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doc:plans [2014/01/13 06:52] – created gkazhoyadoc:plans [2015/05/11 17:06] (current) gkazhoya
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 +//**Note:** this page is deprecated. Please use [[tutorials:advanced:plans|this one]] instead.//
 +
 ====== Writing Plans ====== ====== Writing Plans ======
  
 ==== with-policy ==== ==== with-policy ====
 +{{ :doc:with-policy.png?nolink&300|}}
 +
 +Policies can be used to define generic monitoring behavior that should run concurrently besides main functionality code. Such a policy might be monitoring
 +  * the position of an object inside a robot gripper to see whether it is moving out of the gripper (losing the object)
 +  * the position of an object in the real world, in order to keep the robot's head pointing towards it
 +  * the collision of a robot joint with objects in the real world
 +  * generic events that must definitely interrupt a certain piece of code, but are too specialized to be implemented directly in the monitored code itself
 +
 +A policy consists of multiple parts:
 +  * A name (to identify it)
 +  * A description string (to make its purpose clear)
 +  * A parameter list (which can be used to specialize a generic policy for a situation at hand, without generating a new policy every time the situation changes slightly)
 +  * Code blocks that need to be evaluated during monitoring
 +
 +=== Usage example ===
 +An example of how to define a policy is shown here. The policy accepts two custom parameters, one defining a maximum number and one a match number. During execution of the main ''body'' code (which is really just a loop with an output here), it checks a randomly generated number (ranging from ''0'' to ''max-num'') against the match number ''match-num''. If this is the case (and it will be, eventually), it interrupts (and ends) the main ''body'' code and executes the '':recover'' code, followed by the '':clean-up'' code (both just outputting something here).
  
 <code lisp> <code lisp>
 (define-policy my-policy (max-num match-num) (define-policy my-policy (max-num match-num)
 +  "This is an example policy."
 +  (:init (format t "Initializing policy~%")
 +         t)
   (:check (format t "Checking if random number from 0   (:check (format t "Checking if random number from 0
-                      to ~a equals ~a~%" max-num match-num)+                     to ~a equals ~a~%" max-num match-num)
           (let ((rnd (random max-num)))           (let ((rnd (random max-num)))
             (format t "Got number ~a~%" rnd)             (format t "Got number ~a~%" rnd)
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 </code> </code>
  
 +The calling code for using the policy uses ''with-named-policy'' to refer to the name as specified while defining the policy. The second parameter is a list of parameter values for customizing the policy instance. The rest of the code should be pretty self explanatory.
 <code lisp> <code lisp>
 (top-level (top-level
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              (sleep 2))))              (sleep 2))))
 </code> </code>
 +
 +=== Exception handling ===
 +When policies are used, multiple failures can be signalled. The most meaningful of those are
 +  * ''policy-not-found'': Signalled when a named policy is used that was not defined before.
 +  * ''policy-init-failed'': Signalled when initialization of a policy went wrong (i.e. '':init'' returned ''nil'').
 +  * ''policy-check-condition-met'': The '':check'' condition of the policy returned a non-''nil'' value, '':recover'' was executed and the ''body'' code was interrupted before it could complete execution.
 +
 +If one wants to monitor the triggering of a policy's '':check'' condition, this can be achieved like this:
 +<code lisp>
 +(top-level
 +  (with-failure-handling
 +      ((policy-check-condition-met (f)
 +         (declare (ignore f))is available that the given
 +         (do-custom-handling-here)
 +         (retry))) ;; Or whatever seems appropriate in your use-case e.g. (return)
 +    (with-named-policy 'my-policy (10 5)
 +      (loop do (format t "Main loop cycle.~%")
 +               (sleep 2)))))
 +</code>
 +
 +=== Using multiple policies at once ===
 +When multiple policies are to be used (either a mix of different policies, or the same policy multiple times, each with different parameters), two helpful macros can be used: ''with-policies'' and ''with-named-policies''. Both of these take lists of policies, together with their respective instantiation parameters, as arguments.
 +
 +When policy instances by the names ''my-policy-object'' and ''my-other-policy-object'' should be used, the following code snippet reflects this behaviour. The policy ''my-policy-object'' takes two ''int''s as parameters, while ''is available that the givenmy-other-policy-object'' takes a string as an argument.
 +<code lisp>
 +(with-policies
 +    ((my-policy-object (3 1))
 +     (my-policy-object (100 4))
 +     (my-other-policy-object ("Test")))
 +  (body-code))
 +</code>
 +In this example, the resulting code will be equivalent to the following:
 +<code lisp>
 +(with-policy my-policy-object (3 1)
 +  (with-policy my-policy-object (100 4)
 +    (with-policy my-other-policy-object ("Test")
 +      (body-code))))
 +</code>
 +
 +The same princple applies to ''with-named-policies'', with the only difference being that it does not take policy instances, but policy name symbols as parameters:
 +<code lisp>
 +(with-named-policies
 +    (('my-policy (3 1))
 +     ('my-policy (100 4))
 +     ('my-other-policy ("Test")))
 +  (body-code))
 +</code>
 +This results in the same behavior as:
 +<code lisp>
 +(with-named-policy 'my-policy (3 1)
 +  (with-named-policy 'my-policy (100 4)
 +    (with-named-policy 'my-other-policy ("Test")
 +      (body-code))))
 +</code>
 +
 +=== Built-in Policies ===
 +== timeout-policy ==
 +When a piece of code only has a limited maximum amount of time for execution (and must be aborted after that duration), the ''timeout-policy'' comes in handy.
 +
 +Use it like this:
 +<code lisp>
 +(with-policy cpl:timeout-policy (5.0) ; Timeout after 5.0 seconds (fractions may be used)
 +  (body-code-goes-here))
 +</code>
 +And for catching the check condition when the timeout actually happens:
 +<code lisp>
 +(with-failure-handling
 +    ((policy-check-condition-met (f)
 +      (declare (ignore f))
 +      (handle-error-here-and-maybe-retry)))
 +  (with-policy cpl:timeout-policy (5.0)
 +    (body-code-goes-here)))
 +</code>
 +The ''timeout-policy'' stops the given ''body'' code after a given amount of time (in seconds) if it hasn't finished by then. This helps to add a ''timeout'' functionality to functions that do not inherently support a timeout mechanism (blocking function calls).