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Architecture & System Design › Distributed Systems

BASE

Basically Available, Soft state, Eventual consistency: the counterpart to ACID.

Backend Engineer · Mid-levelData Engineer · Mid-level

Also known as: BASE model, basically available, BASE vs ACID

BASE (Basically Available, Soft state, Eventually consistent) is the optimistic counterpart to ACID: prioritise availability and partition tolerance, accept that state converges eventually rather than instantly. Where ACID transactions promise immediate consistency, BASE systems promise the system stays up and agrees soon.

ACID: consistent now, possibly unavailable (wait or refuse)
BASE: available now, consistent soon (serve, converge)

“Soft state” acknowledges replicas may disagree transiently; “eventually consistent” promises convergence given quiet. DNS, shopping carts, social feeds and caches live here comfortably — the user experience tolerates (or never notices) the lag.

The classic mistakes:

  • BASE for money and inventory. Balances that must never disagree need coordination, not convergence. Match the model to the harm of temporary wrongness.
  • “Eventual” without a bound. Convergence in minutes vs hours changes the design (and the support load). Measure and bound the lag; alert past it.
  • Assuming convergence is automatic. Replicas need anti-entropy (read repair, gossip, version vectors) — “eventual” is engineered, not emergent.
  • Hiding inconsistency from users. Showing disagreeing numbers without acknowledgement erodes trust. Design UI for convergence (timestamps, “updating…”, session consistency).
  • No conflict story. Concurrent writes to the same key need resolution rules; BASE without them is just divergence. Define merge semantics per type.
  • Testing only the converged state. Bugs live in the transient window — flapping UI, double charges, phantom items. Test during convergence, not just after.

When to choose it: availability-first workloads where temporary disagreement is harmless — the CAP theorem’s A-side made practical. For the rest, ACID keeps its crown. See ACID for the other half of the trade.