Write Shape to Base Mechanism Cheat Sheet #

Use this as a default mapping from a normalized path shape to an implementation mechanism family. This is not a proof of the final design. It is a fast first-pass heuristic.

1. Create Target #

Examples:

• create post
• create profile
• create order
• create comment

Default base mechanism:

• insert/create in primary store

Common implementation forms:

• insert row
• append to primary table
• create object in key-value store

When to refine:

• if duplicate retries matter, add idempotency key
• if child object is append-heavy, consider append log/table

2. Overwrite Current Value #

Examples:

• rename file
• edit profile headline
• update document title
• update feature flag rule

Default base mechanism:

• versioned overwrite

Common implementation forms:

• update by version
• optimistic concurrency control
• last-writer-wins only when business semantics allow it

When to refine:

• if stale writes are dangerous, require CAS/version match

3. State Transition #

Examples:

• follow/unfollow
• revoke sharing
• cancel booking
• approve refund
• accept suggestion

Default base mechanism:

• guarded conditional update

Common implementation forms:

• conditional update on current state/version
• lifecycle transition in primary row/object
• optimistic concurrency with state predicate

When to refine:

• if exclusivity matters, combine with claim/lease semantics
• if multiple objects move together, add workflow orchestration

4. Append Event #

Examples:

• audit log event
• activity event
• bid
• metric sample
• span record

Default base mechanism:

• append log or append table

Common implementation forms:

• immutable insert
• event log
• time-series append

When to refine:

• if ordering matters, define ordering scope
• if dedup matters, add event/request id

5. Read Source #

Examples:

• view current order status
• read current document
• read current seat availability
• introspect session

Default base mechanism:

• source read from authoritative state

Common implementation forms:

• primary DB read
• shard owner read
• strongly consistent read path

When to refine:

• if bounded stale is allowed, add replica or cache read rules

6. Read Projection #

Examples:

• home feed
• search results
• top K tags
• dashboard
• leaderboard

Default base mechanism:

• materialized view or search/index read model

Common implementation forms:

• search index
• feed table
• ranking cache
• precomputed aggregation

When to refine:

• if user needs read-your-write, add hybrid source + projection path

7. Projection Write / Update #

Examples:

• increment follower count
• update top tags
• update feed fanout table
• update search index

Default base mechanism:

• counter/projection update

Common implementation forms:

• counter increment/decrement
• async materialized view maintenance
• search indexing worker
• batch recompute

When to refine:

• if exactness matters, prefer idempotent deltas or recompute from source truth

8. Exclusive Claim #

Examples:

• acquire lock
• worker claims job
• claim reminder notification run
• assign one executor to a runnable unit

Default base mechanism:

• claim/lease

Common implementation forms:

• lease record with expiry
• claim token
• fencing token / epoch

When to refine:

• if crash recovery matters, make expiry and fencing explicit

9. Time-Bounded Exclusive Allocation #

Examples:

• hold seat
• hold hotel room
• hold appointment slot

Default base mechanism:

• guarded conditional update with claim/lease semantics

Common implementation forms:

• resource state transition + hold record
• hold expiry index
• fencing/epoch if stale holders can act later

When to refine:

• if commit follows hold, add booking/order workflow object

10. Frontier Advance #

Examples:

• advance campaign recipient frontier
• advance crawler cursor
• advance scheduler next due boundary

Default base mechanism:

• guarded frontier advance

Common implementation forms:

• versioned cursor update
• monotonic pointer/frontier write
• claim-next-batch under current frontier

When to refine:

• if materializing runs, pair with create target

11. Future Constraint to Claimable Run #

Examples:

• due reminder creates notification job
• delayed task becomes runnable
• scheduled run fires

Default base mechanism:

• due-time scan plus create target, often followed by claim/lease

Common implementation forms:

• index by run_at
• scheduler scan
• materialize runnable item
• worker claim token

When to refine:

• if exactly-once due materialization matters, add guarded version update

12. Control Plane Snapshot Publication #

Examples:

• push feature flag config
• publish policy snapshot
• publish validator snapshot
• publish config version to consumers

Default base mechanism:

• versioned snapshot publication

Common implementation forms:

• push or pull-after-notify
• snapshot overwrite
• monotonic version application

When to refine:

• if rollout matters, track published versus applied version separately

Quick Mapping Table #

Interview One-Liner #

After I normalize a path, I pick its write shape first. That usually suggests the first mechanism family: create maps to insert, overwrite maps to versioned overwrite, lifecycle changes map to guarded conditional updates, append facts map to append logs, claim paths map to leases, and feeds/search/top-K map to materialized views.