I'm not sure if I understood correctly, but if you want to integrate a
single kafka producer transaction (or any transaction manager that only
supports local transaction) into a distributed transaction, I think you
can do so as long as all other involved transaction managers support
2-phase commit. In other words, you can include one (and only one)
local-only transaction into a distributed transaction.
The steps the distributed transaction coordinator would have to take to
commit the transaction would be:
1. call prepare on each of the transaction participants that support
2-phase commit
2. if any of them fails, abort all transactions, otherwise, proceed
3. call commit on the one transaction participant that does not support
2-phase commit (kafka producer in this case)
4. if that fails, abort all transactions, otherwise, proceed
5. call commit on all the transaction participants that support 2-phase
commit (since prepare on these succeeded they should not refuse to
commit at this point)
So as to your concern about getting "clearance" (I take it as the
equivalent of the "prepare" call) from the kafka producer, you don't
really need it IMO, as if commit fails on the kafka producer, you can
still abort the remaining transactions.
Of course you can't do that if you have more than one transaction that
doesn't support 2-phase commit in play.
Having said that, the advice these days seems to be to design
distributed systems for eventual consistency, as using distributed
transactions, while tempting, often leads to resource exhaustion as
transaction managers have to go the extra mile to ensure they can commit
any transaction that had prepare return successfully.
Just my 5c. I may be wrong in any of the above, please point it out if so.
Cheers,
Michał
On 19/06/17 14:57, Piotr Nowojski wrote:
Sorry for responding to my own message, but when I sent an original
message/question I was not subscribed to this mailing list and now I
can not respond to Matthias answer directly.
I don't want to share a transaction between multiple Producers
threads/processes, I just would like to resume an interrupted
transaction after a machine crash.
Let me try to phrase the problem differently:
From the perspective of a producer that writes to Kafka, we have the
following situation:
We integrate the producer with transaction in another system. A number
or records should go together atomically (a transaction). Before
committing the transaction, we frequently need to ask for a
"clearance" status, and if we get the "go ahead" we want to commit the
transaction.
Unfortunately, as soon as we get that "clearance", we cannot reproduce
the records any more (the are dropped from the original data stream).
If something fails between the "go ahead" and the committing, we need
to retry the transaction, so we need to come up again with all
records. As a result we have to persist the records before we start
the write transaction. That is a painful overhead to pay, and a lot of
additional operational complexity.
The simplest way to support that pattern without extra overhead would
we could "resume" a transaction:
- Each transaction as a unique Transaction ID
- If a crash of the producer occurs, the transaction is NOT aborted
automatically.
- Instead, the restarted producer process reconnects to the
transaction and decides to commit it or abort it.
- The transaction timeout aborts the transaction after a while if
inactivity.
Maybe this could be easily supported?
Thanks, Piotrek
2017-06-16 17:59 GMT+02:00 Piotr Nowojski <piotr.nowoj...@gmail.com
<mailto:piotr.nowoj...@gmail.com>>:
But isn't it a low hanging fruit at this moment? Isn't that just
an API limitation and wouldn't the backend for transactions
support it with only minor changes to the API (do not fail
automatically dangling transactions on Producer restart)? Flushing
is already there so that _should_ handle the pre-commit. Again,
maybe I'm missing something and for sure I am not familiar with
Kafka's internals.
Piotrek
2017-06-16 15:47 GMT+02:00 Michal Borowiecki
<michal.borowie...@openbet.com
<mailto:michal.borowie...@openbet.com>>:
I don't think KIP-98 is as ambitious as to provide support for
distributed transactions (2 phase commit).
It would be great if I was wrong though :P
Cheers,
Michał
On 16/06/17 14:21, Piotr Nowojski wrote:
Hi, I'm looking into Kafka's transactions API as proposed in
KIP-98. I've read both this KIP-98 document and I looked into
the code that is on the master branch. I would like to use it
to implement some two phase commit mechanism on top of the
Kafka's transactions, that would allow me to tie multiple
systems (some of them might not be Kafka) in one transaction.
Maybe I'm missing something but the problem is I don't see a
way to implement it using proposed Kafka's transactions API.
Even if I have just two processes writing to Kafka topics, I
don't know how can I guarantee that if one's transaction is
committed, the other will also eventually be committed. This
is because if first KafkaProducer successfully commits, but
the second one fails before committing it's data, after
restart the second one's "initTransactions" call will
(according to my understanding of the API) abort previously
non completed transactions. Usually transactional systems
expose API like this
<http://hlinnaka.iki.fi/2013/04/11/how-to-write-a-java-transaction-manager-that-works-with-postgresql/>
<http://hlinnaka.iki.fi/2013/04/11/how-to-write-a-java-transaction-manager-that-works-with-postgresql/>.
Namely there is a known identifier for a transaction and you can
pre-commit
it (void prepare(...) method in before mentioned example) and then
commit
or you can abort this transaction. Usually pre-commit involves flushing
stuff to some temporary files and commit move those files to the final
directory. In case of machine/process failure, if it was before
"pre-commit", we can just rollback all transactions from all of the
processes. However once every process acknowledge that it completed
"pre-commit", each process should call "commit". If some process fails
at
that stage, after restarting this process, I would expect to be able to
restore it's "pre-committed" transaction (having remembered
transaction's
id) and re attempt to commit it - which should be guaranteed to
eventually
succeed.
In other words, it seems to me like the missing features of this API
for me
are:
1. possibility to resume transactions after machine/process crash. At
least
I would expect to be able to commit "flushed"/"pre-committed" data for
such
transactions.
2. making sure that committing already committed transactions doesn't
brake
anything
Or maybe there is some other way to integrate Kafka into such two phase
commit system that I'm missing?
Thanks, Piotrek
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