Consistency Is a Promise

Consistency is not a database setting. Consistency is a promise about reality.

When a system claims consistency, it commits to how truth behaves across time, space, and observers.

I. The Mislabeling of Consistency

Consistency is often reduced to:

ACID levels

replication modes

quorum sizes

isolation levels

These are mechanisms.

Consistency itself decides:

whether two observers may see different truths

whether time can diverge

whether contradiction is tolerated

whether reconciliation is guaranteed

That is a promise.

II. Every Consistency Model Is a Contract

Strong consistency promises:

one truth at a time

immediate visibility

global agreement

Eventual consistency promises:

temporary disagreement

eventual convergence

delayed reconciliation

Weak consistency promises:

nothing beyond best effort

Each promise has consequences.

Undeclared promises are deception.

III. Consistency Determines Legitimacy

If two actors see different states:

who is allowed to act?

whose action is valid?

whose decision counts?

Consistency answers these questions.

Without declared consistency, authority becomes ambiguous.

Ambiguous authority is unlawful.

IV. Consistency Has Temporal Cost

Consistency is paid for with:

latency

availability

throughput

complexity

Choosing a consistency model is choosing which costs are acceptable.

Hidden costs are broken promises.

V. Eventual Consistency Is Not an Excuse

Eventual consistency is lawful only if:

convergence is guaranteed

divergence is bounded

conflicts are resolvable

reconciliation is recorded

“Eventually” without guarantee is evasion.

VI. Consistency Must Be Observable

A lawful system must make visible:

current consistency guarantees

stale vs authoritative data

reconciliation in progress

conflict resolution outcomes

Invisible consistency is unverifiable promise.

VII. Replay Requires Consistency Semantics

Replay must reproduce:

the same divergence

the same convergence

the same conflicts

the same resolutions

If replay collapses inconsistency, audit fails.

VIII. Infrastructure Does Not Define Consistency Law

Databases, replication engines, and consensus protocols do not define promises.

They enforce them.

Consistency law must live in:

kernel

policy

versioned contracts

Not in vendor defaults.

IX. Final Conclusion

Consistency is a promise.

A lawful system:

declares its consistency model explicitly

communicates divergence honestly

guarantees reconciliation where promised

records all inconsistencies and resolutions

replays consistency behavior deterministically

Anything else claims truth while allowing contradiction without accountability.