Live Streaming Platform — 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 live streaming platform 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 10M concurrent viewers · 2–10 s latency · 100K live channels.
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 live streaming platform 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 10M concurrent viewers · 2–10 s latency · 100K live channels. Convert it into peak throughput, retained records, bandwidth, and failure budget; then connect each number to a component.

Dimension	Working estimate	Design implication
Traffic	10M concurrent viewers · 2–10 s latency · 100K live channels	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/channels/{id}/sessionsingest lease

PUBLISH RTMP/SRTmedia packets

GET /v1/live/{id}/manifestauthorized playlist

POST /v1/qoeplayback telemetry

Authoritative records and indexes

channel(channel_id PK, owner, state, ingest_epoch)

segment(channel_id, sequence, rendition, uri, checksum)

manifest(channel_id, version, media_sequence, renditions)

viewer_session(session_id, channel_id, token, region)

Command and event envelopes

# Command envelope
operation: POST /v1/channels/{id}/sessions
request_id: req_01J...
idempotency_key: idem_7f2...
aggregate_key: live-streaming_123
expected_version: 17
payload: {...}

# Durable event
event_id: evt_01J...
aggregate_key: live-streaming_123
event_type: live_streaming_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
Broadcaster	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: Authenticate broadcaster and lease one channel ingest epoch.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents the external caller from also owning the contract and failure behavior of Ingest 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 “Broadcaster reconnects with stale epoch” 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 Broadcaster boundary before extracting or scaling it.
Ingest Edge	Owns authentication, validation, quotas, deadlines, and routing. In this design it participates in: Buffer jitter, timestamp packets, and feed deterministic rendition pipelines.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Broadcaster from also owning the contract and failure behavior of Stream Auth.	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 “One transcoder falls behind wall clock” to Broadcaster.	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 Ingest Edge boundary before extracting or scaling it.
Stream Auth	Preserves durable ordering, replay, and consumer checkpoints. In this design it participates in: Package aligned short segments and publish only checksummed immutable objects.	Decouples bursts and failure from user acknowledgment. It prevents Ingest Edge from also owning the contract and failure behavior of Channel Coordinator.	Call every downstream consumer synchronously. That couples user latency and availability to analytics, notification, indexing, and audit dependencies. Here that simpler choice would couple “Manifest references unavailable segment” to Ingest Edge.	Costs eventual consistency, schema evolution, replay tooling, idempotent consumers, and lag operations. Earn it when bursts or downstream failure must not block acknowledgment. Measure the latency, saturation, lag, or mismatch at the Stream Auth boundary before extracting or scaling it.
Channel Coordinator	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: Advance manifest media sequence atomically and distribute through origin shields/CDNs.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Stream Auth from also owning the contract and failure behavior of Encoder Input Buffer.	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 “Celebrity stream overloads origin” to Stream Auth.	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 Channel Coordinator boundary before extracting or scaling it.
Encoder Input Buffer	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: Players adapt bitrate; QoE and DVR consumers process segments asynchronously.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Channel Coordinator from also owning the contract and failure behavior of Live Transcoders.	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 “CDN serves stream after authorization revocation” to Channel Coordinator.	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 Encoder Input Buffer boundary before extracting or scaling it.
Live Transcoders	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: Authenticate broadcaster and lease one channel ingest epoch.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Encoder Input Buffer from also owning the contract and failure behavior of Segment Packager.	Perform the work before acknowledging the caller. This gives simpler consistency but unbounded tail latency and fragile retries. Here that simpler choice would couple “Broadcaster reconnects with stale epoch” to Encoder Input Buffer.	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 Live Transcoders boundary before extracting or scaling it.
Segment Packager	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: Buffer jitter, timestamp packets, and feed deterministic rendition pipelines.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Live Transcoders from also owning the contract and failure behavior of Live Segment Store.	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 “One transcoder falls behind wall clock” to Live Transcoders.	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 Segment Packager boundary before extracting or scaling it.
Live Segment Store	Owns authoritative versioned state or immutable evidence. In this design it participates in: Package aligned short segments and publish only checksummed immutable objects.	Defines the source of truth and commit boundary. It prevents Segment Packager from also owning the contract and failure behavior of Manifest Service.	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 “Manifest references unavailable segment” to Segment Packager.	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 Live Segment Store boundary before extracting or scaling it.
Manifest Service	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: Advance manifest media sequence atomically and distribute through origin shields/CDNs.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Live Segment Store from also owning the contract and failure behavior of Origin Shield.	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 “Celebrity stream overloads origin” to Live Segment Store.	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 Manifest Service boundary before extracting or scaling it.
Origin Shield	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: Players adapt bitrate; QoE and DVR consumers process segments asynchronously.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Manifest Service from also owning the contract and failure behavior of Multi-CDN Router.	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 “CDN serves stream after authorization revocation” to Manifest Service.	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 Origin Shield boundary before extracting or scaling it.
Multi-CDN Router	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: Authenticate broadcaster and lease one channel ingest epoch.	Removes repeated work from the latency-critical path. It prevents Origin Shield from also owning the contract and failure behavior of Viewer Player.	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 “Broadcaster reconnects with stale epoch” to Origin Shield.	Costs invalidation, staleness, stampede protection, and memory. Add or split it only after the workload (10M concurrent viewers · 2–10 s latency · 100K live channels) proves repeated reads dominate. Measure the latency, saturation, lag, or mismatch at the Multi-CDN Router boundary before extracting or scaling it.
Viewer Player	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: Buffer jitter, timestamp packets, and feed deterministic rendition pipelines.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Multi-CDN Router from also owning the contract and failure behavior of Chat / Reactions.	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 “One transcoder falls behind wall clock” to Multi-CDN Router.	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 Viewer Player boundary before extracting or scaling it.
Chat / Reactions	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: Package aligned short segments and publish only checksummed immutable objects.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Viewer Player from also owning the contract and failure behavior of QoE Stream.	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 “Manifest references unavailable segment” to Viewer Player.	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 Chat / Reactions boundary before extracting or scaling it.
QoE Stream	Preserves durable ordering, replay, and consumer checkpoints. In this design it participates in: Advance manifest media sequence atomically and distribute through origin shields/CDNs.	Decouples bursts and failure from user acknowledgment. It prevents Chat / Reactions from also owning the contract and failure behavior of DVR Archive.	Call every downstream consumer synchronously. That couples user latency and availability to analytics, notification, indexing, and audit dependencies. Here that simpler choice would couple “Celebrity stream overloads origin” to Chat / Reactions.	Costs eventual consistency, schema evolution, replay tooling, idempotent consumers, and lag operations. Earn it when bursts or downstream failure must not block acknowledgment. Measure the latency, saturation, lag, or mismatch at the QoE Stream boundary before extracting or scaling it.
DVR Archive	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: Players adapt bitrate; QoE and DVR consumers process segments asynchronously.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents QoE Stream from also owning the contract and failure behavior of operators and repair tooling.	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 “CDN serves stream after authorization revocation” to QoE Stream.	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 DVR Archive boundary before extracting or scaling it.

Decision 01

Chosen boundary vs simpler monolith

Keep the ingest session stateful per channel but publish immutable short segments so downstream delivery remains cacheable and replayable.

A single deployable with one transactional store is the right starting point for a small live streaming platform workload. The proposed boundary becomes justified when keep the ingest session stateful per channel but publish immutable short segments so downstream delivery remains cacheable and replayable. Separating it contains the named failure “Broadcaster reconnects with stale epoch” without placing unrelated work on the critical path.

Decision 02

Correctness vs availability

A smaller bitrate ladder reduces cost and latency; a larger ladder improves viewer quality under heterogeneous networks.

The design coordinates only where each accepted live streaming platform 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 a control-plane channel owner and data-plane CDN distribution; failover must preserve segment sequence and manifest continuity.

This choice adds dashboards, ownership, schema compatibility, capacity planning, and recovery runbooks. Start with the minimum topology that supports “Package aligned short segments and publish only checksummed immutable objects.” 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 live streaming platform 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

Keep the ingest session stateful per channel but publish immutable short segments so downstream delivery remains cacheable and replayable.

Scale boundary

A smaller bitrate ladder reduces cost and latency; a larger ladder improves viewer quality under heterogeneous networks.

Operational boundary

Use a control-plane channel owner and data-plane CDN distribution; failover must preserve segment sequence and manifest continuity.

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5. Critical flow and algorithm

Authenticate broadcaster and lease one channel ingest epoch.

Buffer jitter, timestamp packets, and feed deterministic rendition pipelines.

Package aligned short segments and publish only checksummed immutable objects.

Advance manifest media sequence atomically and distribute through origin shields/CDNs.

Players adapt bitrate; QoE and DVR consumers process segments asynchronously.

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 live streaming platform 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
Broadcaster reconnects with stale epoch	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.
One transcoder falls behind wall clock	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.
Manifest references unavailable segment	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.
Celebrity stream overloads origin	Isolate the key/tenant/priority, apply admission control, and preserve durable work.	Reconciliation returns zero invariant gaps; lag/replica health and user-visible SLO return to baseline.
CDN serves stream after authorization revocation	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.

Recovery checkpoint

# Recovery checkpoint
scenario: Broadcaster reconnects with stale epoch
operation_id: op_01J...
aggregate_key: live-streaming_123
last_durable_version: 18
projection_checkpoint: partition-07@offset-482901
retry_policy: same_identity / bounded_backoff
reconciliation_status: pending
owner: DVR Archive

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: Broadcaster reconnects with stale epoch — 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 do you reduce glass-to-glass latency?

Keep the ingest session stateful per channel but publish immutable short segments so downstream delivery remains cacheable and replayable. Walk the concrete lifecycle step “Authenticate broadcaster and lease one channel ingest epoch.” Then test it against “Broadcaster reconnects with stale epoch”. State the retry identity, durable checkpoint, recovery signal, and cost of the alternative.

What happens when the encoder disconnects?

A smaller bitrate ladder reduces cost and latency; a larger ladder improves viewer quality under heterogeneous networks. Walk the concrete lifecycle step “Buffer jitter, timestamp packets, and feed deterministic rendition pipelines.” Then test it against “One transcoder falls behind wall clock”. State the retry identity, durable checkpoint, recovery signal, and cost of the alternative.

How is DVR implemented?

Use a control-plane channel owner and data-plane CDN distribution; failover must preserve segment sequence and manifest continuity. Walk the concrete lifecycle step “Package aligned short segments and publish only checksummed immutable objects.” Then test it against “Manifest references unavailable segment”. State the retry identity, durable checkpoint, recovery signal, and cost of the alternative.

How do you prevent stream piracy?

Keep the ingest session stateful per channel but publish immutable short segments so downstream delivery remains cacheable and replayable. Walk the concrete lifecycle step “Advance manifest media sequence atomically and distribute through origin shields/CDNs.” Then test it against “Celebrity stream overloads origin”. State the retry identity, durable checkpoint, recovery signal, and cost of the alternative.

How do you handle a celebrity event?

A smaller bitrate ladder reduces cost and latency; a larger ladder improves viewer quality under heterogeneous networks. Walk the concrete lifecycle step “Players adapt bitrate; QoE and DVR consumers process segments asynchronously.” Then test it against “CDN serves stream after authorization revocation”. 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 live streaming platform 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 live streaming platform 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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Broadcaster

Ingest Edge

Stream Auth

Channel Coordinator

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