Category: Tech Notes

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React and Redux error: “_react.default.createContext is not a function”

I’m adding Redux to an existing React app, converting from Flux to Redux. After wrapping my root component with the <Provider> I got this error and the app doesn’t initialize:

_react.default.createContext is not a function

Doing a quick search found this post suggesting I’ve got a mismatch in versions between newest Redux versions and possibly older versions of React. This is possible as I’m updating an older React app I put together a couple of years ago. Quick way to test this, remove React and react-dom and add them back again.

Right now I have:

"react": "^15.4.1",
"react-dedux": "^0.4.0-beta.4",
"react-dom": "^15.4.1",
"react-redux": "^7.2.0"

Removed React and react-dom and added back latest versions:

"react": "^16.13.1",
"react-dedux": "^0.4.0-beta.4",
"react-dom": "^16.13.1",
"react-redux": "^7.2.0"

Restated my app, fixed!

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Kafka Streams error: “PolicyViolationException: Topic replication factor must be 3”

I’m creating a Streams app to consume a Topic and do a count with results in a KTable, and I’ve got this error:

2020-05-03 15:23:26,373 [streamsapp1-e7955018-aca6-43ce-8967-abd6c6d238d9-StreamThread-1] ERROR org.apache.kafka.streams.processor.internals.InternalTopicManager [] - stream-thread [main] Unexpected error during topic creation for streamsapp1-KSTREAM-AGGREGATE-STATE-STORE-0000000001-changelog.
Error message was: org.apache.kafka.common.errors.PolicyViolationException: Topic replication factor must be 3
2020-05-03 15:23:26,374 [streamsapp1-e7955018-aca6-43ce-8967-abd6c6d238d9-StreamThread-1] ERROR org.apache.kafka.streams.processor.internals.StreamThread [] - stream-thread [streamsapp1-e7955018-aca6-43ce-8967-abd6c6d238d9-StreamThread-1] Encountered the following unexpected Kafka exception during processing, this usually indicate Streams internal errors:
org.apache.kafka.streams.errors.StreamsException: Could not create topic streamsapp1-KSTREAM-AGGREGATE-STATE-STORE-0000000001-changelog.

I think the clue here is that the app is trying to create an intermediate topic, streamsapp1-KSTREAM-AGGREGATE-STATE-STORE-0000000001-changelog, and is failing because the Replication Factor for the intermediate topic does not match the rf of the source topic. This is described in the Streams docs for option config options here.

Adding config property replication.factor=3 to match the rf of the source topic fixes the issue.

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Using confluent cli to start/stop a single node Kafka cluster

Install steps for Confluent Platform are here.

Using confluent cli:

confluent local status
$ confluent local start
The local commands are intended for a single-node development environment
only, NOT for production usage. https://docs.confluent.io/current/cli/index.html
Using CONFLUENT_CURRENT: /tmp/confluent.9Uym9FYU
Starting zookeeper
zookeeper is [UP]
Starting kafka
kafka is [UP]
Starting schema-registry
schema-registry is [UP]
Starting kafka-rest
kafka-rest is [UP]
Starting connect
connect is [UP]
Starting ksql-server
ksql-server is [UP]
Starting control-center
control-center is [UP]
confluent local stop
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awk basics

I’ve tinkered with awk and sed for quick one off tasks before in the past but I always have to go back and check the syntax for awk as it’s one of those things I use only once in a while.

Some quick notes for reference for later:

  • awk splits records in an input file by default by white space chars
  • uses $0 to refer to a whole line, and $1 … $n to refer to each matching token on a line
  • to split using column delimiters other than whitespace use -F

Examples:

Example file – example1.txt:

aaa bbb ccc
ddd eee fff

$ awk {'print $1'} example1.txt

Will print the first matching column:

aaa
ddd

If file has other column delimiters use -F to specify the delimiter, for example, example2.txt:

aaa,bbb,ccc
ddd,eee,fff

$ awk -F , {'print $2'} example2.txt

Will match column 2:

bbb
eee

More info on awk here.

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