[AOLSERVER] AOLserver questions

[Sep Ng]

Hi all,

I've been reading up on aolserver background delivery tricks on OpenACS and I've seen that the patches for the static TCL channel is already in 4.5.1.  In the spirit of improving server performance, I've been wondering if such facility is worth building on the custom app to increase concurrency and scalability.

Most of the time, our aolserver also has to handle incoming requests for multiple jpeg, javascript libraries, and a lot of other things.  Freeing up the connection thread sounds very useful in improving the server scalability so I wanted a little bit of help on getting this to work.

It's been hard trying to wrap my head around using ns_conn channel and what I can actually do with this static TCL thread.  It seems that I should be redefining ns_returnfile to use background delivery.  Could I use it to push a TCL proc that generates given the parameters, the dynamic page to this TCL channel to free up my connections?

Sep

[Tony Bennett (Brown Paper Tickets)]

Look at the scheduling commands at http://panoptic.com/wiki/aolserver/Tcl_API.  You could make an image processing queue that runs in it's own thread and it won't take up any connections.

Tony

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[Sep Ng]

Thanks for the reply.  I am perhaps confused with all of this.  It seems that if I use the scheduling proc, I can start a thread that runs perpetually and does nothing.  Then, I can use tclthread API to transfer control into this and issue some proc that would perform mutex and serve the file to the current ns_conn details and quit.  Am I thinking this right or am I being stupid? :-)

[Tony Bennett (Brown Paper Tickets)]

Scheduling isn't needed. I read your question again and I understand what you're looking for.  You're asking for all the javascript and images on a page to be sent in one request correct?  You'll need to find a way to buffer the output and then parse and change the buffer before it's sent.  It would be nice to have this be part of ns_register_filter postauth.

[Sep Ng]

Generally, what I hope to achieve is that all of these static files will be offloaded into a single connection thread(?)  So, when a request for a static file comes in, I can push it to this sleeping thread and then serve another request while this sleeping thread will look up the image and do ns_returnfile (I guess).  At least, this is how I'm envisioning it right now.

I don't know if I'm looking at it right or not.  Judging from my readings on Gustaf's work, this is how it would operate.  Feel free to correct me if I'm looking at this totally wrong.

[Alex Hisen]

Hi, Sep.

 

If you don’t need to run any tcl code when serving these requests and you still want to serve them using AOLserver (as opposed to some other web server or a CDN), you can take advantage of pools to segregate threads for serving these static resources into their own pool. If those threads never allocate a tcl interpreter, they will be an order of magnitude smaller in footprint than your normal threads and so you can just have a lot of them.

 

We have a module that you might find useful that helps with this:

http://aolserver.am.net/code/modules/ampools.adpx

 

-Alex Hisen

[Sep Ng]

Hi Alex!

Unfortunately, we do have some tcl code that goes on before the image file requests.  So, at the moment, I am thinking of getting those tasks offloaded ASAP from the connection threads to serve valuable requests instead of the static files.  Scalability has been a sticking point for years now and I hope to finally get something working to address this.

Regards.

[Gustaf Neumann]

Dear Sep,

The question whether it is worth to use asynchronous delivery boils
down to a question of usage pattern and desired scalability.
The general problem with serving (large) resources via
classical aolserver is that a connection thread is unable
to handle other threads for the time span of the delivery.
It is important to understand that the time span of the delivery is mostly
determined by the client. A client with little processing power connection
over e.g. a mobile phone can block a connection quite a long time. A
special instance of this is the slow-read attack [2], which is
a special denial-of-service attack.

To serve e.g. 60 concurrent files one would require 60
connection threads. Note that this can happen quite soon when
serving content with several included resources (images, css, js)
the first time to a client. When the server runs out of connection
threads, the requests are queued, which means that the
the user-perceived runtime of a request is actually queueing
time plus execution time.

Background delivery (as described in [2]) is fully integrated in OpenACS
addresses the problem by delegating output spooling (file deliveries)
to a single thread, which can deliver easily several 100 concurrent
downloads by using Tcl's asynchronous I/O operations. Note that
this works not only for static resources, but as well dynamic
requests (e.g. generating long HTML pages from e.g. a database).
We used this approach with very good success since 2006
in large OpenACS installations (with e.g. 2000 simultaneous
active users; "simultaneous active" means here users who
requested pages within a time interval of 5 secs).

In OpenACS, one can use simply ad_returnfile_background [3]
instead of ad_returnfile to make use of background delivery.

The limitations of background delivery are that (a) it just works for
plain http, and (b) that it works for at most 1024 concurrently open
file handles. We addressed (a) by using a reverse proxy in front
of the server, which delivers the files from the backend via https.
The limitation (b) is harder, since it depends on Tcl's usage of the
select()
system call, which allows to wait for events for max. 1024 file
descriptors. Above this limit, it simply crashes. Lifting this limit
in systems like Linux is possible, but requires a privately compiled
libc and linux kernel. You might think, 1024 this is much more
one needs, but we were actually running close to this limit for
lecture casting (video streaming of university lectures).

A better approach is to use NaviServer.'s c-level support.
NaviServer provides lightweight c-implemented
writer-threads using asynchronous I/O similar to
bg-delivery, but not using select(). The writer threads
works seemless with http and https. As with bgdelivery, a single
writer thread can serve a multitude of concurrent deliveries.
When several writer threads are defined, the load is split up
between these. NaviServer can also serve streaming
HTML (multiple ns_write commands) via writer threads.
It also support static and dynamic gzip deliveries see e.g. [3]

When one uses OpenACS with NaviServer it will automatically use
writer-threads when configured. In reference [4] on can see the
difference in response time (actually the time duration spent
in connection threads) in NaviServer. OpenACS.org runs
on NaviServer since Sep 2014. A more detailed discussion
of these properties is in [5], all of this is part of NaviServer 4.99.6.

sorry for the longish reply,
-g

[1]
http://openacs.org/xowiki/Boost_your_application_performance_to_serve_large_files!
[2] http://en.wikipedia.org/wiki/Denial-of-service_attack#Slow_Read_attack
[3]
http://openacs.org/api-doc/proc-view?proc=ad_returnfile_background&source_p=1
[3] http://www.qcode.co.uk/post/121
[4] http://openacs.org/forums/message-view?message_id=4111406
[5]
https://next-scripting.org/xowiki/docs/misc/naviserver-connthreadqueue/index1

Am 19.03.15 um 07:09 schrieb Sep Ng:

[Sep Ng]

Hi Gustaf!  Thank you for the informative response!

I've been thinking of moving to NaviServer but I don't know enough about the transition to make that call yet.  Right now, we're on aolserver and so, I'm trying to see what I can do on this platform.  I do not understand why the delivery doesn't work on https out of the box and requires a reverse proxy. 

I suspect the varied client connection is part of the problem and them sitting on the connection threads is hurting us.  However, we do not serve big files on our server so this has me wondering about the benefits of this change.

I'm not certain if aolserver has any facilities for asynchronous file writing and spooling.  It seems that I will have to build everything by hand.  I had hoped that simply transferring the thread and having it ns_returnfile would be enough to get a simple form of background delivery going but it doesn't look like that's the case.

Regards.

[Gustaf Neumann]

Am 20.03.15 um 05:47 schrieb Sep Ng:

Hi Gustaf!  Thank you for the informative response!

I've been thinking of moving to NaviServer but I don't know enough about the transition to make that call yet.  Right now, we're on aolserver and so, I'm trying to see what I can do on this platform.  I do not understand why the delivery doesn't work on https out of the box and requires a reverse proxy.

bgdelivery takes the socket (file descriptor) of the current connection, but it has no knowledge about SSL. When it hands the file descriptor to the background delivery thread, this can write back to the client just using plain tcl i/o. So, background delivery can certainly write to the file-descriptor, but that won't be accepted by the client trying to decrypt the channel.

I suspect the varied client connection is part of the problem and them sitting on the connection threads is hurting us.

what is hurting you?

 However, we do not serve big files on our server so this has me wondering about the benefits of this change.

whatever big means. connections can "hang" also when writing a few KBs.

I'm not certain if aolserver has any facilities for asynchronous file writing and spooling.

the writer threads are an extension of naviserver over aolserver

 It seems that I will have to build everything by hand.  I had hoped that simply transferring the thread and having it ns_returnfile would be enough to get a simple form of background delivery going but it doesn't look like that's the case.

if your site requires https, one cant use bgdelivery without a reverse proxy.
otherwise, everything is pre-packaged.

-g

[Sep Ng]

Thank you once again for your swift response!

On Friday, March 20, 2015 at 2:33:59 PM UTC+8, Gustaf Neumann wrote:

Am 20.03.15 um 05:47 schrieb Sep Ng:

Hi Gustaf!  Thank you for the informative response!

I've been thinking of moving to NaviServer but I don't know enough about the transition to make that call yet.  Right now, we're on aolserver and so, I'm trying to see what I can do on this platform.  I do not understand why the delivery doesn't work on https out of the box and requires a reverse proxy.

bgdelivery takes the socket (file descriptor) of the current connection, but it has no knowledge about SSL. When it hands the file descriptor to the background delivery thread, this can write back to the client just using plain tcl i/o. So, background delivery can certainly write to the file-descriptor, but that won't be accepted by the client trying to decrypt the channel.

 

I think that is now making more sense now.  Thanks.  I may have to look into this  as well.

I suspect the varied client connection is part of the problem and them sitting on the connection threads is hurting us.

what is hurting you?

 

We have instances where we'd get a high number of concurrent users that the requests are getting queued, but when I look at the logs, there's a lot of static files being served for each login page, let alone other pages being served in aolserver.  So, I'm theorizing that being able to get those static file requests pushed into a single thread and free up the connection threads would help in scalability.

 

 However, we do not serve big files on our server so this has me wondering about the benefits of this change.

whatever big means. connections can "hang" also when writing a few KBs.

Interesting. 

I'm not certain if aolserver has any facilities for asynchronous file writing and spooling.

the writer threads are an extension of naviserver over aolserver

 It seems that I will have to build everything by hand.  I had hoped that simply transferring the thread and having it ns_returnfile would be enough to get a simple form of background delivery going but it doesn't look like that's the case.

if your site requires https, one cant use bgdelivery without a reverse proxy.
otherwise, everything is pre-packaged.

Oh.  We don't use OpenACS as everything here is custom built by me and others before me.  So, it's looking like I'm going to have to roll up my sleeves and get to work.

By the way, I've seen in previous posts of yours that the you did switch from aolserver to naviserver.  How big was the change?  What things did you have to re-write/port to get them running in naviserver?

Regards.

[Gustaf Neumann]

> Am 20.03.15 um 07:48 schrieb Sep Ng:

what is hurting you?

 

> We have instances where we'd get a high number of concurrent users that the requests are getting queued, but when I look at the logs, there's a > lot of static files being served for each login page, let alone other pages being served in aolserver.  So, I'm theorizing that being able to get those > static file requests pushed into a single thread and free up the connection threads would help in scalability.

yes, there is a certain hope, that removing this burden from the
connection threads will improve the situation. Another option to reduce
queuing time is to increase the number of connection threads.
If the bottleneck are slow sql-queries then this pooling stuff will not help.

Often the first task to determine, what the bottleneck is, can be already be difficult.
NaviServer has several introspection means for monitoring. The following 
graph shows queuing times, filter and run times (you won't get
these numbers from aolserver). The graph (from OpenACS.org) shows
that queuing time is on that site typically around 0.1 ms, with peaks in
the range of 16 ms. This is for example quite useful for determining the
right number of running connection threads. naviserver allows to
change this number dynamically without restart....

 

> By the way, I've seen in previous posts of yours that the you did switch from aolserver to naviserver.  How big was the change?  What things did > you have to re-write/port to get them running in naviserver?

We did the move of our main site 4 years ago (now we have around 50 naviserver sites),
but i do not have a detailed writeup of the changes. Most of our changes
went into OpenACS (download OpenACS 5.8.1, search for NaviServer).

what comes to my mind is:
- NaviServer dropped the useless "$conn" argument from several commands
  (like old: "ns_return $conn 200 text/plain ..." -> "ns_return 200 text/plain ..."
- different modules (e.g. for ssl), different config file
- more functionality built-in which was as a module under aolserver
  crypo functions (sha, md5), cache, base-64 encoding, gzip delivery
  (actually, the "ns_cache" function in naviserver usues a single
  command style (ns_cache_eval) and in aolserver subcommand style,
  but we added already a compatibility layer to the naviserver source tree
  which is sufficient for OpenACS
- no ns_share (use nsv instead)
- no "ns_set -persistent"

We did not use the latter two, but this comes sometimes up in the mailing lists.
The move was quite easy for us, but ymmv.

-g

[Sep Ng]

Thank you very much for shedding a lot of light into this.

On Friday, March 20, 2015 at 3:58:19 PM UTC+8, Gustaf Neumann wrote:

> Am 20.03.15 um 07:48 schrieb Sep Ng:

what is hurting you?

 

> We have instances where we'd get a high number of concurrent users that the requests are getting queued, but when I look at the logs, there's a > lot of static files being served for each login page, let alone other pages being served in aolserver.  So, I'm theorizing that being able to get those > static file requests pushed into a single thread and free up the connection threads would help in scalability.

yes, there is a certain hope, that removing this burden from the
connection threads will improve the situation. Another option to reduce
queuing time is to increase the number of connection threads.
If the bottleneck are slow sql-queries then this pooling stuff will not help.

Right now, I do not believe sql queries are the culprit for the sacalability issues.  I have a better understanding on this now.  I think the only real issue from implementation stand point is getting the reverse proxy setup right.

 

Often the first task to determine, what the bottleneck is, can be already be difficult.
NaviServer has several introspection means for monitoring. The following 
graph shows queuing times, filter and run times (you won't get
these numbers from aolserver). The graph (from OpenACS.org) shows
that queuing time is on that site typically around 0.1 ms, with peaks in
the range of 16 ms. This is for example quite useful for determining the
right number of running connection threads. naviserver allows to
change this number dynamically without restart....

 

This chart is something that would benefit us very much.  NaviServer is looking like a target I should be working towards in the future.

 

> By the way, I've seen in previous posts of yours that the you did switch from aolserver to naviserver.  How big was the change?  What things did > you have to re-write/port to get them running in naviserver?

We did the move of our main site 4 years ago (now we have around 50 naviserver sites),
but i do not have a detailed writeup of the changes. Most of our changes
went into OpenACS (download OpenACS 5.8.1, search for NaviServer).

what comes to my mind is:
- NaviServer dropped the useless "$conn" argument from several commands
  (like old: "ns_return $conn 200 text/plain ..." -> "ns_return 200 text/plain ..."
- different modules (e.g. for ssl), different config file
- more functionality built-in which was as a module under aolserver
  crypo functions (sha, md5), cache, base-64 encoding, gzip delivery
  (actually, the "ns_cache" function in naviserver usues a single
  command style (ns_cache_eval) and in aolserver subcommand style,
  but we added already a compatibility layer to the naviserver source tree
  which is sufficient for OpenACS
- no ns_share (use nsv instead)
- no "ns_set -persistent"

We did not use the latter two, but this comes sometimes up in the mailing lists.
The move was quite easy for us, but ymmv.

There seems to be much work to be done and this can't be rolled out quickly.  I will have to spend more time on this when the time comes.

 

-g

[Jeff Rogers]

Sep Ng wrote:
> Thank you very much for shedding a lot of light into this.
>
> On Friday, March 20, 2015 at 3:58:19 PM UTC+8, Gustaf Neumann wrote:
>
> > Am 20.03.15 um 07:48 schrieb Sep Ng:
>
> what is hurting you?
>
> > We have instances where we'd get a high number of concurrent
> users that the requests are getting queued, but when I look at the
> logs, there's a > lot of static files being served for each login
> page, let alone other pages being served in aolserver. So, I'm
> theorizing that being able to get those > static file requests
> pushed into a single thread and free up the connection threads would
> help in scalability.
>
> yes, there is a certain hope, that removing this burden from the
> connection threads will improve the situation. Another option to reduce
> queuing time is to increase the number of connection threads.
> If the bottleneck are slow sql-queries then this pooling stuff will
> not help.
>
> Right now, I do not believe sql queries are the culprit for the
> sacalability issues. I have a better understanding on this now. I
> think the only real issue from implementation stand point is getting the
> reverse proxy setup right.

Another thing to try out if you can distinguish static from dynamic by
the url pattern is to use [ns_pools] to set up the server so that all
static content is served from one threadpool and the slow dynamic pages
from a different pool. These would still be ordinary conn threads (no
background delivery), but it would keep the static requests from one
user from queuing behind the dynamic pages from a different user. I
haven't completely thought through exactly how this would work, but my
first impression is that it would mean longer queuing times for dynamic
requests, but more responsive servicing of static ones, so that pages
would be slower to start but faster once they started.

-J

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[Sep Ng]

This is an intriguing solution.  If it were possible to configure ns_pools to do that.  I'm not too familiar with how this can be accomplished however.

Regards.

[Brad Chick]

AOLserver community,

I need to generate and parse json in my aolserver web app. Seems like it
should be easy enough. But there are no aolserver modules that I can find.

I did find: tcljson
@
https://code.google.com/p/aolserver/source/browse/trunk/modules/tcljson/?r=103

But can't seem to get it to build.

What is the easiest/best way to get aolserver to handle json?

Thanks

--
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[Jeff Rogers]

Brad Chick wrote:
> AOLserver community,
>
> I need to generate and parse json in my aolserver web app. Seems like it
> should be easy enough. But there are no aolserver modules that I can find.
>
> I did find: tcljson
> @
> https://code.google.com/p/aolserver/source/browse/trunk/modules/tcljson/?r=103
>
> But can't seem to get it to build.
>
> What is the easiest/best way to get aolserver to handle json?

Hi Brad,

Check out rl_json - https://github.com/RubyLane/rl_json

It's a plain tcl extension, so it's not aolserver (or naviserver)
specific, but it was written to be used in a naviserver application, so
it integrates perfectly well there.

There are some other packages listed on the Tcler's Wiki at
http://wiki.tcl.tk/13419. There is a json package included in tcllib,
but I would recommend avoiding it as its json2dict conversion is lossier
than it needs to be, making round trips essentially impossible.

-J

[Bernhard van Woerden]

Our take on this was to define a list based string literal called tson to avoid the lossy problem.

https://github.com/qcode-software/qcode-tcl/blob/master/doc/procs/tson2json.md

Bernhard