The Ultimate Open Source Observability Stack (2025 Edition)

“Observability” is often sold as a monolithic $100k/year product. But the reality is that the best observability tools in the world are free and open source. The giants of tech (Google, Netflix, Uber) don’t use out-of-the-box SaaS; they build on top of open standards.

You can too. By assembling the right “Voltron” of tools, you can have a stack that rivals Datadog or New Relic.

Here is the 2025 Blueprints for the Ultimate Open Source Stack.

1. Metrics: Prometheus + VictoriaMetrics

Prometheus is the undisputed king of metrics. It standardizes how everything (internal apps, databases, hardware) exposes data. However, vanilla Prometheus struggles with long-term storage.

The Upgrade: Use VictoriaMetrics. It is a drop-in replacement for Prometheus that is faster, uses less RAM, and handles long-term storage effectively.

• Role: Collects “How much RAM is used?” and “How many requests per second?“

2. Visualization: Grafana

If Prometheus is the database, Grafana is the window. It is the single pane of glass where you overlay your metrics.

• Pro Tip: Use “provisioning” to store your dashboards as code (YAML/JSON) in a Git repo, rather than manually clicking “Save” in the U I. This is GitOps for Monitoring.

3. Uptime & Status: Cluster Uptime

Prometheus is great for “White Box” monitoring (asking the server how it feels). But if the network is down, Prometheus can’t scrape the server. You need “Black Box” monitoring (checking from the outside).

Cluster Uptime fills this gap.

• Role: External health checks (“Is the site up?”) and Public Status Pages.
• Integration: Cluster Uptime exposes its own metrics via /metrics endpoint, so Prometheus can scrape it too!

4. Logs: Grafana Loki

Don’t use ElasticSearch (ELK) unless you really need full-text search on terabytes of documents. It’s heavy and expensive (Java heap!). Loki is “Prometheus for Logs.” It only indexes the metadata (labels), not the log content itself. This makes it insanely cheap and fast.

• Role: Grepping logs across 100 servers instantly. {app="api"} |= "error"

5. Tracing: Jaeger or Tempo

For microservices, you need to follow a request as it hops between services.

• Jaeger: The classic choice.
• Tempo: Grafana’s answer, integrated tightly with Loki.

How It All Fits Together

The magic happens in Correlation.

1. Alert: Cluster Uptime slack bot says “API is Down”.
2. Dashboard: You click the link, opening Grafana.
3. Metrics: You see a spike in “500 Errors” on the API panel (Prometheus).
4. Logs: You highlight that time range in Grafana, and the “Logs” panel below updates (Loki) to show only logs from that specific minute.
5. Root Cause: You see “Connection Refused: Database.”

Total Cost: $0 License Fee. Total Value: Priceless.