Record session statistics on pn.state.session_info

[philippjfr]

Stores various information about user sessions on pn.state.session_info. Currently this records the following fields per session:

• started (float): Timestamp when the session was started

• rendered (float): Timestamp when the session was fully rendered

• ended (float): Timestamp when the session was closed on the server

• user_agent (str): A string identifying the user agent

• Add tests

• Add docs

[jbednar]

Cool! Will the session info ever expire? If not, won't storing that data lead to ever-increasing memory usage over time?

[philippjfr]

Cool! Will the session info ever expire? If not, won't storing that data lead to ever-increasing memory usage over time?

This is true, that said it's about 1.6 KB per session so even at 100k sessions that's not a huge amount. What I might suggest is that it keeps only the most recent N sessions where N is configurable.

[philippjfr]

Okay, I've now reduced it to 0.6 kB per session and added the config.session_history option.

[codecov]

Codecov Report

Merging #1615 into master will decrease coverage by 0.14%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #1615      +/-   ##
==========================================
- Coverage   85.40%   85.25%   -0.15%     
==========================================
  Files         147      147              
  Lines       16610    16660      +50     
==========================================
+ Hits        14185    14204      +19     
- Misses       2425     2456      +31     

Impacted Files	Coverage Δ
panel/command/serve.py	18.08% <0.00%> (-1.02%)	⬇️
panel/io/rest.py	26.04% <20.00%> (-0.93%)	⬇️
panel/io/state.py	45.50% <40.00%> (-0.33%)	⬇️
panel/io/server.py	65.02% <45.00%> (-1.80%)	⬇️
panel/viewable.py	70.38% <70.00%> (-0.54%)	⬇️
panel/config.py	46.39% <100.00%> (+0.18%)	⬆️
panel/io/__init__.py	100.00% <100.00%> (ø)
panel/pane/base.py	89.50% <100.00%> (-0.06%)	⬇️
panel/param.py	89.78% <100.00%> (-0.03%)	⬇️
panel/tests/pane/test_markup.py	99.23% <100.00%> (ø)
... and 1 more

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[jbednar]

What I might suggest is that it keeps only the most recent N sessions where N is configurable.

Sounds good. It should probably also keep the number of sessions as a global value, so that we can tell how much data has been lost due to reaching the session_history limit.