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Audit Log

A simple log of changes, intended to be easily written and non-intrusive.

How it Works

An audit log is the simplest, yet also one of the most effective forms of tracking temporal information. The idea is that any time something significant happens you write some record indicating what happened and when it happened.

An audit log can take many physical forms. The most common form is a file. However a database table also makes a fine audit log. If you use a file you need a format. An ASCII form helps in making it readable to humans without special software. If it's a simple tabular structure, then tab delimited text is simple and effective. More complex structures can be handled nicely by XML.

Audit Log is easy to write but harder to read, especially as it grows large. Occasional ad hoc reads can be done by eye and simple text processing tools. More complicated or repetitive tasks can be automated with scripts. Many scripting languages are well suited to churning though text files. If you use a database table you can save SQL scripts to get at the information.

When you use Audit Log you should always consider writing out both the actual and record dates. They are easy to produce and even though they may be the same 99% of the time, the 1% can save your bacon. As you do this remember that the record date is always the current processing date.

When to use it

The glory of Audit Log is its simplicity. As you compare Audit Log to other patterns such as Temporal Property and Temporal Object you quickly realize that these alternatives add a lot of complexity to an object model, although these are both often better at hiding that complexity than using Effectivity everywhere.

But it's the difficulty of processing Audit Log that is it's limitation. If you are producing bills every week based on combinations of historic data, then all the code to churn through the logs will be slow and difficult to maintain. So it all depends how tightly the accessing of temporal information is integrated into your regular software process. The tighter the integration, the less useful is Audit Log.

Remember that you can use Audit Log in some parts of the model and other patterns elsewhere. You can also use Audit Log for one dimension of time and a different pattern for another dimension. So you might handle actual time history of a property with Temporal Property and use Audit Log to handle the record history.

Example: (Java)

A simple Audit Log can be very simple indeed.

class Customer...
  private String phone;
  public String getPhone() {
    return (phone == null) ? "none" : phone;}
  public void setPhone(String arg, MfDate changeDate) {
    log (changeDate, this, "change of phone", phone, arg);
    phone = arg;
  }
  public void setPhone(String arg) {
    setPhone(arg, MfDate.today());
  }
  private static void log (MfDate validDate, Customer customer, String description, Object oldValue, Object newValue) {
    try {
      logfile().write(validDate.toString() + customer.name() + "\t" + description + 
        "\t" + oldValue + "\t" + newValue + "\t" + MfDate.today() + "\n");
      logfile().flush();
    } catch (IOException e) {throw new ApplicationException ("Unable to write to log");}
  }

Notice that even though the setting method only uses the actual time, I've also added the record date (MfDate.today to the log. I think it's always wise to add both dates as it's easy to do and if you don't add it you can't reconstitute it later.

I'll leave the script for finding out my phone number on some arbitrary date as an exercise for the reader. (Clearly it's too trivial for me to write out here....)

Significant Revisions

07 Mar 04: Imported into P of EAA tree