Integrating SQLAlchemy into a CherryPy application

Quite often, people come on the CherryPy IRC channel asking about the way to use SQLAlchemy with CherryPy. There are a couple of good recipes on the tools wiki but I find them a little complex to begin with. Not to the recipes’ fault, many people don’t necessarily know about CherryPy tools and plugins at that stage.

The following recipe will try to make the example complete whilst as simple as possible to allow folks to start up with SQLAlchemy and CherryPy.

The general idea is to use the plugin mechanism to register functions on an engine basis and enable a tool that will provide an access to the SQLAlchemy session at request time.

Using Jython as a CLI frontend to HBase

HBase, the well known non-relational distributed database, comes with a console program to perform various operations on a HBase cluster. I’ve personally found this tool to be a bit limited and I’ve toyed around the idea of writing my own. Since HBase only comes with a Java driver for direct access and the various RPC interfaces such as Thrift don’t offer the full set of functions over HBase, I decided to go for Jython and to directly use the Java API. This article will show a mock-up of such a tool.

The idea is to provide a simple Python API over the HBase one and couple it with a Python interpreter. This means, it offers the possibility to perform any Python (well Jython) operations whilst operating on HBase itself with an easier API than the Java one.

Note also that the tool uses the WSPBus already described in an earlier article to control the process itself. You will therefore need CherryPy’s latest revision.

To test the tool, you can simply grab the latest copy of HBase and run:

Then you need to configure your classpath so that it includes all the HBase dependencies. To determine them:

Copy the full list of jars and export CLASSPATH with it. (This is from the HBase wiki on Jython and HBase).

Next you have to add an extra jar to the classpath so that Jython supports readline:

Make sure you’ll install libreadline-java as well.

Now, that your environment is setup, save the code above under a script named stave.py and run it as follow:

You can import any Python module available to your Jython environment as well of course.

I will probably extend this tool over time but in the meantime I hope you’ll find it a useful canvas to operate HBase.

Managing your process with the CherryPy’s bus

CherryPy is a successful small web framework which over the years has built up its performances as well as its stability. To do so, Robert Brewer, the main CherryPy’s architect has introduced what is called the Web Site Process Bus (WSPBus). The idea is to manage a Python process by providing it with a bus to which one can publish or subscribe for events. CherryPy’s implementation of the bus comes with a set of pubsub handlers for very basic operations such as responding to system signals, handle thread creation and deletion, drop process privileges and handle PID files. The bus mechanism can help your handling of sub-processes so that they start, run and terminates gracefully. Let’s see how.

Create your bus

First, you need to create a bus instance. This could be as simple as this.

If you want to log through the bus, you will need further work since the bus doesn’t create a logger by default. Let’s see an example.

Not much, just creating a logger and subscribing the bus log channel to an instance method.

Associate the bus with the main process

Before we move on to the management of sub-process, let’s see how we can manage the main Python process already with our bus above.

For this, let’s imagine a bank placing stock orders, those orders will be handled by a broker running in a sub-process.

As you can see, not much again here, we simply associate a bus with the bank object. We also register to the exit channel of the bus so that when we terminated, we can do some cleanup. It’s good use to unregister from the bus.

We don’t actually care where those orders come from so we randomly generate them. The orders are placed every time the bus iterates its loop. This is done by attaching to the main channel of the bus.

We use a process queue to communicate with the broker’s sub-process.

Associate the bus with a sub-process

Handling the sub-process is actually similar to handling the main process. Let’s see the broker implementation for example.

Several things are to be noticed. First we register once again to the bus’ main channel a method that checks the shared queue for incoming data. Whenever the incoming message is “stop”, we exit the bus altogether, thus leaving the sub-process, since it was blocked on the bus loop.

Note that the stop method could be called by the parent process if you needed to programatically stop the sub-process.

Put it all together

Run the code above as follow:

This creates the shared queue, starts the sub-process that runs the broker and finally starts the bank within the main process.

You should see a bunch of messages in the console and if you hit Ctrl-C, this will stop both processes cleanly.

And here we are, we now manage processes and sub-processes with a clean solution. The CherryPy process bus is an elegant add-on to your toolbox that I can only highly advise to consider in the future. The WSPBus implementation is part of the main CherryPy package (CherryPy 3.x), so you’ll have to install it all, even if you don’t require the HTTP framework. But don’t let that hold you back since the HTTP framework isn’t required for the bus to be used.

Happy coding!

The code is here.

CherryPy 3.1 has been released

Yesterday Robert Brewer released version 3.1 of the CherryPy product and I think this calls for a hurray! For those who’ve been using 3.0 they’ll be happy to know that the upgrade will be rather smooth and straightforward. The main API changes have taken place on the engine thanks to the new process bus but the rest of the API is pretty much identical. Overall the fantastic work Robert and many contributors have done was to fix remaining and new bugs making this release the most stable and high-performance of the CherryPy releases. Get it while it’s hot.

CherryPy in the field

Michael Schurter just posted a message on the main CherryPy users mailing-list asking developers using CherryPy to let the project team know about it. I want to support the idea as I would love our dusty success stories page being updated with new entries. I have a feeling that CherryPy is used quite a lot but mainly as a light HTTP framework in administration tools context and those projects quite likely are internals and don’t have much visibility outside of their scope. Nonetheless we’d be interested in knowing where CherryPy is used so please let us know.