Locking

Pessimistic Locking (conservative)

• Acquires an exclusive lock on the resource before reading/writing
• Guarantees no concurrent modification, but introduces contention (multiple threads waiting for the same lock) and reduces throughput

SQL

-- all within the same transaction
SELECT balance FROM accounts WHERE id = 42 FOR UPDATE;   -- acquires lock, balance=100
UPDATE accounts SET balance = balance - 20 WHERE id = 42; -- balance=80
COMMIT;                                                   -- releases lock

DynamoDB

• DynamoDB does not support pessimistic locking natively (no FOR UPDATE equivalent)

HTTP

• HTTP is stateless — no standard mechanism to hold a lock between requests

Optimistic Locking

• Does not hold a lock — instead, each write carries a version or timestamp
• Before writing, check whether the version has changed since the read
• If it changed, the write is rejected and the caller retries or aborts
• Implemented via conditional writes (e.g. UPDATE ... WHERE version = ?)

SQL -

SELECT balance, version FROM accounts WHERE id = 42;  --> balance=100, version=3
UPDATE accounts SET balance = balance - 20, version = 4 WHERE id = 42 AND version = 3;
-- 0 rows affected → someone else changed it, retry from SELECT

DynamoDB -

// 1. GetItem → balance=100, version=3
{
  "TableName": "accounts",
  "Key": {
    "id": { "N": "42" }
  }
}


// 2. Conditional UpdateItem
{
  "TableName": "accounts",
  "Key": {
    "id": { "N": "42" }
  },
  "UpdateExpression": "SET balance = balance - :amt, version = version + :one",
  "ConditionExpression": "version = :v",
  "ExpressionAttributeValues": {
    ":amt": { "N": "20" },
    ":v": { "N": "3" },
    ":one": { "N": "1" }
  }
}

// throws ConditionalCheckFailedException if version changed → retry

HTTP (ETags)

# 1. GET → returns ETag header
curl -i http://api.example.com/accounts/42
# → 200 OK
# → ETag: "v3"
# → {"balance":100}

# 2. PUT with If-Match → only succeeds if ETag still matches
curl -X PUT http://api.example.com/accounts/42 \
  -H 'If-Match: "v3"' \
  -H 'Content-Type: application/json' \
  -d '{"balance":80}'

# → 200 OK (ETag matched, update applied)
# → 412 Precondition Failed (ETag changed → retry from GET)