Binary Logging with CL-Log April 6th, 2011
Patrick Stein

One of the things that my current work does better than anywhere I’ve worked before is logging. When something goes wrong, there is a log file that you can dig through to find all kinds of information about what you were doing and how things were going.

As I move forward programming a game with my UNet library, I want to make sure that I can easily log all the network traffic during testing runs at least.

In looking through the various Lisp logging packages out there, I decided on Nick Levine’s cl-log library.

I installed it in no time with quicklisp.

Then, I set to work trying to figure out how I could use it to log binary data.

Here’s what I ended up with. If you want to do something similar, this should give you a good starting point.

Serializing, unserializing, and categorizing

With my USerial library, I defined a serializer to keep track of the different categories of log messages. And, I made corresponding categories in cl-log.

(make-enum-serializer :log-category (:packet :error :warning :info))

(defcategory :packet)
(defcategory :error)
(defcategory :warning (or :error :warning))
(defcategory :info (or :warning :info))

Specializing the classes

There are two major classes that I specialized: base-message and base-messenger. For my toying around, I didn’t end up adding any functionality to the base-message class. I will show it here though so that you know you can do it.

(defclass serialized-message (base-message)

(defclass serialized-messenger (base-messenger)
  ((filename :initarg :filename :reader serialized-messenger-filename)))

Then, I overrode the messenger-send-message generic function to create a binary header with my USerial library and then write the header and the message out.

(defmethod messenger-send-message ((messenger serialized-messenger)
                                   (message serialized-message))
  (let ((header (make-buffer 16)))
    (serialize* (:uint64 (timestamp-universal-time
                              (message-timestamp message))
                 :log-category (message-category message)
                 :uint64 (buffer-length :buffer (message-description message)))
             :buffer header)
    (with-open-file (stream (serialized-messenger-filename messenger)
                            :direction :output
                            :if-does-not-exist :create
                            :if-exists :append
                            :element-type '(unsigned-byte 8))
      (write-sequence header stream)
      (write-sequence (message-description message) stream))))

Using it

To get things going, I then made a log manager that accepts my serialized-message type and started one of my serialized-messenger instances.

(setf (log-manager)
      (make-instance 'log-manager
                     :message-class 'serialized-message))

(start-messenger 'serialized-messenger :name "binary-logger"
                                       :filename "/tmp/binary-log.dat")

Once these were started, I made a little utility function to make it easy for me to make test messages and then invoked log-message a few times.

(defun make-info (string)
  (serialize :string string :buffer (make-buffer)))

(log-message :warning (make-info "Warning"))
(log-message :info (make-info "This is info"))


In all, it has taken me about four times as long to write blog post as it did to install cl-log with quicklisp, peek through the cl-log documentation and source code enough to figure out how to do this, and write all of the code.

To really use this, I will probably separate out the category of a message from the serialized type of the message. This will probably involve adding a field to the serialized-message class to track the message type, adding an initialize-instance :before method for that class to look through the arguments to pull out the type, and then adding the type as an extra argument to log-message.

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