한국어 README.md

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Presence API

Presence API provides functionalities to deal with real-time presence information. It is an extended version of the 'Active n hours ago' feature of Messenger. Presence API provides a REST API to retrieve friends' current activites and a WebSocket API to listen to friends' and send the user's activity updates.

Presence API is built using AWS CDK with TypeScript.

Architecture

API Architecture Overview Diagram

API Architecture Details Diagram

Components

The system architecture and its relevant cloud deployment details are defined in lib/presence-api-stack.ts. Lambda functions are stored in src/functions with the Lambda layer in src/layer.

AWS RDS (PostgreSQL)

• AWS RDS is connected via AWS RDS Proxy.

• The RDS database schema is defined in user-api.

• The permission for Lambda to access AWS RDS Postgres is set up in lib/presence-api-stack.ts.

AWS Elasticache (Redis)

We use a single REDIS client cluster with four keys - presence_user_connection, presence_connection_user, status, page.

• presence_user_connection stores { user_id: connection_id } and presence_connection_user stores { connection_id: user_id }. They are used to manage connection ids for each user.

• status stores { user_id: timestamp } with timestamp as score. It is used to determine whether a user is online or not.

• presence stores { user_id: page string } where page string is a JSON string of { url: string, title: string }. It is the url and title of tabs users are on.

AWS Lambda

All the presence functionalities are built with AWS Lambda functions.

• connect is invoked via websocket when a user connects to it. It stores the user's connection id in presence_user_connection and presence_connection_user Redis key.

• heartbeat is invoked every minute via websocket from the Chrome extension. It updates the timestamp of status Redis key.

• update_presence is invoked via websocket when user switches the Chrome page. It updates timestamp of status Redis key and page information of page Redis key.

• close_connection is invoked via websocket when user closes websocket connection. It removes user's information (connection id, timestamp, and page information) from all Redis keys.

• timeout is invoked every 3 minutes by AWS Eventbridge to identify offline users and remove their information from Redis.

• get_presence is invoked via REST Api. It returns the presence information of every friend of the user who invokes the function.

• get_user_presence is invoked via REST Api. It returns the presence information of the target user provided by the caller.

AWS API Gateway

• REST API - Provides endpoint for retrieving the current snapshot of presence data of users.

• Websocket API - Provides websocket connection for Chrome extension background.js to send and retrieve real-time presence data.

AWS EventBridge

CloudWatch triggers EventBridge event every 3 minutes to invoke Lambda timeout function. A user who has not been active for the last 3 minutes is treated as offline.

Setup

Environment Variables

In .env set the following environment variables.

AWS_REGION=<AWS region>
COGNITO_POOL_ID=<AWS Cognito User Pool Id>

VPC_ID=<VPC of the backend>
PRIVATE_ROUTE_TABLE1_ID=<Route Table1 id of subnets AWS RDS resides in>
PRIVATE_ROUTE_TABLE2_ID=<Route Table2 id of subnets AWS RDS resides in>
PRIVATE_SUBNET1_ID=<Id of subnet1 AWS RDS resides in>
PRIVATE_SUBNET2_ID=<Id of subnet2 AWS RDS resides in>

SUBNET1_AZ=<Availability zone of subnet1 (e.g. us-west-2a)>
SUBNET2_AZ=<Availability zone of subnet2>

RDS_PROXY_SG_ID=<Security Group of AWS RDS Proxy>
RDS_HOST=<AWS RDS Host>
RDS_PORT=<AWS RDS Port Number>
RDS_USERNAME=<AWS RDS username>
RDS_PASSWORD=<AWS RDS password>
RDS_DB_NAME=<AWS RDS database name>

RDS_PROXY_ARN=<AWS RDS Proxy arn>
RDS_PROXY_NAME=<AWS RDS Proxy name>

LAMBDA_SG_ID=<AWS Lambda Security Group if it exists. 'none' otherwise>
REDIS_SG_ID=<AWS Elasticache Security Group if it exists. 'none' otherwise>
REDIS_PRIMARY_ENDPOINT_ADDRESS=<Elasticache primary endpoint host if it exists. 'none' otherwise>
REDIS_PRIMARY_ENDPOINT_PORT=<Elasticache primary endpoint port if it exists. 'none' otherwise>
REDIS_READER_ENDPOINT_ADDRESS=<Elasticache reader endpoint host if it exists. 'none' otherwise>
REDIS_READER_ENDPOINT_PORT=<Elasticache reader endpoint port if it exists. 'none' otherwise>

CLIENT_URL=<Url of the chat client>

CDK Bootstrap

For initialization, bootstrap AWS CDK by running

cdk bootstrap aws://<AWS Account Id>/<AWS Region>

Refer to https://docs.aws.amazon.com/cdk/latest/guide/bootstrapping.html for more details.

Running Locally

Run Redis on Docker

Run

cd redis-docker
docker-compose up -d

Invoke Lambda Functions

Run cdk synth --no-staging > template.yaml to generate template.yaml.

Check the function identifier from template.yaml and run

/usr/local/bin/sam local invoke [FunctionIdentifier] -e events/[event.json]

Deployment

Run

cdk deploy --outputs-file presence.json