:title: 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 such as Jenkins), 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.

Zuul was designed to handle the workflow of the OpenStack project, but
can be used with any project.

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 DependentPipelineManager 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).  In practice, the OpenStack project observes
something closer to the best case.

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 projects dependencies
---------------------------

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 dependencies can be defined in Zuul configuration by registering a job
in a DependentPipeline of several projects. Whenever a change enters such a
pipeline, it will create references for the other projects as well.  As an
example, given a main project ``acme`` and a plugin ``plugin`` you can
define a job ``acme-tests`` which should be run for both projects:

.. code-block:: yaml

  pipelines:
    - name: gate
      manager: DependentPipelineManager

  projects::
    - name: acme
      gate:
       - acme-tests
    - name: plugin
      gate:
       - acme-tests  # Register job again

Whenever a change enters the ``gate`` pipeline queue, Zuul creates a reference
for it.  For each subsequent change, an additional reference is created for the
changes ahead in the queue.  As a result, you will always be able to fetch the
future state of your project dependencies for each change in the queue.

Based on the pipeline and project definitions above, three changes are
inserted in the ``gate`` pipeline with the associated references:

  ========  ======= ====== =========
  Change    Project Branch Zuul Ref.
  ========  ======= ====== =========
  Change 1  acme    master master/Z1
  Change 2  plugin  stable stable/Z2
  Change 3  plugin  master master/Z3
  ========  ======= ====== =========

Since the changes enter a DependentPipelineManager pipeline, Zuul creates
additional references:

  ====== ======= ========= =============================
  Change Project Zuul Ref. Description
  ====== ======= ========= =============================
  1      acme    master/Z1 acme master + change 1
  ------ ------- --------- -----------------------------
  2      acme    master/Z2 acme master + change 1
  2      plugin  stable/Z2 plugin stable + change 2
  ------ ------- --------- -----------------------------
  3      acme    master/Z3 acme master + change 1
  3      plugin  stable/Z3 plugin stable + change 2
  3      plugin  master/Z3 plugin master + change 3
  ====== ======= ========= =============================

In order to test change 3, you would clone both repositories and simply
fetch the Z3 reference for each combination of project/branch you are
interested in testing. For example, you could fetch ``acme`` with
master/Z3 and ``plugin`` with master/Z3 and thus have ``acme`` with
change 1 applied as the expected state for when Change 3 would merge.
When your job fetches several repositories without changes ahead in the
queue, they may not have a Z reference in which case you can just check
out the branch.