First, the key terms.

Quorum
A set large enough under the protocol’s rules to authorize a decision.
Network partition
A communication failure separating participants into groups.

The idea, explained

Replicas improve resilience but must agree on which writes are committed and in what order. A leader-based protocol commonly replicates log entries to a quorum before committing them. Fault assumptions matter: handling crashed nodes is different from handling malicious participants. Network partitions can prevent a quorum from forming. Consensus improves agreement under a defined model; it does not eliminate every availability, application or operational failure.

STEP 1

State the fault assumptions

A protocol designed for crashes is not automatically resilient to malicious participants. Understand what failures are covered before interpreting a replication claim. Several copies alone do not imply agreement.

STEP 2

Reason about intersecting majorities

With three replicas, any two-member majorities intersect. This helps protocols prevent independent contradictory commitments. The protocol still needs rules for leadership, logs and recovery; counting votes alone is not the whole algorithm.

Follow a worked example.

Three replicas require a majority to commit. If one fails, the remaining two may continue. If the network splits into groups of one and two, the isolated one cannot safely declare conflicting writes committed under that majority rule.

Walk it through, one step at a time

  1. Replicas A, B and C require two participants for the relevant commitment.
  2. A becomes isolated while B and C communicate. A cannot form a majority; B and C may continue under the protocol’s leadership rules.
  3. Refusing A’s isolated writes sacrifices availability there to protect the agreement guarantee.

Where people get stuck

Having multiple copies of data does not by itself establish a consistent replicated history.

Now try it yourself.

Pause before opening each answer. Explain your reasoning in your own words.

1. Check the core idea

Why can a minority of replicas be prevented from accepting committed writes?

Reveal the answer & reasoning

Allowing isolated minorities to commit independently could create conflicting histories. Requiring intersecting quorums protects agreement at the cost of some availability.

2. Apply what you learned

Can a system promise every isolated replica can always commit conflicting writes while preserving one immediate agreed history?

Reveal the answer & reasoning

Not under that partition scenario and guarantee. The system must constrain availability or weaken the claimed consistency behavior.