CODEWITHSUNDEEP
nginxetaghttp-caching
junlin
junlin

Reputation: 2045

Why is the Nginx etag created from last-modified-time and content-length?

Nginx etag source

etag->value.len = ngx_sprintf(etag->value.data, "\"%xT-%xO\"",
                              r->headers_out.last_modified_time,
                              r->headers_out.content_length_n)
                  - etag->value.data;

r->headers_out.etag = etag;

If the file last-modified-time in server is changed but the file content has not been updated, does the etag value will be the same?

Why not the etag value generated by content hash?

Upvotes: 5

Views: 3683

Answers (1)

Kevin Christopher Henry
Kevin Christopher Henry

Reputation: 49012

Why not the etag value generated by content hash?

Unless nginx has documented the reason it's hard to say why.

My speculation is that they did it this way because it's very fast and only takes a constant amount of time. Computing a hash can be a costly operation, with the amount of time needed depending on the size of the response. nginx, with a reputation for simplicity and speed, may not have been wiling to add that overhead.

If the file last-modified-time in server is changed but the file content has not been updated, does the etag value will be the same?

No, it will not be the same and therefore the file will have to be re-served. The result is a slower response than you would get with a hash-based ETag, but the response will be correct.

The bigger concern with this algorithm is that the content could change while the ETag stays the same, in which case the response will be incorrect. This could happen if the file changes (in a way that keeps the same length) faster than the one-second precision of the Last-Modified time. (In theory a hash-based approach has the same issue—that is, it's possible for two different files to produce the same hash—but collisions are so unlikely that it's not a concern in practice.)

So presumably nginx weighed this tradeoff—a faster response, but one that has a slight chance of being incorrect—and decided that it was worth it.

Upvotes: 5

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