Image/Metadata Version. When working with content that changes frequently, servers in caching networks such as Akamai, browser caches, and corporate proxy server caches may store Image Serving responses which may be out of date for periods of time.

id= *val*

val	Version string.

Image Serving includes a versioning mechanism which can help reduce the chance that an outdated cache entry is used by an application. This mechanism involves using req=props to obtain version identifier strings for image data and metadata (such as image map or zoom target data). The version identifier string is then added to cacheable Image Serving requests with the id= command.

When a source image or metadata changes, the corresponding version id value also changes. Including an up-to-date version id value with the id= command ensures that old cache entries can no longer be accessed.

The following table lists the version identifier strings to be used for each req= type:

req= type	version identifier from req=props
img	image.version
map	metadata.version
mask	image.version
targets	metadata.version
tmb	image.version
userdata	metadata.version
xmp	image.version

req= types not listed above either have a short TTL ( attribute::NonImgExpiration) or their responses are not cacheable at all; there is no advantage to including id= with such requests.