More advanced jobs

• We can specify a number of "completions" (default=1)

• This indicates how many times the Job must be executed

• We can specify the "parallelism" (default=1)

• This indicates how many Pods should be running in parallel

• These options cannot be specified with kubectl create job

  (we have to write our own YAML manifest to use them)

---

Scheduling periodic background work

• A Cron Job is a Job that will be executed at specific intervals

  (the name comes from the traditional cronjobs executed by the UNIX crond)

• It requires a schedule, represented as five space-separated fields:

  • minute [0,59]
  • hour [0,23]
  • day of the month [1,31]
  • month of the year [1,12]
  • day of the week ([0,6] with 0=Sunday)

• * means "all valid values"; /N means "every N"

• Example: */3 * * * * means "every three minutes"

• The website https://crontab.guru/ can help to create cron schedules!

---

Creating a Cron Job

• Let's create a simple job to be executed every three minutes

• Careful: make sure that the job terminates!

  (The Cron Job will not hold if a previous job is still running)

.lab[

• Create the Cron Job:

    kubectl create cronjob every3mins --schedule="*/3 * * * *" \
            --image=alpine -- sleep 10
  

• Check the resource that was created:

  kubectl get cronjobs
  

]

---

Cron Jobs in action

• At the specified schedule, the Cron Job will create a Job

• The Job will create a Pod

• The Job will make sure that the Pod completes

  (re-creating another one if it fails, for instance if its node fails)

.lab[

• Check the Jobs that are created:

  kubectl get jobs
  

]

(It will take a few minutes before the first job is scheduled.)

---

class: extra-details

Setting a time limit

• It is possible to set a time limit (or deadline) for a job

• This is done with the field spec.activeDeadlineSeconds

  (by default, it is unlimited)

• When the job is older than this time limit, all its pods are terminated

• Note that there can also be a spec.activeDeadlineSeconds field in pods!

• They can be set independently, and have different effects:

  • the deadline of the job will stop the entire job

  • the deadline of the pod will only stop an individual pod

???

:EN:- Running batch and cron jobs :FR:- Tâches périodiques (cron) et traitement par lots (batch)