Building on varnish-agent2

The varnish-agent2 existing code base is pretty solid,  rather beautiful I might add. These simple words, keep it simple, has been realised by the following rule of thumb.

– Close to 0 configuration
– “Just works”
– Maintainable
– Generic
– Stateless

For those that are keen to get started on the varnish-agent2 code base, I hope that this document will be of some use. I have used a tiny subset of the Concurrent Object Modeling and architectural design mEThod (COMET), particularly the Requirements Modeling, Analysis Modeling and snippets of the actual Design Model.

Lastly, this document mostly serves as a mental note for myself 🙂

Requirements Modeling

The requirement for the vagent2 is to provide an extendible restful interface for varnish cache. In addition, vagent2 act as the point of integration for varnish cache with other systems – e.g. an administrative or monitoring system.

The use cases are simple right now, and is depicted in the use case diagram below.

vagent2 use cases

vagent2 use cases

Analysis Modeling

vagent2 is designed to support the full spectrum of HTTP method types. A user of the vagent2 will issue these HTTP requests and receive JSON data as response where applicable. Furthermore, the vagent2 is built with modules in mind to address a potentially expanding feature set. Lastly, each modules should be able to communicate and reused by another module.

IPC lies at the heart of the varnish-agent2 code base and message passing is the norm here for implementing an event driven model. Each module follows vagent2’s plugin paradigm, and comes equipped with the module’s own private data set. The plugin therefore follows the IPC set of callback methods, such as ipc_start and ipc_run. These methods are assigned to the appropriate functions within each module.

For the module to be exposed as a restful interface, a simple httpd_register method will hook in the module’s reply method of choice, and expose it appropriately.

For any module, the basic dependencies is depicted below.

Module breakdown

basic module dependencies

Static Modeling

varnish-agent2 ships with a few core modules, such as the logger, shmlog access, httpd and ipc modules. vstatus and vadmin provides access to the shmlog via Varnish API. Note that as of writing this blog, varnish 3.0.3 was used.

These aforementioned modules provide the building blocks for managing varnish cache. For an overview of the static models, see the class diagram below.

Static Model

Static overview of vagent2

Dynamic Modeling

Initialisation
The process of initialising a module is rather straightforward. First, add a new init method for the new module in plugins.h, ensure that you have called the init method in main.c, and of course, allocated some memory for the new plugin in main.c too.

This new module must provide an implementation of the new init method. See diagram below depicting vban’s initialisation process.

vban initialisation process

vban initialisation process

Once initialised, by hooking onto struct agent_plugin* structure, the new module will correspond to the IPC life cycle.

plugin->start  = ipc_start;
plugin->ipc->priv = your_private_plugin_data;
plugin->ipc->cb = your_plugin_callback;

plugin->start is called when your plugin starts. Note that you need to assign a start method if you want the IPC to execute your callback.

plug->ipc->priv refers to a persisted data for your plugin. This can be anything, and as a rule of thumb, this is a good place to hold references to other modules.

plug->ipc-cb refers to the callback method of when ipc_run is issued by another module.

A sample execution path

To tie it all together, the collaboration diagram below illustrate the execution path of issuing a ban to the vagent2. Note that ipc is used to reach the vadmin module.

Issue a ban

Issue a ban

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Building Varnish Cache 3.0.3 from source in Mountain Lion, OSX 10.8.2

If you want to install Varnish Cache in OSX, I highly recommend using Homebrew’s recipe for installing Varnish. Easy!

But if you want to build and run Varnish Cache from github, here is a little step-by-step checklist for you.

A shopping list of applications you’ll need for your Mountain Lion:

  • Homebrew 0.9.3
  • Xcode 4.5.2, via Appstore
  • Command Line Tool for Mountain Lion – November 1st 2012

You’ll need the following dependencies via Homebrew

  • automake, note that this has to be version 1.12. I have tested it with 1.13 and you will run into obsolete AM_CONFIG_HEADER. 
  • libtool
  • pcre
  • pkg-config

You’ll need docutils for rst2man

Run the following steps:

  1. brew install the above dependencies
  2. for automake, you will need to switch it to 1.12. See below
  3. install docutils
  4. git checkout 3.0.3 branch of Varnish Cache
  5. run ./autogen.sh
  6. run ./configure
  7. make install

To install another version of automake with Homebrew:

  1. Unlink the existing 1.13 version – brew unlink automake
  2. Get the list of versions available for automake – brew versions automake
  3. Copy the 1.12 information from git checkout…, for example git checkout 1e5eb62 /usr/local/Library/Formula/automake.rb
  4. cd into /usr/local/, or where ever else you have Homebrew storing your formula
  5. Paste the copied git information and this will check out the appropriate version of automake for you
  6. Install 1.12 – brew install automake
  7. Voila! You now have 1.12 version and you can switch between 1.13 or 1.12 by simply doing brew switch automake 1.13