TheAlanWang

Notes

02.Put item into DB

2 min read

put_item

  • write one record(one item) into a table
  • If put an item with the same hash key and same range key:
    • PutItem will overwrite/replace the existing item by default.
    • Not get an error unless you explicitly prevent overwrites.

Add attributes put_item

if bucket_name as HASH key, must add items to store records.

dynamodb = boto3.client("dynamodb", region_name="us-west-2")
 
dynamodb.put_item(
    TableName="S3-object-size-history",
    Item={
        "bucket_name": {"S": "TestBucket"},                 # PK
        "computed_at": {"N": str(int(time.time() * 1000))}, # SK
        "total_size_bytes": {"N": "123456"},                # extra attribute
        "total_objects": {"N": "42"},                       # extra attribute
        "note": {"S": "triggered by s3 event"}              # extra attribute
    }
)

use to create a metadata to store max

meta_key = "__MaxSize__"
 
def put_maxitem(tablename, key, region):
    dynamoDB = boto3.client("dynamodb", region_name=region)
    try:
        dynamoDB.put_item(
            TableName=tablename,
            Item={
                "bucket_name": {"S": key},
                "timestamp": {"N": "0"},
                "max_size_bytes": {"N": "0"},
            }
        )
        print(f"--- Put medadata:{key} in {tablename} ---")
    except ClientError as e:
        print(e)

update metadata

  • use UpdateExpression, ConditionExpression, ExpressionAttributeValues to update
    • ConditionExpression acts as a gate: DynamoDB evaluates it first.
      • If it’s true, the update proceeds;
      • if it’s false, the update is rejected (e.g., ConditionalCheckFailedException).
    • UpdateExpression defines the actual modification
      • (e.g., SET max_size_bytes = :new_size).
    • ExpressionAttributeValues provides the concrete values for placeholders like :new_size (including the DynamoDB type, such as "N" for Number).
def update_maxsize(cur_size):
    try:
        dynamodb.update_item(
            TableName=TABLE_NAME,
            Key={
                "bucket_name": {"S": "__MaxSize__"},
                "timestamp": {"N": "0"}
            },
            UpdateExpression="SET total_size_bytes = :cur_size",
            ConditionExpression="attribute_not_exists(total_size_bytes) OR total_size_bytes < :cur_size",
            ExpressionAttributeValues={":cur_size": {"N": str(cur_size)}},
        )
    except ClientError as e:
        if e.response["Error"]["Code"] == "ConditionalCheckFailedException":
            return
        raise

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