Bill, For your case since once the log is cleaned up to the given offset watermark (or threshold, whatever the name is), future cleaning with the same watermark will effectively be a no-op, so I feel your scenario will be better fit as a one-time admin tool to cleanup the logs rather than customizing the periodic cleaning policy. Does this sound reasonable to you?
Guozhang On Wed, Jan 20, 2016 at 7:09 PM, Bill Warshaw <bill.wars...@appian.com> wrote: > For our particular use case, we would need to. This proposal is really two > separate pieces: custom log compaction policy, and the ability to set > arbitrary key-value pairs in a Topic configuration. > > I believe that Kafka's current behavior of throwing errors when it > encounters configuration keys that aren't defined is meant to help users > not misconfigure their configuration files. If that is the sole motivation > for it, I would propose adding a property namespace, and allow users to > configure arbitrary properties behind that particular namespace, while > still enforcing strict parsing for all other properties. > > On Wed, Jan 20, 2016 at 9:23 PM, Guozhang Wang <wangg...@gmail.com> wrote: > > > So do you need to periodically update the key-value pairs to "advance the > > threshold for each topic"? > > > > Guozhang > > > > On Wed, Jan 20, 2016 at 5:51 PM, Bill Warshaw <bill.wars...@appian.com> > > wrote: > > > > > Compaction would be performed in the same manner as it is currently. > > There > > > is a predicate applied in the "shouldRetainMessage" function in > > LogCleaner; > > > ultimately we just want to be able to swap a custom implementation of > > that > > > particular method in. Nothing else in the compaction codepath would > need > > > to change. > > > > > > For advancing the "threshold transaction_id", ideally we would be able > to > > > set arbitrary key-value pairs on the topic configuration. We have > access > > > to the topic configuration during log compaction, so a custom policy > > class > > > would also have access to that config, and could read anything we > stored > > in > > > there. > > > > > > On Wed, Jan 20, 2016 at 8:14 PM, Guozhang Wang <wangg...@gmail.com> > > wrote: > > > > > > > Hello Bill, > > > > > > > > Just to clarify your use case, is your "log compaction" executed > > > manually, > > > > or it is triggered periodically like the current log cleaning by-key > > > does? > > > > If it is the latter case, how will you advance the "threshold > > > > transaction_id" each time when it executes? > > > > > > > > Guozhang > > > > > > > > > > > > On Wed, Jan 20, 2016 at 1:50 PM, Bill Warshaw < > bill.wars...@appian.com > > > > > > > wrote: > > > > > > > > > Damian, I appreciate your quick response. > > > > > > > > > > Our transaction_id is incrementing for each transaction, so we will > > > only > > > > > ever have one message in Kafka with a given transaction_id. We > > thought > > > > > about using a rolling counter that is incremented on each > checkpoint > > as > > > > the > > > > > key, and manually triggering compaction after the checkpoint is > > > complete, > > > > > but our checkpoints are asynchronous. This means that we would > have > > a > > > > set > > > > > of messages appended to the log after the checkpoint started, with > > > value > > > > of > > > > > the previous key + 1, that would also be compacted down to a single > > > > entry. > > > > > > > > > > Our particular custom policy would delete all messages whose key > was > > > less > > > > > than a given transaction_id that we passed in. I can imagine a > wide > > > > > variety of other custom policies that could be used for retention > > based > > > > on > > > > > the key and value of the message. > > > > > > > > > > On Wed, Jan 20, 2016 at 1:35 PM, Bill Warshaw < > > bill.wars...@appian.com > > > > > > > > > wrote: > > > > > > > > > > > Hello, > > > > > > > > > > > > I'm working on a team that is starting to use Kafka as a > > distributed > > > > > > transaction log for a set of in-memory databases which can be > > > > replicated > > > > > > across nodes. We decided to use Kafka instead of Bookkeeper for > a > > > > > variety > > > > > > of reasons, but there are a couple spots where Kafka is not a > > perfect > > > > > fit. > > > > > > > > > > > > The biggest issue facing us is deleting old transactions from the > > log > > > > > > after checkpointing the database. We can't use any of the > built-in > > > > size > > > > > or > > > > > > time-based deletion mechanisms efficiently, because we could get > > > > > ourselves > > > > > > into a dangerous state where we're deleting transactions that > > haven't > > > > > been > > > > > > checkpointed yet. The current approach we're looking at is > > rolling a > > > > new > > > > > > topic each time we checkpoint, and deleting the old topic once > all > > > > > replicas > > > > > > have consumed everything in it. > > > > > > > > > > > > Another idea we came up with is using a pluggable compaction > > policy; > > > we > > > > > > would set the message key as the offset or transaction id, and > the > > > > policy > > > > > > would delete all messages with a key smaller than that id. > > > > > > I took a stab at implementing the hook in Kafka for pluggable > > > > compaction > > > > > > policies at > > > > > > > > > > > > > > > > > > > > > > > > > > > https://github.com/apache/kafka/compare/trunk...bill-warshaw:pluggable_compaction_policy > > > > > > (rough implementation), and it seems fairly straightforward. One > > > > problem > > > > > > that we run into is that the custom policy class can only access > > > > > > information that is defined in the configuration, and the > > > configuration > > > > > > doesn't allow custom key-value pairs; if we wanted to pass it > > > > information > > > > > > dynamically, we'd have to use some hack like calling Zookeeper > from > > > > > within > > > > > > the class. > > > > > > To get around this, my best idea is to add the ability to specify > > > > > > arbitrary key-value pairs in the configuration, that our client > > could > > > > use > > > > > > to pass information to the custom policy. Does this set off any > > > alarm > > > > > > bells for you guys? If so, are there other approaches we could > > take > > > > that > > > > > > come to mind? > > > > > > > > > > > > > > > > > > Thanks for your time, > > > > > > Bill Warshaw > > > > > > > > > > > > > > > > > > > > > > -- > > > > > <http://appianworld.com> > > > > > This message and any attachments are solely for the intended > > recipient. > > > > If > > > > > you are not the intended recipient, disclosure, copying, use, or > > > > > distribution of the information included in this message is > > prohibited > > > -- > > > > > please immediately and permanently delete this message. > > > > > > > > > > > > > > > > > > > > > -- > > > > -- Guozhang > > > > > > > > > > -- > > > <http://appianworld.com> > > > This message and any attachments are solely for the intended recipient. > > If > > > you are not the intended recipient, disclosure, copying, use, or > > > distribution of the information included in this message is prohibited > -- > > > please immediately and permanently delete this message. > > > > > > > > > > > -- > > -- Guozhang > > > > -- > <http://appianworld.com> > This message and any attachments are solely for the intended recipient. If > you are not the intended recipient, disclosure, copying, use, or > distribution of the information included in this message is prohibited -- > please immediately and permanently delete this message. > -- -- Guozhang
