06. Common Response Headers
Common Response Headers
Response Headers
Response Headers
Headers contain additional data about requests or responses. These are some of the important ones:
Content-Length is a header that must be contained in every response and tells the browser the size of the body in the response. This way the browser knows how many bytes it can expect to receive after the header section and can show you a meaningful progress bar when downloading a file.
Content-Type is also a non-optional header and tells you what type the document has. This way the browser knows which parsing engine to spin up. If it's an image/jpeg, show the image. It’s text/html? Let’s parse it and fire off the necessary, additional HTTP requests. And so on.
Last-Modified is a header that contains the date when the document was last changed. It turned out that the Last-Modified date is not very reliable when trying to figure out if a document has been changed. Sometimes developers will uploaded all files to the server after fixing something, resetting the Last-Modified date on all files even though the contents only changed on a subset. To accommodate this, most servers also send out an ETag. ETag stands for entity tag, and is a unique identifier that changes solely depending on the content of the file. Most servers actually use a hash function like SHA256 to calculate the ETag.
Cache-Control is exactly what it sounds like. It allows the server to control how and for how long the client will cache the response it received. Cache-Control is a complex beast and has a lot of built-in features. 99% of the time, you only need the “cacheability“ and the “max-age”.
If-Modified-Since permits the server to skip sending the actual content of the document if it hasn’t been changed since the date provided in that header. Is there something similar for ETags? Yes there is! The header is called If-None-Match and does exactly that. If the ETag for the document is still matching the ETag sent in the If-None-Match header, the server won’t send the actual document. Both If-None-Match and If-Modified-Since can be present in the same request, but the ETag takes precedence over the If-Modified-Since, as it is considered more accurate.
There are a lot more headers and a lot to explore. If you want to know more, check out the following information on HTTP headers: