Databases: Making a Log of actions, how to handle various references?

Question

hope you all had a happy new year.

So, my question is, what\'s the best way to make a log of actions. Let me explain it with a example, suppose we have these entities:

Answer 1

The following is how I would do it. I have some more comments at the bottom after you have seen the schema.

Log

LogID - unique log ID

Time - date/time of event

LogType - String or ID

(side comment, I would go with an id here so you can use a message table shown below, but if you want quick n dirty you can just just a unique string for each log time (eg "Game Started", "Message Sent", etc)

LogActor

LogID - external key

LogActorType - String or ID (as above, if ID you will need a lookup table)

LogActorID - This is a unique id to the table for the type eg User, Group, Game

Sequence - this is an ordering of the actors.

LogMessage

LogType - exernal key

Message - long string (varchar(max)?)

Language - string(5) so you can key off different language eg "US-en"

Example Data (using your 3 examples)

Log

ID  Time   LogType 
1   1/1/10 1
2   1/1/10 2
3   1/1/10 3

LogActor

LogID LogActorType LogActorID Sequence
1     User         1          1
1     User         2          2
2     User         1          1
2     User         2          2
2     User         2          3
2     Game         1          4
3     User         3          1
3     Group        1          2

LogMessage

LogType Message 
1       {0} Made a new friend {1}
2       {0}, {1}, {2} played a game ({3})
3       {0} joined a group ({1})

User

ID Name
1  User A
2  User B
3  User C

Game

ID Name
1  Name of game

Group

ID Name
1  Name of group

So here are the nice things about this design.

• It is very easy to extend

• It handles multi-language issues independent of the actors

• It is self documenting, the LogMessage table explains exactly what the data you are storing should say.

Some bad things about it.

• You have to do some complicated processing to read the messages.

• You can't just look at the DB and see what has happened.

In my experience the good parts of this kind of a design outweigh the bad bits. What I have done to allow me to do a quick n dirty look at the log is make a view (which I don't use for the application code) that I can look at when I need to see what is going on via the back end.

Let me know if you have questions.

Update - Some example queries

All of my examples are in sqlserver 2005+, let me know if there is a different version you want me to target.

View the LogActor table (There are a number of ways to do this, the best depends on many things including data distribution, use cases, etc) Here are two:

a)

SELECT 
  LogId,
  COLLESCE(U.Name,Ga.Name,Go.Name) AS Name,
  Sequence
FROM LogActor A
LEFT JOIN User U ON A.LogActorID = U.[ID] AND LogActorType = "User"
LEFT JOIN Game Ga ON A.LogActorID = Ga.[ID] AND LogActorType = "Game"
LEFT JOIN Group Go ON A.LogActorID = Go.[ID] AND LogActorType = "Group"
ORDER BY LogID, Sequence

b)

SELECT 
  LogId,
  U.Name AS Name,
  Sequence
FROM LogActor A
INNER JOIN User U ON A.LogActorID = U.[ID] AND LogActorType = "User"
UNION ALL
SELECT 
  LogId,
  Ga.Name AS Name,
  Sequence
FROM LogActor A
INNER JOIN Game Ga ON A.LogActorID = Ga.[ID] AND LogActorType = "Game"
UNION ALL
SELECT 
  LogId,
  Go.Name AS Name,
  Sequence
FROM LogActor A
INNER JOIN Group Go ON A.LogActorID = Go.[ID] AND LogActorType = "Group"
ORDER BY LogID, Sequence

In general I think a) is better than b) For example if you are missing an actor type a) will include it (with a null name). However b) is easier to maintain (because the UNION ALL statements make it more modular.) There are other ways to do this (eg CTE, views, etc). I'm inclined to doing it like b) and from what I've seen that seems to be at least standard practice if not best practice.

So, the last 10 items in the log would looks something like this:

SELECT 
  LogId,
  M.Message,
  COLLESCE(U.Name,Ga.Name,Go.Name) AS Name,
  Time,
  A.Sequence
FROM Log
LEFT JOIN LogActor A ON Log.LogID = A.LogID
LEFT JOIN User U ON A.LogActorID = U.[ID] AND LogActorType = "User"
LEFT JOIN Game Ga ON A.LogActorID = Ga.[ID] AND LogActorType = "Game"
LEFT JOIN Group Go ON A.LogActorID = Go.[ID] AND LogActorType = "Group"
LEFT JOIN LogMessage M ON Log.LogType = M.LogMessage
WHERE LogID IN (SELECT Top 10 LogID FROM Log ORDER BY Date DESC)
ORDER BY Date, LogID, A.Sequence

NB - As you can see, it is easier to select all log items from a date than the last X, because we need a (probably very fast) sub-query for this.