How to cache req.body and then use it in response for coming requests in varnish

[Yuan Zafir]

hi Guillaume，

Thanks for your reply.

I think there should be ambiguity description about my question in my first mail.
Actually what I want is to use varnish as a cache service *without backend servers*,  clients can push/pull the resource to/from varnish cache storage using specific http method, such as POST/GET.
For example, one client can send " POST /string1 "abcd" " to make a string-"abcd" cached into varnish, other clients could then use "GET /string1" request to get this string. If Get request miss the cache, a 404 error could be sent (through synth command in vcl_miss).

I am not sure whether it's feasible to make the string-"abcd" cached into varnish. Do you have any recommendation for this requrement?
Thanks again for your attention!


Zafir Yuan

________________________________
eyz12315 at live.cn

From: Guillaume Quintard<mailto:guillaume at varnish-software.com>
Date: 2019-08-12 14:03
To: Yuan Zafir<mailto:eyz12315 at live.cn>
CC: varnish-misc<mailto:varnish-misc at varnish-cache.org>
Subject: Re: How to cache req.body and then use it in response for coming requests in varnish
Hi,

There are a few things to do here:
- make sure that if req.method == "POST", you return(hash) from vcl_recv (otherwise, the builtin.vcl (https://github.com/varnish/Varnish-Cache/blob/4.1/bin/varnishd/builtin.vcl#L63) will pass)
- but before that, add 'set req.http.x-method = req.method' at the top of vcl_recv, and 'if (bereq.http.x-method == "POST") {set bereq.method = "POST"; }' at the top of vcl_backend_fetch, because for cacheable request, varnish will want to set the method to "GET", so we need to reset it back

On top of that:
- you can use req.hash_always_miss (man vcl) to push new content (and avoid just getting the cached response)
- make sure you only cache when the response is OK, otherwise a wrong POST request will mask the previous valid ones
- if you are acting upon the URL of the resource itself, I'd say it should be a PUT/PATCH, rather than a POST, but that's just pedantry at this point
- realize that the content cached is the response returned by the backend, not the body of the request itself. They may be the same, but maybe not (there are ways to do it though)

Hope that helps
--
Guillaume Quintard


On Sun, Aug 11, 2019 at 10:42 PM Yuan Zafir <eyz12315 at live.cn<mailto:eyz12315 at live.cn>> wrote:
Hi，I am trying to configure varnish as a simple cache service, which means the content posted by clients could be cached as an object. The content chould be returned in responses for the coming GET requests(with same url...). I am not sure wether this can be realized by VCL configuration.

Any recommendation for this situation? Thanks for any help!

I was noticed vmod-bodyaccess can cache req.body and then it could be used in bereq(https://info.varnish-software.com/blog/caching-post-requests-with-varnish), but I do not know how to put it into responses for clients.

thanks!
Zafir
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eyz12315 at live.cn<mailto:eyz12315 at live.cn>
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