Social Post Service — System Design Academy

Complete System Design Tutorial

The design separates the latency-sensitive online path, the correctness-critical durable transition, and asynchronous projection or repair work. Each chapter below explains one decision shown in the diagrams.

1. Requirements and scope

Functional requirements

Execute the primary social post service workflow and return an observable result.
Expose the concrete operations shown below with stable retry identities.
Allow operators to inspect, disable, repair, or replay ambiguous state.
Keep derived analytics and notifications off the correctness-critical path.

Non-functional requirements

Design for 1B users · 500M posts/day · media dominates storage.
Protect the named business invariant under concurrency and partial failure.
Scale by an explicit aggregate/partition key without one global mutex.
Degrade intentionally when cache, queue, store, or regional ownership is unavailable.

Primary invariant: each accepted social post service operation has one stable result and retries do not repeat its business effect.

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2. Capacity assumptions

State assumptions before choosing technology. The supplied workload is 1B users · 500M posts/day · media dominates storage. Convert it into peak throughput, retained records, bandwidth, and failure budget; then connect each number to a component.

Dimension	Working estimate	Design implication
Traffic	1B users · 500M posts/day · media dominates storage	Separate online compute, durable state, and asynchronous consumers.
Peak factor	5–10× average	Admission control, autoscaling headroom, and hot-key protection.
Stored state	write rate × retention × record bytes	Partition count, indexes, compaction, backup, and archival tier.
Network	peak operations × payload size	Batching, compression, edge placement, and egress budget.
Availability	99.9–99.99%	Multi-zone replicas, tested failover, and dependency error budgets.
Recovery	RPO/RTO by invariant	Ack boundary, replay retention, reconciliation, and operator runbooks.

Do not cargo-cult the numbers: show the arithmetic, name the uncertain variable, and say which production metric will replace the interview estimate.

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3. API and data model

POST /v1/media/uploadssigned multipart upload

POST /v1/postspublish media references

PATCH /v1/posts/{id}versioned edit policy

DELETE /v1/posts/{id}tombstone and propagate

Authoritative records and indexes

post(post_id PK, author_id, visibility, text, version, state)

media(media_id PK, owner_id, source_uri, rendition_manifest, scan_state)

follow(follower_id, followee_id, state, version)

moderation(object_id, decision, reason, policy_version)

Command and event envelopes

# Command envelope
operation: POST /v1/media/uploads
request_id: req_01J...
idempotency_key: idem_7f2...
aggregate_key: social-post-service_123
expected_version: 17
payload: {...}

# Durable event
event_id: evt_01J...
aggregate_key: social-post-service_123
event_type: social_post_service_committed
schema_version: 3
trace_id: tr_8ac...

The authoritative aggregate and idempotency record share a transactional or conditional-write boundary. Event streams, search indexes, caches, analytics, and dashboards are rebuildable projections.

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4. Why this architecture and what it costs

The boxes are not a shopping list. Each boundary exists because it owns a different invariant, scaling dimension, data lifecycle, security policy, or failure domain. The table makes those choices explicit and shows when the design should remain simpler.

Component	Owned responsibility	Why separate it	Rejected alternative	Cost and evolution trigger
Creator App	Owns client protocol, request identity, retry, and resume behavior. In this design it participates in: Upload media directly, checksum it, scan it, and generate renditions.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents the external caller from also owning the contract and failure behavior of Media Upload Edge.	Keep this behavior inside an adjacent service. Prefer that simpler option until independent scale, ownership, policy, or failure isolation is measurable. Here that simpler choice would couple “Media upload completes but post never commits” to the external caller.	Costs an interface, deployment, telemetry, ownership, and failure mode. Keep it in-process first; extract when independent scale or reliability pays for the boundary. Measure the latency, saturation, lag, or mismatch at the Creator App boundary before extracting or scaling it.
Media Upload Edge	Owns authentication, validation, quotas, deadlines, and routing. In this design it participates in: Validate ownership of ready media and commit immutable post identity plus version.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Creator App from also owning the contract and failure behavior of Post API.	Put protocol, authentication, quotas, and business transitions in one process. Acceptable at low scale, but it couples external traffic risk to domain releases. Here that simpler choice would couple “Privacy changes while timelines contain the post” to Creator App.	Costs another hop, policy distribution, and a high-blast-radius fleet. Introduce it when many clients or services need one protocol and protection boundary. Measure the latency, saturation, lag, or mismatch at the Media Upload Edge boundary before extracting or scaling it.
Post API	Owns authentication, validation, quotas, deadlines, and routing. In this design it participates in: Publish post-created to timeline, search, notification, and moderation consumers.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Media Upload Edge from also owning the contract and failure behavior of Auth / Privacy.	Put protocol, authentication, quotas, and business transitions in one process. Acceptable at low scale, but it couples external traffic risk to domain releases. Here that simpler choice would couple “Moderation removes a viral cached asset” to Media Upload Edge.	Costs another hop, policy distribution, and a high-blast-radius fleet. Introduce it when many clients or services need one protocol and protection boundary. Measure the latency, saturation, lag, or mismatch at the Post API boundary before extracting or scaling it.
Auth / Privacy	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: Serve text/metadata from post storage and immutable renditions through CDN.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Post API from also owning the contract and failure behavior of Media Scanner.	Embed policy in every caller. Rules then drift by deployment version and emergency changes require fleet-wide redeployment. Here that simpler choice would couple “Fanout consumer replays post-created” to Post API.	Costs an interface, deployment, telemetry, ownership, and failure mode. Keep it in-process first; extract when independent scale or reliability pays for the boundary. Measure the latency, saturation, lag, or mismatch at the Auth / Privacy boundary before extracting or scaling it.
Media Scanner	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: Edits create a new version; deletes create a tombstone that invalidates projections.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Auth / Privacy from also owning the contract and failure behavior of Transcode Workers.	Embed policy in every caller. Rules then drift by deployment version and emergency changes require fleet-wide redeployment. Here that simpler choice would couple “Edit and delete race on the same version” to Auth / Privacy.	Costs an interface, deployment, telemetry, ownership, and failure mode. Keep it in-process first; extract when independent scale or reliability pays for the boundary. Measure the latency, saturation, lag, or mismatch at the Media Scanner boundary before extracting or scaling it.
Transcode Workers	Executes idempotent asynchronous work under lease and retry policy. In this design it participates in: Upload media directly, checksum it, scan it, and generate renditions.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Media Scanner from also owning the contract and failure behavior of Object Storage.	Perform the work before acknowledging the caller. This gives simpler consistency but unbounded tail latency and fragile retries. Here that simpler choice would couple “Media upload completes but post never commits” to Media Scanner.	Costs leases, checkpoints, retries, DLQs, and lag operations. Separate once execution time or burstiness no longer fits the request deadline. Measure the latency, saturation, lag, or mismatch at the Transcode Workers boundary before extracting or scaling it.
Object Storage	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: Validate ownership of ready media and commit immutable post identity plus version.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Transcode Workers from also owning the contract and failure behavior of Post Metadata DB.	Keep this behavior inside an adjacent service. Prefer that simpler option until independent scale, ownership, policy, or failure isolation is measurable. Here that simpler choice would couple “Privacy changes while timelines contain the post” to Transcode Workers.	Costs an interface, deployment, telemetry, ownership, and failure mode. Keep it in-process first; extract when independent scale or reliability pays for the boundary. Measure the latency, saturation, lag, or mismatch at the Object Storage boundary before extracting or scaling it.
Post Metadata DB	Owns authoritative versioned state or immutable evidence. In this design it participates in: Publish post-created to timeline, search, notification, and moderation consumers.	Defines the source of truth and commit boundary. It prevents Object Storage from also owning the contract and failure behavior of Social Graph.	Let each service persist its own interpretation. That removes one clear commit boundary and makes retries, support, and reconciliation ambiguous. Here that simpler choice would couple “Moderation removes a viral cached asset” to Object Storage.	Costs schema migration, replication, backup, and shard ownership. Split storage only when access patterns, retention, or compliance boundaries materially differ. Measure the latency, saturation, lag, or mismatch at the Post Metadata DB boundary before extracting or scaling it.
Social Graph	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: Serve text/metadata from post storage and immutable renditions through CDN.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Post Metadata DB from also owning the contract and failure behavior of Timeline Event Bus.	Keep this behavior inside an adjacent service. Prefer that simpler option until independent scale, ownership, policy, or failure isolation is measurable. Here that simpler choice would couple “Fanout consumer replays post-created” to Post Metadata DB.	Costs an interface, deployment, telemetry, ownership, and failure mode. Keep it in-process first; extract when independent scale or reliability pays for the boundary. Measure the latency, saturation, lag, or mismatch at the Social Graph boundary before extracting or scaling it.
Timeline Event Bus	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: Edits create a new version; deletes create a tombstone that invalidates projections.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Social Graph from also owning the contract and failure behavior of Fanout Service.	Keep this behavior inside an adjacent service. Prefer that simpler option until independent scale, ownership, policy, or failure isolation is measurable. Here that simpler choice would couple “Edit and delete race on the same version” to Social Graph.	Costs an interface, deployment, telemetry, ownership, and failure mode. Keep it in-process first; extract when independent scale or reliability pays for the boundary. Measure the latency, saturation, lag, or mismatch at the Timeline Event Bus boundary before extracting or scaling it.
Fanout Service	Executes idempotent asynchronous work under lease and retry policy. In this design it participates in: Upload media directly, checksum it, scan it, and generate renditions.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Timeline Event Bus from also owning the contract and failure behavior of Search Indexer.	Perform the work before acknowledging the caller. This gives simpler consistency but unbounded tail latency and fragile retries. Here that simpler choice would couple “Media upload completes but post never commits” to Timeline Event Bus.	Costs leases, checkpoints, retries, DLQs, and lag operations. Separate once execution time or burstiness no longer fits the request deadline. Measure the latency, saturation, lag, or mismatch at the Fanout Service boundary before extracting or scaling it.
Search Indexer	Serves a low-latency, disposable projection with authoritative fallback. In this design it participates in: Validate ownership of ready media and commit immutable post identity plus version.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Fanout Service from also owning the contract and failure behavior of Moderation Service.	Read authoritative storage for every request. Simpler initially, but latency and source-of-truth load scale together and a hot key becomes a database incident. Here that simpler choice would couple “Privacy changes while timelines contain the post” to Fanout Service.	Costs invalidation, staleness, stampede protection, and memory. Add or split it only after the workload (1B users · 500M posts/day · media dominates storage) proves repeated reads dominate. Measure the latency, saturation, lag, or mismatch at the Search Indexer boundary before extracting or scaling it.
Moderation Service	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: Publish post-created to timeline, search, notification, and moderation consumers.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Search Indexer from also owning the contract and failure behavior of CDN.	Keep this behavior inside an adjacent service. Prefer that simpler option until independent scale, ownership, policy, or failure isolation is measurable. Here that simpler choice would couple “Moderation removes a viral cached asset” to Search Indexer.	Costs an interface, deployment, telemetry, ownership, and failure mode. Keep it in-process first; extract when independent scale or reliability pays for the boundary. Measure the latency, saturation, lag, or mismatch at the Moderation Service boundary before extracting or scaling it.
CDN	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: Serve text/metadata from post storage and immutable renditions through CDN.	Removes repeated work from the latency-critical path. It prevents Moderation Service from also owning the contract and failure behavior of operators and repair tooling.	Read authoritative storage for every request. Simpler initially, but latency and source-of-truth load scale together and a hot key becomes a database incident. Here that simpler choice would couple “Fanout consumer replays post-created” to Moderation Service.	Costs invalidation, staleness, stampede protection, and memory. Add or split it only after the workload (1B users · 500M posts/day · media dominates storage) proves repeated reads dominate. Measure the latency, saturation, lag, or mismatch at the CDN boundary before extracting or scaling it.

Decision 01

Chosen boundary vs simpler monolith

Upload media directly to object storage using signed URLs, then commit the post metadata.

A single deployable with one transactional store is the right starting point for a small social post service workload. The proposed boundary becomes justified when upload media directly to object storage using signed urls, then commit the post metadata. Separating it contains the named failure “Media upload completes but post never commits” without placing unrelated work on the critical path.

Decision 02

Correctness vs availability

Represent follow edges separately and generate timelines through hybrid fan-out.

The design coordinates only where each accepted social post service operation has one stable result and retries do not repeat its business effect. Everywhere else it accepts bounded staleness or asynchronous propagation. The alternative—strong coordination for every projection—simplifies the mental model but increases tail latency and turns partitions into broad write unavailability.

Decision 03

Operational cost and evolution

Use immutable post IDs and tombstones; propagate deletes and moderation decisions quickly.

This choice adds dashboards, ownership, schema compatibility, capacity planning, and recovery runbooks. Start with the minimum topology that supports “Publish post-created to timeline, search, notification, and moderation consumers.” and extract another service only when measured throughput, blast radius, data lifecycle, or team ownership changes independently.

Why this design: the synchronous path contains only the decisions required to protect each accepted social post service operation has one stable result and retries do not repeat its business effect. Durable projections, notifications, indexing, analytics, and repair are isolated so their load or failure cannot corrupt the acknowledged operation.

Correctness boundary

Upload media directly to object storage using signed URLs, then commit the post metadata.

Scale boundary

Represent follow edges separately and generate timelines through hybrid fan-out.

Operational boundary

Use immutable post IDs and tombstones; propagate deletes and moderation decisions quickly.

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5. Critical flow and algorithm

Upload media directly, checksum it, scan it, and generate renditions.

Validate ownership of ready media and commit immutable post identity plus version.

Publish post-created to timeline, search, notification, and moderation consumers.

Serve text/metadata from post storage and immutable renditions through CDN.

Edits create a new version; deletes create a tombstone that invalidates projections.

Retry-safe mutation skeleton

identity = authenticate(request)
prior = idempotency.get(identity, request.key)
if prior: return prior.result

current = authoritative_store.read(partition_key)
next_state, events = domain.transition(current, request)
authoritative_store.compare_and_set(current.version, next_state)
idempotency.commit(request.key, next_state.result)
outbox.append(events)
return next_state.result

Exactly-once business effect: the network can duplicate requests. Stable identities, conditional state transitions, immutable event IDs, and reconciliation make the user-visible effect happen once.

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6. Caching, hot keys, and backpressure

Cache policy

Cache immutable or slowly changing reads with versioned keys, TTL jitter, short negative entries, and request coalescing.

Hot partition or tenant

Measure per-key and per-tenant skew. Replicate safe reads, isolate the offender, salt only when ordering permits, and enforce budgets.

Backpressure

Bound queues, buffers, connection pools, and concurrency. Shed low-priority work and scale from lag/oldest-item age rather than CPU alone.

Stampede and retry storms

Use single-flight/coalescing, jittered expiry, bounded retries, exponential backoff, and circuit breakers so one dependency failure does not multiply traffic.

Cache outage rule: never allow every miss to fall through simultaneously. Use local protection, request coalescing, circuit breakers, bounded pools, and controlled fallback.

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7. Partitioning and consistency

partition = stable_hash(primary_or_aggregate_key) → shard owner
write = conditional_update(expected_version)
read_projection = versioned and rebuildable

Strong boundary

each accepted social post service operation has one stable result and retries do not repeat its business effect. Use a uniqueness constraint, serializable/conditional write, expected version, or fencing token at this boundary.

Eventual boundary

Analytics, notifications, search, read models, and operational dashboards may lag when freshness is observable and replay exists.

Operation	Consistency	Reason
Authoritative mutation	Conditional / serializable	Protect uniqueness, ownership, money, inventory, order, or legal state transition.
Read after own write	Primary, session token, or cache warm	The caller must observe the result it just committed.
Ordinary read projection	Bounded stale where safe	Replicas and caches improve latency when version/freshness is visible.
Analytics and operations	Eventual + replay	Freshness is less important than isolating the online path.

Route every mutation for one aggregate to one owner/partition epoch.
Use cursors rather than offsets for changing ordered collections.
Retain tombstones until replication and repair windows close.
Move shard ownership through versioned maps and dual-read verification.

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8. Failure drills and recovery

Failure	Runtime behavior	Recovery proof
Media upload completes but post never commits	Stop unsafe progress, serve only explicitly safe degraded behavior, and recover from durable state.	Reconciliation returns zero invariant gaps; lag/replica health and user-visible SLO return to baseline.
Privacy changes while timelines contain the post	Stop unsafe progress, serve only explicitly safe degraded behavior, and recover from durable state.	Reconciliation returns zero invariant gaps; lag/replica health and user-visible SLO return to baseline.
Moderation removes a viral cached asset	Stop unsafe progress, serve only explicitly safe degraded behavior, and recover from durable state.	Reconciliation returns zero invariant gaps; lag/replica health and user-visible SLO return to baseline.
Fanout consumer replays post-created	Stop unsafe progress, serve only explicitly safe degraded behavior, and recover from durable state.	Reconciliation returns zero invariant gaps; lag/replica health and user-visible SLO return to baseline.
Edit and delete race on the same version	Resolve at source of truth with uniqueness, expected version, lease generation, or monotonic cursor.	Reconciliation returns zero invariant gaps; lag/replica health and user-visible SLO return to baseline.

Recovery checkpoint

# Recovery checkpoint
scenario: Media upload completes but post never commits
operation_id: op_01J...
aggregate_key: social-post-service_123
last_durable_version: 18
projection_checkpoint: partition-07@offset-482901
retry_policy: same_identity / bounded_backoff
reconciliation_status: pending
owner: CDN

Every dependency needs a timeout, retry budget, circuit policy, and explicit fail-open, fail-closed, stale, or read-only mode. “Highly available” is incomplete until degraded behavior is named.

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9. Security, regions, and observability

Security and abuse

Authenticate at ingress and authorize the exact resource/tenant on every operation.
Encrypt transport and durable data; tokenize or isolate sensitive material.
Rate-limit by tenant, actor, resource, and network; detect enumeration and anomalous fan-out.
Redact secrets and personal data before logs or analytical storage.
Audit privileged mutations, support access, policy changes, and forced recovery.

Signals that prove correctness

User-visible success rate and p50/p95/p99 latency.
Timeouts, circuit state, saturation, and replica health by dependency.
Queue lag, oldest-item age, retries, and dead-letter growth.
Conflict, duplicate-suppression, and reconciliation mismatch counts.
Business drill: Media upload completes but post never commits — alert before the user-visible invariant is violated.

Regional evolution

Begin multi-zone with tested failover, backups, and reconciliation.
Add global stateless ingress and regional read projections.
Keep one write owner per aggregate or partition where coordination is expensive.
Move ownership with epochs/fencing; avoid silent simultaneous writers.
Use expand/contract schemas, canaries, shadow reads, and reversible cutovers.

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10. Interviewer follow-ups

How are quote-posts modeled?

Upload media directly to object storage using signed URLs, then commit the post metadata. Walk the concrete lifecycle step “Upload media directly, checksum it, scan it, and generate renditions.” Then test it against “Media upload completes but post never commits”. State the retry identity, durable checkpoint, recovery signal, and cost of the alternative.

How do you handle post edits?

Represent follow edges separately and generate timelines through hybrid fan-out. Walk the concrete lifecycle step “Validate ownership of ready media and commit immutable post identity plus version.” Then test it against “Privacy changes while timelines contain the post”. State the retry identity, durable checkpoint, recovery signal, and cost of the alternative.

How do privacy changes propagate?

Use immutable post IDs and tombstones; propagate deletes and moderation decisions quickly. Walk the concrete lifecycle step “Publish post-created to timeline, search, notification, and moderation consumers.” Then test it against “Moderation removes a viral cached asset”. State the retry identity, durable checkpoint, recovery signal, and cost of the alternative.

How does media transcoding work?

Upload media directly to object storage using signed URLs, then commit the post metadata. Walk the concrete lifecycle step “Serve text/metadata from post storage and immutable renditions through CDN.” Then test it against “Fanout consumer replays post-created”. State the retry identity, durable checkpoint, recovery signal, and cost of the alternative.

How do you fight spam?

Represent follow edges separately and generate timelines through hybrid fan-out. Walk the concrete lifecycle step “Edits create a new version; deletes create a tombstone that invalidates projections.” Then test it against “Edit and delete race on the same version”. State the retry identity, durable checkpoint, recovery signal, and cost of the alternative.

Answer pattern: state the invariant, choose a mechanism, walk one failure, name the recovery signal, and explain the cost of the alternative.

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11. How to present this in 45 minutes

Time	What to cover
0–5 min	Clarify scope, workload, SLOs, and the correctness invariant.
5–10 min	Estimate throughput/storage and define concrete APIs and records.
10–20 min	Draw online, durable-write, and asynchronous/repair paths.
20–30 min	Explain the exact lifecycle, partition key, and concurrency control.
30–38 min	Discuss caching, hot keys, replication, and backpressure.
38–43 min	Run failure drills, security, observability, and regional evolution.
43–45 min	Summarize the five most important choices and trade-offs.

Final decision summary

Invariant	each accepted social post service operation has one stable result and retries do not repeat its business effect.
Partitioning	Stable aggregate/key hash with versioned ownership and measured hot-key isolation.
Commit	Expected version or uniqueness at the authoritative state boundary.
Async work	Durable outbox/log, idempotent consumers, checkpoints, and replay.
Recovery	Same operation identity, durable truth lookup, and business reconciliation.

Closing summary

CLOSING “The design protects each accepted social post service operation has one stable result and retries do not repeat its business effect. The online path stays bounded, authoritative state is versioned and partitioned, and asynchronous work is replayable and observable.”

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Creator App

Media Upload Edge

Post API

Auth / Privacy

Complete System Design Tutorial

1. Requirements and scope

Functional requirements

Non-functional requirements

2. Capacity assumptions

3. API and data model

Authoritative records and indexes

Command and event envelopes

4. Why this architecture and what it costs

Chosen boundary vs simpler monolith

Correctness vs availability

Operational cost and evolution

5. Critical flow and algorithm

Retry-safe mutation skeleton

6. Caching, hot keys, and backpressure

7. Partitioning and consistency

Strong boundary

Eventual boundary

8. Failure drills and recovery

Recovery checkpoint

9. Security, regions, and observability

Security and abuse

Signals that prove correctness

Regional evolution

10. Interviewer follow-ups

11. How to present this in 45 minutes

Final decision summary

Closing summary