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Kafka >> mail # dev >> Question about offsets

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Re: Question about offsets
Yes, offsets are now logical 0,1,2,3...

Some details on this change are here:

There was some previous threads on this that discussed the benefits. The
main goals included:
1. Allow key-based retention policy in addition to time-based retention.
This allows the topic to retain the latest value for each message key for
data streams that contain updates.
2. Fix the semantics of compressed messages (which are weird in 0.7 because
the compressed messages essentially have no offset and are unaddressable
hence commit() in the middle of a compressed message set will give
3. Allow odd iteration patterns (skipping ahead N messages, reading
messages backwards, etc)

It is also just nicer for humans and less prone to weird errors.

To answer your questions:

1. Lookups are done using a memory mapped index file kept with the log that
maps logical offsets to byte offsets. This index is sparse (need not
contain every message). Once the begin and end position in the log segment
are found the data transfer happens the same as before.

2. The rule is that all messages have offsets stored with the messages.
These are assigned by the server. So in your example, the offsets of B1, B2
will be sequentially assigned and the wrapper message will have the offset
of the last message in the compressed set.

As a result in 0.8 requesting any offset in the compressed message set will
yield that compressed messageset (you need the whole thing to decompress).
This means the java client now works correctly with commit() and compressed
message sets--if you commit in the middle of a compressed message set and
restart from that point you will read the compressed set again, throw away
the messages with offset lower than your consumers offset, and then start
consuming each message after that.

Hope that makes sense.

We really need to get this stuff documented since it is all floating around
on various wikis and JIRAs.

On Tue, Jan 29, 2013 at 8:38 PM, David Arthur <[EMAIL PROTECTED]> wrote:

> Fiddling with my Python client on 0.8, noticed something has changed with
> offsets.
> It seems that instead of a byte offset in the log file, the offset is now
> a logical one. I had a few questions about this:
> 1) How is the byte offset determined by the broker? Since messages are not
> fixed width, does it use an index or do a simple binary search?
> 2) Regarding compressed MessageSets, how are the offsets incremented?
> Suppose I have the following MessageSet
> MessageSet A
>  - Message A1, normal message
>  - Message A2, normal message
>  - Message A3, compressed MessageSet B
>  - MessageSet B
>     - Message B1
>     - Message B2
> Assuming we start from 0, message A1 gets offset of 0, A2 of 1. Now I am
> unclear how the numbering goes. Would A3 get offset 3, and B1 -> 4 B2 -> 5?
> Or are the offsets inside a compressed MessageSet not used?
> Is it possible to request a message inside a compressed message set?
> Also what about nested compression sets, what if Message B3 is itself a
> compressed MessageSet (not that it makes sense, just curious what would
> happen).
> Thanks!
> -David