Month: October 2022

Kafka – Messages Not Appearing in Topic

I created a few new Kafka topics for a project today — but, in testing, messages sent to the topic weren’t there. I normally echo some string into “” to test messages. Evidently, STDERR wasn’t getting rendered back to my screen this way. I ran the producer script to get the “>” prompt and tried again — voila, a useful error:

[2022-10-31 15:36:23,471] ERROR Error when sending message to topic MyTopic with key: null, value: 4 bytes with error: (
org.apache.kafka.common.InvalidRecordException: Compacted topic cannot accept message without key in topic partition MyTopic-0.

Ohhh — that makes sense! They’ve got an existing process on a different Kafka server, and I just mirrored the configuration without researching what the configuration meant. They use “compact” as their cleanup policy — so messages don’t really age out of the topic. They age out when a newer message with that key gets posted. It’s a neat algorithm that I remember encountering when I first started reading the Kafka documentation … but it’s not something I had a reason to use. The other data we have transiting our Kafka cluster is time-series data where we want all of the info for trending. Having just the most recent, say, CPU utilization on my server isn’t terribly useful. But it makes sense — if I instruct the topic to clean up old data but retain the most recent message for each key … I need to be giving it a key!

Adding a parameter to parse the string into a key/value pair and provide the separator led to data being published to the clients:

echo “test:EchoTest” | /kafka/bin/ –bootstrap-server $(hostname):9092 –topic MyTopic –property “parse.key=true” –property “key.separator=:”

Farm Kitten

This is Ash — our rescue kitten. The Cali-Kitty had four kittens. They were all around on the 27th playing and napping. The morning of the 28th, I woke up really early — just after sunrise. One of the gray kittens was sitting under our garden cart. When he saw me move, he came over to the door and started crying.

This was odd because, just the day before, Anya had observed that we’ve never heard any of these cats talk. I put his food outside (we take it in at night to avoid feeding the raccoons), and he ate while I let the ducks, chickens, and turkeys out. Throughout the day, he was staying near our house, all alone, and crying. But he was afraid of us so dart away any time we opened the door. Finally, we put the small trap out with his food in it. And, in the evening, we had a little gray kitten in the trap.

Scott spent some time getting the kitten to trust him while Anya and I got the birds into their coops. We brought him inside in a larger cage, gave him food and water, and a cat box. Anya sat next to the cage petting him (something that seemed to soothe him). By the next day, we could pick him up and hold him. He purrs when you cuddle and pet him. Anya has been taking him into the bathroom so he has a chance to explore without getting lost somewhere in the house.

Unable to Use JMX Remotely for Kafka Stats

I noticed, today, that our Kafka Manager interface only shows details from one server — the one where we run Kafka Manager. We’ve done everything that we need to do in order to get this working — the port shows as open with nmap, the command to run Kafka includes all of the settings. I’ve even tried setting the JMX hostname, but still there is just one server reporting data

Then I happened across an article online that detailed how JMX actually uses three ports — the configured port 9999 and two other randomly selected and non-configurable ports. I used netstat to list all of the ports in use by the Java PID running my Kafka server and, voila, there were two odd-ball high ports (30000’s and 40000’s). I added those additional ports to the firewall rules and … I’ve got data for all of the Kafka servers!

This is obviously a short-term solution as the two randomly selected ports will be different when I restart the service next time. I’d prefer to leave the firewall in place (i.e. not just open all ports >1024 between the Kafka Manager host and all of the Kafka servers) so might put together a script to identify the “oddball” ports associated to the Java pid and add them to transient firewalld rules. But the last server restart was back in 2021 … so I might just manually add them after the upgrade next week and worry about something ‘better’ next year!

The Ultimate Cat Toy

Neko’s favorite toy was two feathers mounted to a string so they made a V shape and spun like a maple seed as the string was pulled through the air. With a bunch of birds around the farm, though, I didn’t think it would be a good idea to train the kittens to chase after fluttering feathers. Anya has played with the kittens with string, she’s wiggled her fingers along the glass and they bat at them, she’s made a ball for them from her old socks. But today we encountered the ultimate cat toy — the seed pods from a sycamore tree — Scott picked a few up whilst he was mowing the lawn. Wow do the kittens enjoy batting these things all over the place. But you have to be careful walking around the back patio — never know where a ball might be lurking!

KDE Dolphin — Unable to Move Files and Folders

Scott was trying to move some backup files from /a/path/to/backup to /a/path/to-a-different/backup — he’s using Dolphin & has a tab open to each of the folders in question. He chown’d /a/path to his account, chmod’d /a/path so user can read and write. But using the copy/paste option … nothing happens.

I came across a few old (and closed) bugs that seemed to produce errors in this same situation — but the reporters were able to perform their copy/move operations when they used the same tab instead of having one folder open in each tab. It worked … inexplicable, but we have success!

Samba Server Not Appearing In Network Locations (Fedora)

One of our Samba servers — unfortunately the file server — did not show up when browsing the network locations in Dolphin. I map drives from Windows, so it’s not something I’ve really noticed … but Scott is trying to avoid using network mounts and wanted to access everything through “Network”. Several other servers — ones that Scott built — show up, but not the one where we actually store our files.

There’s no salient difference in the samba configurations. But … it turns out that the “newer” way Samba advertises its presence is through mDNS. And I routinely disable the avahi-daemon because, well, I “don’t use” mDNS for anything. Turns out I do use mDNS … so I had to enable and start the avahi-daemon (and restart Samba). Voila, the file server is visible in the list of network places.

Running a Docker Container without *RUNNING* The Container

I needed to get files from a container image that I couldn’t actually start (not enough memory, and finding a box with more memory wasn’t a reasonable option) — fortunately, you can override the container entrypoint to start the container without actually running whatever the container would normally run.

docker run -ti --entrypoint=bash imageName

Yum Does Not Work On CentOS

Since this is the fifth time this month that I’ve spun up some CentOS image and been stymied by the inability to install new packages … I’m going to write down the sed commands that magic the default yum repository configuration to something that’s still functional.

cd /etc/yum.repos.d/
sed -i 's/mirrorlist/#mirrorlist/g' /etc/yum.repos.d/CentOS-*
sed -i 's|#baseurl=|baseurl=|g' /etc/yum.repos.d/CentOS-*