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multiexcerpt-include-macroSnap Execution

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Select one of the following three modes in which the Snap executes:

Validate & Execute: Performs limited execution of the Snap, and generates a data preview during Pipeline validation. Subsequently, performs full execution of the Snap (unlimited records) during Pipeline runtime.
Execute only: Performs full execution of the Snap during Pipeline execution without generating preview data.
Disabled: Disables the Snap and all Snaps that are downstream from it.

Default Value: Execute only
Example: Validate & Execute

Snap type:

Read

Description:

This Snap retrieves data from DynamoDB by executing a DynamoDB Query via the REST API. It processes Snap properties and formats the HTTP request entity body according to the syntax defined in the AWS DynamoDB Query API Reference. It can be used to retrieve data from DynamoDB table as well as Global Secondary Index or Local Secondary Index. Refer to AWS DynamoDB Query API Reference for further details.

Input & Output

Input: This Snap can have an upstream Snap that can pass a document output view such as Mapper or JSON Generator. If the upstream Snap is connected, this Snap executes once per each input document and produces the query results as a stream of documents to the output view. Each input document is used to evaluate expression properties in the Snap.
Output: This Snap can have an downstream Snap that has a document input view such as Mapper or JSON Formatter.

Modes

Ultra pipelines: Supported

Works in Ultra Pipelines.

Spark mode: Not supported in Spark mode.

Prerequisites:

None

Prerequisites:

None

Limitations and Known Issues:

None at this time.

Configurations:

Account & Access

A valid DynamoDB account is required.

Views

Input	This Snap has at most one document input view.
Output	This Snap has exactly one document output view and produces a stream of documents, one for each item matching the query criteria.
Error	This Snap has at most one document error view and produces zero or more documents in the view.

Troubleshooting:

The Snap produces an error document if DynamoDB fails to execute the submitted query. The "request_entity" field in the error document contains the HTTP entity body of the submitted query and may be useful when troubleshooting the pipeline. The Snap processes the Snap properties and prepares the HTTP entity body for the query. The "error_entity" fields in the error document is the error message from the DynamoDB server describing which part of the query has caused the error.

Settings

Label

Required. The name for the Snap. You can modify this to be more specific, especially if you have more than one of the same Snap in your pipeline.

Table name

Required. The name of the DynamoDB table. Table names can be suggested if the expression property is disabled. The input document schema and pipeline parameters can be suggested when the expression property is enabled.

Example: EmployeeEvents, $TableName, _TableName

Default value: None

Secondary index name

The name of Global Secondary Index or Local Secondary Index if used in the query. The Amazon DynamoDB document explains this as following:

"Amazon DynamoDB provides fast access to items in a table by specifying primary key values. However, many applications might benefit from having one or more secondary (or alternate) keys available, to allow efficient access to data with attributes other than the primary key."

Please refer to Improving Data Access with Secondary Indexes for more details.

Example: EffectiveDate-index, $secondary_index, _secondary_index

Default value: None

Projection expression

A string that identifies one or more attributes to retrieve from the table. The attributes in the expression must be separated by commas. If no attribute names are specified, then all attributes will be returned. If any of the requested attributes are not found, they will not appear in the result.

Example: "EffectiveDate, EmpID, EventID, #name" (without double-quote), or $attributes or _attributes

Default value: None

Key condition expression

The condition that specifies the key value(s) for items to be retrieved by the Query action. The condition must perform an equality test on a single partition key value. The condition can also perform one of several comparison tests on a single sort key value. Query can use KeyConditionExpression to retrieve one item with a given partition key value and sort key value, or several items that have the same partition key value but different sort key values. Refer to the KeyConditionExression section in the AWS DynamoDB Query API Reference for more details.

Example: EffectiveDate = :val1

Default value: None

Filter expression

A string that contains conditions that DynamoDB applies after the Query operation, but before the data is returned to the Snap. Items that do not satisfy the filter expression criteria are not returned. A filter expression does not allow key attributes. You cannot define a filter expression based on a partition key or a sort key.

Note: A filter expression is applied after the items have already been read; the process of filtering does not consume any additional read capacity units.
For more information, see the "Filter Expression for Query" section in Working with Queries.

Example: #v >= :num

Default value: None

Expression attribute values

Specify the placeholders for the Expression Attribute Names and/or Expression Attribute Values.ed words as attribute names in expressions.

Info

DynamoDB reserves certain words and they are called reserved names. You must not use reserved words as attribute names in expressions. If you must write an expression that contains a word that conflicts with a DynamoDB reserved word, then you can define an expression attribute name to use in the place of the reserved word. For more information, see Expression Attribute Names.

Similarly, you cannot use certain values as attribute values in expressions and instead must define an expression attribute name. For more information, see Expression Attribute Values.

Attribute: The expression attribute name. Begins with a # for names and : for values.
Type: The attribute type. Blank for Expression Attribute Names. S for string, N for number, and so on, for Values.
Value: The actual literal to be used in the query condition.

Example

If your table contains a column named uuid (a reserved word) and you want to read a value from the uuid column, then you must assign a placeholder for the column name because you cannot use a reserved word directly in your expression. We can define the Attribute #U, leave the Type blank, and enter the value as uuid. In the Snap Filter expression condition, we enter #u=:val1. :val1 is the placeholder for the value in the table which is defined as Attribute :val1 of Type B with Value dGhpcyB0ZXh0.

Attribute	Type	Value
#u		UUID

Action

Enter COUNT to get only the number of items after query execution or DESCRIBE TABLE to get the table metadata at the output view, or QUERY for normal query operations.

Example: QUERY, COUNT, DESCRIBE TABLE

Default value: QUERY

Consistent read

Determines the read consistency model. If set to true, then the operation uses strongly consistent reads. Otherwise, the operation uses eventually consistent reads. Strongly consistent reads are not supported on global secondary indexes. If you query a global secondary index with 'Consistent read' set to true, you will receive an Exception.

Default value: False

Pass through

If true, the input document is passed through to the output view under the 'original' key.

Default value: False

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Examples

Sample query for Global Secondary Index:
Refer to the attached file: DynamoDB Query for GlobalSecondaryIndex.slp

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The 'EmployeeEvents' table has the following items in the above example:

EmpID	Name	EventID	EffectiveDate
{"N":"3"}	{"S":"Paul"}	{"S":"103"}	{"N":"20170703"}
{"N":"2"}	{"S":"John"}	{"S":"102"}	{"N":"20170703"}
{"N":"3"}	{"S":"Paul"}	{"S":"102"}	{"N":"20170703"}
{"N":"20170210"}		{"S":"foobar"}
{"N":"1828"}		{"S":"Preferred Name Change Event"},	{"N":"1475218800000"},
{"N":"1"}		{"S":"101"}	{"N":"20170802"}

The Snap processes Snap properties and sends the following HTTP request entity to DynamoDB:

{
"TableName": "EmployeeEvents",
  "IndexName": "EffectiveDate-index",
  "ProjectionExpression": "EffectiveDate, EmpID, EventID, #nam",
  "KeyConditionExpression": "EffectiveDate = :val1",
  "ExpressionAttributeNames": {
      "#name": "Name"
  },
  "ExpressionAttributeValues": {
      ":val1": {
          "N": "20170703"
      }
  },
  "ReturnConsumedCapacity": "INDEXES"
}

Then, the Snap produces the output documents as following:

EmpID	Name	EventID	EffectiveDate
{"N":"3"}	{"S":"Paul"}	{"S":"103"}	{"N":"20170703"}
{"N":"2"}	{"S":"John"}	{"S":"102"}	{"N":"20170703"}
{"N":"3"}	{"S":"Paul"}	{"S":"102"}	{"N":"20170703"}

2. Sample query for Table:

Refer to the attached file: DynamoDB Query for Table.slp

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The Snap processes Snap properties and sends the following HTTP request entity to DynamoDB:

{
"TableName": "EmployeeEvents",
  "ProjectionExpression": "EffectiveDate, EmpID, EventID, #nam",
  "KeyConditionExpression": "EventID = :val2 AND EmpID > :val3",
  "ExpressionAttributeNames": {
      "#name": "Name"
  },
  "ExpressionAttributeValues": {
      ":val2": {
          "S": "102"
      },
      ":val3": {
          "N": "1"
      }
  },
  "ReturnConsumedCapacity": "INDEXES"
}

Then, the Snap produces the output documents as following:

EmpID	Name	EventID	EffectiveDate
{"N":"2"}	{"S":"John"}	{"S":"102"}	{"N":"20170703"}
{"N":"3"}	{"S":"Paul"}	{"S":"102"}	{"N":"20170703"}

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