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From John Roesler <j...@confluent.io>
Subject Re: [DISCUSS] KIP-213: Second follow-up on Foreign Key Joins
Date Thu, 27 Jun 2019 20:44:26 GMT
Hi Adam,

Hah! Yeah, I felt a headache coming on myself when I realized this
would be a concern.

For what it's worth, I'd also lean toward versioning. It seems more
explicit and more likely to keep us all sane in the long run. Since we
don't _think_ our wire protocol will be subject to a lot of revisions,
we can just use one byte. The worst case is that we run out of numbers
and reserve the last one to mean, "consult another field for the
actual version number". It seems like a single byte on each message
isn't too much to pay.

Since you point it out, we might as well put a version number on the
SubscriptionResponseWrapper as well. It may not be needed, but if we
ever need it, even just once, we'll be glad we have it.

Regarding the instructions field, we can also serialize the enum very
compactly as a single byte (which is the same size a boolean takes
anyway), so it seems like an Enum in Java-land and a byte on the wire
is a good choice.

Agreed on the right and full outer joins, it doesn't seem necessary
right now, although I am happy to see the left join "join" the party,
since as you said, we were so close to it anyway. Can you also add it
to the KIP?

Thanks as always for your awesome efforts on this,
-John

On Thu, Jun 27, 2019 at 3:04 PM Adam Bellemare <adam.bellemare@gmail.com> wrote:
>
> You're stretching my brain, John!
>
> I prefer STRATEGY 1 because it solves the problem in a simple way, and
> allows us to deprecate support for older message types as we go (ie, we
> only support the previous 3 versions, so V5,V4,V3, but not v2 or V1).
>
> STRATEGY 2 is akin to Avro schemas between two microservices - there are
> indeed cases where a breaking change must be made, and forward
> compatibility will provide us with no out other than requiring a full stop
> and full upgrade for all nodes, shifting us back towards STRATEGY 1.
>
> My preference is STRATEGY 1 with instructions as an ENUM, and we can
> certainly include a version. Would it make sense to include a version
> number in  SubscriptionResponseWrapper as well? Currently we don't have any
> instructions in there, as I removed the boolean, but it is certainly
> plausible that it could happen in the future. I don't *think* we'll need
> it, but I also didn't think we'd need it for SubscriptionWrapper and here
> we are.
>
> Thanks for the thoughts, and the info on the right-key. That was
> enlightening, though I can't think of a use-case for it *at this point in
> time*. :)
>
> Adam
>
>
>
> On Thu, Jun 27, 2019 at 12:29 PM John Roesler <john@confluent.io> wrote:
>
> > I think I agree with you, right joins (and therefore full outer joins)
> > don't make sense here, because the result is a keyed table, where the
> > key is the PK of the left-hand side. So, when you have a
> > right-hand-side record with no incoming FK references, you would want
> > to produce a join result like `nullKey: (null, rhsValue)`, but we
> > don't currently allow null keys in Streams. It actually is possible to
> > define them, and therefore to add right- and full-outer foreign-key
> > joins later, but it's non-trivial in a streaming context with
> > continuously updated results. (See the PS if you're curious what I'm
> > thinking). You're correct, right- and full-outer joins are trivial on
> > our current 1:1 table joins because they are equi-joins.
> >
> > Regarding the transition, it sounds like what you're proposing is that
> > we would say, "adding a foreign-key join to your topology requires a
> > full application reset (or a new application id)". This is also an
> > acceptable constraint to place on the feature, but not strictly
> > necessary. Since 2.3, it's now possible to give all the state in your
> > application stable names. This means that it's no longer true that
> > adding a node to your topology graph would break its structure, and it
> > does become possible to add new operators and simply restart the app.
> > Revisiting my prior thought, though, I think the problem is not
> > specific to your feature. For example, adding a new grouped
> > aggregation would produce a new repartition topic, but the repartition
> > topic partitions might get assigned to old nodes in the middle of a
> > rolling bounce, and they would need to just ignore them. This
> > requirement is the same for the repartition topics in the FK join, so
> > it's orthogonal to your design.
> >
> > Back to the first concern, though, I'm not sure I followed the
> > explanation. As a thought experiment, let's imagine that Joe hadn't
> > taken the time to experiment with your feature branch. We wouldn't
> > have noticed the problem until the feature was already released in
> > 2.4. So the wire protocol on that PK->FK subscription topic would have
> > been V1: "FK,PK,HASH,BOOLEAN". Then, Joe would have let us know the
> > problem once they picked up the feature, so we would want to implement
> > your proposed fix and change the wire protocol to V2:
> > "FK,PK,HASH,INSTRUCTIONS" in 2.5. Upon rolling out the update, we
> > would see both 2.4 nodes encountering V2 messages and 2.5 nodes
> > encountering V1 messages. How can they both detect that they are
> > attempting to process a newer or older protocol? If they can detect
> > it, then what should they do?
> >
> > From experience, there are two basic solutions to this problem:
> >
> > STRATEGY1. Add a protocol version to the message (could be a number at
> > the start of the message payload, or it could be a number in the
> > message headers, not sure if it matters much. Payload is probably more
> > compact, since the header would need a name.) In this case, the 2.4
> > worker would know that it's max protocol version is V1, and when it
> > sees the V2 message, it knows that it can't handle it properly. Rather
> > than doing something wrong, it would just not do anything. This means
> > it would stop the task, if not shut down the whole instance. On the
> > other hand, a 2.5 worker would have some defined logic for how to
> > handle all versions (V1 and V2), so once the upgrade is complete, all
> > messages can be processed.
> >
> > STRATEGY2. Make the schema forward-compatible. Basically, we ensure
> > that new fields can only be appended to the message schema, and that
> > older workers using only a prefix of the full message would still
> > behave correctly. Using the example above, we'd instead evolve the
> > schema to V2': "FK,PK,HASH,BOOLEAN,INSTRUCTIONS", and continue to set
> > the boolean field to true for the "new" foreign key. Then, 2.4 workers
> > encountering the a "new FK" message would just see the prefix of the
> > payload that makes sense to them, and they would still continue
> > processing the messages as they always have. Only after the 2.5 code
> > is fully rolled out to the cluster would we be sure to see the desired
> > behavior. Note: in the reverse case, a 2.5 worker knows how to fully
> > parse the new message format, even if it plans to ignore the BOOLEAN
> > field.
> >
> > There are some tradeoffs between these strategies: STRATEGY1 ensures
> > that all messages are only handled by workers that can properly handle
> > them, although it results in processing stalls while there are still
> > old nodes in the cluster. STRATEGY2 ensures that all messages can be
> > processed by all nodes, so there are no stalls, but we can never
> > remove fields from the message, so if there are a lot of revisions in
> > the future, the payloads will become bloated. Also, it's not clear
> > that you can actually pull off STRATEGY2 in all cases. If there's some
> > new kind of message you want to send that has no way to be correctly
> > processed at all under the 2.4 code paths, the prefix thing simply
> > doesn't work. Etc.
> >
> > Also, note that you can modify the above strategies by instead
> > designing the message fields for extensibility. E.g., if you make the
> > instructions field an enum, then you can make sure that the default
> > case is handled sensibly (probably similarly to STRATEGY1, just choke
> > on unknown instructions) and that you never remove an instruction type
> > from the enum in future versions.
> >
> > Does this make sense?
> > -John
> >
> >
> >
> >
> > PS:
> > We can define null keys for streaming tables, but it's tricky.
> >
> > Specifically, you'd want to define some concept of null keys that
> > allows all null keys to be unique, but _also_ to have a fixed
> > identity, so that a particular null-key can be updated later. One
> > example could be to union the existing keyspace with a new
> > null-keyspace, where normal keys are like "key" and null-keys are like
> > "null(identity)". Then given a query like
> > "KTable<String,Integer>.rightJoin(KTable<Integer,Boolean>)", and
> > inputs like:
> > LHS:
> > "a": 1
> > "b": 2
> >
> > RHS:
> > 1: true
> > 3: false
> >
> > a full outer join would produce:
> > "a": (1, true)
> > "b": (2, null)
> > null(3): (null, false)
> >
> > which can be correctly updated later if we get an update on the LHS:
> > PUT("c": 3)
> >
> > We'd emit for the results:
> > DELETE(null(e))
> > EMIT("c": (3, false))
> >
> > Resulting in the correct result table of:
> > "a": (1, true)
> > "b": (2, null)
> > "c": (3, false)
> >
> > As mentioned, this is tricky, and I would avoid it until we have
> > evidence that it's actually useful to cover this part of the design
> > space. Certainly, it would be a separate KIP if it came to that.
> >
> > On Wed, Jun 26, 2019 at 8:57 PM Adam Bellemare <adam.bellemare@gmail.com>
> > wrote:
> > >
> > > Hi John
> > >
> > > Good thinking with regards to upgrade path between versions regarding
> > > over-the-wire instructions in SubscriptionWrapper. At this point in time
> > I
> > > can't think of any new wire message instructions, but I would appreciate
> > as
> > > many eyes on it as possible. I have just included the LEFT join in the
> > last
> > > commit (about 10 min ago) along with INNER join. I do not think that
> > RIGHT
> > > join and OUTER are possible given that there is no LHS key available, so
> > > LHSTable.outerJoinOnForeignKey(RHSTable) wouldn't even make sense. This
> > is
> > > in contrast to the current LHSTable.outerJoin(RHSTable), as they are both
> > > keyed on the same key. I have buffed up the Integration tests and have
> > > tried to make them more readable to ensure that we're covering all the
> > > scenarios. I think that if we can get more eyes on the workflow showing
> > the
> > > various LHS and RHS events and outputs then that may help us validate
> > that
> > > we have all the scenarios covered.
> > >
> > > With regards to the 2.3->2.4 scenario you described, I'm not entirely
> > sure
> > > I follow. If they want to add a FK-join, they will need to rework their
> > > code in the KStreams app and make a new release, since the underlying
> > > topology would be different and new internal topics would need to be
> > > created. In other words, I don't think a rolling upgrade where the user
> > > introduces a FK join would be possible since their topology would
> > > necessitate a full KStreams reset. Is this what you meant?
> > >
> > >
> > >
> > > On Wed, Jun 26, 2019 at 4:10 PM John Roesler <john@confluent.io> wrote:
> > >
> > > > Thanks, Adam!
> > > >
> > > > One unrelated thought that has just now occurred to me is that (unlike
> > > > the equi-joins we currently have), this join logic is potentially
> > > > spread over multiple Streams instances, which in general means that
> > > > the instances may be running different versions of Kafka Streams.
> > > >
> > > > This means that if we discover a bug that requires us to again change
> > > > the wire message (as you did in this proposal update), we need to
> > > > consider what should happen if the PK instance is newer than the FK
> > > > instance, or vice-versa, during a rolling upgrade. We should think
> > > > ahead to this condition and make sure the logic is forward compatible.
> > > >
> > > > Related: what about the initial case, when we release this feature
> > > > (let's say in 2.4)? What will happen if I decide to adopt 2.4 and add
> > > > a FK join together in one upgrade. Thus, the 2.4 member of the cluster
> > > > is producing the SubscriptionWrapper messages, and some 2.3 members
> > > > get the subscription topic assigned to them, but they have no idea
> > > > what to do with it? I'm not sure this is a problem; hopefully they
> > > > just do nothing. If it is a problem, it would be fine to say you have
> > > > to upgrade completely to 2.4 before deploying a FK join.
> > > >
> > > > Just want to make sure we anticipate these issues in case it affects
> > > > the design at all.
> > > >
> > > > Thanks,
> > > > -John
> > > >
> > > > On Wed, Jun 26, 2019 at 2:38 PM Adam Bellemare <
> > adam.bellemare@gmail.com>
> > > > wrote:
> > > > >
> > > > > Sigh... Forgot the link:
> > > > >
> > > >
> > https://cwiki.apache.org/confluence/pages/diffpagesbyversion.action?pageId=74684836&selectedPageVersions=78&selectedPageVersions=74
> > > > >
> > > > > I'll update it when I validate that there are no issues with
> > removing the
> > > > > SubscriptionResponseWrapper boolean.
> > > > >
> > > > > On Wed, Jun 26, 2019 at 3:37 PM Adam Bellemare <
> > adam.bellemare@gmail.com
> > > > >
> > > > > wrote:
> > > > >
> > > > > > >Maybe just call it as (k, leftval, null) or (k, null, rightval)?
> > > > > > Done.
> > > > > >
> > > > > > > if you update the KIP, you might want to send a new "diff
link"
> > to
> > > > this
> > > > > > thread
> > > > > > Here it is:
> > > > > >
> > > > > > > Looking closely at the proposal, can you explain more about
the
> > > > > > propagateIfNull field in SubscriptionResponseWrapper? It sort
of
> > looks
> > > > like
> > > > > > it's always going to be equal to (RHS-result != null).
> > > > > > I believe you are correct, and I missed the forest for the trees.
> > They
> > > > are
> > > > > > effectively the same thing, and I can simply remove the flag.
I
> > will
> > > > code
> > > > > > it up and try it out locally just to be sure.
> > > > > >
> > > > > > Thanks again for your help, it is greatly appreciated!
> > > > > >
> > > > > > On Wed, Jun 26, 2019 at 2:54 PM John Roesler <john@confluent.io>
> > > > wrote:
> > > > > >
> > > > > >> I think the "scenario trace" is very nice, but has one point
that
> > I
> > > > > >> found confusing:
> > > > > >>
> > > > > >> You indicate a retraction in the join output as (k,null)
and a
> > join
> > > > > >> result as (k, leftval, rightval), but confusingly, you also
write
> > a
> > > > > >> join result as (k, JoinResult) when one side is null. Maybe
just
> > call
> > > > > >> it as (k, leftval, null) or (k, null, rightval)? That way
the
> > readers
> > > > > >> can more easily determine if the results meet their expectations
> > for
> > > > > >> each join type.
> > > > > >>
> > > > > >> (procedural note: if you update the KIP, you might want
to send a
> > new
> > > > > >> "diff link" to this thread, since the one I posted at the
> > beginning
> > > > > >> would not automatically show your latest changes)
> > > > > >>
> > > > > >> I was initially concerned that the proposed algorithm would
wind
> > up
> > > > > >> propagating something that looks like a left join (k, leftval,
> > null)
> > > > > >> under the case that Joe pointed out, but after reviewing
your
> > > > > >> scenario, I see that it will emit a tombstone (k, null)
instead.
> > This
> > > > > >> is appropriate, and unavoidable, since we have to retract
the join
> > > > > >> result from the logical view (the join result is a logical
Table).
> > > > > >>
> > > > > >> Looking closely at the proposal, can you explain more about
the
> > > > > >> propagateIfNull field in SubscriptionResponseWrapper?
> > > > > >> It sort of looks like it's always going to be equal to
> > (RHS-result !=
> > > > > >> null).
> > > > > >>
> > > > > >> In other words, can we drop that field and just send back
> > RHS-result
> > > > > >> or null, and then handle it on the left-hand side like:
> > > > > >> if (rhsOriginalValueHash doesn't match) {
> > > > > >>     emit nothing, just drop the update
> > > > > >> } else if (joinType==inner && rhsValue == null)
{
> > > > > >>     emit tombstone
> > > > > >> } else {
> > > > > >>     emit joiner(lhsValue, rhsValue)
> > > > > >> }
> > > > > >>
> > > > > >> To your concern about emitting extra tombstones, personally,
I
> > think
> > > > > >> it's fine. Clearly, we should try to avoid unnecessary
> > tombstones, but
> > > > > >> all things considered, it's not harmful to emit some unnecessary
> > > > > >> tombstones: their payload is small, and they are trivial
to handle
> > > > > >> downstream. If users want to, they can materialize the join
> > result to
> > > > > >> suppress any extra tombstones, so there's a way out.
> > > > > >>
> > > > > >> Thanks for the awesome idea. It's better than what I was
thinking.
> > > > > >> -john
> > > > > >>
> > > > > >> On Wed, Jun 26, 2019 at 11:37 AM Adam Bellemare
> > > > > >> <adam.bellemare@gmail.com> wrote:
> > > > > >> >
> > > > > >> > Thanks John.
> > > > > >> >
> > > > > >> > I'm looking forward to any feedback on this. In the
meantime I
> > will
> > > > > >> work on
> > > > > >> > the unit tests to ensure that we have well-defined
and readable
> > > > > >> coverage.
> > > > > >> >
> > > > > >> > At the moment I cannot see a way around emitting (k,null)
> > whenever
> > > > we
> > > > > >> emit
> > > > > >> > an event that lacks a matching foreign key on the RHS,
except
> > in the
> > > > > >> > (k,null) -> (k,fk) case.
> > > > > >> > If this LHS oldValue=null, we know we would have emitted
a
> > deletion
> > > > and
> > > > > >> so
> > > > > >> > (k,null) would be emitted out of the join. In this
case we don't
> > > > need to
> > > > > >> > send another null.
> > > > > >> >
> > > > > >> > Adam
> > > > > >> >
> > > > > >> > On Wed, Jun 26, 2019 at 11:53 AM John Roesler <
> > john@confluent.io>
> > > > > >> wrote:
> > > > > >> >
> > > > > >> > > Hi Adam,
> > > > > >> > >
> > > > > >> > > Thanks for the proposed revision to your KIP
> > > > > >> > > (
> > > > > >> > >
> > > > > >>
> > > >
> > https://cwiki.apache.org/confluence/pages/diffpagesbyversion.action?pageId=74684836&selectedPageVersions=77&selectedPageVersions=74
> > > > > >> > > )
> > > > > >> > >
> > > > > >> > > in response to the concern pointed out during
code review
> > > > > >> > > (
> > https://github.com/apache/kafka/pull/5527#issuecomment-505137962
> > > > )
> > > > > >> > >
> > > > > >> > > We should have a brief discussion thread (here)
in the mailing
> > > > list to
> > > > > >> > > make sure everyone who wants to gets a chance
to consider the
> > > > > >> > > modification to the design.
> > > > > >> > >
> > > > > >> > > Thanks,
> > > > > >> > > -John
> > > > > >> > >
> > > > > >>
> > > > > >
> > > >
> >

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