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CONCEPT Cited by 1 source

Durable request

Definition

A durable request is one that has met the system's durability requirement — enough followers responsible for the leader's durability have acknowledged receipt that the system can trust the request will survive any subsequent failure and cannot be cancelled.

Sugu Sougoumarane's canonical wiki framing: "The request is in-flight, but has met the durability requirements. This is an implicit, but important stage. We can trust that a durable request will never be canceled." (Source: sources/2026-04-21-planetscale-consensus-algorithms-at-scale-part-6-completing-requests).

The implicit-stage property

Durability is not a follower-visible marker. Unlike the tentative and complete markers — which are explicit messages written down on each follower — durability is a property of the state of the system: "enough of the right followers have ack'd." Only the leader knows when that threshold has been crossed, because only the leader is collecting the acks.

This implicit-ness is the key design observation. Sugu: "This is an implicit, but important stage." The system can rely on it even though nothing broadcasts it.

Consequences of durability

Once a request is durable:

  1. It cannot be cancelled. Completion and cancellation are mutually exclusive; the durable stage is where this invariant first takes hold.
  2. The leader can skip the tentative step for lagging followers. "Once a request becomes durable, the leader is free to transmit that request as complete to followers that have not yet received the message as a single step." Saves one round-trip per lagging follower.
  3. The leader can ack the client. The client-response happens safely at the durable stage, not later at the complete stage — see patterns/early-ack-on-durability for the pattern and its trade-off with quorum reads.
  4. Subsequent electors must propagate it. The elector performing a leadership change cannot cancel a durable request; it must propagate it through to completion under the new leader. (This is the mechanism part 7 of Sugu's series covers — not yet ingested.)

Relation to the three-stage model

Stage Follower marker Can be cancelled? Can be ack'd to client?
Incomplete tentative yes no
Durable tentative (still) no yes (early-ack path)
Complete applied n/a yes (late-ack path)

Durability is the middle stage. Before it, the leader must not ack the client. After it, either ack path is safe.

Why early-ack can be safe here

Under a lock-based election with a leader lease, the leader can ack the client at the durable stage and still guarantee consistent reads — the lease ensures no other leader exists, so leader-local reads are authoritative. Without a lease, early-ack forces reads to go through a quorum round-trip to see the durable-but-not-yet-applied write.

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