[ovs-dev] [PATCH ovn v4] Document process for compatibility between OVS and OVN.

Mark Michelson mmichels at redhat.com
Tue Oct 8 21:11:42 UTC 2019


This document serves to provide an explanation for how OVN will remain
compatible with OVS. It provides instructions for OVN contributors for
how to maintain compatibility even across older versions of OVS when
possible.

It also creates a document to detail compatibility of specific OVN
versions.

Signed-off-by: Mark Michelson <mmichels at redhat.com>
Acked-by: Han Zhou <hzhou8 at ebay.com>
---
v3 -> v4:
* Addressed grammatical fixes
---
v2 -> v3:
* Removed RFC designation
* Added the compatibility document for specific versions of OVS and OVN
* Clarified compatibility concerns prior to the split
* Mentioned possibility of a change to OVS major version as a reason for
  dropping compatibility.
* Clarified where warning messages should be output in the case that
  feature incompatibility is detected.
* Added mention of "tested" and "untested" versions of OVS for
  compatibility.
 ---

 Documentation/automake.mk                          |   2 +
 Documentation/internals/contributing/index.rst     |   1 +
 .../contributing/ovs-ovn-compatibility.rst         | 153 +++++++++++++++++++++
 Documentation/topics/index.rst                     |   1 +
 Documentation/topics/ovs-compatibility.rst         |  70 ++++++++++
 5 files changed, 227 insertions(+)
 create mode 100644 Documentation/internals/contributing/ovs-ovn-compatibility.rst
 create mode 100644 Documentation/topics/ovs-compatibility.rst

diff --git a/Documentation/automake.mk b/Documentation/automake.mk
index f7e1d2628..bfb0a87bc 100644
--- a/Documentation/automake.mk
+++ b/Documentation/automake.mk
@@ -25,6 +25,7 @@ DOC_SOURCE = \
 	Documentation/topics/high-availability.rst \
 	Documentation/topics/integration.rst \
 	Documentation/topics/ovn-news-2.8.rst \
+	Documentation/topics/ovs-compatibility.rst \
 	Documentation/topics/role-based-access-control.rst \
 	Documentation/howto/index.rst \
 	Documentation/howto/docker.rst \
@@ -56,6 +57,7 @@ DOC_SOURCE = \
 	Documentation/internals/contributing/documentation-style.rst \
 	Documentation/internals/contributing/libopenvswitch-abi.rst \
 	Documentation/internals/contributing/submitting-patches.rst \
+	Documentation/internals/contributing/ovs-ovn-compatibility.rst \
 	Documentation/requirements.txt \
 	$(addprefix Documentation/ref/,$(RST_MANPAGES) $(RST_MANPAGES_NOINST))
 FLAKE8_PYFILES += Documentation/conf.py
diff --git a/Documentation/internals/contributing/index.rst b/Documentation/internals/contributing/index.rst
index a46cb046a..7ef57a1e2 100644
--- a/Documentation/internals/contributing/index.rst
+++ b/Documentation/internals/contributing/index.rst
@@ -36,3 +36,4 @@ The below guides provide information on contributing to Open vSwitch itself.
    coding-style-windows
    documentation-style
    libopenvswitch-abi
+   ovs-ovn-compatibility
diff --git a/Documentation/internals/contributing/ovs-ovn-compatibility.rst b/Documentation/internals/contributing/ovs-ovn-compatibility.rst
new file mode 100644
index 000000000..125640fec
--- /dev/null
+++ b/Documentation/internals/contributing/ovs-ovn-compatibility.rst
@@ -0,0 +1,153 @@
+..
+      Copyright (c) 2019 Red Hat, Inc.
+
+      Licensed under the Apache License, Version 2.0 (the "License"); you may
+      not use this file except in compliance with the License. You may obtain
+      a copy of the License at
+
+          http://www.apache.org/licenses/LICENSE-2.0
+
+      Unless required by applicable law or agreed to in writing, software
+      distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+      WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+      License for the specific language governing permissions and limitations
+      under the License.
+
+      Convention for heading levels in Open vSwitch documentation:
+
+      =======  Heading 0 (reserved for the title in a document)
+      -------  Heading 1
+      ~~~~~~~  Heading 2
+      +++++++  Heading 3
+      '''''''  Heading 4
+
+      Avoid deeper levels because they do not render well.
+
+===============================
+Keeping OVN Compatible with OVS
+===============================
+
+This document discusses the general policy of compatibility between OVS and OVN.
+If you are looking for a listing of version compatibility, please see
+:doc:`topics/ovs-compatibility`
+
+OVN has split from OVS. Prior to this split, there were few issues with
+compatibility. All code changes happened at the same time and in the same repo.
+New releases contained the latest OVS and OVN changes. The most common time
+that compatibility typically came into question was during an upgrade; there
+might be a brief period where mismatched versions of OVS and OVN are running.
+That situation was transient.
+
+With OVN and OVS being developed and versioned separately, previous assumptions
+regarding compatibility are no longer valid. It is valid to permanently run a
+version of OVN that is different from the concurrent version of OVS.
+
+OVS and OVN versions
+--------------------
+
+Prior to the split, the release schedule for OVN was bound to the release
+schedule for OVS. OVS releases a new version approximately every 6 months. OVN
+has not yet determined a release schedule, but it is entirely possible that it
+will be different from OVS. Eventually, this will lead to a situation where it
+is very important that we publish which versions of OVN are compatible with
+which versions of OVS. When incompatibilities are discovered, it is important to
+ensure that these are clearly stated.
+
+The split of OVS and OVN happened in the run-up to the release of OVS 2.12. As a
+result, all versions of OVN *must* be compiled against OVS version 2.12 or
+later. Before going further into compatibility, let's explore the ways that OVN
+and OVS can become incompatible.
+
+Compile-time Incompatibility
+----------------------------
+
+The first way that the projects can become incompatible is if the C code for OVN
+no longer can compile.
+
+The most likely case for this would be that an OVN change requires a parallel
+change to OVS. Those keeping up to date with OVN but not OVS will find that OVN
+will no longer compile since it refers to a nonexistent function or out of date
+function in OVS.
+
+Most OVN users will consume OVN via package from their distribution of choice.
+OVN consumes libopenvswitch statically, so even if the version of OVS installed
+on a user's machine is incompatible at compile time, it will not matter.
+
+OVN developers are the only ones that would be inconvenienced by a compile-time
+incompatibility. OVN developers will be expected to regularly update the version
+of OVS they are using. If an OVN developer notices that OVN is not compiling,
+then they should update their OVS code to the latest and try again.
+
+Developers who are making changes to both OVS and OVN at the same time *must*
+contribute the OVS change first and ensure it is merged upstream before
+submitting the OVN change. This way, OVN should never be in a state where it
+will not compile.
+
+The other aspect to consider is compiling an older release of OVN against a
+newer release of OVS. In practice, this is not typically something that someone
+would want to do. However, if it is desired for some reason, then the ABI and
+API guarantees for OVS's libraries should allow for OVN to compile properly. An
+exception to this may be if OVS changes major versions. When OVS changes major
+versions, they reserve the right to make breaking changes to the API and ABI.
+
+Runtime Incompatibility
+-----------------------
+
+The next way that the projects may become incompatible is at runtime. The most
+common way this would happen is if new OpenFlow capabilities are added to OVS as
+part of an OVN change. In this case, if someone updates OVN but does not also
+update OVS, then OVN will not be able to install the OpenFlow rules it wishes
+to.
+
+Unlike with compile-time incompatibilities, we can't wallpaper over the fact
+that the OVS installation is not up to date. The best we can do is make it very
+clear at runtime that a certain feature is not present, and if the feature is
+desired, OVS must be upgraded.
+
+The following is the process that OVN developers should use when making a
+runtime compatibility change to OVS and OVN.
+
+1. Submit the change to OVS first. See the change through until it is merged.
+2. Make the necessary changes to OVN. 
+
+  a. At startup, probe OVS for the existence of the OpenFlow addition. If it
+     is not present, then output an informational message to the logs that 
+     explains which OVN feature(s) cannot be used.
+  b. If a user attempts to explicitly configure the feature that is not usable
+     due to the incompatibility, then output a warning message to the logs. If
+     the lack of the feature can be detected from a utility (such as ovn-nbctl
+     or ovn-sbctl), then the utility should return an error and print to the
+     console why the feature cannot be used.
+  c. Ensure that the code that installs the OpenFlow will only do so if the new
+     feature is present.
+
+Compatibility Statement
+-----------------------
+
+Given the above, the OVN team will try its hardest to maintain any released
+version of OVN with any released version of OVS after version 2.12. Versions of
+OVS prior to 2.12 are not guaranteed to run properly since OVN does not have
+appropriate OpenFlow feature probes in place.
+
+It may seem prudent to only guarantee compatibility with certain releases of
+OVS (e.g. the current and previous versions of OVS). However, dropping
+compatibility would involve actively removing code that ensures runtime safety.
+It seems unwise to do so.
+
+This, however, is a "best effort" policy. The OVN project reserves the right to
+withdraw compatibility support with a previous OVS version, for reasons such as:
+
+- Security risks.
+- Earthshatteringly large changes in OVS (e.g. no longer using OpenFlow or the
+  OVSDB). This likely would coincide with a change in the OVS major version
+  number.
+- Difficulty in safely maintaining compatibility across versions.
+
+In the event that compatibility for a certain version or versions of OVS is
+dropped, the OVN project will clearly document it.
+
+At some point, the compatibility matrix between OVS and OVN will likely become
+too complex to explicitly test every version of OVN against every version of
+OVS. Therefore, we will maintain a list of "tested" and "untested" compabitility
+options for OVS and OVN. "untested" combinations *should* work, but since they
+have not had explicit testing done, they cannot be guaranteed.
diff --git a/Documentation/topics/index.rst b/Documentation/topics/index.rst
index a34925f2e..d3a4eb70a 100644
--- a/Documentation/topics/index.rst
+++ b/Documentation/topics/index.rst
@@ -41,6 +41,7 @@ OVN
    role-based-access-control
    ovn-news-2.8
    testing
+   ovs-compatibility
 
 .. list-table::
 
diff --git a/Documentation/topics/ovs-compatibility.rst b/Documentation/topics/ovs-compatibility.rst
new file mode 100644
index 000000000..50d84803d
--- /dev/null
+++ b/Documentation/topics/ovs-compatibility.rst
@@ -0,0 +1,70 @@
+..
+      Copyright (c) 2019 Red Hat, Inc.
+
+      Licensed under the Apache License, Version 2.0 (the "License"); you may
+      not use this file except in compliance with the License. You may obtain
+      a copy of the License at
+
+          http://www.apache.org/licenses/LICENSE-2.0
+
+      Unless required by applicable law or agreed to in writing, software
+      distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+      WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+      License for the specific language governing permissions and limitations
+      under the License.
+
+      Convention for heading levels in Open vSwitch documentation:
+
+      =======  Heading 0 (reserved for the title in a document)
+      -------  Heading 1
+      ~~~~~~~  Heading 2
+      +++++++  Heading 3
+      '''''''  Heading 4
+
+      Avoid deeper levels because they do not render well.
+
+=================================
+Compatibility between OVN and OVS
+=================================
+
+This document lists versions of OVN and details their compatibility against
+versions of OVS. This document will be updated as new versions of OVS and OVN
+are released.
+
+For a more detailed look into OVN and OVS compatibility, see
+:doc:`internals/contributing/ovs-ovn-compatiblity`.
+
+The following is a template for how a version might be documented:
+
+::
+
+    OVN version X.Y is 100% feature compatible with OVS versions A.B.C and higher.
+
+    - OVN feature "foo" is unavailable in OVS versions A.(B-2).0 until version
+      A.B.C.
+
+    OVN version X.Y has been explicitly tested against
+
+    - OVS version A.(B-3).0 - A.(B-3).4
+    - OVS version A.(B-2).0 - A.(B-2).2
+    - OVS version A.(B-1).0 - A.(B-1).2
+    - OVS version A.B.0 - A.B.C
+    - OVS master A.B.90
+
+    OVN version X.Y has not been tested against OVS versions prior to A.(B-3).0,
+    but it is expected to be able to run as far back as OVS version A.(B-6).0.
+
+    OVN version X.Y is not runtime compatible with OVS versions prior to
+    A.(B-6).0.
+
+OVN master (2.12.90)
+--------------------
+
+OVN master is 100% feature compatible with OVS versions 2.12.0 and higher.
+
+OVN master (version 2.12.90) has been explicitly tested against
+
+- OVS 2.12.0
+- OVS master (2.12.90)
+
+OVN master is not compatible with versions of OVS prior to 2.12.0.
-- 
2.14.5



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