Compatibility details of DynamoDB-compatible ApsaraDB for MongoDB instances

Background information

Amazon DynamoDB is a fully managed NoSQL database service that provides fast and predictable performance with seamless scalability. ApsaraDB for MongoDB is compatible with the DynamoDB protocol. You can select DynamoDB as the protocol type during instance creation to create a DynamoDB-compatible ApsaraDB for MongoDB instance. For more information about how to create a DynamoDB-compatible ApsaraDB for MongoDB instance, see Create a DynamoDB-compatible ApsaraDB for MongoDB instance.

Precautions

• The DynamoDB protocol is supported only for ApsaraDB for MongoDB sharded cluster instances that run MongoDB 4.0.
• You can change the protocol type to DynamoDB for existing ApsaraDB for MongoDB sharded cluster instances to make them compatible with the DynamoDB protocol.
• The DynamoDB protocol is supported only for ApsaraDB for MongoDB sharded cluster instances that reside within VPCs. No authentication is required.

Compatibility details of the DynamoDB protocol

Compatibility details of expressions

• An attribute name that contains .: An attribute name that contains . indicates a scalar attribute or an embedded document. In AWS DynamoDB, if an expression attribute name maps to all elements in a document path, the attribute is handled as a scalar attribute. Otherwise, the attribute is handled as a nested attribute.
  Note Scalar attributes that contain . cannot be processed, such as a query or projection in ApsaraDB for MongoDB.
• ProjectionExpression: Projection expressions support only one-dimensional arrays. When a projection expression contains only one array element, other fields are returned.
• ConditionExpression:
  • The following syntax is available for condition expressions in AWS DynamoDB:

    condition-expression ::=
          operand comparator operand
        | operand BETWEEN operand AND operand
        | operand IN ( operand (',' operand (, ...) ))
        | function
        | condition AND condition
        | condition OR condition
        | NOT condition
        | ( condition )
    
    comparator ::=
        =
        | <>
        | <
        | <=
        | >
        | >=
    
    function ::=
        attribute_exists (path)
        | attribute_not_exists (path)
        | attribute_type (path, type)
        | begins_with (path, substr)
        | contains (path, operand)
        | size (path)

  • In the operand1 comparator operand2 syntax, operand1 must be a path, and operand2 must be an expression attribute value.
  • In the operand1 BETWEEN operand2 AND operand3 syntax, operand1 must be a path, and other operands must be expression attribute values.
  • operand1 IN ( operand2 (',' operand3 (, ...) In )) syntax, operand1 must be a path, and other operands must be expression attribute values.
  • The size(path) function in the function section has the following limits: The attribute specified by path must be of the STRING, Set, or List type. The BINARY or MAP data type is not supported. If the attribute is of the String type, size returns the length of the string. If the attribute is of the Set or List type, size returns the number of elements in the set or list.
• UpdateExpression:
  • The following syntax is available for update expressions in AWS DynamoDB:

    update-expression ::=
        [ SET action [, action] ... ]
        [ REMOVE action [, action] ...]
        [ ADD action [, action] ... ]
        [ DELETE action [, action] ...]
    
    set-action ::=
        path = value
    
    value ::=
        operand
        | operand '+' operand
        | operand '-' operand
    
    operand ::=
        path | function
    
    function ::=
        if_not_exists (path, value)
        | list_append (list1, list2)
    
    remove-action ::=
        path
    
    add-action ::=
        path value
    
    delete-action ::=
        path value

  • set-action:
    • In the SET path = operand syntax, operand cannot be a path.
    • In the SET path = operand1 '+'|'-' operand2 syntax, operand1 must be a path. This indicates that only field increment or decrement is supported in this expression.
    • In the SET path = if_not_exists (path, value) syntax, the two paths must be equal and value must be set to an expression attribute value.
    • In the SET path = if_not_exists (path, value) syntax, partial updates are not supported when multiple conditions are concurrently specified. This indicates that all conditions must be met for this expression to be executed.
    • In the SET path = list_append(list1, list2) syntax, one of the list1 and list2 values must be equal to the path value, and the other one must be an expression attribute value.
  • remove-action: When an element in a list is removed, null is used to replace the removed element. The list size remains unchanged, and the remaining elements are not shifted.

Data type mappings

The supported data types of AWS DynamoDB differ from those of ApsarsDB for MongoDB. For this reason, Alibaba Cloud provides data type mappings between DynamoDB-compatible ApsaraDB for MongoDB instances and ApsaraDB for MongoDB instances. This way, compatibility is maintained for the two sets of data types.

Best practices

You can migrate data from AWS DynamoDB to ApsaraDB for MongoDB. For more information, see Migrate an Amazon DynamoDB database to ApsaraDB for MongoDB by using NimoShake.