Yet Another Cron

A modern Cron replacement that is Docker-friendly

• Free software: MIT license

Features

• "Crontab" is in YAML format;
• Builtin sending of Sentry and Mail outputs when cron jobs fail;
• Flexible configuration: you decide how to determine if a cron job fails or not;
• Designed for running in Docker, Kubernetes, or 12 factor environments:
  • Runs in the foreground;
  • Logs everything to stdout/stderr [1];
• Option to automatically retry failing cron jobs, with exponential backoff;
• Optional HTTP REST API, to fetch status and start jobs on demand;
• Arbitrary timezone support;

[1]	Whereas vixie cron only logs to syslog, requiring a syslog daemon to be running in the background or else you don't get logs!

Status

The project is in beta stage: essential features are complete, and the focus is finding and fixing bugs before the first stable release.

Installation

Install using pip

yacron requires Python >= 3.6 (for systems with older Python, use the binary instead). It is advisable to install it in a Python virtual environment, for example:

python3 -m venv yacronenv
. yacronenv/bin/activate
pip install yacron

Install using pipx

pipx automates creating a virtualenv and installing a python program in the newly created virtualenv. It is as simple as:

pipx install yacron

Install using binary

Alternatively, a self-contained binary can be downloaded from github: https://github.com/gjcarneiro/yacron/releases. This binary should work on any Linux 64-bit system post glibc 2.23 (e.g. Ubuntu:16.04). Python is not required on the target system (it is embedded in the executable).

Usage

Configuration is in YAML format. To start yacron, give it a configuration file or directory path as the -c argument. For example:

yacron -c /tmp/my-crontab.yaml

This starts yacron (always in the foreground!), reading /tmp/my-crontab.yaml as configuration file. If the path is a directory, any *.yaml or *.yml files inside this directory are taken as configuration files.

Configuration basics

This configuration runs a command every 5 minutes:

jobs:
  - name: test-01
    command: echo "foobar"
    shell: /bin/bash
    schedule: "*/5 * * * *"

The command can be a string or a list of strings. If command is a string, yacron runs it through a shell, which is /bin/bash in the above example, but is /bin/sh by default.

If the command is a list of strings, the command is executed directly, without a shell. The ARGV of the command to execute is extracted directly from the configuration:

jobs:
  - name: test-01
    command:
      - echo
      - foobar
    schedule: "*/5 * * * *"

The schedule option can be a string in the traditional crontab format (including @reboot, which will only run the job when yacron is initially executed), or can be an object with properties. The following configuration runs a command every 5 minutes, but only on the specific date 2017-07-19, and doesn't run it in any other date:

jobs:
  - name: test-01
    command: echo "foobar"
    schedule:
      minute: "*/5"
      dayOfMonth: 19
      month: 7
      year: 2017
      dayOfWeek: "*"

Important: by default all time is interpreted to be in UTC, but you can request to use local time instead. For instance, the cron job below runs every day at 19h27 local time because of the utc: false option:

jobs:
  - name: test-01
    command: echo "hello"
    schedule: "27 19 * * *"
    utc: false
    captureStdout: true

Since Yacron version 0.11, you can also request that the schedule be interpreted in an arbitrary timezone, using the timezone attribute:

jobs:
  - name: test-01
    command: echo "hello"
    schedule: "27 19 * * *"
    timezone: America/Los_Angeles
    captureStdout: true

You can ask for environment variables to be defined for command execution:

jobs:
  - name: test-01
    command: echo "foobar"
    shell: /bin/bash
    schedule: "*/5 * * * *"
    environment:
      - key: PATH
        value: /bin:/usr/bin

You can also provide an environment file to define environments for command execution:

jobs:
  - name: test-01
    command: echo "foobar"
    shell: /bin/bash
    schedule: "*/5 * * * *"
    env_file: .env

The env file must be a list of KEY=VALUE pairs. Empty lines and lines starting with # will be ignored.

Variables declared in the environment option will override those found in the env_file.

Specifying defaults

There can be a special defaults section in the config. Any attributes defined in this section provide default values for cron jobs to inherit. Although cron jobs can still override the defaults, as needed:

defaults:
    environment:
      - key: PATH
        value: /bin:/usr/bin
    shell: /bin/bash
    utc: false
jobs:
  - name: test-01
    command: echo "foobar"  # runs with /bin/bash as shell
    schedule: "*/5 * * * *"
  - name: test-02  # runs with /bin/sh as shell
    command: echo "zbr"
    shell: /bin/sh
    schedule: "*/5 * * * *"

Note: if the configuration option is a directory and there are multiple configuration files in that directory, then the defaults section in each configuration file provides default options only for cron jobs inside that same file; the defaults have no effect beyond any individual YAML file.

Reporting

Yacron has builtin support for reporting jobs failure (more on that below) by email, Sentry and shell command (additional reporting methods might be added in the future):

- name: test-01
  command: |
    echo "hello" 1>&2
    sleep 1
    exit 10
  schedule:
    minute: "*/2"
  captureStderr: true
  onFailure:
    report:
      sentry:
        dsn:
          value: example
          # Alternatively:
          # fromFile: /etc/secrets/my-secret-dsn
          # fromEnvVar: SENTRY_DSN
        fingerprint:  # optional, since yacron 0.6
          - yacron
          - "{{ environment.HOSTNAME }}"
          - "{{ name }}"
        extra:
          foo: bar
          zbr: 123
        level: warning
        environment: production
      mail:
        from: [email protected]
        to: [email protected]
        smtpHost: 127.0.0.1
        # optional fields:
        username: "username1"  # set username and password to enable login
        pasword:
          value: example
          # Alternatively:
          # fromFile: /etc/secrets/my-secret-password
          # fromEnvVar: MAIL_PASSWORD
        tls: false  # set to true to enable TLS
        starttls: false  # set to true to enable StartTLS
      shell:
        shell: /bin/bash
        command: ...

Here, the onFailure object indicates that what to do when a job failure is detected. In this case we ask for it to be reported both to sentry and by sending an email.

The captureStderr: true part instructs yacron to capture output from the the program's standard error, so that it can be included in the report. We could also turn on standard output capturing via the captureStdout: true option. By default, yacron captures only standard error. If a cron job's standard error or standard output capturing is not enabled, these streams will simply write to the same standard output and standard error as yacron itself.

It is possible also to report job success, as well as failure, via the onSuccess option.

- name: test-01
  command: echo "hello world"
  schedule:
    minute: "*/2"
  captureStdout: true
  onSuccess:
    report:
      mail:
        from: [email protected]
        to: [email protected]
        smtpHost: 127.0.0.1

Since yacron 0.5, it is possible to customise the format of the report. For mail reporting, the option subject indicates what is the subject of the email, while body formats the email body. For Sentry reporting, there is only body. In all cases, the values of those options are strings that are processed by the jinja2 templating engine. The following variables are available in templating:

• name(str): name of the cron job
• success(bool): whether or not the cron job succeeded
• stdout(str): standard output of the process
• stderr(str): standard error of the process
• exit_code(int): process exit code
• command(str): cron job command
• shell(str): cron job shell
• environment(dict): subprocess environment variables

Example:

- name: test-01
  command: |
    echo "hello" 1>&2
    sleep 1
    exit 10
  schedule:
    minute: "*/2"
  captureStderr: true
  onFailure:
    report:
      mail:
        from: [email protected]
        to: [email protected]
        smtpHost: 127.0.0.1
        subject: Cron job '{{name}}' {% if success %}completed{% else %}failed{% endif %}
        body: |
          {{stderr}}
          (exit code: {{exit_code}})

The shell reporter (since yacron 0.13) executes a user given shell command in the specified shell. It passes all environment variables from the python executable and specifies some additional ones to inform about the state of the job:

• YACRON_FAIL_REASON (str)
• YACRON_FAILED ("1" or "0")
• YACRON_RETCODE (str)
• YACRON_STDERR (str)
• YACRON_STDOUT (str)

A simple example configuration:

- name: test-01
  command: echo "foobar" && exit 123
  shell: /bin/bash
  schedule: "* * * * *"
  onFailure:
    report:
      shell:
        shell: /bin/bash
        command: echo "Error code $YACRON_RETCODE"

Since yacron 0.15, it is possible to send emails formatted as html, by adding the html: true property. For example, here the standard output of a shell command is captured and interpreted as html and placed in the email message:

- name: test-01
  command: echo "hello <b>world</b>"
  schedule: "@reboot"
  captureStdout: true
  onSuccess:
    report:
      mail:
        from: [email protected]
        to: [email protected], [email protected]
        html: true
        smtpHost: 127.0.0.1
        smtpPort: 1025
        subject: This is a cron job with html body

Metrics

Yacron has builtin support for writing job metrics to Statsd:

jobs:
  - name: test01
    command: echo "hello"
    schedule: "* * * * *"
    statsd:
      host: my-statsd.exemple.com
      port: 8125
      prefix: my.cron.jobs.prefix.test01

With this config Yacron will write the following metrics over UDP to the Statsd listening on my-statsd.exemple.com:8125:

my.cron.jobs.prefix.test01.start:1|g  # this one is sent when the job starts
my.cron.jobs.prefix.test01.stop:1|g   # the rest are sent when the job stops
my.cron.jobs.prefix.test01.success:1|g
my.cron.jobs.prefix.test01.duration:3|ms|@0.1

Handling failure

By default, yacron considers that a job has failed if either the process returns a non-zero code or if it generates output to standard error (and standard error capturing is enabled, of course).

You can instruct yacron how to determine if a job has failed or not via the failsWhen option:

failsWhen:
  producesStdout: false
  producesStderr: true
  nonzeroReturn: true
  always: false

producesStdout

If true, any captured standard output causes yacron to consider the job as failed. This is false by default.

producesStderr

If true, any captured standard error causes yacron to consider the job as failed. This is true by default.

nonzeroReturn

If true, if the job process returns a code other than zero causes yacron to consider the job as failed. This is true by default.

always

If true, if the job process exits that causes yacron to consider the job as failed. This is false by default.

It is possible to instruct yacron to retry failing cron jobs by adding a retry option inside onFailure:

- name: test-01
  command: |
    echo "hello" 1>&2
    sleep 1
    exit 10
  schedule:
    minute: "*/10"
  captureStderr: true
  onFailure:
    report:
      mail:
        from: [email protected]
        to: [email protected]
        smtpHost: 127.0.0.1
    retry:
      maximumRetries: 10
      initialDelay: 1
      maximumDelay: 30
      backoffMultiplier: 2

The above settings tell yacron to retry the job up to 10 times, with the delay between retries defined by an exponential backoff process: initially 1 second, doubling for every retry up to a maximum of 30 seconds. A value of -1 for maximumRetries will mean yacron will keep retrying forever, this is mostly useful with a schedule of "@reboot" to restart a long running process when it has failed.

If the cron job is expected to fail sometimes, you may wish to report only in the case the cron job ultimately fails after all retries and we give up on it. For that situation, you can use the onPermanentFailure option:

- name: test-01
  command: |
    echo "hello" 1>&2
    sleep 1
    exit 10
  schedule:
    minute: "*/10"
  captureStderr: true
  onFailure:
    retry:
      maximumRetries: 10
      initialDelay: 1
      maximumDelay: 30
      backoffMultiplier: 2
  onPermanentFailure:
    report:
      mail:
        from: [email protected]
        to: [email protected]
        smtpHost: 127.0.0.1

Concurrency

Sometimes it may happen that a cron job takes so long to execute that when the moment its next scheduled execution is reached a previous instance may still be running. How yacron handles this situation is controlled by the option concurrencyPolicy, which takes one of the following values:

Allow

allows concurrently running jobs (default)

Forbid

forbids concurrent runs, skipping next run if previous hasn’t finished yet

Replace

cancels currently running job and replaces it with a new one

Execution timeout

(new in version 0.4)

If you have a cron job that may possibly hang sometimes, you can instruct yacron to terminate the process after N seconds if it's still running by then, via the executionTimeout option. For example, the following cron job takes 2 seconds to complete, yacron will terminate it after 1 second:

- name: test-03
  command: |
    echo "starting..."
    sleep 2
    echo "all done."
  schedule:
    minute: "*"
  captureStderr: true
  executionTimeout: 1  # in seconds

When terminating a job, it is always a good idea to give that job process some time to terminate properly. For example, it may have opened a file, and even if you tell it to shutdown, the process may need a few seconds to flush buffers and avoid losing data.

On the other hand, there are times when programs are buggy and simply get stuck, refusing to terminate nicely no matter what. For this reason, yacron always checks if a process exited some time after being asked to do so. If it hasn't, it tries to forcefully kill the process. The option killTimeout option indicates how many seconds to wait for the process to gracefully terminate before killing it more forcefully. In Unix systems, we first send a SIGTERM, but if the process doesn't exit after killTimeout seconds (30 by default) then we send SIGKILL. For example, this cron job ignores SIGTERM, and so yacron will send it a SIGKILL after half a second:

- name: test-03
  command: |
    trap "echo '(ignoring SIGTERM)'" TERM
    echo "starting..."
    sleep 10
    echo "all done."
  schedule:
    minute: "*"
  captureStderr: true
  executionTimeout: 1
  killTimeout: 0.5

Change to another user/group

(new in version 0.11)

You can request that Yacron change to another user and/or group for a specific cron job. The field user indicates the user (uid or userame) under which the subprocess must be executed. The field group (gid or group name) indicates the group id. If only user is given, the group defaults to the main group of that user. Example:

- name: test-03
  command: id
  schedule:
    minute: "*"
  captureStderr: true
  user: www-data

Naturally, yacron must be running as root in order to have permissions to change to another user.

Remote web/HTTP interface

(new in version 0.10)

If you wish to remotely control yacron, you can optionally enable an HTTP REST interface, with the following configuration (example):

web:
  listen:
     - http://127.0.0.1:8080
     - unix:///tmp/yacron.sock

Now you have the following options to control it (using HTTPie as example):

Get the version of yacron:

$ http get http://127.0.0.1:8080/version
HTTP/1.1 200 OK
Content-Length: 22
Content-Type: text/plain; charset=utf-8
Date: Sun, 03 Nov 2019 19:48:15 GMT
Server: Python/3.7 aiohttp/3.6.2

0.10.0b3.dev7+g45bc4ce

Get the status of cron jobs:

$ http get http://127.0.0.1:8080/status
HTTP/1.1 200 OK
Content-Length: 104
Content-Type: text/plain; charset=utf-8
Date: Sun, 03 Nov 2019 19:44:45 GMT
Server: Python/3.7 aiohttp/3.6.2

test-01: scheduled (in 14 seconds)
test-02: scheduled (in 74 seconds)
test-03: scheduled (in 14 seconds)

You may also get status info in json format:

$ http get http://127.0.0.1:8080/status Accept:application/json
HTTP/1.1 200 OK
Content-Length: 206
Content-Type: application/json; charset=utf-8
Date: Sun, 03 Nov 2019 19:45:53 GMT
Server: Python/3.7 aiohttp/3.6.2

[
    {
        "job": "test-01",
        "scheduled_in": 6.16588,
        "status": "scheduled"
    },
    {
        "job": "test-02",
        "scheduled_in": 6.165787,
        "status": "scheduled"
    },
    {
        "job": "test-03",
        "scheduled_in": 6.165757,
        "status": "scheduled"
    }
]

Start a job right now:

Sometimes it's useful to start a cron job right now, even if it's not scheduled to run yet, for example for testing:

$ http post http://127.0.0.1:8080/jobs/test-02/start
HTTP/1.1 200 OK
Content-Length: 0
Content-Type: application/octet-stream
Date: Sun, 03 Nov 2019 19:50:20 GMT
Server: Python/3.7 aiohttp/3.6.2

Includes

(new in version 0.13)

You may have a use case where it's convenient to have multiple config files, and choose at runtime which one to use. In that case, it might be useful if you can put common definitions (such as defaults for reporting, shell, etc.) in a separate file, that is included by the other files.

To support this use case, it is possible to ask one config file to include another one, via the include directive. It takes a list of file names: those files will be parsed as configuration and merged in with this file.

Example, your main config file could be:

include:
  - _inc.yaml

jobs:

  - name: my job
    ...

And your included _inc.yaml file could contain some useful defaults:

defaults:
  shell: /bin/bash
  onPermanentFailure:
    report:
      sentry:
        ...