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From "Javed, Haseeb" <javed...@buckeyemail.osu.edu>
Subject Re: How is CorrelationId used for matching request and response
Date Sun, 01 Oct 2017 23:24:01 GMT

I looked in to the code for NetworkClient.java and it all makes sense now. The client fetches
the last request sent out the particular server using InFlightRequest.completeNext(String
source). The correlationId from this request must match with the correlationId in the response
header for them to be in order.


Thanks all for the help.

Haseeb

________________________________
From: Javed, Haseeb <javed.19@buckeyemail.osu.edu>
Sent: Sunday, October 1, 2017 6:39:13 AM
To: dev@kafka.apache.org
Subject: Re: How is CorrelationId used for matching request and response

Thanks all for reaching out.



Ted - I am looking at the 0.11.0 release. Particularly here https://github.com/apache/kafka/blob/0.11.0/clients/src/main/java/org/apache/kafka/common/requests/AbstractResponse.java
[https://avatars3.githubusercontent.com/u/47359?v=4&s=400]<https://github.com/apache/kafka/blob/0.11.0/clients/src/main/java/org/apache/kafka/common/requests/AbstractResponse.java>

apache/kafka<https://github.com/apache/kafka/blob/0.11.0/clients/src/main/java/org/apache/kafka/common/requests/AbstractResponse.java>
github.com
kafka - Mirror of Apache Kafka




In this release, the Server uses the following method in almost all cases (ApiVersionResponses.unsupportedVersionSend(...)
being the only exception)

public Send toSend(String destination, RequestHeader requestHeader) {
    return toSend(destination, requestHeader.apiVersion(), requestHeader.toResponseHeader());
}


Jay - I understand the purpose of correlationId but what I don't understand is how the request/response
matching logic is being implemented. From the code, I see that server always uses the request
header to generate the response header so in effect both request and response headers end
up having the same correlationId. There seems to be no situation where response and request
could possible have different correlationIds.
<https://github.com/apache/kafka/blob/0.11.0/clients/src/main/java/org/apache/kafka/common/requests/AbstractResponse.java>


Haseeb

________________________________
From: Jay Kreps <jay@confluent.io>
Sent: Saturday, September 30, 2017 11:43:30 PM
To: dev@kafka.apache.org
Subject: Re: How is CorrelationId used for matching request and response

Yes the idea of the correlation id is to make it easier for the client to
match a particular response to the request it answers. Kafka’s protocol
allows sending multiple requests without waiting for the response. In
theory you can just rely on ordering, but that can be a bit fragile if the
client has any kind of bug. So this id is an explicit check—a response with
id 42 is the answer to the request you sent with id 42. Hope that helps!

-Jay

On Fri, Sep 29, 2017 at 4:52 PM Ted Yu <yuzhihong@gmail.com> wrote:

> Which release / version are you looking at ?
> In trunk branch, I only see one toSend():
>
>     protected Send toSend(String destination, ResponseHeader header, short
> apiVersion) {
>
>         return new NetworkSend(destination, serialize(apiVersion, header));
>
> On Fri, Sep 29, 2017 at 4:49 PM, Javed, Haseeb <
> javed.19@buckeyemail.osu.edu
> > wrote:
>
> > The Kafka protocol guide mentions that each request and response contains
> > a correlationId which is a user-supplied integer to match requests and
> > corresponding responses. However, when I look at the code in the class
> > AbstractResponse, we have a method defined as following:
> >
> >
> > public Send toSend(String destination, RequestHeader requestHeader) {
> >     return toSend(destination, requestHeader.apiVersion(), requestHeader.
> > toResponseHeader());
> > }
> >
> > So basically we are just using the requestHeader to generate the
> > responseHeader so doesn't this pretty much guarantees that the
> > correlationId for the Request and the Response would always be the same,
> or
> > am I missing something?
> >
> >
> >
>

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