Tech stack

Note: This page refers to our main product repository, not our website.

Frontend

• Web framework/library: React
• State management: Redux + Kea
• Layout/components: Ant Design

Backend

• Framework: Django
• High scale services: Rust
• Databases: PostgreSQL and ClickHouse
• Task queue/event streaming: Redis and Apache Kafka
• Task Worker: Celery, Temporal and Dagster

Testing

• Frontend E2E tests: Playwright
• Backend tests: Pytest and Django's built-in test suite

Additional tools

• CI/CD: GitHub Actions
• Containerization: Docker and Docker Compose
• Linter (frontend): ESLint
• Formatter (backend): Black

Workflow orchestration

We historically used Celery as our task worker. At PostHog’s current scale Celery can be unreliable for larger or long-running workflows, but it remains a practical fit for small, low-latency background tasks (e.g. sending emails or other quick async side-effects). New medium- and large-scale jobs should use Temporal or Dagster instead.

We use both Temporal and Dagster for more complex orchestration, each chosen for their specific strengths.

When to use each tool

We tend to use Celery for lightweight ad-hoc tasks, Dagster for internal data jobs, and Temporal for user-facing workflows. Before choosing between Dagster and Temporal, check the cost note below.

1. Is it a tiny, low-latency, fire-and-forget task (e.g. send email)? → Celery
2. Is it mission-critical with complex failure scenarios? → Temporal
3. Do you need exactly-once guarantees? → Temporal
4. Do you need complex retry policies or long-running workflows? → Temporal
5. Is it primarily about scheduled data transformation? → Dagster or Temporal
6. Do you need rich data lineage and testing? → Dagster

Cost: Temporal vs Dagster

Temporal is dramatically cheaper than Dagster for recurring workloads. Dagster charges $0.02–0.04 per credit (1 credit = 1 asset materialization), while Temporal charges $0.00005 per action (1 action ≈ 1 activity execution). That's a ~300x difference per operation.

In practice, when we migrated experiment metrics precomputation from Dagster to Temporal, the cost went from ~$6k/month to ~$20/month for the same workload. Keep this in mind before adding new Dagster jobs — if Temporal can do the job, it almost certainly should.

Where do we use each?

Celery: Small, fast background tasks (e.g. sending email, minor async operations) Temporal: Batch exports, data warehouse source syncing, AI platform task generation, experiment metrics precomputation Dagster: Exchange rate tracking, one-off production management commands, web analytics data pre-processing