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Introducing Lokahi

Lokahi is a http service uptime checking and notification service. Currently lokahi does very little. Given a URL and a webhook URL, lokahi runs checks every minute on that URL and ensures it's up. If the URL goes down or the health workers have trouble getting to the URL, the service is flagged as down and a webhook is sent out.

Stack

What	Role
Postgres	Database
Go	Language
Twirp	API layer
Protobuf	Serialization
Nats	Message queue
Cobra	CLI

Components

Interrelation graph:

lokahictl

The command line interface, currently outputs everything in JSON. It currently has a few options:

$ ./bin/lokahictl
See https://github.com/Xe/lokahi for more information

Usage:
  lokahictl [command]

Available Commands:
  create      creates a check
  create_load creates a bunch of checks
  delete      deletes a check
  get         dumps information about a check
  help        Help about any command
  list        lists all checks that you have permission to access
  put         puts updates to a check
  run         runs a check
  runstats    gets performance information

Flags:
  -h, --help            help for lokahictl
      --server string   http url of the lokahid instance (default "http://AzureDiamond:hunter2@127.0.0.1:24253")

Use "lokahictl [command] --help" for more information about a command.

Each of these subcommands has help and most of them have additional flags.

lokahid

This is the main API server. It exposes twirp services defined in xe.github.lokahi and xe.github.lokahi.admin. It is configured using environment variables like so:

# Username and password to use for checking authentication
# http://bash.org/?244321
USERPASS=AzureDiamond:hunter2

# Postgres database URL in heroku-ish format
DATABASE_URL=postgres://postgres:hunter2@127.0.0.1:5432/postgres?sslmode=disable

# Nats queue URL
NATS_URL=nats://127.0.0.1:4222

# TCP port to listen on for HTTP traffic
PORT=9001

Every minute, lokahid will scan for every check that is set to run minutely and run them. Running checks any time but minutely is currently unsupported.

healthworker

healthworker listens on nats queue check.run and returns health information about that service.

webhookworker

webhookworker listens on nats queue webhook.egress and sends webhooks based on the input it's given.

Challenges Faced During Development

ORM Issues

Initially, I implemented this using gorm and started to run into a lot of problems when using it in anything but small scale circumstances. Gorm spun up way too many database connections (as many as a new one for every operation!) and quickly exhausted postgres' pool of client. connections.

I rewrote this to use database/sql and sqlx and all of the tests passed the first time I tried to run this, no joke.

Scaling to 50,000 Checks

This one was actually a lot harder than I thought it would be, and not for the reasons I thought it would be. One of the main things that I discovered when I was trying to scale this was that I was putting way too much load on the database way too quickly.

The solution to this was to use bundler to batch-write the most frequently written database items, see here. Even then, database connection count limiting was also needed in order to scale to the full 50,000 checks needed for this to exist as more than a proof of concept.

This service can handle 50,000 HTTP checks in a minute. The only part that gets backed up currently is webhook egress, but that is likely fixable with further optimization on the HTTP checking and webhook egress paths.

Basic Usage

To set up an instance of lokahi on a machine with Docker Compose installed, create a docker compose manifest with the following in it:

version: "3.1"

services:
  # The postgres database where all lokahi data is stored.
  db:
    image: postgres:alpine
    restart: always
    environment:
      POSTGRES_PASSWORD: hunter2
    command: postgres -c max_connections=1000

  # The message queue for lokahid and its workers.
  nats:
    image: nats:1.0.4

  # The service that runs http healthchecks. This is its own service so it can
  # be scaled independently.
  healthworker:
    image: xena/lokahi:latest
    restart: always
    depends_on:
      - "db"
      - "nats"
    environment:
      NATS_URL: nats://nats:4222
      DATABASE_URL: postgres://postgres:hunter2@db:5432/postgres?sslmode=disable
    command: healthworker
  
  # The service that sends out webhooks in response to http healthchecks. This
  # is also its own service so it can be scaled independently.
  webhookworker:
    image: xena/lokahi:latest
    restart: always
    depends_on:
      - "db"
      - "nats"
    environment:
      NATS_URL: nats://nats:4222
      DATABASE_URL: postgres://postgres:hunter2@db:5432/postgres?sslmode=disable
    command: webhookworker

  # The main API server. This is what you port forward to.
  lokahid:
    image: xena/lokahi:latest
    restart: always
    depends_on:
      - "db"
      - "nats"
    environment:
      USERPASS: AzureDiamond:hunter2 # want ideas? https://strongpasswordgenerator.com/
      NATS_URL: nats://nats:4222
      DATABASE_URL: postgres://postgres:hunter2@db:5432/postgres?sslmode=disable
      PORT: 24253
    ports:
      - 24253:24253
      
  # This is a sample webhook server that prints information about incoming 
  # webhooks.
  samplehook:
    image: xena/lokahi:latest
    restart: always
    depends_on:
      - "lokahid"
    environment:
      PORT: 9001
    command: sample_hook
    
  # Duke is a service that gets approximately 50% uptime by changing between up
  # and down every minute. When it's up, it responds to every HTTP request with
  # 200. When it's down, it responds to every HTTP request with 500.
  duke:
    image: xena/lokahi:latest
    restart: always
    depends_on:
      - "samplehook"
    environment:
      PORT: 9001
    command: duke-of-york

Start this with docker-compose up -d.

Configuration

Open ~/.lokahictl.hcl and enter in the following:

server = "http://AzureDiamond:hunter2@127.0.0.1:24253"

Save this and then lokahictl is now configured to work with the local copy of lokahi.

Creating a check

To create a check against duke reporting to samplehook:

$ lokahictl create \
    --every 60 \
    --webhook-url http://samplehook:9001/twirp/github.xe.lokahi.Webhook/Handle \
    --url http://duke:9001 \
    --playbook-url https://github.com/Xe/lokahi/wiki/duke-of-york-Playbook
{
  "id": "a5c7179a-0d3a-11e8-b53d-8faa88cfa70c",
  "url": "http://duke:9001",
  "webhook_url": "http://samplehook:9001/twirp/github.xe.lokahi.Webhook/Handle",
  "every": 60,
  "playbook_url": "https://github.com/Xe/lokahi/wiki/duke-of-york-Playbook"
}

Now attach to samplehook's logs and wait for it:

$ docker-compose -f samplehook
2018/02/09 06:27:15 check id: a5c7179a-0d3a-11e8-b53d-8faa88cfa70c, 
  state: DOWN, latency: 2.265561ms, status code: 500, 
  playbook url: https://github.com/Xe/lokahi/wiki/duke-of-york-Playbook

Webhooks

Webhooks get a HTTP POST of a protobuf-encoded xe.github.lokahi.CheckStatus with the following additional HTTP headers:

Key	Value
`Accept`	`application/protobuf`
`Content-Type`	`application/protobuf`
`User-Agent`	`lokahi/dev (+https://github.com/Xe/lokahi)`

Webhook server implementations should probably store check ID's in a database of some kind and trigger additional logic, such as Pagerduty API calls or similar things. The lokahi standard distribution includes Discord and Slack webhook receivers.

JSON webhook support is not currently implemented, but is being tracked at this github issue.

Call for Contributions

Lokahi is pretty great as it is, but to be even better lokahi needs a bunch of work, experience reports and people willing to contribute to the project.

If making a better HTTP uptime service sounds like something you want to do with your free time, please get involved! Ask questions, fix issues, help newcomers and help us all work together to make the best HTTP uptime service we can.

Social media links for discussion on this article:

Mastodon: https://mst3k.interlinked.me/@cadey/99494112049682603

Reddit: https://www.reddit.com/r/golang/comments/7wbr4o/introducting_lokahi_http_healthchecking_service/

Hacker News: https://news.ycombinator.com/item?id=16338465

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