Merge pull request #570 from concourse/scheduler-behaviour

update docs for scheduler

Author: taylorsilva

images/new-pinning-behavior-1.png

@@ -51,46 +51,17 @@ The subcomponent used to figure out whether a build can be scheduled is called t
 
   The scheduler will schedule a new build if any of the versions produced by
   the algorithm for \reference{schema.get.trigger}{\code{trigger:
-  true}} resources is not equal to the version used by the \bold{previous
-  build} of the job.
+  true}} resources has not been used in any previous build of the job.
 
-  What this means is if the algorithm runs and computes an
-  input version, the scheduler will create a new build as long as that version
-  is different than the previous build's version for that same input. Even if
-  that version has been used by a build 2 months ago, the scheduler will
-  schedule a new build as long as it has not been used by the previous build.
+  What this means is if the algorithm runs and computes an input version, the
+  scheduler will create a new build as long as that version has not been used
+  by any previous build's version for that same input. Even if that version has
+  been used by a build 2 months ago, the scheduler will \bold{not} schedule a
+  new build because that version has been previously used in a build of the
+  job.
 
-  If there are any input versions that are different than the previous build,
-  it will trigger a new build. This can affect some users that are using the
-  pinning functionality to run older versions.
-
-  Let's say you have a job that takes a resource as an input. It currently has
-  two builds, build 1 has ran with version \code{v1} of the resource and build
-  2 has ran with \code{v2}.
-
-  \diagram{images/new-pinning-behavior-1.png}{80%}
-
-  If I pin the input to an older version, a new build is produced because the
-  pinned version \code{v1} is not equivalent to the version of the previous
-  build \code{v2}.
-
-  \diagram{images/new-pinning-behavior-3.png}{90%}
-
-  Then if I unpin the version, another build would be triggered again using the
-  latest version.
-
-  \diagram{images/new-pinning-behavior-4.png}{90%}
-
-  This is because after unpinning the resource, the input version for the next
-  build becomes the latest version \code{v2} which is not equal to the version
-  \code{v1} used by the previous build.
-
-  This is to allow the current state of the builds to always reflect the
-  current state of the versions. That being said, this behavior can be
-  undesirable if the goal is to only run a job using an old version. In this
-  case, build rerunning is the better choice. If you would like to learn more
-  about build rerunning, you can read more about it in the
-  \reference{build-rerunning}{build rerunning section}.
+  If there are any input versions that are different from any previous build,
+  it will trigger a new build.
 }
 
 \section{
@@ -109,9 +80,9 @@ The subcomponent used to figure out whether a build can be scheduled is called t
   }{
     A build finishes for an upstream job (through passed constraints)
   }{
-    Enabling/Disabling a resource version
+    Enabling/Disabling a resource version that has not been used in a previous build
   }{
-    Pinning/Unpinning a resource version
+    Pinning/Unpinning a resource version that has not been used in a previous build
   }{
     Setting a pipeline
   }{