balancer_by_lua: cannot return status codes other than 500 about lua-resty-core HOT 10 CLOSED

@rshriram I mean something like

header_filter_by_lua_block {
    local my_status = ngx.ctx.my_status
    if my_status and ngx.status == 500 then
        ngx.status = my_status
    end
}

Assuming you have set ngx.ctx.my_status to 503 in your balancer_by_lua* handlers.

Generated response headers (and also bodies) can be overridden in the output filters, that's what output filters are all about :)

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@rshriram This is the behavior of the nginx upstream facility and we cannot override that in a 3rd-party module.

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@rshriram Having said that, you can override it in header_filter_by_lua*, BTW.

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@agentzh Thanks for the clarification. To summarize, returning anything other than ngx.error, ngx.ok is not possible in a balancer_by_lua context (I went through the code).

the header_filter_by_lua* -- can it work in balancer_by_lua ? I thought the headers are already generated by the time the request reaches balancer by lua stage. And they can't be overridden. If this works, I would love to use it, as I need to set the host header dynamically based on the choice of upstream instance.

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@rshriram You can always use ngx.ctx to communicate your own data between different phase handlers anyway.

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Would you suggest ngx.var over ngx.ctx, as I thought the latter was costlier.

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@rshriram ngx.var is much more expensive and much more limited than ngx.ctx.

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@rshriram ngx.ctx is not that costly if you do not invoke it in a hot Lua loop (and you shouldn't do that for any real reasons). You can always measure the overhead yourself BTW. Just don't be over panic.

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😲 ! I thought it was the other way around.. And ended up using ngx.var as much as possible, while avoiding ngx.ctx .. Based on this https://www.cryptobells.com/openresty-performance-ngx-ctx-vs-ngx-shared-dict/ .. and now I see your response in google groups (https://groups.google.com/forum/#!msg/openresty-en/Yjaiut8Dv0k/Gc1eZWmLEQAJ)

On Sun, May 1, 2016 at 7:12 PM, Rob Kelledy wrote: 
> "The ngx.ctx lookup requires relatively expensive metamethod calls and it is 
> much slower than explicitly passing per-request data along by your own 
> function arguments. So do not abuse this API for saving your own function 
> arguments because it usually has quite some performance impact." 
> 

Don't worry too much about this paragraph. I added this note just for 
people who want to achieve near-C performance with their Lua code :) 

Sigh!

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@rshriram Better profile with flame graph tools before trying any optimizations. Otherwise you may very well slow things even further :)

No kidding. I've seen such things a lot in real production systems.

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