Open-source monitoring tools compared

The first architectural difference is where state lives. Prometheus pulls metrics over HTTP from targets it finds through service discovery or static configuration, and each server writes to its own local time series database. The Prometheus storage documentation states that local storage is not clustered or replicated, so each Prometheus server is a single failure domain to manage like any other single-node database. Redundancy comes from running a second server with the same scrape configuration or from remote write to external long-term storage. Alerting is a separate component. Alertmanager groups, inhibits and silences alerts, and it can run as a cluster that Prometheus reaches by listing every Alertmanager instance.

Zabbix places a server and a database at the center of the design. The Zabbix 7.4 server stores data in MySQL, MariaDB or PostgreSQL with optional TimescaleDB, and the supported ranges are MySQL or Percona 8.0.30 to 9.7, MariaDB 10.5 to 12.3, PostgreSQL 13 to 18 and TimescaleDB 2.13 to 2.29. SQLite is supported only for proxies, so it never holds the central server data. The native high availability cluster lets several server nodes share one database, with one node active and the others on standby. Failover occurs when the active node exceeds the configurable failover delay, which ranges from 10 seconds to 15 minutes. A short delay reacts faster but risks failing over during a brief network stall, so set it after testing your network. Proxy groups from Zabbix 7.0 onward add a second layer by redistributing monitored hosts when a proxy goes offline.

The practical result is that the shared database becomes the most important failure domain. Losing it stops monitoring for every server node at once, while losing a proxy affects only its hosts until the group rebalances them. Your team owns the database engine, its failover and replication design, backups, restore tests and version upgrades, and these responsibilities should be written into any implementation scope. Before committing, confirm the engine version against the Zabbix requirements, test how long database failover takes compared with the configured failover delay, and ask who performs restores and how often they are rehearsed.

Zabbix and Prometheus are both open source, but they are paid for differently. Zabbix states that its open source software has no license fee, no device limits and no feature gates, and it sells paid technical support subscriptions in Silver, Gold, Platinum, Enterprise and Global tiers. Those tiers differ in coverage and response time: Silver offers 8x5 support with a 1-day response, Gold a 4-hour response, and Platinum, Enterprise and Global 24x7 support with 2-hour, 1.5-hour and 1-hour responses. Prometheus, licensed under Apache-2.0, has no paid enterprise edition of the project itself, so its costs come from infrastructure, storage, implementation and the staff who run it. Check the tier and response time in writing before you assume either one covers your hours.

Nagios Core is open source under GPL v2 and is free to download and run, but it is configured by hand in text files and has a basic web interface. Nagios XI is the commercial product built on the same Core engine. It adds configuration tooling, reporting, access control, advanced dashboards and professional support, and Nagios.org says existing Core configuration imports into it. The license terms and pricing for XI are not covered by the sources used here, so ask Nagios for the license model, the host or device scope it covers, and whether support hours and response targets are written into the quote. Confirm each license against the OSI list before you describe it as open source in internal documents.

Across all three, design, migration, dashboards and templates are mostly one-time costs. Rebuilding checks in Zabbix, writing Prometheus scrape and alerting rules, or moving Nagios object definitions all take effort that does not repeat each year. Hosting, storage, support subscriptions, upkeep and upgrades recur. Prometheus local storage is not clustered or replicated and keeps 15 days of data by default, so long-term retention usually means a remote storage system with its own running cost. Zabbix standard releases are supported for 12 months and LTS releases for five years, which changes how often upgrade work recurs. Ask any vendor or partner for the support tier, who performs upgrades and in what window, the storage and retention sizing behind the estimate, and what the quote excludes. The Zabbix life cycle and release policy is the place to confirm upgrade windows.

Start with the licensing and lifecycle sources, because they settle questions that no feature comparison can. The Zabbix license page confirms the move from GPL-2.0 to AGPL-3.0 starting with Zabbix 7.0, and the Zabbix life cycle and release policy sets out full and limited support windows for each LTS line. Compare those dates with your own patch cadence. Prometheus has no paid edition to price, so its lifecycle question is different: the Prometheus release cycle and long-term support page explains that a minor release generally stops receiving bug fixes after six weeks, and that LTS lines get one year of fixes. Confirm which line your team will pin to and who applies the upgrades. For Nagios Core, the GPL-2.0 license and release history come from the nagioscore README and releases page, while the Nagios Core project page describes the commercial Nagios XI relationship. Ask the vendor directly whether any paid product is required for the features you need.

Next, read the architecture sources. The Prometheus storage documentation states that local storage is not clustered or replicated, so high availability and long retention are separate design decisions, usually handled with remote write and an external long-term store. The Alertmanager docs cover clustering through its cluster flags and advise pointing Prometheus at every Alertmanager instance rather than at a load balancer. For Zabbix, the Zabbix high availability concept page explains active and standby server nodes sharing one database and the configurable failover delay. Check that your database engine is on the supported list in the Zabbix requirements page for your target version, since that list changes between releases.

Use the Prometheus overview as the starting point for the pull model and PromQL, and the nagioscore README for Nagios Core's plugin interface and parent host hierarchy. Before a pilot, record the versions you tested, the support end dates you accepted and the storage and alerting paths you rely on. Confirm each figure against the live page on the day you decide, because release dates and support windows are updated by the projects, not by this page.