Hello, I have changed the API of fast-logger and changed its version to v0.3. fast-logger v0.3 and related packages are alreay on Hackage. Since wai-extra requires v0.2, I believe this change does not effect to Yesod. I have already sent a pull request of wai-extra to catch up this change. The old fast-logger implements the caching algorithm of formatted date which was discussed this ML. The assumption at that time is: 1) formatting time to date string is a heavy job, and 2) issuing gettimeofday() is a light job. So, old algorithm is as follows: - When a formatted date is required, first issues getttimeofday(). Then compare it with a cached time. - If they are equal, return its cached formatted date. - Otherwise, format the new time to a new formatted date, cache them, and return the new formatted date. To my experience, the assumption 2) is not right. getttimeofday() is implemented in user land in Linux 3 but it is a system call in Linux 2. So, I fixed fast-logger so that it can choose caching strategy. The pull request above does not change behavior. That is, its strategy is the algorithm above. I also implemented a new strategy. Designated Haskell thread issues gettimeofday() every second, formats the resut time to a date, and cache it. I know that people here pointed out that a Haskell thread is not waken up correctly in heavy load situation. But I think that generating a incorrect date in a log file is not a big problem. The package to implement these two algorithm is "date-cache". This package can be used not only for logging but also generating the Date: header in HTTP Response. Mighty now uses the new algorithm of date-chace both for logging and generating Date:. --Kazu