Changes of Image Server from #6 to #7

Changed lines at line 4
- request with arbitrary bounding box
- original picture ratio is maintained
- delivering and caching of pictures in dimensions of a certain raster.
- picture fits into bounding box (no dimension will be larger, but can be smaller)
- picture is just scaled down
- allow browser side caching, even for different bounding boxes (using redirection)
- support extern storage of scaled pictures to allow mirroring. (redirection map)
- support explicit precaching (to be copied to a mirror)
1 Specification
1.1 Behaviour
1.1.1 Request
A picture is requested with an picture name and a bounding box. The bounding box specifies the maximal size of the image. The Image Server has a configured raster in which it provides pictures. Lets assume the raster is set to 50 pixel. The picture size is scaled down to fit in the bounding box and then the width is rounded down multiple of 50. (The height is obviously scaled down as well) The needed version of the picture is looked up in the cache and is generated if needed. The file name contains the dimensions and the request is answered using redirection to that file. That allows browser side caching even for slightly different bounding boxes.
1.1.1 Cache
The cache is an important part of the Image Server since it can reduce the server side load and the latency time. The cache has a space limit and the caching stategy is LRU (least recently used).
1.1.1 Mirrors
A mirror should keep pictures in various sizes to save bandwidth or to gain faster access. It is also handy to be able to store the pictures on cheap webspace without CGI support. The Image Server has a mirror map where every mirror has a list of files it has. Furthermore there could be some information about the bandwith the mirror.   If a file is mirrored on more than one mirror is redirected with a probability ratio that correspond to the bandwith ratio.
1.1 Interface
1.1.1 CGI
The request URL is:
{code:none}
\http://<host>/<path>/imgserv.cgi?src=<image src>&width=<width>&height=<height>
{code}
Where __<image src>__ is a complete URL or relative path identifier of the image to request. If the normal request URL of an image would be "images/1.jpg" than the __<image src>__ is "images/1.jpg" too. __<width>__ and __<height>__ are integers greater zero and denote the dimension of the requested image in pixel. Other units are not planed.
Examples:
{code:none}
\http://<host>/<path>/imgserv.cgi?src=images/1.jpg&width=800&height=600
\http://<host>/<path>/imgserv.cgi?src=http://<otherhost>/album1/slides/image1.jpg&width=800&height=600
{code}
1.1.1 Configuration
TODO: describe the configuration file syntax and options
1.1 Design
1.1.1 Raster
We introduced a raster in which the images are scaled. This prevends the cache of filling up with various fine grained scaled versions of the same image. The raster width is an configuration option which belongs to the caching group.
This option must be fixed over an reasonable lifetime of the server. The client shouldn't get different scaled images for the same requested dimension. In particular, the server is not allowed to serve a "good matching" version of an images which is a cache hit.
1.1 initial version
- request with arbitrary bounding box
- original picture ratio is maintained
- delivering and caching of pictures in dimensions of a certain raster
- picture fits into the requested bounding box and configured raster
- picture is just scaled down, never up
- serving scaled images per HTTP redirect to the location of an static image file
1.1 future versions
- support extern storage of scaled images to allow mirroring. (redirection map)
- support explicit precaching (to be copied to a mirror)
1 Design Decisions
1.1 Raster
We introduced a raster in which the images are scaled. This prevends the cache of filling up with various fine grained scaled versions of the same image. The raster width is a configuration option which belongs to the caching group.
1 Specification
1.1 Behaviour
1.1.1 Request
A picture is requested with an picture name and a bounding box. The bounding box specifies the maximal size of the image. The Image Server has a configured raster in which it provides pictures. Lets assume the raster is set to 50 pixel. The picture size is scaled down to fit in the bounding box and then the width is rounded down multiple of 50. (The height is obviously scaled down as well) The needed version of the picture is looked up in the cache and is generated if needed. The file name contains the dimensions and the request is answered using redirection to that file. That allows browser side caching even for slightly different bounding boxes.
1.1.1 Raster
The raster with must be fixed over an reasonable lifetime of the server. The client shouldn't get different scaled images for the same requested dimension. In particular, the server is not allowed to serve a "good matching" version of an images which is a cache hit.
1.1.1 Cache
The cache is an important part of the Image Server since it can reduce the server side load and the latency time. The cache has a space limit and the caching stategy is LRU (least recently used).
1.1.1 Mirrors
A mirror should keep pictures in various sizes to save bandwidth or to gain faster access. It is also handy to be able to store the pictures on cheap webspace without CGI support. The Image Server has a mirror map where every mirror has a list of files it has. Furthermore there could be some information about the bandwith the mirror.   If a file is mirrored on more than one mirror is redirected with a probability ratio that correspond to the bandwith ratio.
1.1 Interface
1.1.1 CGI
The request URL is:
{code:none}
\http://<host>/<path>/imgserv.cgi?src=<image src>&width=<width>&height=<height>
{code}
Where __<image src>__ is a complete URL or relative path identifier of the image to request. If the normal request URL of an image would be "images/1.jpg" than the __<image src>__ is "images/1.jpg" too. __<width>__ and __<height>__ are integers greater zero and denote the dimension of the requested image in pixel. Other units are not planed.
Examples:
{code:none}
\http://<host>/<path>/imgserv.cgi?src=images/1.jpg&width=800&height=600
\http://<host>/<path>/imgserv.cgi?src=http://<otherhost>/album1/slides/image1.jpg&width=800&height=600
{code}
1.1.1 Configuration
TODO: describe the configuration file syntax and options
1 Implementation
1.1.1 Raster