Hello!

On Thu, Nov 17, 2016 at 10:49:58AM -0500, permyakovsv wrote:

> I have further configuration:
> fastcgi_cache fastcgi_cache_products;
> fastcgi_cache_valid 200 302 304 404 25m;
> fastcgi_cache_valid 301 5m;
> fastcgi_cache_valid 500 502 20s;
>
> fastcgi_cache_lock on;
> fastcgi_cache_lock_timeout 25s;
> fastcgi_cache_use_stale updating error timeout
> invalid_header http_500 http_503;
>
> And I see netx picture:
> date http_code http_cache_status
> backend_responce
> ...
> 17/Nov/2016:17:19:54 +0200 200 cs=HIT 0
> 17/Nov/2016:17:19:14 +0200 200 cs=EXPIRED 8.041
> 17/Nov/2016:17:18:07 +0200 200 cs=EXPIRED 3.218
> 17/Nov/2016:17:17:24 +0200 200 cs=EXPIRED 3.291
> 17/Nov/2016:17:17:21 +0200 200 cs=UPDATING 0
> 17/Nov/2016:17:16:14 +0200 200 cs=EXPIRED 3.538
> 17/Nov/2016:17:15:53 +0200 200 cs=EXPIRED 2.977
> 17/Nov/2016:17:14:32 +0200 200 cs=EXPIRED 4.119
> 17/Nov/2016:17:13:49 +0200 200 cs=EXPIRED 4.784
> 17/Nov/2016:17:13:04 +0200 200 cs=HIT 0
> ...
> 17/Nov/2016:16:48:44 +0200 200 cs=HIT 0
> 17/Nov/2016:16:48:37 +0200 200 cs=EXPIRED 4.279
> 17/Nov/2016:16:47:57 +0200 200 cs=EXPIRED 3.936
> 17/Nov/2016:16:47:46 +0200 200 cs=EXPIRED 3.447
> 17/Nov/2016:16:46:53 +0200 200 cs=EXPIRED 3.287
> ...
>
> But I expect only one request to backend, and why first request didn't
> update cache?
> Maybe Anybody faced the same problem?

First of all, the fastcgi_cache_lock directive is unrelated to
updating existing cache entries, it only works when loading _new_
items into cache. See http://nginx.org/r/fastcgi_cache_lock for
details.

In case of updating "fastcgi_cache_use_stale updating" is expected
to work, and "UPDATING" in your logs suggests that it does its job
just fine: it prevents multiple parallel requests to upstream
servers. It clearly prevented multiple requests here:

> 17/Nov/2016:17:17:24 +0200 200 cs=EXPIRED 3.291
> 17/Nov/2016:17:17:21 +0200 200 cs=UPDATING 0

the request at 17:17:21 was served from cache (UPDATING status and
zero upstream response time) as the request finished at 17:17:24
was already started at that time.

Next question is: why requests doesn't update the cache. But
"HIT" in logs clearly indicate that cache is updated, and
correctly used.

What probably looks confusing is that expiration times are clearly
much shorter than configured in proxy_cache_valid directives.
This is quite normal though: as long as there are Cache-Control or
Expires directives, nginx will use them for cache expiration times
instead of proxy_cache_valid.

--
Maxim Dounin
http://nginx.org/

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