# Questions about proxy buffers

**URL:** <https://www.ruby-forum.com/t/questions-about-proxy-buffers/208140>\
**Category:** NGINX\
**Created:** [June 11, 2011, 1:52am UTC](https://www.ruby-forum.com/t/questions-about-proxy-buffers/208140 "2011-06-11T01:52:16Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![luislavena](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/0ea827/32.png) [@luislavena](https://www.ruby-forum.com/u/luislavena)\
**Post date:** [June 11, 2011, 1:52am UTC](https://www.ruby-forum.com/t/questions-about-proxy-buffers/208140/1 "2011-06-11T01:52:16Z")

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Hello,

I have read the documentation of the HTTP Proxy Module regarding  
buffers and would like to understand which is the exact (basic)  
workflow regarding buffers only. Let me ask about some scenarios,  
assuming everything fits in memory and ignoring caching workflows etc.

Let’s suppose proxy\_buffer\_size is 4k and proxy\_buffers is 8 4k.

Say the proxied server sends a response of 2k. Then I guess the entire  
response is buffered, as quickly as possible, and when the proxied  
server is done, then nginx starts sending the buffer to the client. Is  
that right?

The description of proxy\_buffer\_size says “This directive set the  
buffer size, into which will be read the first part of the response,  
obtained from the proxied server.” What is the _first_ part of the  
response? Does it mean the chunk that fits in the first buffer? In  
this case exactly 4k? If that’s correct, what does the module do with  
the “subsequent” parts? does proxy\_buffer\_size have anything to do  
with them? Or are those for the remaining 7 buffers, 4k each?

Let’s say now the proxied server sends a response consisting of 16k.  
Does the module fill 4 buffers, as quickly as possible, and when  
everything is done it sends the 16k to the client? Or does it start  
sending the first buffer while filling the second one?

Here “nginx does not attempt to read the entire answer from the  
proxied server, the maximum size of data which nginx can accept from  
the server is set by directive proxy\_buffer\_size.” should  
proxy\_buffer\_size be proxy\_buffers, or rather a combination of the  
two?

What happens if the proxied server sends a response greater than the  
total buffer sizes, say 50k? Does nginx frees buffers sending data,  
and then buffers another chunk from the proxied server, iterate until  
done?

What’s proxy\_busy\_buffers\_size?

Thanks!!!

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**Author:** ![fxn](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/77aa72/32.png) [@fxn](https://www.ruby-forum.com/u/fxn)\
**Post date:** [June 11, 2011, 2:36am UTC](https://www.ruby-forum.com/t/questions-about-proxy-buffers/208140/2 "2011-06-11T02:36:01Z")

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On 6/11/11, Xavier N. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> What happens if the proxied server sends a response greater than the  
> total buffer sizes, say 50k?

It depends whether or not you have proxy\_buffering enabled.  
With proxy\_buffering nginx tries to receive an entire response as  
quickly as possible. Even uses temporary file if response can’t fit  
into proxy\_buffers.

> Does nginx frees buffers sending data,  
> and then buffers another chunk from the proxied server, iterate until  
> done?

And that’s how nginx does it with disabled proxy\_buffering.

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**Author:** ![fxn](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/77aa72/32.png) [@fxn](https://www.ruby-forum.com/u/fxn)\
**Post date:** [June 12, 2011, 8:29pm UTC](https://www.ruby-forum.com/t/questions-about-proxy-buffers/208140/3 "2011-06-12T20:29:56Z")

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On Sat, Jun 11, 2011 at 4:08 AM, Maxim D. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

Hi Maxim, that was an awesome reply, thank you very much.

> > Let’s say now the proxied server sends a response consisting of 16k.  
> > Does the module fill 4 buffers, as quickly as possible, and when  
> > everything is done it sends the 16k to the client? Or does it start  
> > sending the first buffer while filling the second one?
> 
> It will start sending as long as it has nothing to read for a  
> while (i.e. read() from upstream will return EAGAIN).

That’s a rule of thumb even if we are filling the first data buffer?  
(to distinguish from the first buffer whose primary content are  
headers).

I have in mind chunked responses from the proxied server in that  
question. Imagine we are producing a HTML response whose head is kinda  
static content, but whose dynamic body is costly. If the app sends a  
chunk with the head of the document, and after that some amount of  
time passes after the bytes from the second chunk come (because they  
are costly to produce), would in general nginx send that head to the  
client?

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**Author:** ![fxn](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/77aa72/32.png) [@fxn](https://www.ruby-forum.com/u/fxn)\
**Post date:** [June 11, 2011, 4:09am UTC](https://www.ruby-forum.com/t/questions-about-proxy-buffers/208140/4 "2011-06-11T04:09:10Z")

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Hello!

On Sat, Jun 11, 2011 at 01:50:59AM +0200, Xavier N. wrote:

> I have read the documentation of the HTTP Proxy Module regarding  
> buffers and would like to understand which is the exact (basic)  
> workflow regarding buffers only. Let me ask about some scenarios,  
> assuming everything fits in memory and ignoring caching workflows etc.

Just a side note: you may want to read this relatively recent  
thread:

> **[How do proxy\_module response buffering options work?](http://mailman.nginx.org/pipermail/nginx/2011-April/026435.html)**

> Let’s suppose proxy\_buffer\_size is 4k and proxy\_buffers is 8 4k.
> 
> Say the proxied server sends a response of 2k. Then I guess the entire  
> response is buffered, as quickly as possible, and when the proxied  
> server is done, then nginx starts sending the buffer to the client. Is  
> that right?

Basically, yes. “Bascially” here assumes proxy\_buffering on (the  
default), and “response” being interpreted as “response body”, as  
header is processed separately and may be sent without buffering  
(this will be usually prevented by other options like  
postpone\_output though).

> The description of proxy\_buffer\_size says “This directive set the  
> buffer size, into which will be read the first part of the response,  
> obtained from the proxied server.” What is the _first_ part of the  
> response?

This is a buffer where response header will be read and parsed.  
It may also happen to contain some parts of response body as long  
as they were read from kernel along with header.

This buffer is also used in unbuffered mode, but this is  
completely different story.

> Does it mean the chunk that fits in the first buffer? In  
> this case exactly 4k?

No.

> If that’s correct, what does the module do with  
> the “subsequent” parts? does proxy\_buffer\_size have anything to do  
> with them? Or are those for the remaining 7 buffers, 4k each?

Subsequent parts of the response are read to proxy\_buffers.

> Let’s say now the proxied server sends a response consisting of 16k.  
> Does the module fill 4 buffers, as quickly as possible, and when  
> everything is done it sends the 16k to the client? Or does it start  
> sending the first buffer while filling the second one?

It will start sending as long as it has nothing to read for a  
while (i.e. read() from upstream will return EAGAIN).

> Here “nginx does not attempt to read the entire answer from the  
> proxied server, the maximum size of data which nginx can accept from  
> the server is set by directive proxy\_buffer\_size.” should  
> proxy\_buffer\_size be proxy\_buffers, or rather a combination of the  
> two?

There were a stray newline before the sentence in question, I just  
removed it from wiki. It’s about work with buffering switched  
off.

> What happens if the proxied server sends a response greater than the  
> total buffer sizes, say 50k? Does nginx frees buffers sending data,  
> and then buffers another chunk from the proxied server, iterate until  
> done?

Yes. With some nuances though: as long as sending is not possible  
(client is slow) nginx will spool data to disk in an attempt to  
free backend as soon as possible. Disk buffering may be limited  
(or even completely disabled) with proxy\_max\_temp\_file\_size  
directive.

> What’s proxy\_busy\_buffers\_size?

This directive specifies how many data may be in “busy” buffers,  
i.e. already passed to output filter chain but still not sent.  
It limits amount of busy buffers and leaves some buffers available  
for reading upstream response and buffering it to disk.

Maxim D.

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<div class="post-metadata">

**Author:** ![fxn](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/77aa72/32.png) [@fxn](https://www.ruby-forum.com/u/fxn)\
**Post date:** [June 13, 2011, 11:57pm UTC](https://www.ruby-forum.com/t/questions-about-proxy-buffers/208140/5 "2011-06-13T23:57:34Z")

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Hello!

On Sun, Jun 12, 2011 at 08:28:24PM +0200, Xavier N. wrote:

> > while (i.e. read() from upstream will return EAGAIN).
> 
> That’s a rule of thumb even if we are filling the first data buffer?  
> (to distinguish from the first buffer whose primary content are  
> headers).

Buffers which aren’t fully filled aren’t sent with proxy\_buffering  
on.

> I have in mind chunked responses from the proxied server in that  
> question. Imagine we are producing a HTML response whose head is kinda  
> static content, but whose dynamic body is costly. If the app sends a  
> chunk with the head of the document, and after that some amount of  
> time passes after the bytes from the second chunk come (because they  
> are costly to produce), would in general nginx send that head to the  
> client?

No.

Maxim D.