| A smart implementation of ServerLoadMonitor.
What a browser does:
connected disconnected connected ...
____________ ____________
| | | |
| | | |
_____| |__________| |______
[---cT---] [---dT---] [---cT---] ...
Where cT is the connectedTime and dT is the disconnectedTime.
We impose some limits: a maximum number of symultaneously connected
browsers maxWaitingThreads, and the maximum number of
connections per second maxHitsPerSecond.
We attempt to keep the actual waitingThreads and hitsPerSecond within
bounds by vairying connectedTime and disconnectedTime.
The system is in one of 3 modes: USAGE_LOW, USAGE_HIGH and USAGE_DIGG. The
boundary between USAGE_LOW and USAGE_HIGH is called threadOut. The boundary
between USAGE_HIGH and USAGE_DIGG is called hitOut.
The system starts in USAGE_LOW mode. This mode uses constant values of
connectedTime=60 secs and disconnectedTime=0 secs. We could use much bigger
values for connectedTime (like infinite) however the servlet spec does not
enable servlet engines to inform us if the browser goes away so we check by
asking the browser to reconnect periodically.
In USAGE_LOW mode we measure the number of clients using the number of
concurrently connected browsers (waitingThreads), when this goes above
maxWaitingThreads we move into USAGE_HIGH mode.
On entering USAGE_HIGH mode, the settings (initially) change to
connectedTime=49 secs and disconnectedTime=1 sec. As the load increases the
connectedTime decreases linearly from 49 secs down to prevent the hits per
second from going above maxHitsPerSecond. If the connectedTime goes below
1sec then the mode switches to USAGE_DIGG. If the connectedTime goes above
49 secs then mode switches to USAGE_LOW.
Note: there is some danger of an overlap where the system toggles between
USAGE_HIGH and USAGE_LOW. We need some way to prevent this from happening.
On entering USAGE_DIGG mode, the connectedTime changes to 0 secs, and the
disconnectedTime changes to 2 secs (to keep the round trip time at 2 secs).
The disconnectedTime alters to prevent the hitsPerSecond from going above
maxHitsPerSecond (In USAGE_HIGH mode the connectedTime was altered).
When the disconnectedTime would go under 2 secs, we switch back to USAGE_HIGH
mode.
author:
Joe Walker [joe at getahead dot org] | 