Dynamodb Table Design

DynamoDB Table Design

This skill takes a formalized access patterns document and produces a complete DynamoDB single-table design. It maps every access pattern to a concrete DynamoDB operation with explicit key conditions.

When to Use

• The user has an access patterns .md file (from dynamodb-access-patterns or manually written)
• The user wants to design PK/SK structures for their entities
• The user wants to decide which GSIs they need
• The user wants to verify that every access pattern is covered by the table design
• The user wants to add a new entity to an existing table design

Pipeline Position

  1. Access Patterns  -->  [2. Table Design]  -->  3. Query Interfaces
  (dynamodb-access-       (this skill)           (dynamodb-query-
   patterns)                                      interfaces)

Input: Access patterns .md file Output: Table design .md file consumed by dynamodb-query-interfaces

Prerequisites

Before starting, read the access patterns file. If it doesn't exist or is incomplete, tell the user to run the dynamodb-access-patterns skill first.

If the user already has a table in production and wants to add entities, also read their existing table design file (if any) to understand current keys and GSIs.

Design Workflow

Step 1: Read and Validate Access Patterns

Read the access patterns file and verify:

1. Every access pattern has an ID (AP-XXX)
2. Every pattern specifies input parameters and result type
3. Multi-entity patterns are explicitly marked
4. Write patterns specify transaction requirements

If anything is missing, ask the user to clarify before proceeding.

Step 2: Group Entities into Item Collections

This is the core of single-table design. Analyze the access patterns to determine which entities should share a partition key.

Rules for grouping:

1. Entities fetched together in a multi-entity read MUST share a PK.

   • If AP-003 fetches User + Orders together, they must share a PK.

2. 1:N relationships where the parent is always known go under the parent's PK.

   • Orders belong to a User -> PK = USER#userId

3. Entities accessed only by their own ID get their own PK.

   • A Product looked up by productId -> PK = PRODUCT#productId

4. Global collections (list all, leaderboard) use a static PK.

   • All products catalog -> PK = PRODUCTS

5. N:N or deeply nested relationships use composite PKs.

   • Items in a specific cart -> PK = USER#userId#CART#cartId

Present the proposed grouping to the user:

Based on your access patterns, here's how I'd group entities:

Item Collection 1: User scope
  PK = USER#<userId>
  Contains: User, Order, Balance, Preferences
  Supports: AP-001, AP-002, AP-003, AP-005

Item Collection 2: Product catalog
  PK = PRODUCTS
  Contains: Product (catalog entries)
  Supports: AP-006, AP-007

Item Collection 3: Product detail
  PK = PRODUCT#<productId>
  Contains: Product (full record), Review
  Supports: AP-008, AP-009, AP-010

Does this grouping make sense? Any patterns I'm missing?

Step 3: Design Primary Keys

For each item type, define the PK and SK.

Key Design Principles:

Separator Convention

Use # as the separator between key segments. This is the universal DynamoDB convention.

PK = ENTITY_PREFIX#identifier
SK = ENTITY_PREFIX#identifier

PK Patterns

SK Patterns

Sort Key Design for Range Queries

If an access pattern needs sorting or range queries, the sort field MUST be encoded in the SK:

# Sort by date (newest first -- use ScanIndexForward=false)
SK = ORDER#2024-01-15T10:30:00Z#01HORDERID

# Sort by score (string-encoded numbers must be zero-padded)
SK = SCORE#000150#01HPLAYERID    (score=150, padded to 6 digits)

# Sort by status + date (hierarchical sort)
SK = ORDER#SHIPPED#2024-01-15T10:30:00Z#01HORDERID

Zero-padding rule: When numbers are used in string sort keys, they MUST be zero-padded to a fixed width so lexicographic order matches numeric order. Decide the maximum expected value and pad accordingly.

Descending order trick: To sort descending without ScanIndexForward=false, invert the value:

SK = SCORE#<(MAX_SCORE - actualScore) zero-padded>#<id>

Step 4: Identify GSI Requirements

For each access pattern NOT covered by the table's PK/SK, determine if a GSI is needed.

You need a GSI when:

GSI Design Rules:

1. Reuse GSIs aggressively. DynamoDB allows max 20 GSIs per table. Each GSI costs additional storage and write capacity. Design GSI keys to be overloaded across entity types when possible.

2. Name GSIs generically. Use GSI1, GSI2, GSI3 for overloaded indexes. Use descriptive names only for single-purpose indexes (e.g., email-index).

3. GSI key attributes follow the naming convention:

   • Overloaded: GSI1PK (String), GSI1SK (String)
   • Single-purpose: The actual attribute name (e.g., email as PK, createdAt as SK)

4. Choose the right projection:

   • ALL -- projects all attributes (default, simplest, most expensive)
   • KEYS_ONLY -- only key attributes (cheapest, requires table fetch for other attributes)
   • INCLUDE -- specific attributes (middle ground)

5. Sparse indexes are powerful. If only some items have the GSI attributes, only those items appear in the index. Use this to create filtered views.

Present GSI decisions to the user:

I need the following GSIs:

GSI1 (overloaded, String/String):
  - User lookup by email: GSI1PK=EMAIL#<email>, GSI1SK=EMAIL#<email>
  - Order leaderboard: GSI1PK=ORDERS, GSI1SK=ORDER#<total-inverted>#<orderId>
  Projection: ALL

No additional GSIs needed. Total: 1 GSI.

Does this look right? Are there access patterns I should reconsider?

Step 5: Map Every Access Pattern to a DynamoDB Operation

This is the accountability step. EVERY access pattern from the input file must be mapped to a concrete DynamoDB operation.

For each access pattern, define:

Example:

AP-001: Get user by userId
  Operation: Query
  Table: Main
  PK: USER#<userId>
  SK: = USER#<userId>
  Params: Limit=1

AP-002: List orders for user, newest first
  Operation: Query
  Table: Main
  PK: USER#<userId>
  SK: begins_with(ORDER#)
  Params: ScanIndexForward=false, Limit=20, paginated

AP-010: Create order
  Operation: TransactWriteItems
  Items:
    1. Put: PK=USER#<userId>, SK=ORDER#<timestamp>#<orderId> (Condition: attribute_not_exists(pk))
    2. Put: PK=ORDER#<orderId>, SK=ORDER#<orderId> (Condition: attribute_not_exists(pk))
    3. Update: PK=USER#<userId>, SK=ORDER_COUNT (SET count = count + 1)

Step 6: Validate the Design

Read reference/validation-checklist.md and run through every check with the user before finalizing.

Output Format

Persist the output as a markdown file. Suggest docs/dynamodb/table-design.md or alongside the access patterns file.

Read reference/output-template.md for the exact file structure to use.

Common Design Patterns

Read reference/design-patterns.md for a catalog of reusable single-table design patterns. Reference these when making key design decisions and present relevant patterns to the user.

Rules for the Agent

1. Read the access patterns file first. Never design keys without documented access patterns.
2. Map EVERY access pattern. If a pattern can't be mapped, it's a design problem -- don't skip it.
3. Prefer the base table over GSIs. Only add a GSI when the base table genuinely can't serve the pattern.
4. Be interactive. Present groupings and key designs to the user for approval before finalizing.
5. Show concrete examples. For each entity, include a sample JSON item.
6. Document decisions. When there are trade-offs (e.g., duplication vs. extra query), explain both options and document the choice.
7. Stay language-agnostic. Use DynamoDB operation names (Query, GetItem, TransactWriteItems) not SDK-specific function names.
8. Challenge hot partitions. If a static PK like PRODUCTS will receive heavy traffic, discuss sharding strategies with the user.
9. Consider write amplification. If an entity is written to 3 item collections, that's 3x write cost. Make sure it's justified.
10. Persist the file. Always write the output to a .md file.

Next Step

Once the table design file is complete, tell the user:

Table design is documented at [file path].

The next step is to generate the query interface -- method signatures
for every access pattern, ready to be implemented in your language of choice.

To continue, use the `dynamodb-query-interfaces` skill with this file as input.