Logging to CouchDB from C

Setting up a centralized form of logging can be a pain. CouchDB can help make it easy.

Prerequisites

First, grab the log4net library. It's common, proven, and makes logging straightforward.

Second, pull down PostLog from github and build

$ git clone https://statianzo@github.com/statianzo/PostLog.git
$ cd PostLog
$ msbuild PostLog.sln

PostLog is an HttpAppender for log4net. It will make an HTTP request for logging events.

Finally, with an instance of CouchDB running, create a database for logging against.

$ curl -X PUT http://localhost:5984/testlog

Use log4net like normal

If your project currently uses, you're in business. If not, take the following example:

using System;
using log4net.Config;
using log4net;

namespace CouchDBAppenderExample
{
  class MainClass
  {
    public static void Main (string[] args)
    {
      XmlConfigurator.Configure();
      ILog logger = LogManager.GetLogger(typeof(MainClass));
      Console.WriteLine ("Press enter to do it");
      Console.ReadLine();
      try {
        throw new InvalidOperationException("Don't do that!");
      }
      catch(InvalidOperationException e) {
        logger.Error("It blew up", e);
      }
      Console.WriteLine("Done");
      Console.ReadLine();
    }
  }
}

A typical log4net usage. An exception occurs and it gets logged.

Configure log4net

PostLog's HttpAppender is configured in the log4net app.config section. Several options are available.

• uri The location to send the HTTP Request
• method The HTTP method to be used (defaults to POST)
• useragent The value of the useragent header
• format The format to serialize the log data. Options are form, json, and xml
• xmlrootname root element tag for xml

A sample configuration is as follows:

<?xml version="1.0"?>
<configuration>
  <configSections>
    <section name="log4net"
             type="log4net.Config.Log4NetConfigurationSectionHandler, log4net" />
  </configSections>

  <log4net>
    <appender name="HttpAppender" type="PostLog.HttpAppender, PostLog">
      <uri value="http://localhost:5984/testlog" />
      <formatterType value="PostLog.JsonBodyFormatter, PostLog" />
      <parameter>
        <name value="date" />
        <layout type="log4net.Layout.RawTimeStampLayout" />
      </parameter>
      <parameter>
        <name value="level" />
        <layout type="log4net.Layout.PatternLayout">
          <conversionPattern value="%level" />
        </layout>
      </parameter>
      <parameter>
        <name value="message" />
        <layout type="log4net.Layout.PatternLayout">
          <conversionPattern value="%message" />
        </layout>
      </parameter>
      <parameter>
        <name value="exception" />
        <layout type="log4net.Layout.ExceptionLayout" />
      </parameter>
    </appender>
    <root>
      <level value="DEBUG" />
      <appender-ref ref="HttpAppender" />
    </root>
  </log4net>
</configuration>

With the new appender in place, your log statements should start hitting CouchDB.

Views

Now that your log statements are in Couch, you probably want to look at them.

Let's create a file named logging.json. The file has a view to see all error log statements.

{
  "_id":"_design/logging",
  "language": "javascript",
  "views":
  {
    "error": {
      "map": "function(doc) { if (doc.level === 'ERROR')  emit(doc.date, doc) }"
    },
  }
}

Send it to Couch with curl:

$ curl -T - http://localhost:5984/testlog/_design/logging < logging.json

Now we can view all error statements by calling

$ curl http://localhost:5984/testlog/_design/logging/_view/error

Example

An example project is available on github at: https://github.com/statianzo/CouchDBAppenderExample