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In distributed systems, such as a network of Google Compute Engine instances, it is challenging to reliably schedule tasks because any individual instance may become unavailable due to autoscaling or network partitioning.
Google Cloud Platform provides a managed Cloud Scheduler service. Using this service for scheduling and Google Cloud Pub/Sub for distributed messaging, you can build an application to reliably schedule tasks across a fleet of Compute Engine instances.
This sample illustrates how to build a solution. For a full description of the design pattern used in this sample, see Reliable Task Scheduling on Compute Engine with Cloud Scheduler.
For an example of how to start and stop VMs, see Scheduling Compute Instances with Cloud Scheduler.
This sample contains two components:
Instructions for configuring Cloud Scheduler to send cron messages to Cloud Pub/Sub topics.
A utility that runs on Compute Engine. This utility monitors a Cloud Pub/Sub topic. When it detects a new message, it runs the corresponding command locally on the server.
You specify the cron messages to send and their timing in the Cloud Scheduler configuration. When Cloud Scheduler fires a scheduled event, the cron message is passed to the corresponding previously created Cloud Pub/Sub topic.
The utility running on the Compute Engine instances receives cron messages from Cloud Pub/Sub and runs the specified commands that are normally run by cron. To do so, it performs the following actions:
This sample includes the reusable wrapper code of the utility, an example of its use, and a sample script that it runs.
The overview for configuring and running this sample is as follows:
If you don’t already have one, create a Google Account.
Create a Developers Console project.
Create an App Engine app. This is required by Cloud Scheduler:
$ gcloud app create --region=us-central
7. Enable the Cloud Scheduler API:
$ gcloud services enable cloudscheduler.googleapis.com
Ensure that the following is installed if not already on your system:
Important: This tutorial uses several billable components of Google Cloud Platform. To estimate the cost of running this sample:
f1-microGoogle Compute Instance for 15 minutes of one day while you test the sample. After which, you delete the project, releasing all resources. That's 0.25 hours per month.
To clone the GitHub repository to your computer, run the following command:
$ git clone https://github.com/GoogleCloudPlatform/reliable-task-scheduling-compute-engine-sample
Change directories to the
reliable-task-scheduling-compute-engine-sampledirectory. The exact path depends on where you placed the directory when you cloned the sample files from GitHub.
$ cd reliable-task-scheduling-compute-engine-sample
gcloudcommand-line tool to use the project you created in Prerequisites.
$ gcloud config set project
Where you replacewith the identifier of your cloud project.
Create the Pub/Sub topic that you will push messages to.
$ gcloud pubsub topics create test
The topic is now listed under
gcloud pubsub topics list. You can also see the topic in the console:
Big Data > Pub/Sub
Next, we configure Cloud Scheduler to push a message containing the string
test jobevery minute to the Pub/Sub topic
testthat we just created.
gcloud scheduler jobs create pubsub test-job --schedule="* * * * *" \ --topic=test --message-body="test job"
scheduleis specified in unix-cron format. A
*in every field means the job runs every minute, every hour, every day of the month, every month, every day of the week. More simply put, it runs once per minute.
The job is now visible in
gcloud scheduler jobs list. You can also see the jobs in the console:
Tools > Cloud Scheduler
Execution logs for the job are visible via the Logs link for each job.
The utility running on a Compute Engine instance monitors a set of Cloud Pub/Sub topic subscriptions and runs commands on that instance each time it receives a message. By configuring which topics the utility monitors, you can control the jobs that run on each instance. Separating the scheduling logic from the utility logic using Cloud Pub/Sub messaging gives you the ability to schedule all of your jobs with Cloud Scheduler, and then configure the utility on each instance to listen to only the job messages that apply to that instance.
In the sample implementation of the utility, the topic to subscribe to is set as a variable in
TOPIC = 'test'
This value is used when the code creates an
Executorobject to monitor a Cloud Pub/Sub topic.
test_executor = Executor(topic=TOPIC, project=PROJECT, task_cmd=logger, subname='logger_sample_task')
If you only need the utility to monitor a single topic, you can simply change the value of
TOPICin this script. To have the utility monitor multiple topics, you need to instantiate multiple
For this runthrough of the sample, leave
'test'so you can verify your results as described in the following sections.
In this sample, the utility acts as a wrapper to run commands that can be configured as cron jobs. These commands are specified in the
test_executor.pyfile. For this example, the command runs a script
logger_sample_task.pythat simply prints output to
script_path = os.path.abspath(os.path.join(os.getcwd(), 'logger_sample_task.py')) sample_task = "python -u %s" % script_path
To modify the sample to run your own tasks, update the command syntax in the
For your first time running the sample, leave this set to the
logger_sample_taskscript so you can verify your results as described in the following sections.
The utility script runs on your Compute Engine instances and subscribes to the Cloud Pub/Sub topics you specified in Cloud Scheduler. When the utility script receives a message, it runs the corresponding job locally. To make this possible, install the utility script on each instance where you want durable cron jobs to run. The script files are in the
Create a Compute Engine instance with Cloud Pub/Sub scope. In the following example, the instance name is
$ gcloud compute instances create cronworker \ --machine-type f1-micro \ --scopes https://www.googleapis.com/auth/pubsub,https://www.googleapis.com/auth/logging.write \ --zone us-central1-a
gce/test_executor.pyto change the project constant:
PROJECT = 'your-project-id'
your-project-idwith the identifier of your cloud project.
Copy the utility script files to the new instance.
$ gcloud compute scp --recurse gce cronworker:~/ --zone=us-central1-a
SSH into the Compute Engine instance. The following steps are run on the instance over the SSH session.
$ gcloud compute ssh cronworker \ --zone us-central1-a
Update the apt-get package lists on the instance.
$ sudo apt-get update
pipand the Python development libraries on the instance.
$ sudo apt-get install -y python-pip python-dev
Install the Python client library for accessing Google APIs on the instance with Python Pip.
$ sudo pip install --upgrade google-api-python-client oauth2client pytz
Change directories on the instance to the directory where you uploaded the utility script files.
$ cd gce
Run the utility file script,
$ python test_executor.py
After you start the utility, it checks for messages on the specified Cloud Pub/Sub topic (
'test', by default). When it checks the subscription and receives a message, it runs a sample task that simply prints output to
stdout. If you are still connected to the instance using SSH, you’ll see output like the following when the task runs.
Doing work... 1 Doing work... 2 Doing work... 3 … Doing work... 20
The utility script records its activity using Google Cloud Logging. After a cron job has had time to run, you can view the
stdoutoutput of the job in the Logs Viewer for Google Compute Engine.
Open the Developers Console and select your project from the list.
From the navigational menu, select Stackdriver > Logging > Logs.
Expand the dropdown box displaying GAE Application and select GCE VM Instance > cronworker.
Expand the dropdown box displaying All Logs, and select task_runner to display the logged messages from the executor utility, and testsampletask_task to display the output of the sample task running on the Compute Engine instance. Click Ok to confirm.
You can also see the topic in the Pub/Sub console now lists the subscription.
Now that you have tested the sample, delete the cloud resources you created to prevent further billing for them on your account.
Delete the Compute Engine instance.
$ gcloud compute instances delete cronworker --zone=us-central1-a
Delete the Cloud Scheduler job.
You can delete the job from the Cloud Scheduler section of the Developers Console.
Delete the Cloud Pub/Sub topic. You can delete the topic and associated subscriptions from the Cloud Pub/Sub section of the Developers Console.
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