WORKED SOLUTIONS — after attempting the exercises.
1 Replaceable release code differs from durable records/uploads; identify independent recovery owners.
2 All listed bad config values fail the closed contract. Exact bool/int semantics matter.
3 Failedv 3 keepsv 2;rollbackv 1;state hash unchanged. Wrong hash/version reuse/schema 2 fail.
This is local pointer/contract checking,not a real web-app health check or migration recovery.
4 Restored count 2,total 70000,integrityok,logical hash matches snapshot;current stays 3,total 75000.
Promoting snapshot omits post-snapshotC/5000 unless separately reconciled. Small measured seconds
are one local observation,not a production RTO. Same-device temp backup is not offsite recovery.
5 Success 18/20=.90. Sorted 19 thof 20=1000 ms. Allowed failures 20*(1-.95)=1;remaining 1-2=-1.
Duplicate IDs violate final-job grain; unknown payload fields fail safe-log contract.
6 A commits 30000;B simulated failure rolls back;rerun appliesB,duplicatesA twice,rejectsC.
Rerun counts applied 1,duplicate 2,rejected 1;unique 2,total 70000. Only this DB effect is atomic.
Runnable extension (from kit directory):
from contextlib import closing
from lab import queue_connection,apply_event,bounded_retry
with closing(queue_connection()) as c:
    apply_event(c,{'event_id':'A','amount_cents':30000})
    apply_event(c,{'event_id':'B','amount_cents':40000})
    apply_event(c,{'event_id':'D','amount_cents':5000})
    assert apply_event(c,{'event_id':'D','amount_cents':5000})=='duplicate'
    assert c.execute('SELECT value FROM total').fetchone()[0]==75000
attempts=[]
def timeout():
    attempts.append(1)
    raise TimeoutError('Fictional transient timeout')
try:
    bounded_retry(timeout,3)
except TimeoutError:
    pass
else:
    raise AssertionError('Third timeout must remain a failure')
assert len(attempts)==3

7 Baseline 720*2+10*3+5*10=1520 cents. Egress 50=>1970,threshold 1600 alert;100=>2470,above 2000 budget.
Invented monthly-if-forgotten cost,not provider prices or two-hour cost. Expiry 2 his review metadata,
not enforced deletion. Budget notifications do not guarantee a hard cap. Review every retained resource.
8 Local compile/tests can pass while container/hostedCI/IaC remain unexecuted. Record each separately.
9 Good memo separates fixture SLO breach,rollback mechanics and snapshot restoration evidence.
Do not infer a shared root cause or production incident resolution from independent drills.
Keep current account/upload data intact; a code rollback is not a full DB restore.
