A couple follow-ups from Ismael's previous comments. 1. We have removed the individual message CRC field from the message format. Because this field can already change on the broker in some situations, we feel it is probably not wise to let clients depend on it, and removing it saves some space and the redundant verification on the broker. We have also proposed to deprecate the checksum() APIs which are exposed in RecordMetadata and ConsumerRecord. 2. We changed the message timestamp field to be variable length. The values are computed relative to the timestamp of the first message in the set. We considered my previous suggestion to make the first message timestamp absolute with later messages relative to their previous timestamp, but ultimately felt it was simpler to stick with the same pattern that is used for relative offsets. To enable this, we added an initial timestamp field to the message set.
Thanks, Jason On Mon, Feb 6, 2017 at 6:01 PM, Apurva Mehta <apu...@confluent.io> wrote: > Hello, > > I have gone ahead and updated the KIP wiki with a summary of the changes to > the RPC protocol. The KIP wiki should now have _all_ the public facing > changes being proposed. > > The proposed changes were always in the Google doc, and now we are simply > making good on our promise to copy them over to the wiki since the design > is almost finalized. > > Thanks, > Apurva > > On Mon, Feb 6, 2017 at 4:02 PM, Jason Gustafson <ja...@confluent.io> > wrote: > > > Hey Tom, > > > > Re; complexity. This is always a tradeoff with new features. The changes > > we've made during the design and review process have greatly simplified > the > > implementation for clients, and especially for the consumer, but there is > > nontrivial work needed here to support transactions on the producer. I'm > > not sure how it could be otherwise and we've spent a ton of time thinking > > about this. It's also worth mentioning that there's a relatively low bar > to > > support the idempotent producer while keeping the client thin (it > requires > > support for one new request type and some sequence bookkeeping). > > > > Ultimately, we have to decide whether the improved semantics are worth > the > > cost of the complexity. In my opinion, they are. The benefit of having > > exactly-once processing in streaming applications is huge. And to manage > > the complexity, we've intentionally used patterns that were already > > familiar in the codebase (e.g. our approach to maintaining transaction > > state through a coordinator is copied from how consumer offsets are > > managed). We've moved as much work from the clients to the broker as > > possible, and we have resisted at every turn complicating the client APIs > > even where it may have simplified some internals. > > > > -Jason > > > > On Mon, Feb 6, 2017 at 2:55 PM, Apurva Mehta <apu...@confluent.io> > wrote: > > > > > Hi Tom, > > > > > > I updated the KIP with a note our plans for performance testing: > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP- > > > 98+-+Exactly+Once+Delivery+and+Transactional+Messaging#KIP-98- > > > ExactlyOnceDeliveryandTransactionalMessaging-Performance > > > > > > Thanks for pointing that out. > > > > > > Regards, > > > Apurva > > > > > > > > > > > > > > > > > > On Mon, Feb 6, 2017 at 7:40 AM, Tom Crayford <tcrayf...@heroku.com> > > wrote: > > > > > > > I think the updated wiki page makes sense with respect to ACLs, there > > > seems > > > > to be little potential for abuse there (other than the noted and > known > > > > issues). > > > > > > > > I am going to note that this is a major complexity increase for > Kafka, > > > and > > > > that I'm concerned about performance impact (the JVM is quite… > pedantic > > > > about method size, for example, and even adding conditionals to > larger > > > > methods could impact this). The KIP doesn't note plans for > performance > > > > testing. > > > > > > > > I'm also concerned about the impact on non-JVM client libraries - > > > writing a > > > > client for Kafka is already a very complicated undertaking, and this > > adds > > > > to that complexity significantly. > > > > > > > > However, the approach seems ok enough. It does also violate the > "Kafka > > > has > > > > dumb brokers and smart clients" (which I realize is in direct > > > contradiction > > > > of the previous statement about client implementation being hard). > I'd > > > love > > > > to see some discussion in either the doc or the wiki as to why much > of > > > this > > > > transactional work isn't a client-side part of Kafka Streams. > > > > > > > > On Sat, Feb 4, 2017 at 3:38 AM, Jason Gustafson <ja...@confluent.io> > > > > wrote: > > > > > > > > > One additional note on the authorization. The WriteTxnMarker API is > > > > > restricted to inter-broker usage, so it requires Cluster > > authorization > > > > > (just like other inter-broker APIs). I've updated the document and > > wiki > > > > to > > > > > reflect this. > > > > > > > > > > Also, I have renamed GroupCoordinatorRequest to > > FindCoordinatorRequest > > > > > since there is no group for transactional producers. Let me know if > > > there > > > > > are any concerns. > > > > > > > > > > -Jason > > > > > > > > > > On Fri, Feb 3, 2017 at 2:35 PM, Jason Gustafson < > ja...@confluent.io> > > > > > wrote: > > > > > > > > > > > Hi Tom, > > > > > > > > > > > > I said this in the voting thread, but can the authors include a > > > section > > > > > >> about new ACLs if there are going to be ACLs for > TransactionalId. > > > It's > > > > > >> mentioned in the google doc, but I think new ACLs should be in a > > KIP > > > > > >> directly. > > > > > > > > > > > > > > > > > > We've updated the wiki. Can you take a look and let us know if > you > > > have > > > > > > additional concerns? > > > > > > > > > > > > Thanks, > > > > > > Jason > > > > > > > > > > > > On Fri, Feb 3, 2017 at 1:52 PM, Rajini Sivaram < > > > > rajinisiva...@gmail.com> > > > > > > wrote: > > > > > > > > > > > >> Hi Jason, > > > > > >> > > > > > >> Thank you for the responses. Agree that authorizing > > > transactional.id > > > > in > > > > > >> the > > > > > >> producer requests will be good enough for version 1. And making > it > > > > > tighter > > > > > >> in future based on delegation tokens sounds good too. > > > > > >> > > > > > >> Regards, > > > > > >> > > > > > >> Rajini > > > > > >> > > > > > >> > > > > > >> On Fri, Feb 3, 2017 at 8:04 PM, Jason Gustafson < > > ja...@confluent.io > > > > > > > > > >> wrote: > > > > > >> > > > > > >> > Hey Rajini, > > > > > >> > > > > > > >> > Thanks for the questions. Responses below: > > > > > >> > > > > > > >> > > > > > > >> > > 1. Will the transaction coordinator check topic ACLs based > on > > > the > > > > > >> > > requesting client's credentials? Access to transaction logs, > > > > topics > > > > > >> being > > > > > >> > > added for transaction etc? > > > > > >> > > > > > > >> > > > > > > >> > Good question. I think it makes sense to check topic Write > > > > permission > > > > > >> when > > > > > >> > adding partitions to the transaction. I'll add this to the > > > document. > > > > > >> > Perhaps authorization to the transaction log itself, however, > > can > > > be > > > > > >> > assumed from having access to the ProducerTransactionalId > > > resource? > > > > > This > > > > > >> > would be similar to how access to __consumer_offsets is > assumed > > if > > > > the > > > > > >> > client has access to the Group resource. > > > > > >> > > > > > > >> > 2. If I create a transactional produce request (by hand, not > > using > > > > the > > > > > >> > > producer API) with a random PID (random, hence unlikely to > be > > in > > > > > use), > > > > > >> > will > > > > > >> > > the broker append a transactional message to the logs, > > > preventing > > > > > LSO > > > > > >> > from > > > > > >> > > moving forward? What validation will broker do for PIDs? > > > > > >> > > > > > > >> > > > > > > >> > Yes, that is correct. Validation of the TransactionalId to PID > > > > binding > > > > > >> is a > > > > > >> > known gap in the current proposal, and is discussed in the > > design > > > > > >> document. > > > > > >> > Now that I'm thinking about it a bit more, I think there is a > > good > > > > > case > > > > > >> for > > > > > >> > including the TransactionalId in the ProduceRequest (I think > Jun > > > > > >> suggested > > > > > >> > this previously). Verifying it does not ensure that the > included > > > PID > > > > > is > > > > > >> > correct, but it does ensure that the client is authorized to > use > > > > > >> > transactions. If the client wanted to do an "endless > transaction > > > > > >> attack," > > > > > >> > having Write access to the topic and an authorized > > transactionalID > > > > is > > > > > >> all > > > > > >> > they would need anyway even if we could authorize the PID > > itself. > > > > This > > > > > >> > seems like a worthwhile improvement. > > > > > >> > > > > > > >> > For future work, my half-baked idea to authorize the PID > binding > > > is > > > > to > > > > > >> > leverage the delegation work in KIP-48. When the PID is > > generated, > > > > we > > > > > >> can > > > > > >> > give the producer a token which is then used in produce > requests > > > > (say > > > > > an > > > > > >> > hmac covering the TransactionalId and PID). > > > > > >> > > > > > > >> > > > > > > >> > > 3. Will every broker check that a client sending > transactional > > > > > produce > > > > > >> > > requests at least has write access to transaction log topic > > > since > > > > it > > > > > >> is > > > > > >> > not > > > > > >> > > validating transactional.id (for every produce request)? > > > > > >> > > > > > > >> > 4. I understand that brokers cannot authorize the > transactional > > > id > > > > > for > > > > > >> > each > > > > > >> > > produce request since requests contain only the PID. But > since > > > > there > > > > > >> is a > > > > > >> > > one-to-one mapping between PID and transactional.id, and a > > > > > >> connection is > > > > > >> > > never expected to change its transactional.id, perhaps it > is > > > > > >> feasible to > > > > > >> > > add authorization and cache the results in the Session? > > Perhaps > > > > not > > > > > >> for > > > > > >> > > version 1, but feels like it will be good to close the > > security > > > > gap > > > > > >> here. > > > > > >> > > Obviously it would be simpler if transactional.id was in > the > > > > > produce > > > > > >> > > request if the overhead was acceptable. > > > > > >> > > > > > > >> > > > > > > >> > I think my response above addresses both of these. We should > > > include > > > > > the > > > > > >> > TransactionalId in the ProduceRequest. Of course it need not > be > > > > > >> included in > > > > > >> > the message format, so I'm not too concerned about the > > additional > > > > > >> overhead > > > > > >> > it adds. > > > > > >> > > > > > > >> > Thanks, > > > > > >> > Jason > > > > > >> > > > > > > >> > > > > > > >> > On Fri, Feb 3, 2017 at 6:52 AM, Ismael Juma < > ism...@juma.me.uk> > > > > > wrote: > > > > > >> > > > > > > >> > > Comments inline. > > > > > >> > > > > > > > >> > > On Thu, Feb 2, 2017 at 6:28 PM, Jason Gustafson < > > > > ja...@confluent.io > > > > > > > > > > > >> > > wrote: > > > > > >> > > > > > > > >> > > > Took me a while to remember why we didn't do this. The > > > timestamp > > > > > >> that > > > > > >> > is > > > > > >> > > > included at the message set level is the max timestamp of > > all > > > > > >> messages > > > > > >> > in > > > > > >> > > > the message set as is the case in the current message > format > > > (I > > > > > will > > > > > >> > > update > > > > > >> > > > the document to make this explicit). We could make the > > message > > > > > >> > timestamps > > > > > >> > > > relative to the max timestamp, but that makes > serialization > > a > > > > bit > > > > > >> > awkward > > > > > >> > > > since the timestamps are not assumed to be increasing > > > > sequentially > > > > > >> or > > > > > >> > > > monotonically. Once the messages in the message set had > been > > > > > >> > determined, > > > > > >> > > we > > > > > >> > > > would need to go back and adjust the relative timestamps. > > > > > >> > > > > > > > > >> > > > > > > > >> > > Yes, I thought this would be a bit tricky and hence why I > > > > mentioned > > > > > >> the > > > > > >> > > option of adding a new field at the message set level for > the > > > > first > > > > > >> > > timestamp even though that's not ideal either. > > > > > >> > > > > > > > >> > > Here's one idea. We let the timestamps in the messages be > > > varints, > > > > > >> but we > > > > > >> > > > make their values be relative to the timestamp of the > > previous > > > > > >> message, > > > > > >> > > > with the timestamp of the first message being absolute. > For > > > > > >> example, if > > > > > >> > > we > > > > > >> > > > had timestamps 500, 501, 499, then we would write 500 for > > the > > > > > first > > > > > >> > > > message, 1 for the next, and -2 for the final message. > Would > > > > that > > > > > >> work? > > > > > >> > > Let > > > > > >> > > > me think a bit about it and see if there are any problems. > > > > > >> > > > > > > > > >> > > > > > > > >> > > It's an interesting idea. Comparing to the option of having > > the > > > > > first > > > > > >> > > timestamp in the message set, It's a little more space > > efficient > > > > as > > > > > we > > > > > >> > > don't have both a full timestamp in the message set _and_ a > > > varint > > > > > in > > > > > >> the > > > > > >> > > first message (which would always be 0, so we avoid the > extra > > > > byte) > > > > > >> and > > > > > >> > > also the deltas could be a little smaller in the common > case. > > > The > > > > > main > > > > > >> > > downside is that it introduces a semantics inconsistency > > between > > > > the > > > > > >> > first > > > > > >> > > message and the rest. Not ideal, but maybe we can live with > > > that. > > > > > >> > > > > > > > >> > > Ismael > > > > > >> > > > > > > > >> > > > > > > >> > > > > > > > > > > > > > > > > > > > > > > > > > > >
