I seem to be late to the party on this one - can you summarize what happened to the log files - and wire protocol - as a result of this change?
I have some questions regarding what implications this change has: 1) is it required or optional, iotw can the physical offset mechanism still be used? 2) if required, is it going to change the ability of kafka for ultra-efficient i/o (currently, near zero memory is used, sendfile keeps things fast and tidy) 3) What additional capabilities does this give - for example can one do negative indexing and positive indexing from a given offset? I am wondering, is this a specific instance of a more general class of indexing? By negative indexing, I mean for example at Tagged we wanted the capability to retrieve the message at m messages ago from the top of the message queue. To implement this, we patched kafka itself to create a shadow kafka topic that for every message received in topic A, would write the offset of said message from topic A into the shadow topic A'. Since offsets are fixed width, the topic A' is effectively an index and to find m messages ago is simple math --> read from offset n - m*(fixed message size) where n is the current offset of A' and m is num messages ago which will result in an offset in topic A.. Can this new ability you have implemented provide that kind of functionality? On Mon, Oct 8, 2012 at 12:16 PM, Jay Kreps (JIRA) <j...@apache.org> wrote: > > [ > https://issues.apache.org/jira/browse/KAFKA-506?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel] > > Jay Kreps resolved KAFKA-506. > ----------------------------- > > Resolution: Fixed > > Committed. > > > Store logical offset in log > > --------------------------- > > > > Key: KAFKA-506 > > URL: https://issues.apache.org/jira/browse/KAFKA-506 > > Project: Kafka > > Issue Type: Bug > > Affects Versions: 0.8 > > Reporter: Jay Kreps > > Assignee: Jay Kreps > > Fix For: 0.8 > > > > Attachments: KAFKA-506-phase-2.patch, > KAFKA-506-phase-2-v1.patch, KAFKA-506-phase-2-v2.patch, > KAFKA-506-phase-2-v3.patch, KAFKA-506-phase-2-v4.patch, > KAFKA-506-phase-2-v5.patch, KAFKA-506-phase-2-v5.patch, > KAFKA-506-v1-draft.patch, KAFKA-506-v1.patch, > KAFKA-506-v4-changes-since-v3.patch > > > > > > Currently we only support retention by dropping entire segment files. A > more nuanced retention policy would allow dropping individual messages from > a segment file by recopying it. This is not currently possible because the > lookup structure we use to locate messages is based on the file offset > directly. > > To fix this we should move to a sequential, logical offset (0,1,2,3,...) > which would allow deleting individual messages (e.g. 2) without deleting > the entire segment. > > It is desirable to make this change in the 0.8 timeframe since we are > already doing data format changes. > > As part of this we would explicitly store the key field given by the > producer for partitioning (right now there is no way for the consumer to > find the value used for partitioning). > > This combination of features would allow a key-based retention policy > that would clean obsolete values either by a user defined key. > > The specific use case I am targeting is a commit log for local state > maintained by a process doing some kind of near-real-time processing. The > process could log out its local state changes and be able to restore from > this log in the event of a failure. However I think this is a broadly > useful feature. > > The following changes would be part of this: > > 1. The log format would now be > > 8 byte offset > > 4 byte message_size > > N byte message > > 2. The offsets would be changed to a sequential, logical number rather > than the byte offset (e.g. 0,1,2,3,...) > > 3. A local memory-mapped lookup structure will be kept for each log > segment that contains the mapping from logical to physical offset. > > I propose to break this into two patches. The first makes the log format > changes, but retains the physical offset. The second adds the lookup > structure and moves to logical offset. > > Here are a few issues to be considered for the first patch: > > 1. Currently a MessageSet implements Iterable[MessageAndOffset]. One > surprising thing is that the offset is actually the offset of the next > message. I think there are actually several uses for the current offset. I > would propose making this hold the current message offset since with > logical offsets the next offset is always just current_offset+1. Note that > since we no longer require messages to be dense, it is not true that if the > next offset is N the current offset is N-1 (because N-1 may have been > deleted). Thoughts or objections? > > 2. Currently during iteration over a ByteBufferMessageSet we throw an > exception if there are zero messages in the set. This is used to detect > fetches that are smaller than a single message size. I think this behavior > is misplaced and should be moved up into the consumer. > > 3. In addition to adding a key in Message, I made two other changes: (1) > I moved the CRC to the first field and made it cover the entire message > contents (previously it only covered the payload), (2) I dropped support > for Magic=0, effectively making the attributes field required, which > simplifies the code (since we are breaking compatibility anyway). > > -- > This message is automatically generated by JIRA. > If you think it was sent incorrectly, please contact your JIRA > administrators > For more information on JIRA, see: http://www.atlassian.com/software/jira >
