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WARNING WARNING WARNING WARNING WARNING

PLEASE NOTE: This document applies to the HEAD of the source tree

If you are using a released version of Kubernetes, you should refer to the docs that go with that version.

Documentation for other releases can be found at releases.k8s.io.

Example: Job with Work Queue with Pod Per Work Item

In this example, we will run a Kubernetes Job with multiple parallel worker processes. You may want to be familiar with the basic, non-parallel, use of Job first.

In this example, as each pod is created, it picks up one unit of work from a task queue, completes it, deletes it from the queue, and exits.

Here is an overview of the steps in this example:

  1. Start a message queue service. In this example, we use RabbitMQ, but you could use another one. In practice you would set up a message queue service once and reuse it for many jobs.
  2. Create a queue, and fill it with messages. Each message represents one task to be done. In this example, a message is just an integer that we will do a lengthy computation on.
  3. Start a Job that works on tasks from the queue. The Job starts several pods. Each pod takes one task from the message queue, processes it, and repeats until the end of the queue is reached.

Starting a message queue service

This example uses RabbitMQ, but it should be easy to adapt to another AMQP-type message service.

In practice you could set up a message queue service once in a cluster and reuse it for many jobs, as well as for long-running services.

Start RabbitMQ as follows:

$ kubectl create -f examples/celery-rabbitmq/rabbitmq-service.yaml
service "rabbitmq-service" created
$ kubectl create -f examples/celery-rabbitmq/rabbitmq-controller.yaml
replicationController "rabbitmq-controller" created

We will only use the rabbitmq part from the celery-rabbitmq example.

Testing the message queue service

Now, we can experiment with accessing the message queue. We will create a temporary interactive pod, install some tools on it, and experiment with queues.

First create a temporary interactive Pod.

# Create a temporary interactive container
$ kubectl run -i --tty temp --image ubuntu:14.04
Waiting for pod default/temp-loe07 to be running, status is Pending, pod ready: false
... [ previous line repeats several times .. hit return when it stops ] ...

Note that your pod name and command prompt will be different.

Next install the amqp-tools so we can work with message queues.

# Install some tools
root@temp-loe07:/# apt-get update
.... [ lots of output ] ....
root@temp-loe07:/# apt-get install -y curl ca-certificates amqp-tools python dnsutils
.... [ lots of output ] ....

Later, we will make a docker image that includes these packages.

Next, we will check that we can discover the rabbitmq service:

# Note the rabitmq-service has a DNS name, provided by Kubernetes:

root@temp-loe07:/# nslookup rabbitmq-service
Server:		10.0.0.10
Address:	10.0.0.10#53

Name:	rabbitmq-service.default.svc.cluster.local
Address: 10.0.147.152

# Your address will vary.

If Kube-DNS is not setup correctly, the previous step may not work for you. You can also find the service IP in an env var:

# env | grep RABBIT | grep HOST
RABBITMQ_SERVICE_SERVICE_HOST=10.0.147.152
# Your address will vary.

Next we will verify we can create a queue, and publish and consume messages.

# In the next line, rabbitmq-service is the hostname where the rabbitmq-service
# can be reached.  5672 is the standard port for rabbitmq.

root@temp-loe07:/# BROKER_URL=amqp://guest:guest@rabbitmq-service:5672
# If you could not resolve "rabbitmq-service" in the previous step,
# then use this command instead:
# root@temp-loe07:/# BROKER_URL=amqp://guest:guest@$RABBITMQ_SERVICE_SERVICE_HOST:5672

# Now create a queue:

root@temp-loe07:/# /usr/bin/amqp-declare-queue --url=$BROKER_URL -q foo -d
foo

# Publish one message to it:

root@temp-loe07:/# /usr/bin/amqp-publish --url=$BROKER_URL -r foo -p -b Hello

# And get it back.

root@temp-loe07:/# /usr/bin/amqp-consume --url=$BROKER_URL -q foo -c 1 cat && echo
Hello
root@temp-loe07:/#

In the last command, the amqp-consume tool takes one message (-c 1) from the queue, and passes that message to the standard input of an an arbitrary command. In this case, the program cat is just printing out what it gets on the standard input, and the echo is just to add a carriage return so the example is readable.

Filling the Queue with tasks

Now lets fill the queue with some "tasks". In our example, our tasks are just strings to be printed.

In a practice, the content of the messages might be:

  • names of files to that need to be processed
  • extra flags to the program
  • ranges of keys in a database table
  • configuration parameters to a simulation
  • frame numbers of a scene to be rendered

In practice, if there is large data that is needed in a read-only mode by all pods of the Job, you will typically put that in a shared file system like NFS and mount that readonly on all the pods, or the program in the pod will natively read data from a cluster file system like HDFS.

For our example, we will create the queue and fill it using the amqp command line tools. In practice, you might write a program to fill the queue using an amqp client library.

$ /usr/bin/amqp-declare-queue --url=$BROKER_URL -q job1  -d
job1
$ for f in apple banana cherry date fig grape lemon melon
do
  /usr/bin/amqp-publish --url=$BROKER_URL -r job1 -p -b $f
done

So, we filled the queue with 8 messages.

Create an Image

Now we are ready to create an image that we will run as a job.

We will use the amqp-consume utility to read the message from the queue and run our actual program. Here is a very simple example program:

#!/usr/bin/env python

# Copyright 2015 The Kubernetes Authors All rights reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

# Just prints standard out and sleeps for 10 seconds.
import sys
import time
print("Processing " + sys.stdin.lines())
time.sleep(10)

Download example

Now, build an an image. If you are working in the source tree, then change directory to examples/job/work-queue-1. Otherwise, make a temporary directory, change to it, download the Dockerfile, and worker.py. In either case, build the image with this command: `

$ docker build -t job-wq-1 .

For the Docker Hub, tag your app image with your username and push to the Hub with the below commands. Replace <username> with your Hub username.

docker tag job-wq-1 <username>/job-wq-1
docker push <username>/job-wq-1

If you are using Google Container Registry, tag your app image with your project ID, and push to GCR. Replace <project> with your project ID.

docker tag job-wq-1 gcr.io/<project>/job-wq-1
gcloud docker push gcr.io/<project>/job-wq-1

Defining a Job

Here is a job definition. You'll need to make a copy of the Job and edit the image to match the name you used, and call it ./job.yaml.

apiVersion: batch/v1
kind: Job
metadata:
  name: job-wq-1
spec:
  completions: 8
  parallelism: 2
  template:
    metadata:
      name: job-wq-1
    spec:
      containers:
      - name: c
        image: gcr.io/<project>/job-wq-1
      restartPolicy: OnFailure

Download example

In this example, each pod works on one item from the queue and then exits. So, the completion count of the Job corresponds to the number of work items done. So we set, .spec.completions: 8 for the example, since we put 8 items in the queue.

Running the Job

So, now run the Job:

$ kubectl create -f ./job.yaml

Now wait a bit, then check on the job.

$ ./kubectl describe jobs/job-wq-1 
Name:		job-wq-1
Namespace:	default
Image(s):	gcr.io/causal-jigsaw-637/job-wq-1
Selector:	app in (job-wq-1)
Parallelism:	4
Completions:	8
Labels:		app=job-wq-1
Pods Statuses:	0 Running / 8 Succeeded / 0 Failed
No volumes.
Events:
  FirstSeen	LastSeen	Count	From	SubobjectPath	Reason			Message
  ─────────	────────	─────	────	─────────────	──────			───────
  27s		27s		1	{job }			SuccessfulCreate	Created pod: job-wq-1-hcobb
  27s		27s		1	{job }			SuccessfulCreate	Created pod: job-wq-1-weytj
  27s		27s		1	{job }			SuccessfulCreate	Created pod: job-wq-1-qaam5
  27s		27s		1	{job }			SuccessfulCreate	Created pod: job-wq-1-b67sr
  26s		26s		1	{job }			SuccessfulCreate	Created pod: job-wq-1-xe5hj
  15s		15s		1	{job }			SuccessfulCreate	Created pod: job-wq-1-w2zqe
  14s		14s		1	{job }			SuccessfulCreate	Created pod: job-wq-1-d6ppa
  14s		14s		1	{job }			SuccessfulCreate	Created pod: job-wq-1-p17e0

All our pods succeeded. Yay.

Alternatives

This approach has the advantage that you do not need to modify your "worker" program to be aware that there is a work queue.

It does require that you run a message queue service. If running a queue service is inconvenient, you may want to consider one of the other job patterns.

This approach creates a pod for every work item. If your work items only take a few seconds, though, creating a Pod for every work item may add a lot of overhead. Consider another example, that executes multiple work items per Pod.

In this example, we used use the amqp-consume utility to read the message from the queue and run our actual program. This has the advantage that you do not need to modify your program to be aware of the queue. A different example, shows how to communicate with the work queue using a client library.

Caveats

If the number of completions is set to less than the number of items in the queue, then not all items will be processed.

If the number of completions is set to more than the number of items in the queue, then the Job will not appear to be completed, even though all items in the queue have been processed. It will start additional pods which will block waiting for a mesage.

There is an unlikely race with this pattern. If the container is killed in between the time that the message is acknowledged by the amqp-consume command and the time that the container exits with success, or if the node crashes before the kubelet is able to post the success of the pod back to the api-server, then the Job will not appear to be complete, even though all items in the queue have been processed.

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