The best open source alternative to Splunk is Grafana, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 50 alternatives to Splunk and 15 is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Splunk are Prometheus (Free, Open Source), Graylog (Free, Open Source), Logstash (Freemium, Open Source) and Graphite (Free, Open Source).
Prometheus is an open-source systems monitoring and alerting toolkit originally built at SoundCloud. Since its inception in 2012, many companies and organizations have adopted Prometheus, and the project has a very active developer and user community.
Graylog is an open source syslog implementation that stores your logs in Elasticsearch. It consists of a server and web interface written in Java that accepts your syslog messages via TCP or UDP (or GELF messages) and stores it in the database.
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Graylog vs Splunk opinions
Most users think Graylog is a great alternative to Splunk.
Pretty easy to setup once you figure out the log capture scripts. Then it's very powerful and realiable.
Prelude is a Universal Security Information & Event Management (SIEM) system. It collects, normalizes, sorts, aggregates, correlates and reports all security-related events independently of the product brand or license giving rise to such events.
Loki is a horizontally-scalable, highly-available, multi-tenant log aggregation system inspired by Prometheus. It is designed to be very cost effective and easy to operate. It does not index the contents of the logs, but rather a set of labels for each log stream.