Learning Agility and Tool Proficiency Questions

def list_s3_files(prefix):
    # return list of s3 keys
    pass

def compute_checksum(s3_key):
    # use S3 ETag if single-part, else download and md5
    pass

def claim_manifest_row(file_path, checksum, worker_id):
    # INSERT new manifest or UPDATE if checksum changed
    # Use conditional transaction to set status='PROCESSING', process_job_id=worker_id
    pass

def process_file_to_staging(s3_key, staging_table):
    # download or stream, transform, write parquet to staging S3 or load directly to staging_table
    pass

def merge_staging_to_target(staging_table, target_table, key_cols, cols):
    # execute MERGE ... USING staging_table ON key_cols ...
    pass

def mark_processed(file_path, checksum, worker_id):
    # set status='PROCESSED', processed_at=now(), process_job_id=worker_id
    pass

def ingest_worker(hour_prefix):
    files = list_s3_files(hour_prefix)
    for key in files:
        checksum = compute_checksum(key)
        claimed = claim_manifest_row(key, checksum, WORKER_ID)
        if not claimed:
            continue  # already processing or up-to-date
        try:
            staging = f"stg_{uuid4().hex}"
            process_file_to_staging(key, staging)
            merge_staging_to_target(staging, "dw.table", key_cols=["id"], cols=[...])
            mark_processed(key, checksum, WORKER_ID)
        except Exception as e:
            # set manifest status=FAILED, log and allow retry
            set_failed(file_path=key, worker=WORKER_ID, err=str(e))
            raise

from confluent_kafka import Consumer, Producer, KafkaError

# Simplified: confluent_kafka's Producer supports idempotence, but transactional API is limited in Python.
conf_c = {'bootstrap.servers': 'broker:9092', 'group.id': 'grp', 'enable.auto.commit': False}
conf_p = {'bootstrap.servers': 'broker:9092', 'enable.idempotence': True}

consumer = Consumer(conf_c)
producer = Producer(conf_p)
consumer.subscribe(['input-topic'])

def process_and_write(value, key):
    # idempotent write to external DB using upsert by idempotency key
    idempotency_key = key  # or derive from payload
    upsert_to_db(idempotency_key, value)  # ensure DB has unique constraint

while True:
    msg = consumer.poll(1.0)
    if msg is None:
        continue
    if msg.error():
        continue
    try:
        process_and_write(msg.value(), msg.key())
        # commit offset after successful external write
        consumer.commit(message=msg)
    except Exception:
        # handle retry / poison messages / logging
        pass

WITH recent AS (
  SELECT
    user_id,
    event_type,
    event_ts,
    ROW_NUMBER() OVER (PARTITION BY user_id ORDER BY event_ts DESC) AS rn
  FROM events
  WHERE event_ts >= now() - INTERVAL '30 days'
)
SELECT user_id, event_type, event_ts
FROM recent
WHERE rn = 1;

SELECT DISTINCT ON (user_id) user_id, event_type, event_ts
FROM events
WHERE event_ts >= now() - INTERVAL '30 days'
ORDER BY user_id, event_ts DESC;

SELECT u.user_id, r.event_type, r.event_ts
FROM users u
LEFT JOIN (
  -- use the previous DISTINCT ON or CTE result as r
) r USING (user_id);