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The fluvio consume command

fluvio consume​

The fluvio consume command is a way to read the contents of records in a Fluvio topic from a command-line environment.

The consume command will only read from one of those partitions, defaulting to partition 0.

The following fluvio consume examples come after the fluvio produce examples.

Examples​

Consume all records​

When consuming, we need to specify a starting offset from which to begin reading. We can use the --from-beginning (-B) flag in order to read everything from the very beginning. Here we'll also use the --disable-continuous (-d) flag in order to exit after all the records have been read:

$ fluvio consume my-topic -B -d
This is my first record ever
This is my second record ever
Alice In Wonderland
Bruce Wayne
Santa Claus

Notice that all the records are printed by value only: the records with keys have not had their keys printed! This is the default behavior of the consumer. To see how to print the keys of key/value records, see the next example!

Consume key/value records​

If we want to see both the keys and values of the records in the topic, you can use the --key-value flag:

$ fluvio consume my-topic -dB --key-value
[null] This is my first record ever
[null] This is my second record ever
[alice] Alice In Wonderland
[batman] Bruce Wayne
[santa] Santa Claus

Records that were not given a key are printed with [null].

Consume using a SmartModule​

Fluvio SmartModules are WASM modules that can edit the contents of a stream inline, before the records of that stream are delivered to a consumer. One way to use SmartModules is to supply the WASM module with the --smartmodule-path flag to the fluvio consume command.

The simplest SmartModule is the filter example, which filters records from the stream based on whether they contain the letter a or not. You can find the full example code in our GitHub repo and compile it to test out yourself.

Once you have compiled your SmartModule Filter and have a .wasm file for it, you can apply it by sending the binary to the cluster when you start your CLI consumer:

$ fluvio consume my-topic -B --smartmodule-path="fluvio_wasm_filter.wasm"

Alternatively, to avoid sending the SmartModule binary to the cluster with each fluvio consume session, you can have the cluster store it for you:

$ fluvio smartmodule create --wasm-file="fluvio_wasm_filter.wasm" my_filter

Then you can apply the SmartModule by name:

$ fluvio consume my-topic -B --smartmodule="my_filter"

Consume from a topic with multiple partitions​

As of today, the Fluvio CLI Consumer can only consume records from a single partition at a time. When running fluvio consume topic-name, the CLI will read records from partition 0 by default. Let's look at how we can read records from the different partitions in a topic by using the --partition (-p) flag.

Start out by creating a new topic with multiple partitions using fluvio topic create.

$ fluvio topic create consume-multi -p 3

Let's create a text file with some records we would like to send. Each line of the text file will be treated as one record.

# Put the following records into a text file using your favorite editor
$ cat records.txt
one
two
three
four
five
six
seven
eight
nine

Then, produce the test data to the topic.

$ fluvio produce "consume-multi" -f records.txt

After producing some data, let's take a look at how the records got distributed among our partitions using fluvio partition list.

$ fluvio partition list
 TOPIC          PARTITION  LEADER  REPLICAS  RESOLUTION  HW  LEO  LSR  FOLLOWER OFFSETS
 consume-multi  0          5001    []        Online      3   3    0    []
 consume-multi  1          5001    []        Online      3   3    0    []
 consume-multi  2          5001    []        Online      3   3    0    []

We can see by the high watermark (HW) and log-end-offset (LEO) that 3 records were sent to each partition. Let's look at how to consume from each partition.

To consume from a specific partition, use the --partition (-p) flag on fluvio consume.

$ fluvio consume "consume-multi" -B --partition 0
one
four
seven

To consume from partition 1:

$ fluvio consume "consume-multi" -B --partition 1
two
five
eight

And from partition 2:

$ fluvio consume "consume-multi" -B --partition 2
three
six
nine

Consume from all partitions​

At times, it is useful to see all records from all partitions from a single consumer. Using the example above:

$ fluvio partition list
 TOPIC          PARTITION  LEADER  REPLICAS  RESOLUTION  HW  LEO  LSR  FOLLOWER OFFSETS
 consume-multi  0          5001    []        Online      3   3    0    []
 consume-multi  1          5001    []        Online      3   3    0    []
 consume-multi  2          5001    []        Online      3   3    0    []

Each partition has 3 records. Now let's consume from all partitions:

$ fluvio consume "consume-multi" -B -A
one
four
seven
two
three
five
six
eight
nine
tip

There is no order guarantee between partitions.

Sometimes, the default Consumer printout might not work for your needs. As of Fluvio 0.9.6 you can now use the --format string to describe how the Consumer should print your records!

The format string will replace placeholders such as {{key}}, {{value}}, {{partition}}(added in Fluvio 0.9.9 ), {{offset}} and {{time}} (added in Fluvio 0.9.25) with the actual contents for each record. One possible use for this is formatting each record as a CSV row:

$ fluvio consume my-topic -B --format="{{time}},{{partition}},{{offset}},{{key}},{{value}}"
2022-05-04T15:35:49.244Z,0,0,null,This is my first record ever
2022-05-04T15:35:49.244Z,0,1,null,This is my second record ever
2022-05-04T15:52:19.963Z,0,2,alice,Alice In Wonderland
2022-05-04T15:52:28.875Z,0,3,batman,Bruce Wayne
2022-05-04T15:53:37.099Z,0,4,santa,Santa Claus