Explain like I'm five · SRE

What is a Bulkhead?

The bulkhead pattern, told as a story about a park split into zones that each use only their own share of resources.

trouble spreads from one ride to the next, and the nexttrouble in one ride spreads until the whole park stops

Trouble in one ride spreads until the whole park stops.The trouble crosses over with nothing to stop it, again and again.

ONE GENERATORevery ride shares the one generator, the one staff poolwhen every ride shares the same resource, one of them can use it all up

When every ride shares the same resource, one of them can use it all up.One generator, one staff pool — everyone is tied to the same thing.

ZONE 1ZONE 2ZONE 3ZONE 4split into zones — one zone's trouble doesn't touch the othersjust like a ship's watertight bulkheads

Split into zones — one zone's trouble doesn't touch the others.Just like a ship's watertight bulkheads.

Split into zonesput up a wall
Each zone gets its own shareits own staff, its own power
Only one zone is affectedthe others stay untouched
Give important zones moreprotect the critical ones first
ZONE 1 — fullZONE 2ZONE 3ZONE 4only one zone's resources run out — the other three stay normal

Only one zone's resources run out — the other three stay normal.The map shows at a glance which zone has the problem.

nine zones... hard to keep up withsplit too finely and management gets tedious while resources go to waste — the right size matters

Split too finely and management gets tedious while resources go to waste.Finding the right zone size matters.

In one breath

Bulkhead = splitting the park into zones that each use only their own share of resources, so trouble in one zone leaves the others untouched.

Named after a ship's watertight bulkheads. It splits resources — thread pools, connection pools, server instances — by zone (tenant, service) so one overload doesn't cascade into a cascading failure across everything. Split too finely, though, and management overhead and wasted resources grow.

When grown-ups say it

Bulkhead pattern
Designing the park into separate zones. Keeps one zone's trouble from crossing into another.
Isolation
Walling zones off from each other. The core thing a bulkhead does.
Resource pool
A shared supply, like a generator or a staff roster. Leave it unsplit, and one user can take it all.
Thread pool isolation
Giving each zone its own set of workers (threads). The most common way to apply a bulkhead.
Tenant isolation
Splitting resources by guest group (tenant). One tenant's surge doesn't endanger another.
Cascading failure
One ride's trouble spreading to the next, and the next. The exact thing a bulkhead is built to stop.
Circuit breaker
The red button that stops sending guests somewhere broken. Often paired with bulkheads, which split things into zones. → the red button you press when something breaks
Graceful degradation
Staying open with the rest, even if part is broken. Splitting into zones with a bulkhead makes this easier to pull off.
Next story
The Red Button You Press When Something Breaks →
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