How we built a logging platform for GitHub Actions with ClickHouse

Blacksmith built a GitHub Actions logging platform to help developers investigate CI failures by searching across logs rather than reviewing individual job outputs. It selected ClickHouse for storage because of its low operational overhead, strong compression, affordable scaling, fast aggregation performance, and ability to support joins with other observability data, avoiding the complexity of an ELK stack and message queue. To enable efficient substring searches across millions of log lines, the system uses ClickHouse’s n-gram Bloom filter indexes, materialized columns to infer log severity, and row-level TTL policies for customer-specific retention. Logs are securely streamed from Firecracker VMs through host-side daemons, batched and inserted asynchronously into ClickHouse, with retries and ingestion-time deduplication using ReplacingMergeTree settings. The platform also includes a lightweight Lucene-inspired query language supporting text, property, logical, and comparison searches that compiles to SQL, while ClickHouse aggregation queries power log trends and histograms; the service is available to existing Blacksmith customers.

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