OpenACS

Here is some background information for my experiments with the delivery methods.

For this experiment, I compared 5 different means for this kind of

communications

 - ns_http over HTTP (the standard setup, which is used in OpenACS 5.10)

 - ns_http over HTTPS

 - ns_conn over HTTP using persistent connections

 - ns_conn over HTTPS using persistent connections

 - ns_udp using UDP

I tested the is in 2-node cluster to make measurements simple consisting

of the canonical server and one node listening on the following protocols/ports:

 - http://127.0.0.1:8101

 - https://127.0.0.1:8444

 - udp://127.0.0.1:8101

The first test sends per call 1000 intra-server commands from the canonical server

to the 2nd node over the various delivery methods:

  set times 1000

  lappend _ ns_http-[time {::acs::CS_127.0.0.1_8101 message set x ns_http} $times]

  lappend _ ns_https-[time {::acs::CS_127.0.0.1_8444 message set x ns_https} $times]

  lappend _ ns_connchan-http-[time {::acs::CS_127.0.0.1_8101 message -delivery connchan  set x ns_http} $times]

  lappend _ ns_connchan-https-[time {::acs::CS_127.0.0.1_8444 message -delivery connchan set x ns_https} $times]

  lappend _ ns_udp-[time {::acs::CS_127.0.0.1_8101 message -delivery udp set x udp} $times]

  join $_ \n

This leads to the following results:

  ns_http            564.027083 microseconds per iteration

  ns_https          1483.478916 microseconds per iteration

  ns_connchan-http   147.688541 microseconds per iteration

  ns_connchan-https   68.480875 microseconds per iteration

  ns_udp             198.343416 microseconds per iteration

Since the commands are sent in sequence, the variant with the

persistent HTTP connection is the fastest, although this is Tcl

implemented. The slowest is the version with HTTPS via ns_http without

persistent connections. We see a factor of 20 in terms of performance.

When using ns_udp with the "-noreply" option, we have would have

a "fire and forget" solution, which might be ok when the packet loss

rate is low. That would lead to 54 microseconds.

Clearly, the numbers for persistent connections look the best, but it has

as well some disadvantages compared to other solutions:

- the server has to keep a socket open to every node (but no

 connection thread)

- the keepalive setting of the server must set sufficiently long to

 gain advantage of persistent connections (e.g. 5 sec keepalive,

 heart beat frequency of 1s)

- Since the whole communication goes over a single connection, it is

 necessary to serialize the requests to avoid that multiple

 connection threads write concurrently to the same connection and

 interfere with each other

- It is probably necessary to have a separate thread handling the

 outgoing intra-server talk (implementing cmd queuing,

 async-handling, heart-beat, etc.). Since this has to be a Tcl-thread

 it will use up some memory (similar to a connection thread).

- This intra-server talk thread requires queuing and event handling we

 have so far just in xotcl-core, so when implemented, it will require

 the xotcl-core package (maybe this can be put later to acs-core).

As a second experiment, I've implemented a simple heart-beat service

inside the request monitor that checks the liveliness of the nodes

every second. So, in contrary to the back to back commands of the

first experiment, these are single calls. Here are some random

values for the 5 delivery methods:

[27/Dec/2022:20:29:34.171376][::throttle] Notice: -cluster: http://127.0.0.1:8101 set x ns_http sent total 2.907ms

[27/Dec/2022:20:29:34.182241][::throttle] Notice: -cluster: https://127.0.0.1:8444 set x ns_https sent total 10.798ms

[27/Dec/2022:20:29:34.183475][::throttle] Notice: -cluster: http://127.0.0.1:8101 set x ns_connchan sent total 1.161m

[27/Dec/2022:20:29:34.183657][::throttle] Notice: -cluster: https://127.0.0.1:8444 set x https-connchan sent total 0.086ms

[27/Dec/2022:20:29:34.188564][::throttle] Notice: -cluster: udp://127.0.0.1:8101 set x udp sent total 4.861ms

[27/Dec/2022:20:30:25.494080][::throttle] Notice: -cluster: http://127.0.0.1:8101 set x ns_http sent total 2.049ms

[27/Dec/2022:20:30:25.516306][::throttle] Notice: -cluster: https://127.0.0.1:8444 set x ns_https sent total 21.903ms

[27/Dec/2022:20:30:25.517239][::throttle] Notice: -cluster: http://127.0.0.1:8101 set x ns_connchan sent total 0.814ms

[27/Dec/2022:20:30:25.522957][::throttle] Notice: -cluster: https://127.0.0.1:8444 set x https-connchan sent total 0.33ms

[27/Dec/2022:20:30:25.534274][::throttle] Notice: -cluster: udp://127.0.0.1:8101 set x udp sent total 11.099ms

[27/Dec/2022:20:31:54.993455][::throttle] Notice: -cluster: http://127.0.0.1:8101 set x ns_http sent total 2.431ms

[27/Dec/2022:20:31:55.003036][::throttle] Notice: -cluster: https://127.0.0.1:8444 set x ns_https sent total 9.499ms

[27/Dec/2022:20:31:55.010100][::throttle] Notice: -cluster: http://127.0.0.1:8101 set x ns_connchan sent total 6.981ms

[27/Dec/2022:20:31:55.010585][::throttle] Notice: -cluster: https://127.0.0.1:8444 set x https-connchan sent total 0.322ms

[27/Dec/2022:20:31:55.017764][::throttle] Notice: -cluster: udp://127.0.0.1:8101 set x udp sent total 7.13ms

We see in essence the same pattern. The approach with the persistent

connections looks here the best as well. It is not clear to me, why

HTTPS over connchan is the best, but the communication seems ok. Maybe

some buffering/nagle algorithm is responsible for this. We see as well

that the round-trip takes typically single to double-digit

milliseconds. So when a single HTTP request to nsd triggers multiple

cache-flush operations to multiple nodes, this will take some

time. When e.g., the request issues 5 cash-flush operations, which are

sent to 5 nodes, and every request with take 1ms, the cache flushing

will make the original request about 25ms slower. This might also be

an argument for a separate thread doing these operations

asynchronously.

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