![]() ![]() This is made possible by the protocol adaptors that the File Fabric provides to enable public and private storage cloud to be accessible over common protocols. If the server provides ETags, then clients must use ETags rather than dates to do cache validation and lost-update protection.CloudDAV is the File Fabric implementation of WebDAV with a unique twist, it works over any Storage Cloud mapped to a File Fabric account whether that Storage Cloud is WebDAV enabled or not. The cases in which WebDAV uses ETags are discussed later. The WebDAV Working Group has repeatedly discussed making ETags required functionality for all WebDAV-compliant servers. ![]() Webdav client cache update#Not only are ETags useful for saving bandwidth, they are quite necessary to prevent all lost update cases. Unless there's a very strong reason not to (I can't think of any), WebDAV servers should support ETags and return the ETag header in response to every GET request. If-None-Match: * is the opposite, meaning the server should apply the operation only if the addressed resource cannot match any token (that means it does not exist). Thus, If-Match: * means that the request can succeed only if the addressed resource matches any token (that means it exists). Both headers can also have the special value *, which matches any token. The other header that uses ETags is If-Match, which has the opposite syntax: If the If-Match header is on a request, the server must only apply the operation if all the clauses in the header are "true" in that they match the resource being addressed.īoth the If-Match and If-None-Match headers can take multiple values separated with commas. Listing 3-15 Request for cached resource. If none of those ETags match the server's instance, the server responds with its instance body (see Listing 3-15). If any of these ETags match the server's instance, the server responds with 412 Precondition Failed. The client puts all possible matching ETags (if multiple instances for the same URL are cached) in the If-None-Match header. Webdav client cache download#The client returns the ETag to the server when it checks whether it needs to download the resource again. ![]() The ETag header simply includes the ETag value for the current instance of the resource. The server provides the ETag to the client with the ETag header on every successful GET response. The alternative is generally to use the last-modified date of a resource to see if it has changed, but this approach is less reliable, and ETags should be used whenever they are available. The primary use of ETags is to avoid downloading a file that hasn't changed since the last time it was downloaded. When the ETag changes, the entity in the cache becomes obsolete and should be discarded or updated. Any Web resource cache should store ETags in order to do cache invalidation. Whenever a cachable entity is downloaded, the server can provide an ETag in the ETag header (the format of the ETag header is simple it contains one entity tag). We'll ignore weak ETags for the rest of this book because WebDAV servers should and do generally implement strong ETags.ĮTags are generated by the server and are opaque to the client. Clients can detect a weak ETag because it must begin with "W/ ". Two entities from the same URL with the same weak ETag should be mostly the same, whereas two entities from the same URL with the same strong ETag must be exactly the same. This is done by having the server provide ETags for all GET response bodies that can be cached, and the client sends the ETag back in requests for the same resource. ![]() It's useful for the client to be able to ask a Web server if an entity that the client previously cached is still up to date. An entity is the information transferred as the payload of a request or response. ETags identify a particular instance of an entity. ![]()
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