2016-04-11 3:01 GMT+02:00 Jun Rao <j...@confluent.io>: > Thinking about ApiVersionRequest a bit more. There are quite a few things > special about it. In the ideal case, (1) its version should never change; >
The only thing we know of the future is that we dont know anything, we can't think of every possible future use case, that's why need to be able to evolve interfaces as requirements and use-cases change. This is the gist of KIP-35, and hampering KIP-35 itself, by not letting it also evolve, does not seem right to me at all. > (2) it needs to be done before authentication (either SSL/SASL); (3) it is > required to be issued at the beginning of each connection but never needs > to be issued again on the same connection. So, instead of modeling it as a > regular request, it seems a cleaner approach is to just bake that into the > initial connection handshake even before the authentication layer. So the > sequencing in a connection will be api discovery, authentication, followed > by regular requests. I am not sure we can still add this in a backward > compatible way now (e.g., not sure what the initial bytes from an SSL > connection will look like). Even if we can do this in a backward compatible > way, it's probably non-trivial amount of work though. > I have the luxory of not knowing the broker internals, so I can only discuss this on a conceptual design level. In its simplest form each API request type has a NeedsAuth flag and the broker protocol request layer simply checks if the current session is Authenticated before processing the request: If not the session is closed and an error is logged. The only two API requests that dont have the NeedsAuth flag would be SaslHandshakeRequest and ApiVersionRequest, the latter could also use filtering to only return the same two requests in ApiVersionResponse before the client is authenticated (as not to "leak" information). If authentication is not configured on the broker all sessions are deemed authenticated by default. Re backwards compatibility: My suggestion is to keep the current special SASL handshake protocol on the SASL_PLAIN/SASL_SSL endpoints, but use the new in-band Kafka SaslHandshakeRequest API on the PLAIN/SSL endpoints. This way the broker is backwards compatible with older clients that only supports the special SASL protocol, and newer cliets are also backwards compatible with older brokers that only supports the special SASL protocol. Newer clients connecting to new brokers will be configured to use non-SASL ports and use the in-band Kafka SaslHandshakeRequest to authenticate. Using the existing standard Kafka protocol and the new future-proof functionality of ApiVersionRequest allows the in-band authentication mechanisms and semantics to naturally evolve over time without breaking existing clients. > > We started KIP-35 with supporting a client to know if a version is > supported by the broker. It's now evolved into supporting a client to > implement multiple versions of the protocol and dynamically pick a version > supported by the broker. The former is likely solvable without > ApiVersionRequest. How important is the latter? What if the C client just > follows the java client model by implementing one version of protocol per C > client release (which seems easier to implement)? > We've discussed this at length and it is not an option for librdkafka, nor kafka-python, and probably other clients as well, due to usability/UX and maintenance reasons. (There's even discussion of making the Java client more version agnostic!) > > Thanks, > > Jun > > > On Fri, Apr 8, 2016 at 4:20 PM, Jun Rao <j...@confluent.io> wrote: > > > Magnus, > > > > A while back, we had another proposal for the broker to just send the > > correlation id and an empty payload if it receives an unsupported version > > of the request. I didn't see that in the rejected section. It seems > simpler > > than the current proposal where the client has to issue an > > ApiVersionRequest on every connection. Could you document the reason why > we > > rejected it? > > > > Thanks, > > > > Jun > > > > On Tue, Apr 5, 2016 at 1:47 PM, Ashish Singh <asi...@cloudera.com> > wrote: > > > >> On Fri, Apr 1, 2016 at 1:32 AM, Ismael Juma <ism...@juma.me.uk> wrote: > >> > >> > Two more things: > >> > > >> > 3. We talk about backporting of new request versions to stable > branches > >> in > >> > the KIP. In practice, we can't do that until the Java client is > changed > >> so > >> > that it doesn't blindly use the latest protocol version. Otherwise, if > >> new > >> > request versions were added to 0.9.0.2, the client would break when > >> talking > >> > to a 0.9.0.1 broker (given Jason's proposal, it would fail a bit more > >> > gracefully, but that's still not good enough for a stable branch). It > >> may > >> > be worth making this clear in the KIP (yes, it is a bit orthogonal and > >> > doesn't prevent the KIP from being adopted, but good to avoid > >> confusion). > >> > > >> Good point. Adding this note and also adding a note that Kafka has not > >> backported an api version so far. > >> > >> > > >> > 4. The paragraph below is a bit confusing. It starts talking about > 0.9.0 > >> > and trunk and then switches to 0.9.1. Is that intentional? > >> > > >> Yes. > >> > >> > > >> > "Deprecation of a protocol version will be done by marking a protocol > >> > version as deprecated in protocol documentation. Documentation shall > >> also > >> > be used to indicate a protocol version that must not be used, or for > any > >> > such information.For instance, say 0.9.0 had protocol versions [0] for > >> api > >> > key 1. On trunk, version 1 of the api key was added. Users running off > >> > trunk started using version 1 of the api and found out a major bug. To > >> > rectify that version 2 of the api is added to trunk. For some reason, > >> it is > >> > now deemed important to have version 2 of the api in 0.9.1 as well. To > >> do > >> > so, version 1 and version 2 both of the api will be backported to the > >> 0.9.1 > >> > branch. 0.9.1 broker will return 0 as min supported version for the > api > >> and > >> > 2 for the max supported version for the api. However, the version 1 > >> should > >> > be clearly marked as deprecated on its documentation. It will be > >> client's > >> > responsibility to make sure they are not using any such deprecated > >> version > >> > to the best knowledge of the client at the time of development (or > >> > alternatively by configuration)." > >> > > >> > Ismael > >> > > >> > > >> > > >> > On Fri, Apr 1, 2016 at 9:22 AM, Ismael Juma <ism...@juma.me.uk> > wrote: > >> > > >> > > A couple of questions: > >> > > > >> > > 1. The KIP says "Specific version may be deprecated through protocol > >> > > documentation but must still be supported (although it is fair to > >> return > >> > an > >> > > error code if the specific API supports it).". It may be worth > >> expanding > >> > > this a little more. For example, what does it mean to support the > >> API? I > >> > > guess this means that the broker must not disconnect the client and > >> the > >> > > broker must return a valid protocol response. Given that it says > that > >> it > >> > is > >> > > "fair" (I would probably replace "fair" with "valid") to return an > >> error > >> > > code if the specific API supports it, it sounds like we are saying > >> that > >> > we > >> > > don't have to maintain the semantic behaviour (i.e. we could > _always_ > >> > > return an error for a deprecated API?). Is this true? > >> > > > >> > > 2. ApiVersionQueryRequest seems a bit verbose, why not > >> ApiVersionRequest? > >> > > > >> > > Ismael > >> > > > >> > > >> > >> > >> > >> -- > >> > >> Regards, > >> Ashish > >> > > > > >
