Explain like I'm five · SRE

What is Eventual Consistency?

Eventual consistency, told as a story about a stock board that takes a moment to catch up with the warehouse.

just arrivedit just came in…STOCK BOARDsold outyou said it was sold out?new stock just arrived, but the counter's board still shows the old number

New stock just arrived, but the counter's board still shows the old number.A guest asks, 'you said it was sold out?'

COUNTER Asold outCOUNTER B5 leftstill catching up…syncing the copies to match takes a brief moment

Why: syncing the copies to match takes a brief moment.Syncing the replicas takes a brief moment.

COUNTER Asold out (old)wait just a littleCOUNTER A5 left (matches)even if it's off at first, wait a little and it ends up matching

Even if it's off at first, wait a little and it ends up matching.In between, it can be slightly different.

Use it for low-stakes infolike numbers on a display board
Don't use it for moneyuse strong consistency instead
Measure how late it getsyou need to know the replication lag
Tell guests up front'it just changed, so it may lag a bit'
t=0 update5 leftt=1 counter Asold out (stale)t=3 counter A5 left (matches)it takes a little time before every number matches

Timeline: t=0 update → t=1 still stale → t=3 matches.It takes a little time before every number matches.

OUR PROMISEusually matches within 3smay briefly differ before thatso when does it actually match?without knowing how long 'eventually' takes, guests get more confused — so you promise a time

Without knowing how long 'eventually' takes, guests get more confused.So you need to promise a time.

In one breath

Eventual consistency: replicas won't match right away, but wait a little and they end up the same — in between, they can briefly differ.

This contrasts with strong consistency, where every replica always matches. Knowing the replication lag lets you tell guests when things will line up. It's usually avoided for money, and used instead for low-stakes things like caches or view counts.

When grown-ups say it

Eventual Consistency
The promise that things match up eventually, even if a bit late. Mostly used for low-stakes information.
Strong Consistency
Always showing the latest, correct answer. Used wherever money is involved. → you can't have both correctness and always-open
Replication Lag
The time it takes a replica to catch up to the original. → keeping the same guest list in two warehouses
Read-your-writes Consistency
The promise that at least your own change shows up for you right away. Less confusing for guests than plain eventual consistency.
Vector Clock
A label-like tool for telling which replica is more recent. The details live elsewhere — just know the name for now.
Conflict Resolution
The rule for settling conflicting values into one. Last-write-wins, manual merge, and others are common approaches.
Caching
A memo board with answers written down in advance. → the memo board with answers written in advance
CAP Theorem
What you get when CAP's trade-off picks availability. → you can't have both correctness and always-open
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