GitHub: a billion repositories, and a push that lands on a majority or not at all

How a code host at GitHub's scale works: a billion repositories on five replicas each, where Spokes accepts a reference update only when a strict majority applies it with matching checksums, so there is no primary to fail over; a 2026 move, now rolling out, from a repository-wide lock to a quorum per reference, with Azure Blob Storage as a virtual replica next; reads sent to the nearest in-sync replica and fileservers drained before reboots; one Kafka event per push fanned out to consumers with their own retry rules; webhooks with a 10-second timeout, per-host isolation and delivery ids; 124 million Actions jobs a day on fresh VMs with per-account queues, fair share and leases with epochs; and code search sharded by Git blob id, with permissions checked twice.