Tag Archives: quirk

Kafka quirks: topics with names too long

http://clipart-library.com/clipart/8cznLELEi.htm

IMPORTANT: This bug is fixed in versions 2.1.2, 2.2.2, 2.3.0 and newer.

One of those bugs that you normally don’t come across… until you do. Officially, Kafka topic names can be up to 249 characters. If you try to create a topic with a name longer than that, it will reject it:

$ kafka-topics --bootstrap-server localhost:9092 --create --partitions 1 --replication-factor 1 --topic [topic name with 250 characters]
Error while executing topic command : org.apache.kafka.common.errors.InvalidTopicException: Topic name is illegal, it can't be longer than 249 characters, topic name: [topic name with 250 characters]

Pretty self-explanatory, right? Well, it’s not that simple…

249 characters? Actually… 209

While officially advertised as 249, in reality, you should not create topics with names longer than 209 characters. Why? One of the comments in the Jira ticket report explains it easily:

This limit ends up not being quite right since topic deletion ends up renaming the directory to the form topic-partition.uniqueId-delete as can be seen in LogManager.asyncDelete.

If we do the math:

  • 249 –
  • 1 character for the “.”
  • 32 characters for the uniqueId (a.k.a. UUID)
  • 7 characters for “-delete”

The result is 209. That is the longest your topic name can be if you don’t want to hit the bug.

What if I use a topic name longer than 209?

Well, terrible things… if you try to delete the topic. Otherwise nothing. But if you do try to delete the topic, the first error that you will encounter is this:

ERROR Error while renaming dir for [your long topic name here]-0 in log dir /var/lib/kafka/data (kafka.server.LogDirFailureChannel)
java.nio.file.FileSystemException: /var/lib/kafka/data/[your long topic name here]-0 -> /var/lib/kafka/data/[your long topic name here]-0.abb0d0dcdfd94e2b9222335d8bcaebcd-delete: File name too long

What is worse, your broker will keep failing upon restart, trying to remove the folder that is still pending to get deleted.

Get me out of this trap!

There is hope though if you can use a bit of SSH and connect to the broker that is failing (or maybe all of them are failing because they were all hosting partitions for the deleted topic).

Go to the directory where Kafka stores its data (log.dirs=/var/lib/kafka/data normally) and remove the folders for the partitions associated with the topic you want to get rid of.

Once that is deleted, it is time to mangle a bit with Zookeeper data. Be very careful here, if you make a mistake you will probably lose data (and maybe the whole cluster).

Log in into Zookeeper using zookeeper-shell and execute a couple of delete instructions to get rid of all traces of the faulty topic.

$ zookeeper-shell localhost:2181
rmr /admin/delete_topics/[your long topic name here]
rmr /brokers/topics/[your long topic name here]
rmr /config/topics/[your long topic name here]
quit

Now, restart your broker(s) and they should have completely forgotten that this topic ever existed…

What about AWS Kafka (MSK)?

As of this writing, MSK supports three versions:

  • 1.1.1: this version is just “too old” (published in July, 2018) and I wouldn’t recommend using it. It suffers this bug too.
  • 2.1.1: definitively suffers this bug (personally experienced it…).
  • 2.3.1: is not affected by this bug.

Therefore, my recommendation is to run Kafka 2.3.1 and take advantage of this and other bug fixes and features.

Kafka quirks: tombstones that refuse to disappear

From https://www.clipart.email/clipart/tombstone-rip-clipart-83182.html

Recently in one of the clients I consult for, I came across a strange situation: tombstone records that “refused” to disappear.

The scenario was quite simple:

  1. Kafka Streams application that materializes some state (in RocksDB).
  2. From time to time, a punctuation kicks it, pulls all the accumulated records and sends them somewhere.
  3. Upon success, it deletes all the records and calls it a day.

However, when consuming the changelog topic, I notice that there were lots of tombstone records. Having some of them made sense, that is how a “delete” should be represented in a changelog topic. However, having so many that hadn’t cleared out was unexpected.

I applied a few strategies/changes until I finally made them “gone”.

Step 1 – Roll your segments more often

Compaction only happens when the file that contains your topic/partition data is rolled. Therefore, it is important to adjust when that happens if you want to influence the compaction process:

  • segment.ms: the segment can stay open for up to this value. By default, that is 7 days.
  • segment.bytes: the segment can stay open up to this number of bytes. The default here is 1 GB, which is too big for low traffic topics.

The defaults for these two settings have “big data” stamped on them. If you don’t have a “big data” topic, chances are the process won’t be responsive enough for you.

I tried setting them up to 60,000 ms (1 min) and 1,048,576 (1 MB) respectively… with no luck. Nothing changed; tombstones were still there.

Step 2 – Tolerate less dirtiness

It is also possible that, even if your segments are rolling regularly, the log compaction thread doesn’t pick up your topic/partition file because it is not dirty enough, meaning the ratio between entries that are candidates for compaction and those that aren’t is not meeting the configured threshold.

min.cleanable.dirty.ratio controls this threshold and it is 0.5 by default, meaning you need at least 50% of your topic/partition file with “dirty” entries for compaction to run. Anything below that, the thread doesn’t find it worth doing compaction on it.

My next step was to set this value to 0.01. This quite aggressive and I wouldn’t recommend it for most topics, unless you have low volume and you really, really want to keep your topic/partition spotless.

However, this didn’t do the trick either…

Step 3 – Be less nice with your replaying consumers

When a consumer is replaying a topic from the beginning, it might encounter this problem:

  1. Offset X contains a record with Key K and Value V.
  2. A few records “later” (maybe millions…), record with Key K again, bu with a Value null, AKA tombstone.
  3. If the consumer reads the first record, but compaction runs and gets rid of the second record (the tombstone), the consumer will never know that the record with Key K has been deleted.

To compensate for this scenario, Kafka has a config setting called delete.retention.ms that controls how long tombstones should be kept around for the benefit of these consumers. Its default: 1 day.

This is very useful, but it will also keep tombstones around unnecessarily if you don’t expect any replaying consumer to read a given topic or, at least, to take as long as 1 day.

My next attempt was to configure this down to 60,000 ms (1 minute)… but still not working.

Step 4 – It’s not a feature… it’s a bug

I ran out of options here so I thought that maybe this is one of those rare and unfortunate occasions when I hit one of Kafka bugs. Fired up a quick search on Google and… voila!

Tombstones can survive forever: https://issues.apache.org/jira/browse/KAFKA-8522

Long story short, under certain circumstances, tombstones will get their “timeouts” renew regularly, meaning they will not honor delete.retention.ms and stick around.

The only walkaround that seems to work is to set delete.retention.ms to zero, forcing the tombstones to be deleted immediately, instead of sticking around for the benefit of consumers replaying the topic.

However, this solution must be used with great care. For the scenario described at the beginning, a Kafka Streams app and a changelog topic, using this option can have unexpected side effects during the Restore phase, when the app reads its changelog topics to restore its state. If, while doing so, compaction kicked in, it might miss the tombstone records for entries that it has already consumed, keeping entries in its key/value store that should have been removed.

Unfortunately, until the bug is fixed, if your app needs all these tombstones evicted from the changelog, this seems to be the only option.