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A Cron-like system for running tasks

Project description

A cron-like system to run your application tasks across any node, instead of one special snowflake. This is done by keeping your tasks in MySQL and publishing them over AMQP to workers that will run your tasks and eventually save the results back into the DB. This was started as a hackathon project at SeatGeek

Requirements

  • Python 2.7
  • RabbitMQ 3.x
  • MySQL 5.x

Installation

Using PIP via PyPi:

pip install cronq

Using PIP via Github:

pip install git+git://github.com/seatgeek/cronq.git@0.27.0#egg=cronq

Adding to your requirements.txt file (run pip install -r requirements.txt afterwards):

git+ssh://git@github.com/seatgeek/python-cronq.git@0.27.0#egg=cronq

Usage

There are various workers that are used by Cronq, as well as a web admin.

cronq-runner

The runner executes tasks, and should be run on hosts that will actually perform work. There is no limit to the number of runners that can execute.

The runner requires /var/log/cronq/ to exist and be writable by the user executing the runner. If it is not, logs will be written to /tmp.

# setup rabbitmq connection info
export RABBITMQ_URL=amqp://guest:guest@localhost/

# specify the rabbitmq queue to listen to
export CRONQ_QUEUE=cronq_jobs # `cronq_jobs` is the default queue

# log job output to stdout *as well as* /var/log/cronq
export CRONQ_RUNNER_LOG_TO_STDOUT=1

# run commands
cronq-runner

When run, cronq-runner will:

  • Setup a rabbitmq connection
  • Listen to the cronq_jobs queue
  • Retrieve commands from the queue
  • Publish a message saying the command is started
  • Run the command in a shell subprocess
  • Publish a message on success/failure to the cronq exchange and cronq_results queue. This is not configurable.
  • Listen for more messages

cronq-injector

The cronq-injector command will non-destructively create any necessary cronq tables, though it will need a database to perform this action against. Please note that if you do not have tables created, it is helpful to run the injector first.

The injector is used to retrieve jobs from the database and publish them to AMQP. Jobs are published in the following format:

# where job is a database record
{
    'name': job.name,
    'command': unicode(job.command),
    'id': job.id,
}

You can ostensibly run as many injectors as necessary. MySQL isolation levels are used to attain locks on job records.

# setup rabbitmq connection info
export RABBITMQ_URL=amqp://guest:guest@localhost/

# specify the database connection string
export DATABASE_URL=mysql+pymysql://cronq:cronq@localhost/cronq

# run the comand injector
cronq-injector

cronq-injector perform a 1 second sleep between job injections, but may perform an unlimited number of job injections in that time period.

Note that jobs are not queued up at the exact time you specify in the database. Rather, jobs that matches the following heuristic are queued one-at-a-time until no jobs are left to be queued for that injection cycle:

Job.next_run < NOW() OR Job.run_now = 1

cronq-results

The results aggregator listens to the cronq_results queue for the results of cronq-runner executions. You can run as many of these as possible, as they will retrieve results one-at-a-time from rabbitmq.

# setup rabbitmq connection info
export RABBITMQ_URL=amqp://guest:guest@localhost/

# specify the database connection string
export DATABASE_URL=mysql+pymysql://cronq:cronq@localhost/cronq

# run the results-aggregator
cronq-results

These results can be viewed for particular commands within the web-admin, or by inspecting the database.

cronq-pruner

As time goes on, you will wish to prune your database of old events. A naive pruner is packaged with cronq, and it has the ability to prune events by a range or to keep at most N recent events

cronq-web

The web view is a WSGI app run from cronq.web:app and requires only database access. The following is an example for running the web admin using webscale technologies:

# install libevent-dev
sudo apt-get install libevent-dev

# install required python libraries
sudo pip install greenlet gevent gunicorn

# specify the database connection string
export DATABASE_URL=mysql+pymysql://cronq:cronq@localhost/cronq

# if you have an aggregated log dashboard, you can provide a search url
# template. it will be used in the web dashboard for linking to logs
# the following strings will be replaced:
#
# {job_id} : replaced with the job's job_id
# {run_id} : replaced with the job's run_id
# {start_time} : replaced with job's start time in ISO format url quoted
#                ex - "2016-08-15T08%3A00%3A11.000Z"
# {end_time} : replaced with job's end time, or now if job is still running, in ISO format url quoted
               ex - "2016-08-15T08%3A00%3A31.999Z"
export CRONQ_LOG_URL_TEMPLATE="https://logs.service/search?run_id={run_id}&from={start_time}&to={end_time}"

# run the web admin
gunicorn --access-logfile - -w 2 --bind :5000 --worker-class gevent cronq:make_application\(\) --error-logfile - --log-file -

# access the panel on http://127.0.0.1:5000

The web admin will list available commands, their result history, and a button to allow you to immediately schedule a job.

Categories Api

The web admin exposes a category endpoint which allows you to replace a set of jobs with a single API call

curl -v 'localhost:5000/api/category/example' -f -XPUT -H 'content-type: application/json' -d '
{
    "category": "example",
    "jobs": [{
        "name": "Test Job",
        "schedule": "R/2013-05-29T00:00:00/PT1M",
        "command": "sleep 10",
        "routing_key": "slow"
    }]
}'

This adds / updates a job named Test Job in the example category. The time format is ISO 8601. Any jobs no longer defined for the example category will be removed. This allows you to script job additions / removes in your VCS.

Configuration Validation

Before posting cronq configuration, you may wish to validate that the file is in the correct format. To do so, you can use the included cronq-validator console script.

# looks for a cronq.config in the current directory by default
cronq-validator

# can be scoped to a specific file
cronq-validator --config cronq.json

This will check both json syntax as well as whether the contents pass the Cronq jsonschema. We also include a cronq.validator module which contains the following methods:

  • cronq_schema: Returns the jsonschema as a python dictionary
  • validate: Takes a path to a config file and runs validation. May raise ValidationError.

License

BSD

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