event-stream

A system to accept event streams and take actions when certain conditions are met.

Setup steps

start redis docker run -d --name container-redis -p 127.0.0.1:6379:6379 --rm redis
set node version nvm use (optional)
install deps npm install
start npm start

generating events

node load.js

matching tag

curl --location --request POST 'http://localhost:3333/events' \
--header 'content-type: application/json' \
--data-raw '{
    "user-name": "Nitin",
    "location": "Mumbai",
    "text": "Hi",
    "tags": ["happy"]
}'

matching location

curl --location --request POST 'http://localhost:3333/events' \
--header 'content-type: application/json' \
--data-raw '{
    "user-name": "Nitin",
    "location": "Pune",
    "text": "Hi",
    "tags": []
}'

10th event by user

for run in {1..10}; do
  curl --location --request POST 'http://localhost:3333/events' \
  --header 'content-type: application/json' \
  --data-raw '{
      "user-name": "Nitin",
      "location": "Delhi",
      "text": "Hi!!!",
      "tags": []
  }'
  sleep 0.2
done

Conditions and actions

Conditions

Simply put, conditions are functions which take event and return true when that condition is fulfilled. This works well when conditions are stateless, meaning we can determine if the condition is fulfilled just by looking at that event.

But like in the case when we wanted 10th event from a user to trigger an action, we wanted some context along with event

So conditions are function that take a context and return a function which then takes an event and return a Promise which would either resolve to true or false

So every condition function looks like the following

const condition = (context) => (event) => Promise.resolve(true/false)

Incase of 10th event from a user,context has a function incr which helps us determine the count of events by user till that point, it looks as following

const every10thEvent =
  ({ incr }) =>
  (event) =>

  incr(`counts::${event["user-name"]}`)
  .then(count => count && count % 10 === 0);

actions

Actions are function which look like following and are called when corresponding condition is fulfilled

const action = (event) => Promise.resolve(event);

note on the implementation of every10thEvent condition

In this case we wanted to trigger an action when we receive the 10th event from the user, but this condition is every 10th event.

nitinrathi/event-stream