The Numb-Nuts Tutorial to the Celery Distributed Task Queue (using Python)

Celery is a distributed queue that is very easy to pick-up. I’ll do two quick examples: one that sends a job and returns and another that sends a job and then retrieves a result. I’m going to use SQLite for this example (which is interfaced via SQLAlchemy). Since Celery seems to have some issues importing SQLite under Python 3, we’ll use Python 2.7 . Make sure that you install the “celery” and “sqlalchemy” Python packages.

Without Results

Define the tasks module and save it as

import celery

_BACKEND_URI = 'sqla+sqlite:///tasks.sqlite'
_APP = celery.Celery('sqlite_queue_without_results', broker=_BACKEND_URI)

def some_task(incoming_message):
    return "ECHO: {0}".format(incoming_message)

Start the server:

$ celery -A sqlite_queue_without_results worker --loglevel=info

Execute the following Python to submit a job:

import sqlite_queue_without_results

_ARG1 = "Test message"

That’s it. You’ll see something similar to the following in the server window:

Celery (Without Result)

With Results

This time, when we define the tasks module, we’ll provide Celery a results backend. Call this module

import celery

_RESULT_URI = 'db+sqlite:///results.sqlite'
_BACKEND_URI = 'sqla+sqlite:///tasks.sqlite'

_APP = celery.Celery('sqlite_queue_with_results', broker=_BACKEND_URI, backend=_RESULT_URI)

def some_task(incoming_message):
    return "ECHO: {0}".format(incoming_message)

Start the server:

$ celery -A sqlite_queue_with_results worker --loglevel=info

Execute the following Python to submit a job:

import sqlite_queue_with_results

_ARG1 = "Test message"
r = sqlite_queue_with_results.some_task.delay(_ARG1)
value = r.get(timeout=2)

print("Result: [{0}]".format(value))

Since we’re using a traditional DBMS (albeit a fast, local one) to store our results, we’ll be internally polling for a state change and then fetching the result. Therefore, it is a more costly operation and I’ve used a two-second timeout to accommodate this.

The server output will be similar to the following:

Celery (With Result)

The client output will look like:

Result: [ECHO: Test message]

Celery has many more features not explored by this tutorial, including:

  • exception propagation
  • custom task states (including providing metadata that can be read by the client)
  • task ignore/reject/retry responses
  • HTTP-based tasks (for calling your tasks in another place or language)
  • task routing
  • periodic/scheduled tasks
  • workflows
  • drawing visible graphs in order to inspect behavior

For more information, see the user guide.

Installing Xcode Command Line Tools for Mavericks (Problems)

I had a perfectly running development environment under Mavericks 10.9.1 . I’m not, by nature, someone who would prefer to use a Mac, but sometimes we have to take what we’re given, and, at least, it’s Unix-based.

I was surviving without having to install Xcode, until recently when I had to investigate Apple’s illegally-modified “pngcrush” utility. I required Xcode in order to get the iPhone optimizations. Otherwise, I just got the standard version of the open-source utility. So, I installed it.

Yesterday, I had to install/build the Python “cryptography” module, which requires a C build-environment. Now, I had some cc/gcc discrepancies, and one unsupported command-line argument. Obviously, it’s Xcode. So, I uninstalled it. I innocently also installed the 10.9.1->10.9.2 Mavericks update at the same time.

Catastrophe. Now, the same stuff is broken, and I get warnings every single time I invoke Brew:

Warning: No developer tools installed.
You should install the Command Line Tools.
Run `xcode-select --install` to install them.

When I run xcode-select, I get a dialog that says the command-line tools are required, one button for installing them, and another for the full Xcode install. When I click to install the tools, I got the EULA and then a progress-bar that said “Finding Software”, only to give me a message:

Can't install the software because it is not currently available from the Software Update server.

I had to physically go and download the dmg package:

However, when I dragged the pkg file into Applications and ran it, I inevitably ran into the following message, every time:

The installation failed.

The Installer can't locate the data it needs to install the software. Check your install media or Internet connection and try again, or contact the software manufacturer for assistance.

It turns out that it expected to be run directly from the dmg container. It looks like everything is working now (with only the command-line tools, and not requiring the whole Xcode install).

Though I’m still investigating the build errors I now having, the emphasis of this post is how to remedy the Xcode/tools errors that I was seeing.


Julia Language

Julia Language

The Julia dynamic, technical language, for efficient, distributed development. It also built on LLVM, and has the beautiful IJulia interface. From the homepage:

  • Multiple dispatch: providing ability to define function behavior across many combinations of argument types
  • Dynamic type system: types for documentation, optimization, and dispatch
  • Good performance, approaching that of statically-compiled languages like C
  • Built-in package manager
  • Lisp-like macros and other metaprogramming facilities
  • Call Python functions: use the PyCall package
  • Call C functions directly: no wrappers or special APIs
  • Powerful shell-like capabilities for managing other processes
  • Designed for parallelism and distributed computation
  • Coroutines: lightweight “green” threading
  • User-defined types are as fast and compact as built-ins
  • Automatic generation of efficient, specialized code for different argument types
  • Elegant and extensible conversions and promotions for numeric and other types
  • Efficient support for Unicode, including but not limited to UTF-8
  • MIT licensed: free and open source