Re: [ovs-dev] [RFC v2] Document process for compatibility between OVS and OVN.

[Han Zhou]

On Thu, Sep 12, 2019 at 2:00 PM Mark Michelson  wrote:
>
> On 9/12/19 4:51 PM, Han Zhou wrote:
> > Thanks Mark for writing this up.
> >
> > On Fri, Sep 6, 2019 at 2:08 PM Mark Michelson  > > wrote:
> >  >
> >  > 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.
> >  >
> >  > Note that the document currently makes reference to some non-existent
> >  > items. For instance, it refers to an ovs-compat directory in the OVN
> >  > project. Also, it refers to some macros for testing the OVS version.
> >  > These will be added in a future commit if the process documented
here is
> >  > approved.
> >  >
> >  > I'm submitting this as an RFC, since I imagine there are some
details I
> >  > have not thought about, and there will also be some great suggestions
> >  > from others on this matter.
> >  >
> >  > Signed-off-by: Mark Michelson  > >
> >  > ---
> >  >  Documentation/automake.mk 

> > |   1 +
> >  >  Documentation/internals/contributing/index.rst |   1 +
> >  >  .../contributing/ovs-ovn-compatibility.rst | 129
> > +
> >  >  3 files changed, 131 insertions(+)
> >  >  create mode 100644
> > Documentation/internals/contributing/ovs-ovn-compatibility.rst
> >  >
> >  > diff --git a/Documentation/automake.mk 
> > b/Documentation/automake.mk 
> >  > index f7e1d2628..d9bd4be1f 100644
> >  > --- a/Documentation/automake.mk 
> >  > +++ b/Documentation/automake.mk 
> >  > @@ -56,6 +56,7 @@ 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 0..d40b58c32
> >  > --- /dev/null
> >  > +++ b/Documentation/internals/contributing/ovs-ovn-compatibility.rst
> >  > @@ -0,0 +1,129 @@
> >  > +..
> >  > +  Copyright (c) 2019 Nicira, 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
> >  > +===
> >  > +
> >  > +OVN has split from OVS. Prior to this split, there was no issue with
> >  > +compatibility. All code changes happened at the same time and in the
> > same repo.
> >  > +New releases contained the latest OVS and OVN changes. But now these
> > assumptions
> >  > +are no longer valid, and so care must be taken to maintain
> > compatibility.
> >  > +
> >  > +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. 

Re: [ovs-dev] [RFC v2] Document process for compatibility between OVS and OVN.

[Mark Michelson]

On 9/12/19 4:51 PM, Han Zhou wrote:

Thanks Mark for writing this up.

On Fri, Sep 6, 2019 at 2:08 PM Mark Michelson > wrote:

 >
 > 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.
 >
 > Note that the document currently makes reference to some non-existent
 > items. For instance, it refers to an ovs-compat directory in the OVN
 > project. Also, it refers to some macros for testing the OVS version.
 > These will be added in a future commit if the process documented here is
 > approved.
 >
 > I'm submitting this as an RFC, since I imagine there are some details I
 > have not thought about, and there will also be some great suggestions
 > from others on this matter.
 >
 > Signed-off-by: Mark Michelson >

 > ---
 >  Documentation/automake.mk    
    |   1 +

 >  Documentation/internals/contributing/index.rst     |   1 +
 >  .../contributing/ovs-ovn-compatibility.rst         | 129 
+

 >  3 files changed, 131 insertions(+)
 >  create mode 100644 
Documentation/internals/contributing/ovs-ovn-compatibility.rst

 >
 > diff --git a/Documentation/automake.mk  
b/Documentation/automake.mk 

 > index f7e1d2628..d9bd4be1f 100644
 > --- a/Documentation/automake.mk 
 > +++ b/Documentation/automake.mk 
 > @@ -56,6 +56,7 @@ 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 0..d40b58c32
 > --- /dev/null
 > +++ b/Documentation/internals/contributing/ovs-ovn-compatibility.rst
 > @@ -0,0 +1,129 @@
 > +..
 > +      Copyright (c) 2019 Nicira, 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
 > +===
 > +
 > +OVN has split from OVS. Prior to this split, there was no issue with
 > +compatibility. All code changes happened at the same time and in the 
same repo.
 > +New releases contained the latest OVS and OVN changes. But now these 
assumptions
 > +are no longer valid, and so care must be taken to maintain 
compatibility.

 > +
 > +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 

Re: [ovs-dev] [RFC v2] Document process for compatibility between OVS and OVN.

[Ben Pfaff]

On Thu, Sep 12, 2019 at 04:06:14PM -0400, Mark Michelson wrote:
> On 9/12/19 3:25 PM, Ben Pfaff wrote:
> > It's possible that we could introduce features for logical flows that
> > allow different behavior based on whether a given node supports a
> > particular feature.  I've thought about this in the past, but it
> > didn't become necessary yet.
> 
> I'm curious how this would operate. I tend to think of logical flows as
> describing a feature at a high level. It's then up to ovn-controller to
> translate this as appropriate. So I would think the logical flow would not
> be interested in what the node's datapath supports, since the node is free
> to translate the logical flow as it sees fit.
> 
> I guess this would be useful if the presence of a feature on a node might
> affect subsequent logical flows beyond the current one being evaluated,
> though.

The kind of compatibility I have in mind would cover two kinds of
situations:

1. If ovn-northd has to deal with cases where ovn-controller might not
   be up-to-date on some nodes, then it might need to enable some
   features conditionally.  In the upgrade order we originally
   prescribed, this will not happen because ovn-controller is upgraded
   first, but I remember hearing that there is some kind of pressure
   to support upgrading in the opposite order.

2. If ovn-controller has to deal with cases where OVS or its datapath
   might not be up-to-date in some important way, and there's some
   kind of fallback that can be implemented for older versions.

I envision this as being the moral equivalent of #ifdefs for logical
flows.

It's better if it's not needed.
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Re: [ovs-dev] [RFC v2] Document process for compatibility between OVS and OVN.

[Han Zhou]

Thanks Mark for writing this up.

On Fri, Sep 6, 2019 at 2:08 PM Mark Michelson  wrote:
>
> 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.
>
> Note that the document currently makes reference to some non-existent
> items. For instance, it refers to an ovs-compat directory in the OVN
> project. Also, it refers to some macros for testing the OVS version.
> These will be added in a future commit if the process documented here is
> approved.
>
> I'm submitting this as an RFC, since I imagine there are some details I
> have not thought about, and there will also be some great suggestions
> from others on this matter.
>
> Signed-off-by: Mark Michelson 
> ---
>  Documentation/automake.mk  |   1 +
>  Documentation/internals/contributing/index.rst |   1 +
>  .../contributing/ovs-ovn-compatibility.rst | 129
+
>  3 files changed, 131 insertions(+)
>  create mode 100644
Documentation/internals/contributing/ovs-ovn-compatibility.rst
>
> diff --git a/Documentation/automake.mk b/Documentation/automake.mk
> index f7e1d2628..d9bd4be1f 100644
> --- a/Documentation/automake.mk
> +++ b/Documentation/automake.mk
> @@ -56,6 +56,7 @@ 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 0..d40b58c32
> --- /dev/null
> +++ b/Documentation/internals/contributing/ovs-ovn-compatibility.rst
> @@ -0,0 +1,129 @@
> +..
> +  Copyright (c) 2019 Nicira, 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
> +===
> +
> +OVN has split from OVS. Prior to this split, there was no issue with
> +compatibility. All code changes happened at the same time and in the
same repo.
> +New releases contained the latest OVS and OVN changes. But now these
assumptions
> +are no longer valid, and so care must be taken to maintain compatibility.
> +
> +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 

Re: [ovs-dev] [RFC v2] Document process for compatibility between OVS and OVN.

[Mark Michelson]

On 9/12/19 3:25 PM, Ben Pfaff wrote:

On Fri, Sep 06, 2019 at 05:08:17PM -0400, Mark Michelson wrote:

+===
+Keeping OVN Compatible with OVS
+===
+
+OVN has split from OVS. Prior to this split, there was no issue with
+compatibility. All code changes happened at the same time and in the same repo.
+New releases contained the latest OVS and OVN changes. But now these 
assumptions
+are no longer valid, and so care must be taken to maintain compatibility.


"no issue with compatibility" significantly overstates the case, since
a distributed system has to be upgraded in some order (and different
parties actually wanted different orders!).  I agree with the larger
point that the split increases the difficulty.


+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.
+
+When compiling older releases of OVN, it should be able to compile against 
newer
+versions of OVS due to API and ABI guarantees in OVS's libaries.


The above section doesn't observe the difference between minor and
major newer versions.  I believe that our current ABI/API guarantee is
for different minro versions within a major release.  It is not
necessarily the case that a newer major release has compatible ABI/API
with an older one.  (I think that we use library naming to make this
clear.)


+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
+updage 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 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.
+  c. Ensure that the code that installs the OpenFlow will only do so if the new
+ feature is present.


It's possible that we could introduce features for logical flows that
allow different behavior based on whether a given node supports a
particular feature.  I've thought about this in the past, but it
didn't become necessary yet.



I'm curious how this would operate. I tend to think of logical flows as 
describing a feature at a high level. It's then up to ovn-controller to 
translate this as appropriate. So I would think the logical flow would 
not be interested in what the node's datapath supports, since the node 
is free to translate the logical flow as it sees fit.


I guess this would be useful if the presence of a feature on a node 
might affect subsequent logical flows beyond the current one being 
evaluated, though.

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Re: [ovs-dev] [RFC v2] Document process for compatibility between OVS and OVN.

[Ben Pfaff]

On Fri, Sep 06, 2019 at 05:08:17PM -0400, Mark Michelson wrote:
> +===
> +Keeping OVN Compatible with OVS
> +===
> +
> +OVN has split from OVS. Prior to this split, there was no issue with
> +compatibility. All code changes happened at the same time and in the same 
> repo.
> +New releases contained the latest OVS and OVN changes. But now these 
> assumptions
> +are no longer valid, and so care must be taken to maintain compatibility.

"no issue with compatibility" significantly overstates the case, since
a distributed system has to be upgraded in some order (and different
parties actually wanted different orders!).  I agree with the larger
point that the split increases the difficulty.

> +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.
> +
> +When compiling older releases of OVN, it should be able to compile against 
> newer
> +versions of OVS due to API and ABI guarantees in OVS's libaries.

The above section doesn't observe the difference between minor and
major newer versions.  I believe that our current ABI/API guarantee is
for different minro versions within a major release.  It is not
necessarily the case that a newer major release has compatible ABI/API
with an older one.  (I think that we use library naming to make this
clear.)

> +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
> +updage 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 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.
> +  c. Ensure that the code that installs the OpenFlow will only do so if the 
> new
> + feature is present.

It's possible that we could introduce features for logical flows that
allow different behavior based on whether a given node supports a
particular feature.  I've thought about this in the past, but it
didn't become necessary yet.
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Re: [ovs-dev] [RFC v2] Document process for compatibility between OVS and OVN.

[Russell Bryant]

On Fri, Sep 6, 2019 at 5:08 PM Mark Michelson  wrote:
>
> 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.
>
> Note that the document currently makes reference to some non-existent
> items. For instance, it refers to an ovs-compat directory in the OVN
> project. Also, it refers to some macros for testing the OVS version.
> These will be added in a future commit if the process documented here is
> approved.
>
> I'm submitting this as an RFC, since I imagine there are some details I
> have not thought about, and there will also be some great suggestions
> from others on this matter.
>
> Signed-off-by: Mark Michelson 
> ---
>  Documentation/automake.mk  |   1 +
>  Documentation/internals/contributing/index.rst |   1 +
>  .../contributing/ovs-ovn-compatibility.rst | 129 
> +
>  3 files changed, 131 insertions(+)
>  create mode 100644 
> Documentation/internals/contributing/ovs-ovn-compatibility.rst
>
> diff --git a/Documentation/automake.mk b/Documentation/automake.mk
> index f7e1d2628..d9bd4be1f 100644
> --- a/Documentation/automake.mk
> +++ b/Documentation/automake.mk
> @@ -56,6 +56,7 @@ 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 0..d40b58c32
> --- /dev/null
> +++ b/Documentation/internals/contributing/ovs-ovn-compatibility.rst
> @@ -0,0 +1,129 @@
> +..
> +  Copyright (c) 2019 Nicira, Inc.

Copy/paste oversight?  I don't think a Copyright notice is necessary,
but if you leave it, s/Nicira/Red Hat/

> +
> +  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
> +===
> +
> +OVN has split from OVS. Prior to this split, there was no issue with
> +compatibility. All code changes happened at the same time and in the same 
> repo.

I think some compatibility issues were always there since it was still
possible to run separate versions of OVN vs OVS, or even separate
versions within the same deployment during upgrades.  I'd say it's now
important to be even more explicit about compatibility since OVN and
OVS are now released and versioned independently.

> +New releases contained the latest OVS and OVN changes. But now these 
> assumptions
> +are no longer valid, and so care must be taken to maintain compatibility.
> +
> +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 

Re: [ovs-dev] [RFC v2] Document process for compatibility between OVS and OVN.

[Numan Siddique]

Hi Mark,

Few comments below.


On Sat, Sep 7, 2019 at 2:38 AM Mark Michelson  wrote:

> 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.
>
> Note that the document currently makes reference to some non-existent
> items. For instance, it refers to an ovs-compat directory in the OVN
>

I think you need to update the commit message as "ovs-compat" directory
approach is removed in RFC v2.


> project. Also, it refers to some macros for testing the OVS version.
> These will be added in a future commit if the process documented here is
> approved.
>
> I'm submitting this as an RFC, since I imagine there are some details I
> have not thought about, and there will also be some great suggestions
> from others on this matter.
>
> Signed-off-by: Mark Michelson 
> ---
>  Documentation/automake.mk  |   1 +
>  Documentation/internals/contributing/index.rst |   1 +
>  .../contributing/ovs-ovn-compatibility.rst | 129
> +
>  3 files changed, 131 insertions(+)
>  create mode 100644
> Documentation/internals/contributing/ovs-ovn-compatibility.rst
>
> diff --git a/Documentation/automake.mk b/Documentation/automake.mk
> index f7e1d2628..d9bd4be1f 100644
> --- a/Documentation/automake.mk
> +++ b/Documentation/automake.mk
> @@ -56,6 +56,7 @@ 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 0..d40b58c32
> --- /dev/null
> +++ b/Documentation/internals/contributing/ovs-ovn-compatibility.rst
> @@ -0,0 +1,129 @@
> +..
> +  Copyright (c) 2019 Nicira, 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
> +===
> +
> +OVN has split from OVS. Prior to this split, there was no issue with
> +compatibility. All code changes happened at the same time and in the same
> repo.
> +New releases contained the latest OVS and OVN changes. But now these
> assumptions
> +are no longer valid, and so care must be taken to maintain compatibility.
> +
> +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
> 

[ovs-dev] [RFC v2] Document process for compatibility between OVS and OVN.

[Mark Michelson]

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.

Note that the document currently makes reference to some non-existent
items. For instance, it refers to an ovs-compat directory in the OVN
project. Also, it refers to some macros for testing the OVS version.
These will be added in a future commit if the process documented here is
approved.

I'm submitting this as an RFC, since I imagine there are some details I
have not thought about, and there will also be some great suggestions
from others on this matter.

Signed-off-by: Mark Michelson 
---
 Documentation/automake.mk  |   1 +
 Documentation/internals/contributing/index.rst |   1 +
 .../contributing/ovs-ovn-compatibility.rst | 129 +
 3 files changed, 131 insertions(+)
 create mode 100644 
Documentation/internals/contributing/ovs-ovn-compatibility.rst

diff --git a/Documentation/automake.mk b/Documentation/automake.mk
index f7e1d2628..d9bd4be1f 100644
--- a/Documentation/automake.mk
+++ b/Documentation/automake.mk
@@ -56,6 +56,7 @@ 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 0..d40b58c32
--- /dev/null
+++ b/Documentation/internals/contributing/ovs-ovn-compatibility.rst
@@ -0,0 +1,129 @@
+..
+  Copyright (c) 2019 Nicira, 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
+===
+
+OVN has split from OVS. Prior to this split, there was no issue with
+compatibility. All code changes happened at the same time and in the same repo.
+New releases contained the latest OVS and OVN changes. But now these 
assumptions
+are no longer valid, and so care must be taken to maintain compatibility.
+
+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