Why We Replaced Rails with Elixir (And Regretted It)

"We rewrote our Rails app in Elixir—and it cost us 6 months of our lives."

Elixir promised us:
⚡ Blazing performance under load
🧩 Fault-tolerant systems that self-heal
💸 Lower server costs with lightweight processes

But after a year in production, we found the dark side of the BEAM. Here’s why we ultimately migrated back to Rails—and what we’d do differently today.

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1. The Promises vs. The Reality

Promise: "Performance at Scale"

• Expected: 10x faster response times under load.
• Reality: Our CRUD endpoints were slower due to:
  • N+1 queries (Ecto doesn’t auto-include like ActiveRecord)
  • JSON parsing bottlenecks (no Oj equivalent)

# N+1 hell in Elixir
users = Repo.all(User)
Enum.map(users, fn user ->
  posts = Repo.all(from p in Post, where: p.user_id == ^user.id)
  %{user: user, posts: posts}
end)

Promise: "Fault Tolerance"

• Expected: Self-healing systems.
• Reality: BEAM’s supervision trees don’t fix bad code.
  • A bug in our payment service still crashed—it just restarted mid-transaction.

Promise: "Lower Costs"

• Expected: 50% fewer servers.
• Reality: Our PostgreSQL bill doubled because:
  • Phoenix’s connection pooling was less efficient than Puma’s.
  • We needed more CPU to match Rails’ JSON rendering speed.

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2. The 3 Biggest Pain Points

1. Ecosystem Gaps

What We Missed from Rails:

• ActiveAdmin → Had to build admin panels from scratch.
• Rails’ Migrations → Ecto migrations lacked change reversibility.
• Gems (like Devise) → Elixir libs were less battle-tested.

2. Hiring Struggles

• 10x fewer Elixir devs than Rubyists.
• Onboarding took 3x longer—even for senior engineers.

3. Debugging Complexity

• BEAM’s observability tools (:observer, recon) were powerful but opaque.
• No equivalent to byebug—debugging meant tracing processes manually.

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3. What Actually Worked Well

✅ Real-time features (Phoenix Channels crushed ActionCable)
✅ Background jobs (Oban was more reliable than Sidekiq)
✅ High-throughput APIs (once we optimized JSON parsing)

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4. When Elixir Is the Right Choice

🔹 Real-time apps (chat, gaming, live dashboards)
🔹 High-uptime requirements (99.999% SLA)
🔹 Teams with BEAM experience

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5. What We’d Do Differently

1. Hybrid approach: Keep Rails for CRUD, use Elixir for real-time components.
2. Benchmark first: Prove Elixir’s ROI before full migration.
3. Train the team: Invest in 3 months of Elixir mentorship.

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"But Elixir Is the Future!"

Maybe—but future-proofing has costs. Our lesson?

1. Don’t rewrite—augment instead.
2. Measure bottlenecks before jumping stacks.
3. Honor team DNA (ours was Ruby through and through).

Tried a stack switch? Share your story below—triumphs or regrets.