英文:
What's the most efficient ETag generation given the following possibilities?
问题
我最近一直在使用Go来缓存HTTP响应，并且我正在尝试找出生成正确ETag的最有效方法。
目前我有以下可用的数据：
- 当我渲染模板时，模板的名称。
- 传递给模板或JSON响应的动态数据。
- 整个响应体。
- 响应体的长度。
- 我可能遗漏的其他内容？
经过一些思考，我得出结论，如果我将模板的名称和生成的动态数据组合起来，理论上应该可以创建一个合法的唯一ETag，并且开销最小。但是，如果我想返回大约30kb的HTML数据库结果，我不知道这样做会有多复杂。
我正在使用Go的标准库中的crc32例程来从传递给它的数据生成ETag。
是否有更好的方法来生成ETag，甚至缓存动态数据？我不能仅仅监视文件的最后修改时间，因为数据可能在文件未更改的情况下发生变化。
英文:
I've been fooling around with caching http responses lately in Go and I'm trying to figure out the most efficient way possible to generate proper ETags.
Right now I have the following data available:
- The name of a template being rendered for when I ever render templates.
- The dynamic data being passed into a template or response for json responses.
- The entire response body.
- The length of the body.
- Something that I might be missing?
After some thinking I came to the conclusion that if I combine the name of the template and the dynamic data being produced this should in theory create a legit unique ETag with the least amount of overhead but I don't know how nasty this will get if I start wanting to return like 30kb of html worth of database results.
I'm using a crc32 routine from Go's stdlib to generate the ETag from the data I pass into it.
Is there a better way to generate ETags, or even cache dynamic data? I can't just monitor the last-modified time of a file because the the data can change without the file changing.
答案1
得分: 1
一般来说，你希望使用一些计算成本较低的东西作为ETag。这是因为如果客户端发送了一个条件请求（例如通过If-None-Match HTTP请求头），你可以决定是否适合发送一个304 Not Modified响应，而无需对页面进行所有处理。
例如，如果你有某种页面内容的修订标识符，那么它可能是一个很好的ETag。
如果你需要做所有必要的工作来渲染页面，只为了生成一个ETag，那么你可以使用渲染页面内容的哈希值，或者根本不使用ETag。
英文:
In general, you want to use something that is cheap to calculate as an ETag. The reason for this is that if the client sends a conditional request (e.g. via the If-None-Match HTTP request header), you can decide whether it is appropriate to send a 304 Not Modified response without having to do all the processing for the page.
For example, if you have some kind of revision identifier for the content of a page, then that might make a good ETag.
If you will need to do all the work necessary to render the page just to generate an ETag, then you may as well just use a hash of the rendered page content, or no ETag at all.
答案2
得分: 0
如果您正在寻找一种高效的ETag机制，并且不需要强加密的功能，我建议使用CRC-32。为了防止碰撞，您可以结合一些东西，比如模板名称、数据长度和CRC：
func etag(name string, data []byte) string {
crc := crc32.ChecksumIEEE(data)
return fmt.Sprintf(`W/"%s-%d-%08X"`, name, len(data), crc)
}
这个将生成一个类似W/"tpl-17-3074C885"的ETag。
英文:
If you're looking for an efficient ETag mechanism and don't need something cryptographically strong, I would suggest using CRC-32. To help prevent collisions, you could combine a few things like the template name, length of the data, and the crc:
func etag(name string, data []byte) string {
crc := crc32.ChecksumIEEE(data)
return fmt.Sprintf(`W/"%s-%d-%08X"`, name, len(data), crc)
}
This will produce an etag like W/"tpl-17-3074C885".
1: http://en.wikipedia.org/wiki/HTTP_ETag "Wikipedia entry on ETag"
2: http://play.golang.org/p/PyPMccIjYs "Playground link demonstrating etag with crc"
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