Thats a great point, Dana. Thanks for bringing this up.
Ewen raised the same concern and suggested something called "feature
detection" where the broker would advertise support for specific features
that clients may need and by that avoid a linear model (it also allows for
advertising features such as compression codecs that are not part of the
protocol at all). I hope he'll jump in to explain :)
Do you have other suggestions?
Gwen
On Mon, Mar 14, 2016 at 6:22 PM, Dana Powers <dana.pow...@gmail.com> wrote:
> Is a linear protocol int consistent with the current release model? It
> seems like that would break down w/ the multiple release branches that are
> all simultaneously maintained? Or is it implicit that no patch release can
> ever bump the protocol int? Or maybe the protocol int gets some extra
> "wiggle" on minor / major releases to create unallocated version ints that
> could be used on future patch releases / backports?
>
> I think the protocol version int does make sense for folks deploying from
> trunk.
>
> -Dana
>
> On Mon, Mar 14, 2016 at 6:13 PM, Jay Kreps <j...@confluent.io> wrote:
>
> > Yeah I think that is the point--we have a proposal for a new protocol
> > versioning scheme and a vote on it but it doesn't actually describe
> > how versioning will work yet! I gave my vague impression based on this
> > thread, but I want to make sure that is correct and get it written
> > down before we adopt it.
> >
> > -Jay
> >
> > On Mon, Mar 14, 2016 at 5:58 PM, Gwen Shapira <g...@confluent.io> wrote:
> > > On Mon, Mar 14, 2016 at 5:31 PM, Jay Kreps <j...@confluent.io> wrote:
> > >
> > >> Couple of missing things:
> > >>
> > >> This KIP doesn't have a proposal on versioning it just gives different
> > >> options, it'd be good to get a concrete proposal in the KIP. Here is
> my
> > >> understanding of what we are proposing (can someone sanity check and
> if
> > >> correct, update the kip):
> > >>
> > >> 1. We will augment the existing api_version field in the header
> with
> > a
> > >> protocol_version that will begin at some initial value and
> increment
> > by
> > >> 1
> > >> every time we make a changes to any of the api_versions (question:
> > >> including internal apis?).
> > >>
> > >
> > > Jay, this part was not in the KIP and was never discussed.
> > > Are you proposing adding this? Or is it just an assumption you made?
> > >
> > >
> > >
> > >> 2. The protocol_version will be added to the metadata request
> > >> 3. We will also add a string that this proposal is calling
> > VersionString
> > >> which will describe the build of kafka in some way. The clients
> > should
> > >> not
> > >> under any circumstances do anything with this string other than
> > print it
> > >> out to the user.
> > >>
> > >> One thing I'm not sure about: I think currently metadata sits in the
> > client
> > >> for 10 mins by default. Say a client bootstraps and then a server is
> > >> downgraded to an earlier version, won't the client's metadata version
> > >> indicate that that client handles a version it doesn't actually handle
> > any
> > >> more? We need to document how clients will handle this.
> > >>
> > >> Here are some comments on other details:
> > >>
> > >> 1. As a minor thing I think we should avoid naming the fields
> > VersionId
> > >> and VersionString which sort of implies they are both used for
> > >> versioning.
> > >> I think we should call them something like ProtocolVersion and
> > >> BuildDescription, with BuildDescription being totally unspecified
> > other
> > >> than that it is some kind of human readable string describing a
> > >> particular
> > >> Kafka build. We really don't want a client attempting to use this
> > >> string in
> > >> any way as that would always be the wrong thing to do in the
> > versioning
> > >> scheme we are proposing, you should always use the protocol
> version.
> > >> 2. Does making the topics field in the metadata request nullable
> > >> actually make sense? We have a history of wanting to add magical
> > values
> > >> rather than fields. Currently topics=[a] means give me information
> > about
> > >> topic a, topics=[] means give me information about all topics, and
> we
> > >> are
> > >> proposing topics=null would mean don't give me topics. I don't
> have a
> > >> strong opinion.
> > >> 3. I prefer Jason's proposal on using a conservative metadata
> version
> > >> versus the empty response hack. However I think that may actually
> > >> exacerbate the downgrade scenario I described.
> > >> 4. I agree with Jason that we should really look at the details of
> > the
> > >> implementation so we know it works--implementing server support
> > without
> > >> actually trying it is kind of risky.
> > >>
> > >> As a meta comment: I'd really like to encourage us to think of the
> > protocol
> > >> as a document that includes the following things:
> > >>
> > >> - The binary format, error codes, etc
> > >> - The request/response interaction
> > >> - The semantics of each request in different cases
> > >> - Instructions on how to use this to implement a client
> > >>
> > >> This document is versioned with the protocol number and is the source
> of
> > >> truth for the protocol.
> > >>
> > >> Part of any protocol change needs to be an update to the instructions
> on
> > >> how to use that part of the protocol. We should be opinionated. If
> there
> > >> are two options there should be a reason, and then we need to document
> > both
> > >> and say exactly when to use each.
> > >>
> > >> I think we also need to get a "how to" document on protocol changes
> > just so
> > >> people know what they need to do to add a new protocol feature.
> > >>
> > >> -Jay
> > >>
> > >> On Mon, Mar 14, 2016 at 4:51 PM, Jason Gustafson <ja...@confluent.io>
> > >> wrote:
> > >>
> > >> > >
> > >> > > Are you suggesting this as a solution for the problem where a
> KIP-35
> > >> > aware
> > >> > > client sends a higher version of metadata req, say v2, to a KIP-35
> > >> aware
> > >> > > broker that only supports up to v1.
> > >> >
> > >> >
> > >> > Yes, that's right. In that case, the client first sends v1, finds
> out
> > >> that
> > >> > the broker supports v2, and then sends v2 (if it has any reason to
> do
> > >> so).
> > >> >
> > >> > We had a bit of discussion on such scenarios, and it seemed to be a
> > >> chicken
> > >> > > and egg problem that is hard to avoid. Your suggestion definitely
> > makes
> > >> > > sense, however it falls under the purview of clients.
> > >> >
> > >> >
> > >> > That basically means clients should figure it out for themselves?
> > Might
> > >> be
> > >> > nice to have a better answer.
> > >> >
> > >> > KIP-35 only aims on adding support for getting the version info
> from a
> > >> > > broker. This definitely can be utilized by our clients. However,
> > that
> > >> can
> > >> > > follow KIP-35 changes. Does this sound reasonable to you?
> > >> >
> > >> >
> > >> > It may be OK, but I'm a little concerned about offering a feature
> > that we
> > >> > don't support ourselves. Sometimes it's not until implementation
> that
> > we
> > >> > find out whether it really works as expected. And if we're
> eventually
> > >> > planning to support it, I feel we should think through some of the
> > cases
> > >> a
> > >> > bit more. For example, the upgrade and downgrade cases that Becket
> > >> > mentioned earlier. It doesn't feel too great to support this feature
> > >> unless
> > >> > we can offer guidance on how to use it.
> > >> >
> > >> > -Jason
> > >> >
> > >> >
> > >> >
> > >> > On Mon, Mar 14, 2016 at 4:20 PM, Ashish Singh <asi...@cloudera.com>
> > >> wrote:
> > >> >
> > >> > > Hi Jason,
> > >> > >
> > >> > > On Mon, Mar 14, 2016 at 4:04 PM, Jason Gustafson <
> > ja...@confluent.io>
> > >> > > wrote:
> > >> > >
> > >> > > > Perhaps clients should always send the oldest version of the
> > metadata
> > >> > > > request which supports KIP-35 when initially connecting to the
> > >> cluster.
> > >> > > > Depending on the versions in the response, it can upgrade to a
> > more
> > >> > > recent
> > >> > > > version. Then maybe we don't need the empty response hack?
> > >> > > >
> > >> > > Are you suggesting this as a solution for the problem where a
> KIP-35
> > >> > aware
> > >> > > client sends a higher version of metadata req, say v2, to a KIP-35
> > >> aware
> > >> > > broker that only supports up to v1.
> > >> > >
> > >> > > We had a bit of discussion on such scenarios, and it seemed to be
> a
> > >> > chicken
> > >> > > and egg problem that is hard to avoid. Your suggestion definitely
> > makes
> > >> > > sense, however it falls under the purview of clients.
> > >> > >
> > >> > > >
> > >> > > > One thing that's not clear to me is whether the ultimate goal of
> > this
> > >> > KIP
> > >> > > > is to have our clients support multiple broker versions. It
> would
> > be
> > >> a
> > >> > > > little weird to have this feature if our own clients don't use
> it.
> > >> > > >
> > >> > > KIP-35 only aims on adding support for getting the version info
> > from a
> > >> > > broker. This definitely can be utilized by our clients. However,
> > that
> > >> can
> > >> > > follow KIP-35 changes. Does this sound reasonable to you?
> > >> > >
> > >> > > >
> > >> > > > -Jason
> > >> > > >
> > >> > > > On Mon, Mar 14, 2016 at 3:34 PM, Ashish Singh <
> > asi...@cloudera.com>
> > >> > > wrote:
> > >> > > >
> > >> > > > > On Mon, Mar 14, 2016 at 2:12 PM, Ismael Juma <
> ism...@juma.me.uk
> > >
> > >> > > wrote:
> > >> > > > >
> > >> > > > > > On Mon, Mar 14, 2016 at 8:45 PM, Gwen Shapira <
> > g...@confluent.io
> > >> >
> > >> > > > wrote:
> > >> > > > > > >
> > >> > > > > > > > I don't see how it helps. If the client is communicating
> > >> with a
> > >> > > > > broker
> > >> > > > > > > that
> > >> > > > > > > > does not support KIP-35, that broker will simply close
> the
> > >> > > > > connection.
> > >> > > > > > If
> > >> > > > > > > > the broker supports KIP-35, then it will provide the
> > broker
> > >> > > > version.
> > >> > > > > I
> > >> > > > > > > > don't envisage a scenario where a broker does not
> support
> > >> > KIP-35,
> > >> > > > but
> > >> > > > > > > > implements the new behaviour of sending an empty
> > response. Do
> > >> > > you?
> > >> > > > > > > >
> > >> > > > > > > > Are you sure about that? Per KIP-35, the broker supplies
> > the
> > >> > > > version
> > >> > > > > in
> > >> > > > > > > response to Metadata request, not in response to anything
> > else.
> > >> > > > > > > If the client sends producer request version 42
> > (accidentally
> > >> or
> > >> > > due
> > >> > > > to
> > >> > > > > > > premature upgrade) to KIP-35-compactible broker - we want
> to
> > >> see
> > >> > an
> > >> > > > > empty
> > >> > > > > > > packet and not a connection close.
> > >> > > > > > > Sending a broker version was deemed impractical IIRC.
> > >> > > > > > >
> > >> > > > > >
> > >> > > > > > OK, so this is a different case than the one Ashish
> described
> > >> > > ("client
> > >> > > > > that
> > >> > > > > > wants to support broker versions that do not provide broker
> > >> version
> > >> > > in
> > >> > > > > > metadata and broker versions that provides version info in
> > >> > > metadata").
> > >> > > > > So,
> > >> > > > > > you are suggesting that if a client is communicating with a
> > >> broker
> > >> > > that
> > >> > > > > > implements KIP-35 and it receives an empty response, it will
> > >> assume
> > >> > > > that
> > >> > > > > > the broker doesn't support the request version and it won't
> > try
> > >> to
> > >> > > > parse
> > >> > > > > > the response? I think it would be good to explain this kind
> of
> > >> > thing
> > >> > > in
> > >> > > > > > detail in the KIP.
> > >> > > > > >
> > >> > > > > Actually even in this case and the case I mentioned, closing
> > >> > connection
> > >> > > > > should be fine. Lets think about possible reasons that could
> > lead
> > >> to
> > >> > > this
> > >> > > > > issue.
> > >> > > > >
> > >> > > > > 1. Client has incorrect mapping of supported protocols for a
> > broker
> > >> > > > > version.
> > >> > > > > 2. Client misread broker version from metadata response.
> > >> > > > > 3. Client constructed unsupported protocol version by mistake.
> > >> > > > >
> > >> > > > > In all the above cases irrespective of what broker does,
> client
> > >> will
> > >> > > keep
> > >> > > > > sending wrong request version.
> > >> > > > >
> > >> > > > > At this point, I think sending an empty packet instead of
> > closing
> > >> > > > > connection is a nice to have and not mandatory requirement.
> > Like in
> > >> > the
> > >> > > > > above case, a client can catch parsing error and be sure that
> > there
> > >> > is
> > >> > > > > something wrong in the protocol version it is sending.
> However,
> > a
> > >> > > generic
> > >> > > > > connection close does not really provide any information on
> > >> probable
> > >> > > > cause.
> > >> > > > >
> > >> > > > > What do you guys suggest?
> > >> > > > >
> > >> > > > > >
> > >> > > > > > Ismael
> > >> > > > > >
> > >> > > > >
> > >> > > > >
> > >> > > > >
> > >> > > > > --
> > >> > > > >
> > >> > > > > Regards,
> > >> > > > > Ashish
> > >> > > > >
> > >> > > >
> > >> > >
> > >> > >
> > >> > >
> > >> > > --
> > >> > >
> > >> > > Regards,
> > >> > > Ashish
> > >> > >
> > >> >
> > >>
> >
>
