:title: Project Gating

.. _project_gating:

Project Gating
==============

Traditionally, many software development projects merge changes from
developers into the repository, and then identify regressions
resulting from those changes (perhaps by running a test suite with a
continuous integration system), followed by more patches to fix those
bugs.  When the mainline of development is broken, it can be very
frustrating for developers and can cause lost productivity,
particularly so when the number of contributors or contributions is
large.

The process of gating attempts to prevent changes that introduce
regressions from being merged.  This keeps the mainline of development
open and working for all developers, and only when a change is
confirmed to work without disruption is it merged.

Many projects practice an informal method of gating where developers
with mainline commit access ensure that a test suite runs before
merging a change.  With more developers, more changes, and more
comprehensive test suites, that process does not scale very well, and
is not the best use of a developer's time.  Zuul can help automate
this process, with a particular emphasis on ensuring large numbers of
changes are tested correctly.

Testing in parallel
-------------------

A particular focus of Zuul is ensuring correctly ordered testing of
changes in parallel.  A gating system should always test each change
applied to the tip of the branch exactly as it is going to be merged.
A simple way to do that would be to test one change at a time, and
merge it only if it passes tests.  That works very well, but if
changes take a long time to test, developers may have to wait a long
time for their changes to make it into the repository.  With some
projects, it may take hours to test changes, and it is easy for
developers to create changes at a rate faster than they can be tested
and merged.

Zuul's :value:`dependent pipeline manager<pipeline.manager.dependent>`
allows for parallel execution of test jobs for gating while ensuring
changes are tested correctly, exactly as if they had been tested one
at a time.  It does this by performing speculative execution of test
jobs; it assumes that all jobs will succeed and tests them in parallel
accordingly.  If they do succeed, they can all be merged.  However, if
one fails, then changes that were expecting it to succeed are
re-tested without the failed change.  In the best case, as many
changes as execution contexts are available may be tested in parallel
and merged at once.  In the worst case, changes are tested one at a
time (as each subsequent change fails, changes behind it start again).

For example, if a core developer approves five changes in rapid
succession::

  A, B, C, D, E

Zuul queues those changes in the order they were approved, and notes
that each subsequent change depends on the one ahead of it merging:

.. blockdiag::

  blockdiag foo {
    node_width = 40;
    span_width = 40;
    A <- B <- C <- D <- E;
  }

Zuul then starts immediately testing all of the changes in parallel.
But in the case of changes that depend on others, it instructs the
test system to include the changes ahead of it, with the assumption
they pass.  That means jobs testing change *B* include change *A* as
well::

  Jobs for A: merge change A, then test
  Jobs for B: merge changes A and B, then test
  Jobs for C: merge changes A, B and C, then test
  Jobs for D: merge changes A, B, C and D, then test
  Jobs for E: merge changes A, B, C, D and E, then test

Hence jobs triggered to tests A will only test A and ignore B, C, D:

.. blockdiag::

  blockdiag foo {
    node_width = 40;
    span_width = 40;
    master -> A -> B -> C -> D -> E;
    group jobs_for_A {
        label = "Merged changes for A";
        master -> A;
    }
    group ignored_to_test_A {
        label = "Ignored changes";
        color = "lightgray";
        B -> C -> D -> E;
    }
  }

The jobs for E would include the whole dependency chain: A, B, C, D, and E.
E will be tested assuming A, B, C, and D passed:

.. blockdiag::

  blockdiag foo {
    node_width = 40;
    span_width = 40;
    group jobs_for_E {
        label = "Merged changes for E";
        master -> A -> B -> C -> D -> E;
    }
  }

If changes *A* and *B* pass tests (green), and *C*, *D*, and *E* fail (red):

.. blockdiag::

  blockdiag foo {
    node_width = 40;
    span_width = 40;

    A [color = lightgreen];
    B [color = lightgreen];
    C [color = pink];
    D [color = pink];
    E [color = pink];

    master <- A <- B <- C <- D <- E;
  }

Zuul will merge change *A* followed by change *B*, leaving this queue:

.. blockdiag::

  blockdiag foo {
    node_width = 40;
    span_width = 40;

    C [color = pink];
    D [color = pink];
    E [color = pink];

    C <- D <- E;
  }

Since *D* was dependent on *C*, it is not clear whether *D*'s failure is the
result of a defect in *D* or *C*:

.. blockdiag::

  blockdiag foo {
    node_width = 40;
    span_width = 40;

    C [color = pink];
    D [label = "D\n?"];
    E [label = "E\n?"];

    C <- D <- E;
  }

Since *C* failed, Zuul will report its failure and drop *C* from the queue,
keeping D and E:

.. blockdiag::

  blockdiag foo {
    node_width = 40;
    span_width = 40;

    D [label = "D\n?"];
    E [label = "E\n?"];

    D <- E;
  }

This queue is the same as if two new changes had just arrived, so Zuul
starts the process again testing *D* against the tip of the branch, and
*E* against *D*:

.. blockdiag::

  blockdiag foo {
    node_width = 40;
    span_width = 40;
    master -> D -> E;
    group jobs_for_D {
        label = "Merged changes for D";
        master -> D;
    }
    group ignored_to_test_D {
        label = "Skip";
        color = "lightgray";
        E;
    }
  }

.. blockdiag::

  blockdiag foo {
    node_width = 40;
    span_width = 40;
    group jobs_for_E {
        label = "Merged changes for E";
        master -> D -> E;
    }
  }


Cross Project Testing
---------------------

When your projects are closely coupled together, you want to make sure
changes entering the gate are going to be tested with the version of
other projects currently enqueued in the gate (since they will
eventually be merged and might introduce breaking features).

Such relationships can be defined in Zuul configuration by placing
projects in a shared queue within a dependent pipeline.  Whenever
changes for any project enter a pipeline with such a shared queue,
they are tested together, such that the commits for the changes ahead
in the queue are automatically present in the jobs for the changes
behind them.  See :ref:`project` for more details.

A given dependent pipeline may have as many shared change queues as
necessary, so groups of related projects may share a change queue
without interfering with unrelated projects.
:value:`Independent pipelines <pipeline.manager.independent>` do
not use shared change queues, however, they may still be used to test
changes across projects using cross-project dependencies.

.. _dependencies:

Cross-Project Dependencies
--------------------------

Zuul permits users to specify dependencies across projects.  Using a
special footer in Git commit messages, users may specify that a change
depends on another change in any repository known to Zuul.

Zuul's cross-project dependencies behave like a directed acyclic graph
(DAG), like git itself, to indicate a one-way dependency relationship
between changes in different git repositories.  Change A may depend on
B, but B may not depend on A.

To use them, include ``Depends-On: <change-url>`` in the footer of a
commit message.  For example, a change which depends on a GitHub pull
request (PR #4) might have the following footer::

  Depends-On: https://github.com/example/test/pull/4

And a change which depends on a Gerrit change (change number 3)::

  Depends-On: https://review.example.com/3

Changes may depend on changes in any other project, even projects not
on the same system (i.e., a Gerrit change may depend on a GitHub pull
request).

.. note::

   An older syntax of specifying dependencies using Gerrit change-ids
   is still supported, however it is deprecated and will be removed in
   a future version.

Dependent Pipeline
~~~~~~~~~~~~~~~~~~

When Zuul sees changes with cross-project dependencies, it serializes
them in the usual manner when enqueuing them into a pipeline.  This
means that if change A depends on B, then when they are added to a
dependent pipeline, B will appear first and A will follow:

.. blockdiag::
  :align: center

  blockdiag crd {
    orientation = portrait
    span_width = 30
    class greendot [
        label = "",
        shape = circle,
        color = green,
        width = 20, height = 20
    ]

    A_status [ class = greendot ]
    B_status [ class = greendot ]
    B_status -- A_status

    'Change B\nURL: .../4' <- 'Change A\nDepends-On: .../4'
  }

If tests for B fail, both B and A will be removed from the pipeline, and
it will not be possible for A to merge until B does.


.. note::

   If changes with cross-project dependencies do not share a change
   queue then Zuul is unable to enqueue them together, and the first
   will be required to merge before the second is enqueued.

Independent Pipeline
~~~~~~~~~~~~~~~~~~~~

When changes are enqueued into an independent pipeline, all of the
related dependencies (both normal git-dependencies that come from
parent commits as well as cross-project dependencies) appear in a
dependency graph, as in a dependent pipeline. This means that even in
an independent pipeline, your change will be tested with its
dependencies.  Changes that were previously unable to be fully tested
until a related change landed in a different repository may now be
tested together from the start.

All of the changes are still independent (you will note that the whole
pipeline does not share a graph as in a dependent pipeline), but for
each change tested, all of its dependencies are visually connected to
it, and they are used to construct the git repositories that Zuul uses
when testing.

When looking at this graph on the status page, you will note that the
dependencies show up as grey dots, while the actual change tested shows
up as red or green (depending on the jobs results):

.. blockdiag::
  :align: center

  blockdiag crdgrey {
    orientation = portrait
    span_width = 30
    class dot [
        label = "",
        shape = circle,
        width = 20, height = 20
    ]

    A_status [class = "dot", color = green]
    B_status [class = "dot", color = grey]
    B_status -- A_status

    "Change B\nURL: .../4" <- "Change A\nDepends-On: .../4"
  }

This is to indicate that the grey changes are only there to establish
dependencies.  Even if one of the dependencies is also being tested, it
will show up as a grey dot when used as a dependency, but separately and
additionally will appear as its own red or green dot for its test.


Multiple Changes
~~~~~~~~~~~~~~~~

A change may list more than one dependency by simply adding more
``Depends-On:`` lines to the commit message footer.  It is possible
for a change in project A to depend on a change in project B and a
change in project C.

.. blockdiag::
  :align: center

  blockdiag crdmultichanges {
    orientation = portrait
    span_width = 30
    class greendot [
        label = "",
        shape = circle,
        color = green,
        width = 20, height = 20
    ]

    C_status [ class = "greendot" ]
    B_status [ class = "greendot" ]
    A_status [ class = "greendot" ]
    C_status -- B_status -- A_status

    A [ label = "Repo A\nDepends-On: .../3\nDepends-On: .../4" ]
    group {
        orientation = portrait
        label = "Dependencies"
        color = "lightgray"

        B [ label = "Repo B\nURL: .../3" ]
        C [ label = "Repo C\nURL: .../4" ]
    }
    B, C <- A
  }

Cycles
~~~~~~

If a cycle is created by use of cross-project dependencies, Zuul will
abort its work very early.  There will be no message in Gerrit and no
changes that are part of the cycle will be enqueued into any pipeline.
This is to protect Zuul from infinite loops.