Open Stack

On 02/10/2015 05:26 PM, Boris Pavlovic wrote:
> James,
> 
> Awesome! Amazing! You guys rock!=)

Thanks Boris! Just trying to keep up with all of the awesome developers
we have out there!

> On Wed, Feb 11, 2015 at 1:26 AM, James E. Blair <corvus at inaugust.com> wrote:
> 
>> Hi,
>>
>> We have added support for cross-repo dependencies (CRD) in Zuul.  The
>> important bits:
>>
>> * To use them, include "Depends-On: <gerrit-change-id>" in the footer of
>>   your commit message.  Use the full Change-ID ('I' + 40 characters).
>>
>> * These are one-way dependencies only -- do not create a cycle.
>>
>> * This is what all the grey dots and lines are in the check pipeline.
>>
>> Cross-Repo Dependencies Explained
>> =================================
>>
>> There are two behaviors that might go by the name "cross-repo
>> dependencies".  We call them one-way and multi-way.
>>
>> Multi-way CRD allow for bidirectional links between changes.  For
>> instance, A depends on B and B depends on A.  The theory there is that
>> both would be tested together and merged as a unit.  This is _not_ what
>> we have implemented in Zuul.  Discussions over the past two years have
>> revealed that this type of behavior could cause problems for continuous
>> deploments as it means that two components must be upgraded
>> simultaneously.  Not supporting this behavior is a choice we have made.
>>
>> One-way CRD behaves like a directed acyclic graph (DAG), like git
>> itself, to indicate a one-way dependency relationship between changes in
>> different git repos.  Change A may depend on B, but B may not depend on
>> A.  This is what we have implemented in Zuul.
>>
>> Gate Pipeline
>> =============
>>
>> When Zuul sees CRD changes, 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 the gate pipeline, B will appear first
>> and A will follow.  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 that if changes with CRD do not share a change queue (such as the
>> "integrated gate" then Zuul is unable to enqueue them together, and the
>> first will be required to merge before the second is enqueued.
>>
>> Check Pipeline
>> ==============
>>
>> When changes are enqueued into the check pipeline, all of the related
>> dependencies (both normal git-dependencies that come from parent commits
>> as well as CRD changes) appear in a dependency graph, as in gate.  This
>> means that even in the check pipeline, your change will be tested with
>> its dependency.  So changes that were previously unable to be fully
>> tested until a related change landed in a different repo may now be
>> tested together from the start.
>>
>> All of the changes are still independent (so you will note that the
>> whole pipeline does not share a graph as in gate), but for each change
>> tested, all of its dependencies are visually connected to it, and they
>> are used to construct the git references 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.  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.
>>
>> (If you don't see grey dots on the status page, reload the page to get
>> the latest version.)
>>
>> Multiple Changes
>> ================
>>
>> A Gerrit change ID may refer to multiple changes (on multiple branches
>> of the same project, or even multiple projects).  In these cases, Zuul
>> will treat all of the changes with that change ID as dependencies.  So
>> if you say that a tempest change Depends-On a change ID that has changes
>> in nova master and nova stable/juno, then when testing the tempest
>> change, both nova changes will be applied, and when deciding whether the
>> tempest change can merge, both changes must merge ahead of it.
>>
>> A change may depend on more than one Gerrit change ID as well.  So it is
>> possible for a change in tempest to depend on a change in devstack and a
>> change in nova.  Simply add more "Depends-On:" lines to the footer.
>>
>> Cycles
>> ======
>>
>> If a cycle is created by use of CRD, 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.  I hope that we can improve this to at least
>> leave a message in Gerrit in the future.  But in the meantime, please be
>> cognizant of this and do not create dependency cycles with Depends-On
>> lines.
>>
>> Examples
>> ========
>>
>> The following two infra changes have been tested together because of the
>> Depends-On: line in the commit message of the first:
>>
>>   https://review.openstack.org/#/c/152508/
>>   https://review.openstack.org/#/c/152504/
>>
>> In fact, you can see earlier test results failing until it was rechecked
>> after CRD went into production (around 2015-02-10 15:20 UTC).
>>
>> This devstack change depended on a grenade change:
>>
>>   https://review.openstack.org/#/c/154575/
>>   https://review.openstack.org/#/c/153702/
>>
>> And with CRD, was able to be tested in check together as well as being
>> enqueued into the gate at the same time as its dependency.
>>
>> Note that in this example, the Gerrit change ID that 154575 depends on
>> is actually associated with two changes on two separate branches.  In
>> cases such as this, Zuul treats all instances as dependencies (so both
>> changes would be tested together, and both must merge before 154575
>> may).
>>
>> Further Questions
>> =================
>>
>> This is a fairly substantial new feature, and we may still have a bit to
>> learn about it.
>>
>> If you have any further questions, feel free to ask here or in the
>> #openstack-infra channel on Freenode.
>>
>> We will update the infra manual soon to reflect this change.
>>
>> -Jim
>>
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>>
> 
> 
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