Python task queue using Redis
… image:: https://github.com/closeio/tasktiger/actions/workflows/test.yaml/badge.svg?event=push
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TaskTiger is a Python task queue using Redis.
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… _great engineers: http://jobs.close.com
… contents:: Contents
Per-task fork
TaskTiger forks a subprocess for each task, This comes with several benefits:
Memory leaks caused by tasks are avoided since the subprocess is terminated
when the task is finished. A hard time limit can be set for each task, after
which the task is killed if it hasn’t completed. To ensure performance, any
necessary Python modules can be preloaded in the parent process.
Unique queues
TaskTiger has the option to avoid duplicate tasks in the task queue. In some
cases it is desirable to combine multiple similar tasks. For example, imagine
a task that indexes objects (e.g. to make them searchable). If an object is
already present in the task queue and hasn’t been processed yet, a unique
queue will ensure that the indexing task doesn’t have to do duplicate work.
However, if the task is already running while it’s queued, the task will be
executed another time to ensure that the indexing task always picks up the
latest state.
Task locks
TaskTiger can ensure to never execute more than one instance of tasks with
similar arguments by acquiring a lock. If a task hits a lock, it is requeued
and scheduled for later executions after a configurable interval.
Task retrying
TaskTiger lets you retry exceptions (all exceptions or a list of specific
ones) and comes with configurable retry intervals (fixed, linear,
exponential, custom).
Flexible queues
Tasks can be easily queued in separate queues. Workers pick tasks from a
randomly chosen queue and can be configured to only process specific queues,
ensuring that all queues are processed equally. TaskTiger also supports
subqueues which are separated by a period. For example, you can have
per-customer queues in the form process_emails.CUSTOMER_ID and start a
worker to process process_emails and any of its subqueues. Since tasks
are picked from a random queue, all customers get equal treatment: If one
customer is queueing many tasks it can’t block other customers’ tasks from
being processed. A maximum queue size can also be enforced.
Batch queues
Batch queues can be used to combine multiple queued tasks into one. That way,
your task function can process multiple sets of arguments at the same time,
which can improve performance. The batch size is configurable.
Scheduled and periodic tasks
Tasks can be scheduled for execution at a specific time. Tasks can also be
executed periodically (e.g. every five seconds).
Structured logging
TaskTiger supports JSON-style logging via structlog, allowing more
flexibility for tools to analyze the log. For example, you can use TaskTiger
together with Logstash, Elasticsearch, and Kibana.
The structlog processor tasktiger.logging.tasktiger_processor can
be used to inject the current task id into all log messages.
Reliability
TaskTiger atomically moves tasks between queue states, and will re-execute
tasks after a timeout if a worker crashes.
Error handling
If an exception occurs during task execution and the task is not set up to be
retried, TaskTiger stores the execution tracebacks in an error queue. The
task can then be retried or deleted manually. TaskTiger can be easily
integrated with error reporting services like Rollbar.
Admin interface
A simple admin interface using flask-admin exists as a separate project
(tasktiger-admin_).
… _tasktiger-admin: https://github.com/closeio/tasktiger-admin
It is easy to get started with TaskTiger.
Create a file that contains the task(s).
… code:: python
def my_task():
print(‘Hello’)
Queue the task using the delay method.
… code:: python
In [1]: import tasktiger, tasks
In [2]: tiger = tasktiger.TaskTiger()
In [3]: tiger.delay(tasks.my_task)
Run a worker (make sure the task code can be found, e.g. using PYTHONPATH).
… code:: bash
% PYTHONPATH=. tasktiger
{“timestamp”: “2015-08-27T21:00:09.135344Z”, “queues”: null, “pid”: 69840, “event”: “ready”, “level”: “info”}
{“task_id”: “6fa07a91642363593cddef7a9e0c70ae3480921231710aa7648b467e637baa79”, “level”: “debug”, “timestamp”: “2015-08-27T21:03:56.727051Z”, “pid”: 69840, “queue”: “default”, “child_pid”: 70171, “event”: “processing”}
Hello
{“task_id”: “6fa07a91642363593cddef7a9e0c70ae3480921231710aa7648b467e637baa79”, “level”: “debug”, “timestamp”: “2015-08-27T21:03:56.732457Z”, “pid”: 69840, “queue”: “default”, “event”: “done”}
A TaskTiger object keeps track of TaskTiger’s settings and is used to
decorate and queue tasks. The constructor takes the following arguments:
connection
Redis connection object. The connection should be initialized with
decode_responses=True to avoid encoding problems on Python 3.
config
Dict with config options. Most configuration options don’t need to be
changed, and a full list can be seen within TaskTiger’s __init__
method.
Here are a few commonly used options:
ALWAYS_EAGER
If set to True, all tasks except future tasks (when is a future
time) will be executed locally by blocking until the task returns. This is
useful for testing purposes.
BATCH_QUEUES
Set up queues that will be processed in batch, i.e. multiple jobs are taken
out of the queue at the same time and passed as a list to the worker
method. Takes a dict where the key represents the queue name and the value
represents the batch size. Note that the task needs to be declared as
batch=True. Also note that any subqueues will be automatically treated
as batch queues, and the batch value of the most specific subqueue name
takes precedence.
ONLY_QUEUES
If set to a non-empty list of queue names, a worker only processes the
given queues (and their subqueues), unless explicit queues are passed to
the command line.
setup_structlog
If set to True, sets up structured logging using structlog when
initializing TaskTiger. This makes writing custom worker scripts easier
since it doesn’t require the user to set up structlog in advance.
Example:
… code:: python
import tasktiger
from redis import Redis
conn = Redis(db=1, decode_responses=True)
tiger = tasktiger.TaskTiger(connection=conn, config={
‘BATCH_QUEUES’: {
# Batch up to 50 tasks that are queued in the my_batch_queue or any
# of its subqueues, except for the send_email subqueue which only
# processes up to 10 tasks at a time.
‘my_batch_queue’: 50,
‘my_batch_queue.send_email’: 10,
},
})
TaskTiger provides a task decorator to specify task options. Note that simple
tasks don’t need to be decorated. However, decorating the task allows you to
use an alternative syntax to queue the task, which is compatible with Celery:
… code:: python
import tasktiger
tiger = tasktiger.TaskTiger()
@tiger.task()
def my_task(name, n=None):
print(‘Hello’, name)
… code:: python
In [1]: import tasks
In [2]: tasks.tiger.delay(my_task, args=(‘John’,), kwargs={‘n’: 1})
In [3]: tasks.my_task.delay(‘John’, n=1)
Tasks support a variety of options that can be specified either in the task
decorator, or when queueing a task. For the latter, the delay method must
be called on the TaskTiger object, and any options in the task decorator
are overridden.
… code:: python
@tiger.task(queue=‘myqueue’, unique=True)
def my_task():
print(‘Hello’)
… code:: python
tiger.delay(my_task, queue=‘otherqueue’)
When queueing a task, the task needs to be defined in a module other than the
Python file which is being executed. In other words, the task can’t be in the
__main__ module. TaskTiger will give you back an error otherwise.
The following options are supported by both delay and the task decorator:
queue
Name of the queue where the task will be queued.
hard_timeout
If the task runs longer than the given number of seconds, it will be
killed and marked as failed.
unique
Boolean to indicate whether the task will only be queued if there is no
similar task with the same function, arguments, and keyword arguments in the
queue. Note that multiple similar tasks may still be executed at the same
time since the task will still be inserted into the queue if another one
is being processed. Requeueing an already scheduled unique task will not
change the time it was originally scheduled to execute at.
unique_key
If set, this implies unique=True and specifies the list of kwargs to use
to construct the unique key. By default, all args and kwargs are serialized
and hashed.
lock
Boolean to indicate whether to hold a lock while the task is being executed
(for the given args and kwargs). If a task with similar args/kwargs is queued
and tries to acquire the lock, it will be retried later.
lock_key
If set, this implies lock=True and specifies the list of kwargs to
use to construct the lock key. By default, all args and kwargs are
serialized and hashed.
max_queue_size
A maximum queue size can be enforced by setting this to an integer value.
The QueueFullException exception will be raised when queuing a task if
this limit is reached. Tasks in the active, scheduled, and queued
states are counted against this limit.
when
Takes either a datetime (for an absolute date) or a timedelta
(relative to now). If given, the task will be scheduled for the given
time.
retry
Boolean to indicate whether to retry the task when it fails (either because
of an exception or because of a timeout). To restrict the list of failures,
use retry_on. Unless retry_method is given, the configured
DEFAULT_RETRY_METHOD is used.
retry_on
If a list is given, it implies retry=True. The task will be only retried
on the given exceptions (or its subclasses). To retry the task when a hard
timeout occurs, use JobTimeoutException.
retry_method
If given, implies retry=True. Pass either:
(f, args), where f takes the retry number as the firstThe function needs to return the desired retry interval in seconds, or raise
StopRetry to stop retrying. The following built-in functions can be
passed for common scenarios and return the appropriate tuple:
fixed(delay, max_retries)
Returns a method that returns the given delay (in seconds) or raises
StopRetry if the number of retries exceeds max_retries.
linear(delay, increment, max_retries)
Like fixed, but starts off with the given delay and increments it
by the given increment after every retry.
exponential(delay, factor, max_retries)
Like fixed, but starts off with the given delay and multiplies it
by the given factor after every retry.
For example, to retry a task 3 times (for a total of 4 executions), and wait
60 seconds between executions, pass retry_method=fixed(60, 3).
runner_class
If given, a Python class can be specified to influence task running behavior.
The runner class should inherit tasktiger.runner.BaseRunner and implement
the task execution behavior. The default implementation is available in
tasktiger.runner.DefaultRunner. The following behavior can be achieved:
Execute specific code before or after the task is executed (in the forked
child process), or customize the way task functions are called in either
single or batch processing.
Note that if you want to execute specific code for all tasks,
you should use the CHILD_CONTEXT_MANAGERS configuration option.
Control the hard timeout behavior of a task.
Execute specific code in the main worker process after a task failed
permanently.
This is an advanced feature and the interface and requirements of the runner
class can change in future TaskTiger versions.
The following options can be only specified in the task decorator:
batch
If set to True, the task will receive a list of dicts with args and
kwargs and can process multiple tasks of the same type at once.
Example: [{"args": [1], "kwargs": {}}, {"args": [2], "kwargs": {}}]
Note that the list will only contain multiple items if the worker
has set up BATCH_QUEUES for the specific queue (see the Configuration
section).
schedule
If given, makes a task execute periodically. Pass either:
(f, args), where f takes the current datetime as the firstThe schedule function must return the next task execution datetime, or
None to prevent periodic execution. The function is executed to determine
the initial task execution date when a worker is initialized, and to determine
the next execution date when the task is about to get executed.
For most common scenarios, the below mentioned built-in functions can be passed:
periodic(seconds=0, minutes=0, hours=0, days=0, weeks=0, start_date=None, end_date=None)
Use equal, periodic intervals, starting from start_date (defaults to
2000-01-01T00:00Z, a Saturday, if not given), ending at end_date (or
never, if not given). For example, to run a task every five minutes
indefinitely, use schedule=periodic(minutes=5). To run a task every
every Sunday at 4am UTC, you could use
schedule=periodic(weeks=1, start_date=datetime.datetime(2000, 1, 2, 4)).
cron_expr(expr, start_date=None, end_date=None)
start_date, to specify the periodic task start date. It defaults to
2000-01-01T00:00Z, a Saturday, if not given.
end_date, to specify the periodic task end date. The task repeats
forever if end_date is not given.
For example, to run a task every hour indefinitely,
use schedule=cron_expr("0 * * * *"). To run a task every Sunday at
4am UTC, you could use schedule=cron_expr("0 4 * * 0").
In some cases the task retry options may not be flexible enough. For example,
you might want to use a different retry method depending on the exception type,
or you might want to like to suppress logging an error if a task fails after
retries. In these cases, RetryException can be raised within the task
function. The following options are supported:
method
Specify a custom retry method for this retry. If not given, the task’s
default retry method is used, or, if unspecified, the configured
DEFAULT_RETRY_METHOD. Note that the number of retries passed to the
retry method is always the total number of times this method has been
executed, regardless of which retry method was used.
original_traceback
If RetryException is raised from within an except block and
original_traceback is True, the original traceback will be logged (i.e.
the stacktrace at the place where the caught exception was raised). False by
default.
log_error
If set to False and the task fails permanently, a warning will be logged
instead of an error, and the task will be removed from Redis when it
completes. True by default.
Example usage:
… code:: python
from tasktiger.exceptions import RetryException
from tasktiger.retry import exponential, fixed
def my_task():
if not ready():
# Retry every minute up to 3 times if we’re not ready. An error will
# be logged if we’re out of retries.
raise RetryException(method=fixed(60, 3))
try:
some_code()
except NetworkException:
# Back off exponentially up to 5 times in case of a network failure.
# Log the original traceback (as a warning) and don't log an error if
# we still fail after 5 times.
raise RetryException(method=exponential(60, 2, 5),
original_traceback=True,
log_error=False)
The tasktiger command is used on the command line to invoke a worker. To
invoke multiple workers, multiple instances need to be started. This can be
easily done e.g. via Supervisor. The following Supervisor configuration file
can be placed in /etc/supervisor/tasktiger.ini and runs 4 TaskTiger workers
as the ubuntu user. For more information, read Supervisor’s documentation.
… code:: bash
[program:tasktiger]
command=/usr/local/bin/tasktiger
process_name=%(program_name)s_%(process_num)02d
numprocs=4
numprocs_start=0
priority=999
autostart=true
autorestart=true
startsecs=10
startretries=3
exitcodes=0,2
stopsignal=TERM
stopwaitsecs=600
killasgroup=false
user=ubuntu
redirect_stderr=false
stdout_logfile=/var/log/tasktiger.out.log
stdout_logfile_maxbytes=250MB
stdout_logfile_backups=10
stderr_logfile=/var/log/tasktiger.err.log
stderr_logfile_maxbytes=250MB
stderr_logfile_backups=10
Workers support the following options:
-q, --queues
If specified, only the given queue(s) are processed. Multiple queues can be
separated by comma. Any subqueues of the given queues will be also processed.
For example, -q first,second will process items from first,
second, and subqueues such as first.CUSTOMER1, first.CUSTOMER2.
-e, --exclude-queues
If specified, exclude the given queue(s) from processing. Multiple queues can
be separated by comma. Any subqueues of the given queues will also be
excluded unless a more specific queue is specified with the -q option.
For example, -q email,email.incoming.CUSTOMER1 -e email.incoming will
process items from the email queue and subqueues like
email.outgoing.CUSTOMER1 or email.incoming.CUSTOMER1, but not
email.incoming or email.incoming.CUSTOMER2.
-m, --module
Module(s) to import when launching the worker. This improves task performance
since the module doesn’t have to be reimported every time a task is forked.
Multiple modules can be separated by comma.
Another way to preload modules is to set up a custom TaskTiger launch script,
which is described below.
-h, --host
Redis server hostname (if different from localhost).
-p, --port
Redis server port (if different from 6379).
-a, --password
Redis server password (if required).
-n, --db
Redis server database number (if different from 0).
-M, --max-workers-per-queue
Maximum number of workers that are allowed to process a queue.
--store-tracebacks/--no-store-tracebacks
Store tracebacks with execution history (config defaults to True).
In some cases it is convenient to have a custom TaskTiger launch script. For
example, your application may have a manage.py command that sets up the
environment and you may want to launch TaskTiger workers using that script. To
do that, you can use the run_worker_with_args method, which launches a
TaskTiger worker and parses any command line arguments. Here is an example:
… code:: python
import sys
from tasktiger import TaskTiger
try:
command = sys.argv[1]
except IndexError:
command = None
if command == ‘tasktiger’:
tiger = TaskTiger(setup_structlog=True)
# Strip the “tasktiger” arg when running via manage, so we can run e.g.
# ./manage.py tasktiger --help
tiger.run_worker_with_args(sys.argv[2:])
sys.exit(0)
TaskTiger provides access to the Task class which lets you inspect queues
and perform various actions on tasks.
Each queue can have tasks in the following states:
queued: Tasks that are queued and waiting to be picked up by the workers.active: Tasks that are currently being processed by the workers.scheduled: Tasks that are scheduled for later execution.error: Tasks that failed with an error.To get a list of all tasks for a given queue and state, use
Task.tasks_from_queue. The method gives you back a tuple containing the
total number of tasks in the queue (useful if the tasks are truncated) and a
list of tasks in the queue, latest first. Using the skip and limit
keyword arguments, you can fetch arbitrary slices of the queue. If you know the
task ID, you can fetch a given task using Task.from_id. Both methods let
you load tracebacks from failed task executions using the load_executions
keyword argument, which accepts an integer indicating how many executions
should be loaded.
Tasks can also be constructed and queued using the regular constructor, which
takes the TaskTiger instance, the function name and the options described in
the Task options section. The task can then be queued using its delay
method. Note that the when argument needs to be passed to the delay
method, if applicable. Unique tasks can be reconstructed using the same
arguments.
The Task object has the following properties:
id: The task ID.
data: The raw data as a dict from Redis.
executions: A list of failed task executions (as dicts). An execution
dict contains the processing time in time_started and time_failed,
the worker host in host, the exception name in exception_name and
the full traceback in traceback.
serialized_func, args, kwargs: The serialized function name with
all of its arguments.
func: The imported (executable) function
The Task object has the following methods:
cancel: Cancel a scheduled task.
delay: Queue the task for execution.
delete: Remove the task from the error queue.
execute: Run the task without queueing it.
n_executions: Queries and returns the number of past task executions.
retry: Requeue the task from the error queue for execution.
update_scheduled_time: Updates a scheduled task’s date to the given date.
The current task can be accessed within the task function while it’s being
executed: In case of a non-batch task, the current_task property of the
TaskTiger instance returns the current Task instance. In case of a
batch task the current_tasks property must be used which returns a list of
tasks that are currently being processed (in the same order as they were passed
to the task).
Example 1: Queueing a unique task and canceling it without a reference to the
original task.
… code:: python
from tasktiger import TaskTiger, Task
tiger = TaskTiger()
task = Task(tiger, send_mail, args=[‘email_id’], unique=True)
task.delay(when=datetime.timedelta(minutes=5))
task = Task(tiger, send_mail, args=[‘email_id’], unique=True)
task.cancel()
Example 2: Inspecting queues and retrying a task by ID.
… code:: python
from tasktiger import TaskTiger, Task
QUEUE_NAME = ‘default’
TASK_STATE = ‘error’
TASK_ID = ‘6fa07a91642363593cddef7a9e0c70ae3480921231710aa7648b467e637baa79’
tiger = TaskTiger()
n_total, tasks = Task.tasks_from_queue(tiger, QUEUE_NAME, TASK_STATE)
for task in tasks:
print(task.id, task.func)
task = Task.from_id(tiger, QUEUE_NAME, TASK_STATE, TASK_ID)
task.retry()
Example 3: Accessing the task instances within a batch task function to
determine how many times the currently processing tasks were previously
executed.
… code:: python
from tasktiger import TaskTiger
tiger = TaskTiger()
@tiger.task(batch=True)
def my_task(args):
for task in tiger.current_tasks:
print(task.n_executions())
The --max-workers-per-queue option uses queue locks to control the
number of workers that can simultaneously process the same queue. When using
this option a system lock can be placed on a queue which will keep workers
from processing tasks from that queue until it expires. Use the
set_queue_system_lock() method of the TaskTiger object to set this lock.
TaskTiger comes with Rollbar integration for error handling. When a task errors
out, it can be logged to Rollbar, grouped by queue, task function name and
exception type. To enable logging, initialize rollbar with the
StructlogRollbarHandler provided in the tasktiger.rollbar module. The
handler takes a string as an argument which is used to prefix all the messages
reported to Rollbar. Here is a custom worker launch script:
… code:: python
import logging
import rollbar
import sys
from tasktiger import TaskTiger
from tasktiger.rollbar import StructlogRollbarHandler
tiger = TaskTiger(setup_structlog=True)
rollbar.init(ROLLBAR_API_KEY, APPLICATION_ENVIRONMENT,
allow_logging_basic_config=False)
rollbar_handler = StructlogRollbarHandler(‘TaskTiger’)
rollbar_handler.setLevel(logging.ERROR)
tiger.log.addHandler(rollbar_handler)
tiger.run_worker_with_args(sys.argv[1:])
Error’d tasks occasionally need to be purged from Redis, so TaskTiger
exposes a purge_errored_tasks method to help. It might be useful to set
this up as a periodic task as follows:
… code:: python
from tasktiger import TaskTiger, periodic
tiger = TaskTiger()
@tiger.task(schedule=periodic(hours=1))
def purge_errored_tasks():
tiger.purge_errored_tasks(
limit=1000,
last_execution_before=(
datetime.datetime.utcnow() - datetime.timedelta(weeks=12)
)
)
Tests can be run locally using the provided docker compose file. After installing docker, tests should be runnable with:
… code :: bash
docker-compose run --rm tasktiger pytest
Tests can be more granularly run using normal pytest flags. For example:
… code :: bash
docker-compose run --rm tasktiger pytest tests/test_base.py::TestCase
#. Make sure the code has been thoroughly reviewed and tested in a realistic production environment.
#. Update setup.py and CHANGELOG.md. Make sure you include any breaking changes.
#. Run python setup.py sdist and twine upload dist/<PACKAGE_TO_UPLOAD>.
#. Push a new tag pointing to the released commit, format: v0.13 for example.
#. Mark the tag as a release in GitHub’s UI and include in the description the changelog entry for the version.
An example would be: https://github.com/closeio/tasktiger/releases/tag/v0.13.