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fn guard

Runs rest (the steps after step k) and, when it fails, the compensation comp of step k with v (the step's result). A compensation is retried with backoff (SAGA_UNDO_RETRIES); when it never succeeds the saga is stuck and the result is CompensationFailed{ cause, step } (§11.3). Compensations that are already done (a takeover replays the saga) are skipped.

Example:

oxl✓ checked in CI
@error data Declined { why: str }

service Ledger {
  @public @idempotent rpc openAcct(id: u64) -> u64
  @public @idempotent rpc closeAcct(id: u64) -> ()
  @public @idempotent rpc charge(id: u64) -> u64 | Declined
}

impl Ledger {
  rpc openAcct(id) {
    print("open {id}")
    id
  }
  rpc closeAcct(id) { print("close {id}") }
  rpc charge(id) {
    if id > 1 { fail Declined{ why: "limit" } }
    id
  }
}

saga flow(a: u64, b: u64) -> u64 {
  Ledger.openAcct(a)   undo x => Ledger.closeAcct(x)
  -> Ledger.openAcct(b)   undo y => Ledger.closeAcct(y)
  -> Ledger.charge(b)
}

fn main() io, net, time, db {
  match flow(1, 2) {
    u64 v => print("done {v}")
    Declined d => print("declined {d.why}")
    CompensationFailed c => print("stuck")
    NetError e => print("net error")
  }
}
Output
open 1
open 2
close 2
close 1
declined limit
pub(api) fn guard<A: Serializable, U, T, effect F, effect G>(k: i64, v: A, comp: fn(A) -> U F, rest: fn(A) -> T G) -> T | CompensationFailed  net, db, time, F, G

Since. 1.0.0-beta

Parameters

NameType
ki64
vA
compfn(A) -> U F, rest: fn(A) -> T G

Returns

T

Errors

Effects

none declared (pure unless the body says otherwise)

Defined in: std/saga/saga.oxl:575

See also