Why still get into srcache_store handler even if $srcache_expire=0?

[wonderbeyond]

I just put my srcache settings directly under server block:

server {
    set $canonical_host www.example.com;

    # ...

    srcache_default_expire 0;
    set_escape_uri $pagecache_key $canonical_host$uri$is_args$args;
    srcache_fetch GET /pagecache key=$pagecache_key;
    srcache_store PUT /pagecache key=$pagecache_key&exptime=$srcache_expire;
    include srcache_status_headers;

    # ...
}

You see, I use srcache_default_expire, So the $srcache_expire will default to 0,
I expect my upstream setting their appropriate exptime by max-age.

However, my srcache_store get involved even if $srcache_expire==0,
and then cause an extra useless redis request.

location = /redis_put {
    # tested OK with SSDB
    internal;
    set_unescape_uri $exptime $arg_exptime;

    redis2_query setx $redis_key $echo_request_body $exptime;

    redis2_pass pagecache_ssdb;
}

[agentzh]

@wonderbeyond I think you should use the srcache_store_skip directive for that. Zero exptime does not affect whether srcache_store is run.

[wonderbeyond]

I known srcache_store_skip, but it's difficult for complex use case. Since it requires a variable, not a logic block(eg: lua code).
If I depend multiple variables to determine whether to skip, map directive is incapable.
Since some vars are only ready after upstream response, set_by_lua is alse hard.

[agentzh]

@wonderbeyond You can always use access_by_lua* or rewrite_by_lua* to evaluate the value of your custom nginx variable for srcache_skip. Don't use map; its lazy evaluation can also lead to surprises.

[agentzh]

@wonderbeyond Given that $srcache_exptime is not mandatory. The user may or may not use it for its srcache subrequests and ngx_srcache indeed has no control over that and it even has no idea about that. For this reason, I don't think it's a good idea for ngx_srcache to always assume $srcache_exptime is actually used and make srcache_store behave completely upon its value. The user may want to forcibly cache things by hard-coding a fixed exptime in its srcache_store request line, for example.

[wonderbeyond]

@agentzh thank you, I've understood that $srcache_expire is only a convenience by srcache which respects upstream's Cache-Control.

I think it's perfect if my cache layer can interact with upstream, so I've made use of $srcache_expire.

You recommend me:

use access_by_lua* or rewrite_by_lua* to evaluate the value of of your custom nginx variable for srcache_skip.

However, since I need to combine factors from upstream response to evaluate the value, access_by_lua* and rewrite_by_lua* won't work. So what's the possible place?

[agentzh]

@wonderbeyond It's possible with header_filter_by_lua* if the info is in the upstream response header. But be careful with output filter ordering (better use OpenResty to be sure).

[wonderbeyond]

@agentzh Thanks, I will give a try.

I will set ngx.var.srcache_nostore=true in header_filter_by_lua_block{}, and expect srcache_store_skip $srcache_nostore works...

PS: I'm using original Nginx plused plugins coming from OpenResty.

[wonderbeyond]

So late I am back here again.

---

Below is my new configuration sample:

server {
    set $canonical_host www.example.com;

    # ...

    set_escape_uri $pagecache_key $canonical_host$uri$is_args$args;
    set $pagecache_exptime '';

    header_filter_by_lua_block {
        # MY DOUBT: this block runs in `output-header-filter` phase.
        # Can I ensure this block always run after `$srcache_expire` is evaluated???
        if not ngx.var.pagecache_exptime or ngx.var.pagecache_exptime == '' then
            ngx.var.pagecache_exptime = ngx.var.srcache_expire
        end
    }

    srcache_fetch GET /pagecache key=$pagecache_key;
    srcache_store PUT /pagecache key=$pagecache_key&exptime=$pagecache_exptime;

    location /foo {
        # I've set $pagecache_exptime, so I **dont respect upstream's Cache-Control**
        set $pagecache_exptime 1y;
    }

    location /bar {
        # I dont set `$pagecache_exptime` and **respect upstream's Cache-Control**
    }

    # ...
}

I leveraged the auto-evaluated $srcache_expire to determine my exptime, But a specific location can force a new exptime by setting $pagecache_exptime.

I compute my exptime in header_filter_by_lua_block, But Can I ensure this block always run after $srcache_expire is evaluated???

PS: I'm not using 100% OpenResty, but a self-built nginx using many components form OpenResty.