Bigtable Basics

Bigtable Schema Design Guide for Agents

This document provides guidelines for designing performant schemas.

Table of Contents

• Key concepts [L15-L40]

• Defining the Row Key Template [L42-L63]

• Structured Row Keys [L65-L106]

• Row Key Design & Hotspotting [L108-L128]

• Counters for real-time metrics [L130-L135]

• Materialized views [L137-L178]

• Performance Checklist (Agent Verification) [L180-L197]

Key concepts

• Row key: Bigtable stores data lexicographically by row key. For best performance queries should be designed to filter by row key in its entirety or prefix. Point lookups by row key or reading ranges starting with a key will be the most performant. Row keys can have multiple parts combined using a delimiter, typically following a hierarchical format such as category#subcategory#productID as in apparel#shoes#0123. Bigtable doesn't support multi-row transactions but changes within a row are transactional. When designing schemas put data that needs to be updated transactionally within the same row.

• Column Families: Group data that is accessed together within a row. Defined as part of the schema. Contents of a family can easily be deleted in bulk with a single command for a given row key.

• Column Qualifiers: Defined at write time. Each row can have as many unique qualifiers within the row size limits (256 MB) with no limit on number of qualifiers per table. Qualifiers can be used in two ways: 1. as attributes in a JSON document e.g. zipcode, city, state, street address or 2. to store data like affinity scores e.g. 0.9, 0.7 for different products or web pages they visited e.g. home, search, cart.

• Timestamps: Are used for versioning. They are not system timestamps. They are user-defined and often used for event times like a sensor reading, address change timestamp or date a social media post was written. They can be used to expire items using TTL or move them to cold storage for cost savings as well as time-travel queries to find the "as of" state of a record.

Defining the Row Key Template

Since Bigtable treats row keys as opaque bytes, defining a Row Key Template is a manual design process based on your use case. To ensure consistency when programmatically interacting with data, your application code must implement a mechanism (such as string formatting or concatenation) to construct and parse these keys.

1. The Template Format

Define your keys using a placeholder syntax: {tenant_id}#{entity_type}#{reversed_timestamp}#{uuid}

2. Implementation Pattern

Use centralized factory functions to construct keys.

• Java: String.format("%s#%s#%d#%s", tenantId, entity, Long.MAX_VALUE - ts, uuid)

• Go: fmt.Sprintf("%s#%s#%d#%s", tenantID, entity, math.MaxInt64-ts, uuid)

3. Delimiter Selection

Use #, :, or |. Ensure delimiters don't appear in the field data.

Structured Row Keys

Bigtable supports Structured Row Keys to define the structure of your row keys. This metadata helps external tools (like BigQuery) and the Bigtable SQL interface understand how to parse your keys.

Why use Structured Row Keys?

• Automatic Parsing: SQL queries can reference individual segments by name instead of using string functions.

• Integration: Improves the experience when querying Bigtable from BigQuery or Spark.

• Validation: Helps prevent malformed keys.

Managing via gcloud

You can define the structure when creating a table or update an existing one:

gcloud bigtable instances tables update {table_id} \
    --instance=${BIGTABLE_INSTANCE} \
    --row-key-schema-definition-file={row_key_schema_definition_file}

Where {row_key_schema_definition_file} is a YAML file. A template is provided in assets/row_key_schema.yaml. You can copy this template to create your schema definition.

Row Key Design & Hotspotting

If row keys are autoincrement or are prefixed by date or timestamp, all writes will hit a single node, creating a "hotspot" and taxing the overall system performance. Bigtable's in-memory tier addresses hotspotting for reads (e.g. trending content on social media) but keys should be designed by keeping writes in mind.

Distribution Strategy

To ensure high performance, agents must validate that row keys are designed for high cardinality.

• Avoid: Sequential timestamps at the start of the key.

• Prefer: Prefixes to divide up the key space or reversed timestamps (e.g., tenantID#reversedTimestamp#objectID).

Field Salting Example

If a user must use a low-cardinality prefix, recommend "salting" the key: salt = hash(original_key) % number_of_nodes new_row_key = salt + "#" + original_key

Counters for real-time metrics

For frequent updates on single row metrics e.g. number of ad views, social media post likes, API calls or daily unique viewers (using data sketches like HLL), create an aggregate family to use Bigtable counters for much higher throughput and lower latency compared to read-modify-write.

Materialized views

For real-time analytics

Materialized views can be used for real-time aggregations across one or more rows for any type of data including aggregate families (note that approximate count distinct sketches will need to be aggregated using HLL_COUNT.MERGE). Frequently used metrics can be pre-aggregated efficiently as these views are incrementally maintained, then further filtered and aggregated at read time using SQL. Telemetry, merchant analytics, ad performance monitoring and real-time features for machine learning are some common use cases. Below is an example query that can be used as a materialized view that returns hourly count of messages in each chat room for a messaging application that has chatroom's unique identifier as the first row key token.

SELECT SPLIT(_key, '#')[0] AS chatroom, TIMESTAMP_TRUNC(_timestamp, HOUR) AS time_bucket,
COUNT(_key) AS total_messages FROM UNPACK((SELECT * FROM messages(WITH_HISTORY=>TRUE)))
GROUP BY 1, 2

For secondary indexing

Materialized views can be used as asynchronous global secondary indexes. Given the wide range of SQL functions supported, even geospatial index (using S2_CELLIDFROMPOINT) and inverted index use cases can be served with materialized views. Below is an example of an inverted index that allows fast search for rows that have occurrences of a given word in any of its cells in user_profile family or in a particular qualifier .

SELECT
u.value AS indexed_value,
u.key AS indexed_qualifier,
ARRAY_AGG(_key) AS user_keys
FROM users, UNNEST(MAP_ENTRIES(user_profile)) u
GROUP BY 1,2

Filtering this index view for just indexed_value returns all occurrences in the form of an array of row keys from the users table while using both indexed_value and indexed_qualifier returns results for only that qualifier.

Performance Checklist (Agent Verification)

When reviewing or generating schema-related code, verify the following:

• [ ] Row Key Size: Must be < 4KB (Ideal: 10–100 bytes). Large keys increase memory pressure and disk usage.

• [ ] Uniqueness: Ensure row keys are globally unique. Duplicate keys will overwrite existing data.

• [ ] Character Set: Use ^[a-zA-Z0-9\-_#]+$. Stick to alphanumeric, underscores, and hashes. Zero pad all numbers to ensure correct string sorting.

• [ ] Column Qualifier Size: Keep < 16 KB to minimize storage footprint.

• [ ] Column Family Count: Limit to < 100 families. Keep names short.

• [ ] Cell Field Size: Keep < 10 MB (100 MB is the hard limit). Larger cells slow down retrieval.

• [ ] Row Size: Keep < 100 MB. Note that Bigtable enforces a hard limit of 256 MB at read time.