ETag vs Header Expires

Question

I ve looked around but haven t been able to figure out if I should use both an ETag and an Expires Header or one or the other   What I m trying to do is make sure that my flash files  and other images and what not only get updated when there is a change to those files   I don t want to do anything special like changing the filename or putting some weird chars on the end of the url to make it not get cached   Also  is there anything I need to do programatically on my end in my PHP scripts to support this or is it all Apache

User · Answer

In my view  With Expire Header  server can tell the client when my data would be stale  while with Etag  server would check the etag value for client  each request

User · Answer

ETag is used to determine whether a resource should use the copy one  and Expires Header like Cache-Control is told the client that before the cache decades  client should fetch the local resource   In modern sites  There are often offer a file named hash  like app 98a3cf23 js  so that it s a good practice to use Expires Header  Besides this  it also reduce the cost of network   Hope it helps

User · Answer

They are slightly different - the ETag does not have any information that the client can use to determine whether or not to make a request for that file again in the future   If ETag is all it has  it will always have to make a request   However  when the server reads the ETag from the client request  the server can then determine whether to send the file  HTTP 200  or tell the client to just use their local copy  HTTP 304    An ETag is basically just a checksum for a file that semantically changes when the content of the file changes   The Expires header is used by the client  and proxies caches  to determine whether or not it even needs to make a request to the server at all   The closer you are to the Expires date  the more likely it is the client  or proxy  will make an HTTP request for that file from the server   So really what you want to do is use BOTH headers - set the Expires header to a reasonable value based on how often the content changes   Then configure ETags to be sent so that when clients DO send a request to the server  it can more easily determine whether or not to send the file back   One last note about ETag - if you are using a load-balanced server setup with multiple machines running Apache you will probably want to turn off ETag generation   This is because inodes are used as part of the ETag hash algorithm which will be different between the servers   You can configure Apache to not use inodes as part of the calculation but then you d want to make sure the timestamps on the files are exactly the same  to ensure the same ETag gets generated for all servers

User · Answer

They are slightly different - the ETag does not have any information that the client can use to determine whether or not to make a request for that file again in the future   If ETag is all it has  it will always have to make a request   However  when the server reads the ETag from the client request  the server can then determine whether to send the file  HTTP 200  or tell the client to just use their local copy  HTTP 304    An ETag is basically just a checksum for a file that semantically changes when the content of the file changes   The Expires header is used by the client  and proxies caches  to determine whether or not it even needs to make a request to the server at all   The closer you are to the Expires date  the more likely it is the client  or proxy  will make an HTTP request for that file from the server   So really what you want to do is use BOTH headers - set the Expires header to a reasonable value based on how often the content changes   Then configure ETags to be sent so that when clients DO send a request to the server  it can more easily determine whether or not to send the file back   One last note about ETag - if you are using a load-balanced server setup with multiple machines running Apache you will probably want to turn off ETag generation   This is because inodes are used as part of the ETag hash algorithm which will be different between the servers   You can configure Apache to not use inodes as part of the calculation but then you d want to make sure the timestamps on the files are exactly the same  to ensure the same ETag gets generated for all servers

User · Answer

Etag and Last-modified headers are validators   They help the browser and or the cache  reverse proxy  to understand if a file page  has changed  even if it preserves the same name   Expires and Cache-control are giving refresh information   This means that they inform  the browser and the reverse in-between proxies  up to what time or for how long  they may keep the page file at their cache   So the question usually is which one validator to use  etag or last-modified  and which refresh infomation header to use  expires or cache-control

User · Answer

ETag is used to determine whether a resource should use the copy one  and Expires Header like Cache-Control is told the client that before the cache decades  client should fetch the local resource   In modern sites  There are often offer a file named hash  like app 98a3cf23 js  so that it s a good practice to use Expires Header  Besides this  it also reduce the cost of network   Hope it helps

User · Answer

One additional thing I would like to mention that some of the answers may have missed is the downside to having both ETags and Expires Cache-control in your headers    Depending on your needs it may just add extra bytes in your headers which may increase packets which means more TCP overhead  Again  you should see if the overhead of having both things in your headers is necessary or will it just add extra weight in your requests which reduces performance   You can read more about it on this excellent blog post by Kyle Simpson  http   calendar perfplanet com 2010 bloated-request-response-headers

User · Answer

Etag and Last-modified headers are validators   They help the browser and or the cache  reverse proxy  to understand if a file page  has changed  even if it preserves the same name   Expires and Cache-control are giving refresh information   This means that they inform  the browser and the reverse in-between proxies  up to what time or for how long  they may keep the page file at their cache   So the question usually is which one validator to use  etag or last-modified  and which refresh infomation header to use  expires or cache-control

User · Answer

Another summary   You need to use both  ETags are a  server side  information  Expires are a  Client side  caching    Use ETags except if you have a load-balanced server  They are safe and will let clients know they should get new versions of your server files every time you change something on your side  Expires must be used with caution  as if you set a expiration date far in the future but want to change one of the files immediatelly  a JS file for instance   some users may not get the modified version until a long time

User · Answer

One additional thing I would like to mention that some of the answers may have missed is the downside to having both ETags and Expires Cache-control in your headers    Depending on your needs it may just add extra bytes in your headers which may increase packets which means more TCP overhead  Again  you should see if the overhead of having both things in your headers is necessary or will it just add extra weight in your requests which reduces performance   You can read more about it on this excellent blog post by Kyle Simpson  http   calendar perfplanet com 2010 bloated-request-response-headers

User · Answer

Expires and Cache-Control are  strong caching headers   Last-Modified and ETag are  weak caching headers   First the browser check Expires Cache-Control to determine whether or not to make a request to the server  If have to make a request  it will send Last-Modified ETag in the HTTP request  If the Etag value of the document matches that  the server will send a 304 code instead of 200  and no content  The browser will load the contents from its cache

User · Answer

Another summary   You need to use both  ETags are a  server side  information  Expires are a  Client side  caching    Use ETags except if you have a load-balanced server  They are safe and will let clients know they should get new versions of your server files every time you change something on your side  Expires must be used with caution  as if you set a expiration date far in the future but want to change one of the files immediatelly  a JS file for instance   some users may not get the modified version until a long time

User · Answer

They are slightly different - the ETag does not have any information that the client can use to determine whether or not to make a request for that file again in the future   If ETag is all it has  it will always have to make a request   However  when the server reads the ETag from the client request  the server can then determine whether to send the file  HTTP 200  or tell the client to just use their local copy  HTTP 304    An ETag is basically just a checksum for a file that semantically changes when the content of the file changes   The Expires header is used by the client  and proxies caches  to determine whether or not it even needs to make a request to the server at all   The closer you are to the Expires date  the more likely it is the client  or proxy  will make an HTTP request for that file from the server   So really what you want to do is use BOTH headers - set the Expires header to a reasonable value based on how often the content changes   Then configure ETags to be sent so that when clients DO send a request to the server  it can more easily determine whether or not to send the file back   One last note about ETag - if you are using a load-balanced server setup with multiple machines running Apache you will probably want to turn off ETag generation   This is because inodes are used as part of the ETag hash algorithm which will be different between the servers   You can configure Apache to not use inodes as part of the calculation but then you d want to make sure the timestamps on the files are exactly the same  to ensure the same ETag gets generated for all servers

User · Answer

Expires and Cache-Control are  strong caching headers   Last-Modified and ETag are  weak caching headers   First the browser check Expires Cache-Control to determine whether or not to make a request to the server  If have to make a request  it will send Last-Modified ETag in the HTTP request  If the Etag value of the document matches that  the server will send a 304 code instead of 200  and no content  The browser will load the contents from its cache

User · Answer

They are slightly different - the ETag does not have any information that the client can use to determine whether or not to make a request for that file again in the future   If ETag is all it has  it will always have to make a request   However  when the server reads the ETag from the client request  the server can then determine whether to send the file  HTTP 200  or tell the client to just use their local copy  HTTP 304    An ETag is basically just a checksum for a file that semantically changes when the content of the file changes   The Expires header is used by the client  and proxies caches  to determine whether or not it even needs to make a request to the server at all   The closer you are to the Expires date  the more likely it is the client  or proxy  will make an HTTP request for that file from the server   So really what you want to do is use BOTH headers - set the Expires header to a reasonable value based on how often the content changes   Then configure ETags to be sent so that when clients DO send a request to the server  it can more easily determine whether or not to send the file back   One last note about ETag - if you are using a load-balanced server setup with multiple machines running Apache you will probably want to turn off ETag generation   This is because inodes are used as part of the ETag hash algorithm which will be different between the servers   You can configure Apache to not use inodes as part of the calculation but then you d want to make sure the timestamps on the files are exactly the same  to ensure the same ETag gets generated for all servers

User · Answer

In my view  With Expire Header  server can tell the client when my data would be stale  while with Etag  server would check the etag value for client  each request

[http] ETag vs Header Expires

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