Although Nginx supports proxy caching it doesn’t provide support for the HTTP Vary header out of the box. This is a problem if you want to use Nginx to proxy different versions of the same URI which Vary on Content-Language or proxy different representations of a RESTful resource specified via the Accept header.
Fortunately it’s relatively easy to add support for the Vary header using the Nginx Lua module and a small amount of Lua, which is much easier than building and maintaining a 3rd party module and doesn’t greatly impact performance.
First, we define a dictionary in the nginx config which will store a mapping from URIs to Vary headers:
lua_shared_dict uriToVary 10m;
Next we define the default location in the nginx config, which will use the lua ProxyRequest function to make a subrequest to /proxy_request then store the Vary header in the response in the dictionary.
location / {
# make subrequest to /proxy_request, then store response headers
content_by_lua '
local vary = require("vary")
local response = vary.ProxyRequest()
';
}
function ProxyRequest()
-- make subrequest and capture response
local response = ngx.location.capture("/proxy_request", {
method = GetRequestMethod(ngx.var.request_method), body = ngx.req.get_body_data()})
-- forward HTTP headers from response
for k,v in pairs(response.header) do
ngx.header[k] = v
end
ngx.shared.uriToVary:set(ngx.var.request_uri, response.header["Vary"])
-- forward status and body from response
ngx.status = response.status
ngx.print(response.body)
return response
end
Finally, we define the /proxy_request location which will use the lua RewriteCache function to combine the uri with the vary headers to generate the final cache key used by the proxy_cache module.
location /proxy_request {
internal;
# set defaults
set $noCache 1;
set $cacheBypass 1;
set $cacheKey nil;
# rewrite using stored data
rewrite_by_lua '
local vary = require("vary")
vary.RewriteCache()
';
# proxy request
proxy_cache_bypass $cacheBypass;
proxy_no_cache $noCache;
proxy_cache_key $cacheKey;
proxy_cache API_CACHE;
proxy_pass $proxy$request_uri;
}
function RewriteCache()
local varyOn = ngx.shared.uriToVary:get(ngx.var.request_uri)
local cacheKey = nil
-- if vary unknown for this uri, bypass cache and do not cache
if varyOn == nil then
return
end
cacheKey = ngx.var.request_uri .. GenerateCacheKey(varyOn, ngx.req.get_headers())
ngx.var.noCache = 0
ngx.var.cacheKey = cacheKey
ngx.var.cacheBypass = 0
end
function GenerateCacheKey(varyOnStr, requestHeaders)
local cacheKey = ""
for part in string.gmatch(varyOnStr, "([^,%s+]+)") do
if requestHeaders[part] then
cacheKey = cacheKey .. ":" .. requestHeaders[part]
end
end
return cacheKey
end
The first time a URI is requested the cache will be bypassed, but the Vary header from the response will be stored in the shared dictionary. The second time the URI is requested the cache key will be generated from the URI and the appropriate request headers specified in the vary header and the response will be cached. When the URI is subsequently requested with the same set of headers it will be served from the cache.
Note that when the shared dictionary is full it will evict old entries using an LRU scheme. Nginx will generate “ngx_slab_alloc() failed” errors when this occurs, but these can safely be ignored.
Thanks to @jonastryggvi for working with me on the Vary support and @CCPGames for allowing me to blog about it.
