DynamoDB and its Data Pane Operations - 3

This is the 3rd and the last article in the series for Data Pane 1 (read-write) operations and Data Pane 2 (read-write) operations.

In this article, I am going to show you such operations on DynamoDB scan. Some of the criteria which can be used in the scan operations are - Filter Expression, with options, such as max items, --starting-token, Count and ScannedCount and Reserved words.

In this article, I am not explaining the terminology and the concepts used in DynamoDB Tables. Please visit my previous articles for this - DynamoDB and its Control Pane Operations - 1 and DynamoDB and its Control Pane Operations - 2.

Since the scan operation involves scanning the whole table and not just a single item, there is no Key condition expression.

Scan

• Retrieves all items in the specified table or index.

• You can retrieve entire items, or just a subset of their attributes.

Filter Expression

• It removes some items from the Result Set returned by the Query.

• This can be compared in SQL to this is the part of the WHERE clause that acts on the non-Key attributes.

Max items

• If we want to limit the number of items then we use the --max-items option.

ScannedCount and Count

• Counts the items in the results

• If you don't use a filter expression, ScannedCount and Count have the same value.

Let’s get started!

Please visit my GitHub Repository for DynamoDB articles on various topics being updated on constant basis.

Pre-requisites:

• AWS user account with admin access, not a root account.
• Cloud9 IDE with AWS CLI.

Objectives:

1. Create a DynamoDB table

2. Add 3 items to the table

3. Scan the table to get a subset of the attributes

4. Scan the table to limit the items number to 1 using --max-items parameter.

5. Issue the scan request again, this time passing the NextToken value into the --starting-token parameter

6. Dealing with Reserved words

Resources Used:

Amazon DynamoDB Developer Guide

scan cli

Reserved words

Steps for implementation to this project:

• Copy the and paste the code in your AWS Cloud9 command prompt.

1. Create a DynamoDB table

aws dynamodb create-table \
    --table-name Movies \
    --attribute-definitions \
        AttributeName=Title,AttributeType=S \
        AttributeName=Review,AttributeType=S \
    --key-schema \
        AttributeName=Title,KeyType=HASH \
        AttributeName=Review,KeyType=RANGE \
    --provisioned-throughput \
        ReadCapacityUnits=10,WriteCapacityUnits=5
aws dynamodb wait table-exists --table-name Movies

2. Add 3 items to the table

aws dynamodb put-item \
    --table-name Movies \
    --item '{
           "Title": {"S": "Abhimaan"},
           "Review": {"S": "Excellent"},
           "Actor": {"S": "Big B"},
           "PostedBy": {"S": "User A"},
           "Songs": {"L": [ {"S": "Song1"},
                            {"S": "Song2"}
                          ]
                    }
            }'
aws dynamodb put-item \
    --table-name Movies \
    --item '{
           "Title": {"S": "Abhimaan"},
           "Review": {"S": "Terrific"},
           "Actor": {"S": "Jaya"},
           "PostedBy": {"S": "User B"},
           "Songs": {"L": [ {"S": "Song1"},
                            {"S": "Song2"}
                          ]
                    }
            }'
aws dynamodb put-item \
    --table-name Movies \
    --item '{
           "Title": {"S": "Abhimaan"},
           "Review": {"S": "Too Good"},
           "Actor": {"S": "Bindu"},
           "PostedBy": {"S": "User A"},
           "Songs": {"L": [ {"S": "Song1"},
                            {"S": "Song2"}
                          ]
                    }
            }'  

3. Scan the table to get a subset of the attributes

• Scan scanned all 3 items (ScannedCount) in the table -
  "ScannedCount": 3

• But the Filter Expression reduced our result set size down to 2 items (Count) - "Count": 2

• Hence, you should see 2 items in the result set, as the non-key attribute PostedBy references 2 items in Movies table.

• --item '{
  "PostedBy": {"S": "User A"}

• Refer the Movies table to check
  

aws dynamodb scan \
    --table-name Movies \
    --filter-expression 'PostedBy = :user' \
    --expression-attribute-values '{
        ":user" : {"S": "User A"}
    }' \
    --return-consumed-capacity TOTAL

4. Scan the table to limit the items number to 1 using --max-items parameter.

aws dynamodb scan \
    --table-name Movies \
    --filter-expression 'PostedBy = :user' \
    --expression-attribute-values '{
        ":user" : {"S": "User A"}
    }' \
    --max-items 1 \
    --return-consumed-capacity TOTAL

• the scan response will show a NextToken which we can then issue to a next scan call to pick up the 2nd item.

NextToken

"NextToken": "eyJFeGNsdXNpdmVTdGFydEtleSI6IG51bGwsICJib3RvX3RydW5jYXRlX2Ftb3VudCI6IDF9"

5. Issue the scan request again, this time passing the NextToken value into the --starting-token parameter

aws dynamodb scan \
    --table-name Movies \
    --filter-expression 'PostedBy = :user' \
    --expression-attribute-values '{
        ":user" : {"S": "User A"}
    }' \
    --max-items 1 \
    --starting-token eyJFeGNsdXNpdmVTdGFydEtleSI6IG51bGwsICJib3RvX3RydW5jYXRlX2Ftb3VudCI6IDF9 \
    --return-consumed-capacity TOTAL

6. Dealing with Reserved words

• In the Filter Expression, if the attribute name is actually a DynamoDB Reserved Word, it will throw an error - An error occurred (ValidationException) when calling the Scan operation: Invalid FilterExpression: Attribute name is a reserved keyword; reserved keyword: Number

• Because the Number attribute name is actually a DynamoDB Reserved Word.

• This can be solved by putting a placeholder which is a # (pound sign) in the FilterExpression and provide the actual attribute name in the --expression-attribute-names
  

aws dynamodb scan \
    --table-name <Table Name> \
    --filter-expression 'Pictures >= :pictures AND #Number >= :comment' \
    --expression-attribute-values '{
        ":pictures" : {"N": "1"},
        ":Number" : {"S": "Nice"}
    }' \
    --expression-attribute-names '{"#Number" : "number"}' \
    --return-consumed-capacity TOTAL

Cleanup

• Delete the table Movies

What we have done so far

We have successfully demonstrated Data Pane operations on DynamoDB scan, using these criteria - Filter Expression, with options, such as max items, --starting-token, Count and ScannedCount, Reserved words.