Technical Design Document: [Feature or system name]

In-depth technical design document for a significant feature, service or migration, in the style of engineering design docs used at large software companies: context, goals and non-goals, detailed design, alternatives, cross-cutting concerns, testing, rollout and cost. For a short proposal, use the lean Design Doc template instead. Replace every [bracketed] value and every row marked Example.

Document control

Approvals

Revision history

Contents

1. Context and scope
2. Goals and non-goals
3. Overview of the design
4. Detailed design
5. Alternatives considered
6. Cross-cutting concerns
7. Testing strategy
8. Rollout and migration
9. Operations and support
10. Cost
11. Open questions
12. References and glossary

1. Context and scope

1.1 Background

[Describe the current system and how it behaves today. Example: "Search on the catalogue uses SQL LIKE queries against the product table. With 1.2 million products, p95 search latency is 2.8 s and full-text relevance is poor: 31% of searches end without a click."]

1.2 Problem statement

[The specific problem this design solves, with evidence: metrics, incidents, customer feedback, upcoming load.]

1.3 Scope

2. Goals and non-goals

2.1 Goals

2.2 Non-goals

• [Example: Supporting search across marketplace partner catalogues.]
• [Example: Replacing the product database as the source of truth.]
• [Example: Multi-language stemming beyond English and German in this phase.]

3. Overview of the design

[Summary. Example: "We introduce a search index (OpenSearch) populated by change events from the product database through an indexer service. The search API queries the index and falls back to the existing SQL search if the index is unavailable."]

4. Detailed design

4.1 APIs

{
  "q": "trail running shoes",
  "filters": {"brand": ["Acme"], "size": ["42"]},
  "pageSize": 24
}

Compatibility: [how existing clients are affected; versioning approach; deprecation timeline].

4.2 Data storage

Consistency model: [for example "the index is eventually consistent with the database; G-03 bounds the delay; reads that require exact stock levels still go to the database"].

4.3 Code and module changes

4.4 Key flows

The indexing flow and its failure handling:

4.5 Failure modes and degradation

4.6 Capacity

Index throughput needed during a full rebuild:

[State load assumptions, node sizing and headroom. Example: "3 data nodes with 8 vCPU and 32 GB each handle 300 queries/s at 35% CPU in the load test; we provision for 2x peak."]

5. Alternatives considered

Reasons for the choice: [Example: "B fails the latency goal at our data volume; C is excluded by the data residency policy; A meets all goals at acceptable operational cost."]

6. Cross-cutting concerns

6.1 Security

[Authentication and authorisation of new endpoints, input validation and query injection risk, secrets handling, network exposure, dependency review. Example: "The index is reachable only from the search API and indexer subnets; queries are built with the client library, never by string concatenation."]

6.2 Privacy

[Personal data processed or stored, lawful basis, retention, deletion propagation. Example: "No personal data is indexed. Seller names are public business data."]

6.3 Observability

6.4 Accessibility

[Impact on user-facing surfaces. Example: "API only. Client teams keep announcing result counts to screen readers when results change."]

6.5 Internationalisation

[Languages, analysers, collation, number and date formats. Example: "English and German analysers in phase 1; queries in other languages fall back to the standard analyser."]

6.6 Reliability and data integrity

[Idempotency, ordering, versioning of documents, reconciliation job that compares database and index counts daily.]

7. Testing strategy

8. Rollout and migration

8.1 Rollout plan

8.2 Data migration

[Initial backfill procedure, duration, verification (counts and sampled document comparison), and how changes during backfill are captured.]

8.3 Launch checklist

9. Operations and support

10. Cost

11. Open questions

12. References and glossary

Standard and licence note

Based on common industry practice for engineering design documents, as used by large software organisations: context, goals and non-goals, design, alternatives, cross-cutting concerns and rollout. Template licence: MIT.