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copied!<p>ETag is similar to the Last-Modified header. It's a mechanism to determine change by the client.</p> <p>Arguably, an ETag that JUST HAPPENS to be the Last Modified date (i.e. the same text) meets all the criteria necessary for an ETag. It simply needs to be a unique value representing the state of a resource. Not unique across the entire domain of resources, simply within the resource.</p> <p>Now, technically, an ETag has "infinite" resolution compared to a Last-Modified header. Last-Modified only changes at a granularity of 1 second, whereas an ETag can be sub second.</p> <p>You can implement both ETag and Last-Modified, or simply one or the other (or none, of course). If you Last-Modified is not sufficient, then consider an ETag.</p> <p>Mind, I would not set ETag for "every" resource. Basically, I wouldn't set it for anything that has no expectation of being cached (dynamic content notably). There's no point in that case, just wasted work.</p> <p>Edit: I see your edit, and clarify.</p> <p>MD5 is fine. The only downside is calculating MD5 all the time. Running MD5 on, say, a 200K PDF file, is expensive. Running MD5 on a resource that has no expectation of being cached is simply wasteful (i.e. dynamic content).</p> <p>The trick is simply that whatever mechanism you use, it should be as cheap as Last-Modified typically is. Last-Modified is, again, typically, a property of the resource, and usually very cheap to access.</p> <p>ETags should be similarly cheap. If you are using MD5, and you can cache/store the association between the resource and the MD5 hash, then that's a fine solution. However, recalculating the MD5 each time the ETag is necessary, is basically counter to the idea of using ETags to improve overall server performance.</p>

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