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Flow Project Template (Word Count)

This repository is an example and template for new Flow projects. Try it in your browser by selecting <> Code > Codespaces > New codespace, using GitHub Codespaces.

Not part of an organization with access to Codespaces? No problem. Follow the instructions to set up a local devcontainer and proceed in the same manner. Note, however, that Mac M1 is not currently supported for local development.

Wondering what Flow is? See 📖 Documentation or :octocat: github.com/estuary/flow. tl;dr: it makes it easy to continuously move and transform your data across all the places you want it.

Through the magic of VSCode devcontainers ✨, this repository comes with a PostgreSQL database that you interact with.

Continuous Materializations in PostgreSQL

PostgreSQL is super great, and supports materialized views, but it doesn't offer continuous materialized views. So... let's build one with Flow?

How many times have you managed text documents in PostgreSQL and thought to yourself:

"Gee-whiz, self, I wish I had a table of word counts that was always up-to-date!"

... basically never, right? Well, let's do it anyway! This example composes:

  • Change data capture (CDC) from a documents table.
  • A derivation which computes word and document frequency updates.
  • A materialization back into a word_counts table.

🧪 Verify Tests

All cutting-edge word count projects should have tests. Let's make sure the words are, um, counted:

$ flowctl test --source word-counts.flow.yaml
... snip ...
Running  1  tests...
✔️ word-counts.flow.yaml :: acmeCo/tests/word-counts-from-documents

Ran 1 tests, 1 passed, 0 failed

🚢 Run It Locally

Start a local, temporary Flow data plane:

$ flowctl temp-data-plane 
export BROKER_ADDRESS=http://localhost:8080
export CONSUMER_ADDRESS=http://localhost:9000

A data plane is a long-lived, multi-tenant, scale-out component that usually runs in a data center. Fortunately it also shrinks down to your laptop. Note the exported addresses above; you'll need those.

⚙️ Deploy the catalog to your data plane:

$ export BROKER_ADDRESS=http://localhost:8080
$ export CONSUMER_ADDRESS=http://localhost:9000
$ flowctl deploy --wait-and-cleanup --source flow.yaml
... snip ...
Deployment done. Waiting for Ctrl-C to clean up and exit.

🔍 Flow is now watching the documents table, and materializing to word_counts:

$ psql --host localhost
psql (13.5 (Debian 13.5-0+deb11u1), server 13.2 (Debian 13.2-1.pgdg100+1))
Type "help" for help.

flow=# insert into documents (body) values ('The cat in the hat.'), ('hat Cat CAT!');
INSERT 0 2
flow=# select word, count, doc_count from word_counts;
 word | count | doc_count
------+-------+-----------
 cat  |     3 |         2
 hat  |     2 |         2
 in   |     1 |         1
 the  |     2 |         1
(4 rows)
flow=# update documents set body = 'cat Cat CAT!' where id = 2;
UPDATE 1
flow=# select word, count, doc_count from word_counts order by word;
 word | count | doc_count
------+-------+-----------
 cat  |     4 |         2
 hat  |     1 |         1
 in   |     1 |         1
 the  |     2 |         1
(4 rows)
flow=# delete from documents ;
DELETE 2
flow=# select word, count, doc_count from word_counts order by word;
 word | count | doc_count
------+-------+-----------
 cat  |     0 |         0
 hat  |     0 |         0
 in   |     0 |         0
 the  |     0 |         0
(4 rows)