celery 笔记 [ things should be known when talking about celery ]

从我的 blog 转过来的，希望多多交流啦

http://litaotao.github.io/hello-celery/

1. hello, celery

three questions:
- what is celery?
- why do we need celery?
- what features does celery have?

1.1 what is celery?

First, celery is a distributed task queue [ a distributed system to process messages].
Second, it provides operations with the tools required to maintain such a system.
- flower
- jobtastic
Third, it was built for Dijango Apps in the early, but scaled since version 0.8.
Fourth, it focus on real-time processing, while also supporting task scheduling.

1.2 why do we need celery?

run something in the background.
run something after the web request is done.
making something is done by executing it asynchronously and using retries.
schedule periodic task.
distributed computing.
parallel executing.

1.3 what features does celery have?

simple: write in python, easily configure.
high available: retry mechanism, independent broker HA.
fast: process millions of tasks one minute.
flexible: Almost every part of Celery can be extended or used on its own, Custom pool implementations, serializers, compression schemes, logging, schedulers, consumers, producers, autoscalers, broker transports and much more.
monitoring: A stream of monitoring events is emitted by workers and is used by built-in and external tools to tell you what your cluster is doing – in real-time.
auto reloading: In development workers can be configured to automatically reload source code as it changes.
workflows: Simple and complex workflows can be composed using a set of powerful primitives we call the “canvas”, including grouping, chaining, chunking and more.
auto scaling: Dynamically resizing the worker pool depending on load, or custom metrics specified by the user, used to limit memory usage in shared hosting/cloud environments or to enforce a given quality of service.
Time & Rate Limits: You can control how many tasks can be executed per second/minute/hour, or how long a task can be allowed to run, and this can be set as a default, for a specific worker or individually for each task type.
Scheduling: You can specify the time to run a task in seconds or a datetime, or or you can use periodic tasks for recurring events based on a simple interval, or crontab expressions supporting minute, hour, day of week, day of month, and month of year.
Resource leak protection:
User components:

2 how does celery work?

2.1 some terms in message queue and celery

celery task: a serialized task info which will be scheduled to done by celery,

it contains some meta data, like bellow:

{"id": "4cc7438e-afd4-4f8f-a2f3-f46567e7ca77",
 "task": "celery.task.PingTask",
 "args": [],
 "kwargs": {},
 "retries": 0,
 "eta": "2009-11-17T12:30:56.527191"
}

producer: the one who send task messages to the broker,

it is a celery client.

consumer: fetch task messages from queues and process them.
broker: the one who receives task messages from producer and route different

to different queues.

exchange: exchange, or a router, the one who really do message receiving from producer

dispatching message to queues. two things should be taken in mind about exchange:
- one message from producer can be routed to zero or more queues;
- you can set route rules for the exchange, called bindings;

queue: buffer for store task messages.

2.2 the celery workflow

2.3 the workflow deployment

3. install celery

two possible ways to install celery.

install the latest stable celery and its dependences

chenshan@mac007:~/Desktop$sudo pip install -U celery
...
...
...
Installing collected packages: celery, kombu, billiard, amqp

the celery dependences info is listed here: Does Celery have many dependencies

install celery with bundles

Celery also defines a group of bundles that can be used to install Celery and the dependencies for a given feature.

You can specify these in your requirements or on the pip comand-line by using brackets. Multiple bundles can be specified by separating them by commas. for example, using the command pip install "celery[librabbitmq,redis,auth,msgpack]" to install celery with librabbitmq, redis, auth, msgpack libriaries.

all bundles command can be find here: bundles