**Exponential Backoff Delay**
[Napier / Dash Automation, LLC]

So I needed an exponential backoff delay for reconnecting to our Websocket Server when our servers go down.  I decided to implement it using a simple coroutine and the "redelay" command (reconnect delay).  Exponential Backoff is used to schedule reconnects to a server so that when your server connection is re-established every instance of your scripts won't reconnect at the same exact time.  If you want to learn more I found http://blog.johnryding.com/post/78544969349/how-to-reconnect-web-sockets-in-a-realtime-web-app%|%How to Reconnect Web Sockets in a Realtime Web App without Flooding the Server%|% to be very helpful.

Basically it will generate a delay in ms for which is a random number between 1 second and the maximum delay.  Each time you call the coroutine it will increase the maximum and return a new delay to use.  You can set a name for the redelay or call it normally.  You can also set the maximum seconds (default is 30) by using the second argument.

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namespace eval redelay { variable count 0 }

proc redelay {  {name {}} {max 30} } {
  if {$name eq {}} { append name ::redelay::task [incr ::redelay::count] }
  return [coroutine $name ::redelay::delay $max]
}

proc ::redelay::delay { max } {
  yield [info coroutine]
  set attempts 1
  while 1 {
    set interval [expr { min($max, (pow(2, $attempts) - 1)) * 1000 }]
    set delay [expr { int(rand()*($interval-1+1)+1) }]
    incr attempts
    if {[yield $delay] ne {}} { return }
  }
}

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Here is an example of it being used:


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set d [redelay]
set delay [$d]
puts $delay
set delay [$d]
puts $delay
set delay [$d]
puts $delay
$d break

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