Atomic Counters

Counters are everywhere — likes, view counts, inventory, rate-limit buckets. The naïve implementation is read, add one, save — and it’s wrong the moment two requests land at the same time. This recipe shows the right way: use Esix’s increment and decrement, which translate to MongoDB’s $inc operator and run atomically on the server. Esix features used: increment, decrement on QueryBuilder.

What You’ll Build

POST   /posts/:id/view → record a view (anyone can call)
POST   /posts/:id/like → like a post (authenticated)
DELETE /posts/:id/like → undo a like

Each endpoint runs in a single round trip and is safe under arbitrary concurrency.

The Post Model

Two counters on the document — no userId lists, no per-action collection. If you also need to know who liked a post, model that with a separate Like collection alongside; for the count itself, a number is enough.

// src/models/post.ts
import { BaseModel } from 'esix'

export default class Post extends BaseModel {
  public title = ''
  public body = ''
  public viewCount = 0
  public likeCount = 0
}

Why Not Read-Modify-Write?

This pattern looks fine and is wrong:

// ❌ Don't do this.
const post = await Post.find(id)
post.viewCount += 1
await post.save()

If two requests run this at the same time, both read the same viewCount, both write viewCount + 1, and one of the increments vanishes. The fix is to push the increment all the way down to MongoDB:

// ✅ Atomic — translates to $inc.
await Post.where('id', id).increment('viewCount')

increment returns the number of documents modified (0 when nothing matched), which is enough for the route handler to tell “found and updated” from “not found”.

View Counter

A view endpoint is the simplest case — single field, single increment:

// src/routes/posts/views.ts
import { Router } from 'express'
import Post from '../../models/post'

const router = Router({ mergeParams: true })

router.post('/view', async (request, response) => {
  const modified = await Post.where('id', request.params.id).increment(
    'viewCount'
  )

  if (modified === 0) {
    return response.status(404).json({ error: 'Post not found' })
  }

  response.status(204).end()
})

export default router

Like and Unlike

Likes are symmetric: one route increments, the mirror route decrements. The optional second argument lets you bump by something other than 1:

// src/routes/posts/likes.ts
import { Router } from 'express'
import Post from '../../models/post'
import { requireAuth } from '../../auth/require-auth'

const router = Router({ mergeParams: true })

router.post('/like', requireAuth, async (request, response) => {
  const modified = await Post.where('id', request.params.id).increment(
    'likeCount'
  )

  if (modified === 0) {
    return response.status(404).json({ error: 'Post not found' })
  }

  response.status(204).end()
})

router.delete('/like', requireAuth, async (request, response) => {
  const modified = await Post.where('id', request.params.id).decrement(
    'likeCount'
  )

  if (modified === 0) {
    return response.status(404).json({ error: 'Post not found' })
  }

  response.status(204).end()
})

export default router

Note: decrement accepts any positive number — decrement('likeCount', 5) would subtract five — and it doesn’t clamp at zero. If you need a floor, guard the call site or use a MongoDB conditional update.

Bumping by More Than One

Both helpers take an optional second argument. Some everyday examples:

// Award 10 points for finishing a challenge.
await User.where('id', userId).increment('score', 10)

// Charge a one-off fee against an account balance.
await Account.where('id', accountId).decrement('balance', 25)

Bulk Increments

increment runs against the whole where clause, so you can bump many documents at once. This is great for batch reconciliation:

// Mark every active subscription as having been billed once more.
const updated = await Subscription
  .where('status', 'active')
  .increment('billingCycles')

updated is the count of documents $inc actually touched — useful when you want to log “processed N records” or feed a progress UI.

Pattern Notes

Counters belong on the parent. Don’t store viewCount in a sidecar collection — that just turns a one-document write into a join.
One increment per request. Avoid calling increment inside a loop; if you need to bump by N, pass N as the second argument.
Distinguish “missing” from “no-op”. increment returns 0 for both an empty where and a non-existent id — your route should return 404 in either case.

What’s Next

REST API for a Blog — fold these counter endpoints into a full CRUD router.
Aggregation Dashboard — expose totals and averages of the counters you’re now safely incrementing.

Express

Next.js

Atomic Counters

What You’ll Build

The Post Model

Why Not Read-Modify-Write?

View Counter

Like and Unlike

Bumping by More Than One

Bulk Increments

Pattern Notes

What’s Next

​What You’ll Build

​The Post Model

​Why Not Read-Modify-Write?

​View Counter

​Like and Unlike

​Bumping by More Than One

​Bulk Increments

​Pattern Notes

​What’s Next

What You’ll Build

The Post Model

Why Not Read-Modify-Write?

View Counter

Like and Unlike

Bumping by More Than One

Bulk Increments

Pattern Notes

What’s Next