Digital Wallet — 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 digital wallet 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 100K transfers/s · zero lost money · multi-currency.
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: no scarce resource or monetary effect is created twice, lost after acknowledgment, or committed without an audit trail.

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2. Capacity assumptions

State assumptions before choosing technology. The supplied workload is 100K transfers/s · zero lost money · multi-currency. Convert it into peak throughput, retained records, bandwidth, and failure budget; then connect each number to a component.

Dimension	Working estimate	Design implication
Traffic	100K transfers/s · zero lost money · multi-currency	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/wallets/{id}/transfers

POST /v1/wallets/{id}/topups

POST /v1/wallets/{id}/withdrawals

GET /v1/wallets/{id}/statement?cursor=

Authoritative records and indexes

account(wallet_id, currency, state, limits_version)

ledger_entry(txn_id, account_id, debit, credit)

transfer(transfer_id PK, idempotency_key, state, version)

rail_attempt(transfer_id, provider_ref, attempt, outcome)

Command and event envelopes

# Command envelope
operation: POST /v1/wallets/{id}/transfers
request_id: req_01J...
idempotency_key: idem_7f2...
aggregate_key: digital-wallet_123
expected_version: 17
payload: {...}

# Durable event
event_id: evt_01J...
aggregate_key: digital-wallet_123
event_type: digital_wallet_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
Wallet Client	Owns client protocol, request identity, retry, and resume behavior. In this design it participates in: Authenticate owner, enforce KYC/limits, and resolve idempotency result.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents the external caller from also owning the contract and failure behavior of Wallet API.	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 “Client retries completed transfer” to the external caller.	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 Wallet Client boundary before extracting or scaling it.
Wallet API	Owns authentication, validation, quotas, deadlines, and routing. In this design it participates in: Reserve available balance and append balanced ledger entries in one transaction.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Wallet Client from also owning the contract and failure behavior of Identity / KYC.	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 “Funding rail returns unknown outcome” to Wallet Client.	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 Wallet API boundary before extracting or scaling it.
Identity / KYC	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: For internal transfer commit both sides; external rails create an uncertain attempt first.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Wallet API from also owning the contract and failure behavior of Limits Engine.	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 “Ledger entries do not balance” to Wallet 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 Identity / KYC boundary before extracting or scaling it.
Limits Engine	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: Project balances/statements and publish notification/risk events from outbox.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Identity / KYC from also owning the contract and failure behavior of Idempotency 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 “AML hold races withdrawal” to Identity / KYC.	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 Limits Engine boundary before extracting or scaling it.
Idempotency Store	Owns authoritative versioned state or immutable evidence. In this design it participates in: Reconcile provider settlement files and repair projection mismatches from ledger truth.	Defines the source of truth and commit boundary. It prevents Limits Engine from also owning the contract and failure behavior of Transfer Orchestrator.	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 “Balance projection lags recent credit” to Limits Engine.	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 Idempotency Store boundary before extracting or scaling it.
Transfer Orchestrator	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: Authenticate owner, enforce KYC/limits, and resolve idempotency result.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Idempotency Store from also owning the contract and failure behavior of Double-Entry Ledger.	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 “Client retries completed transfer” to Idempotency 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 Transfer Orchestrator boundary before extracting or scaling it.
Double-Entry Ledger	Owns authentication, validation, quotas, deadlines, and routing. In this design it participates in: Reserve available balance and append balanced ledger entries in one transaction.	Defines the source of truth and commit boundary. It prevents Transfer Orchestrator from also owning the contract and failure behavior of Balance Projection.	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 “Funding rail returns unknown outcome” to Transfer Orchestrator.	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 Double-Entry Ledger boundary before extracting or scaling it.
Balance Projection	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: For internal transfer commit both sides; external rails create an uncertain attempt first.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Double-Entry Ledger from also owning the contract and failure behavior of Funding Rail Adapter.	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 “Ledger entries do not balance” to Double-Entry Ledger.	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 Balance Projection boundary before extracting or scaling it.
Funding Rail Adapter	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: Project balances/statements and publish notification/risk events from outbox.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Balance Projection from also owning the contract and failure behavior of Withdrawal Adapter.	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 “AML hold races withdrawal” to Balance Projection.	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 Funding Rail Adapter boundary before extracting or scaling it.
Withdrawal Adapter	Owns authoritative versioned state or immutable evidence. In this design it participates in: Reconcile provider settlement files and repair projection mismatches from ledger truth.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Funding Rail Adapter from also owning the contract and failure behavior of Outbox Stream.	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 “Balance projection lags recent credit” to Funding Rail Adapter.	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 Withdrawal Adapter boundary before extracting or scaling it.
Outbox Stream	Preserves durable ordering, replay, and consumer checkpoints. In this design it participates in: Authenticate owner, enforce KYC/limits, and resolve idempotency result.	Decouples bursts and failure from user acknowledgment. It prevents Withdrawal Adapter from also owning the contract and failure behavior of Risk / AML.	Call every downstream consumer synchronously. That couples user latency and availability to analytics, notification, indexing, and audit dependencies. Here that simpler choice would couple “Client retries completed transfer” to Withdrawal Adapter.	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 Outbox Stream boundary before extracting or scaling it.
Risk / AML	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: Reserve available balance and append balanced ledger entries in one transaction.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Outbox Stream from also owning the contract and failure behavior of Reconciliation.	Embed policy in every caller. Rules then drift by deployment version and emergency changes require fleet-wide redeployment. Here that simpler choice would couple “Funding rail returns unknown outcome” to Outbox 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 Risk / AML boundary before extracting or scaling it.
Reconciliation	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: For internal transfer commit both sides; external rails create an uncertain attempt first.	Proves correctness and supports controlled recovery. It prevents Risk / AML from also owning the contract and failure behavior of Notifications.	Infer correctness from request logs. Logs do not prove durable business state, detect silent drift, or safely repair ambiguous outcomes. Here that simpler choice would couple “Ledger entries do not balance” to Risk / AML.	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 Reconciliation boundary before extracting or scaling it.
Notifications	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: Project balances/statements and publish notification/risk events from outbox.	Separates a distinct scale, ownership, policy, or failure boundary. It prevents Reconciliation from also owning the contract and failure behavior of Audit / Statements.	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 “AML hold races withdrawal” to Reconciliation.	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 Notifications boundary before extracting or scaling it.
Audit / Statements	Owns one bounded domain decision, transition, or operational capability. In this design it participates in: Reconcile provider settlement files and repair projection mismatches from ledger truth.	Proves correctness and supports controlled recovery. It prevents Notifications from also owning the contract and failure behavior of operators and repair tooling.	Infer correctness from request logs. Logs do not prove durable business state, detect silent drift, or safely repair ambiguous outcomes. Here that simpler choice would couple “Balance projection lags recent credit” to Notifications.	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 Audit / Statements boundary before extracting or scaling it.

Decision 01

Chosen boundary vs simpler monolith

The immutable double-entry ledger is truth; balances are projections with version checkpoints and can be rebuilt.

A single deployable with one transactional store is the right starting point for a small digital wallet workload. The proposed boundary becomes justified when the immutable double-entry ledger is truth; balances are projections with version checkpoints and can be rebuilt. Separating it contains the named failure “Client retries completed transfer” without placing unrelated work on the critical path.

Decision 02

Correctness vs availability

Transfer idempotency belongs at the command boundary, not the HTTP retry layer alone, because clients and workers can both repeat work.

The design coordinates only where no scarce resource or monetary effect is created twice, lost after acknowledgment, or committed without an audit trail. 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

External top-up and withdrawal rails are sagas with uncertain outcomes; reserve funds and reconcile provider references before final state.

This choice adds dashboards, ownership, schema compatibility, capacity planning, and recovery runbooks. Start with the minimum topology that supports “For internal transfer commit both sides; external rails create an uncertain attempt first.” 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 no scarce resource or monetary effect is created twice, lost after acknowledgment, or committed without an audit trail. Durable projections, notifications, indexing, analytics, and repair are isolated so their load or failure cannot corrupt the acknowledged operation.

Correctness boundary

The immutable double-entry ledger is truth; balances are projections with version checkpoints and can be rebuilt.

Scale boundary

Transfer idempotency belongs at the command boundary, not the HTTP retry layer alone, because clients and workers can both repeat work.

Operational boundary

External top-up and withdrawal rails are sagas with uncertain outcomes; reserve funds and reconcile provider references before final state.

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5. Critical flow and algorithm

Authenticate owner, enforce KYC/limits, and resolve idempotency result.

Reserve available balance and append balanced ledger entries in one transaction.

For internal transfer commit both sides; external rails create an uncertain attempt first.

Project balances/statements and publish notification/risk events from outbox.

Reconcile provider settlement files and repair projection mismatches from ledger truth.

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

Add a cache only for a measured repeated read; do not place mutable correctness state in a disposable layer.

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

no scarce resource or monetary effect is created twice, lost after acknowledgment, or committed without an audit trail. 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
Client retries completed transfer	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.
Funding rail returns unknown outcome	Query/retry with the same operation identity; do not issue a second business effect.	Reconciliation returns zero invariant gaps; lag/replica health and user-visible SLO return to baseline.
Ledger entries do not balance	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.
AML hold races withdrawal	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.
Balance projection lags recent credit	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: Client retries completed transfer
operation_id: op_01J...
aggregate_key: digital-wallet_123
last_durable_version: 18
projection_checkpoint: partition-07@offset-482901
retry_policy: same_identity / bounded_backoff
reconciliation_status: pending
owner: Audit / Statements

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: Client retries completed transfer — 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 pending balances work?

The immutable double-entry ledger is truth; balances are projections with version checkpoints and can be rebuilt. Walk the concrete lifecycle step “Authenticate owner, enforce KYC/limits, and resolve idempotency result.” Then test it against “Client retries completed transfer”. State the retry identity, durable checkpoint, recovery signal, and cost of the alternative.

How are transfers made idempotent?

Transfer idempotency belongs at the command boundary, not the HTTP retry layer alone, because clients and workers can both repeat work. Walk the concrete lifecycle step “Reserve available balance and append balanced ledger entries in one transaction.” Then test it against “Funding rail returns unknown outcome”. State the retry identity, durable checkpoint, recovery signal, and cost of the alternative.

How do chargebacks affect balances?

External top-up and withdrawal rails are sagas with uncertain outcomes; reserve funds and reconcile provider references before final state. Walk the concrete lifecycle step “For internal transfer commit both sides; external rails create an uncertain attempt first.” Then test it against “Ledger entries do not balance”. State the retry identity, durable checkpoint, recovery signal, and cost of the alternative.

How do you support multiple currencies?

The immutable double-entry ledger is truth; balances are projections with version checkpoints and can be rebuilt. Walk the concrete lifecycle step “Project balances/statements and publish notification/risk events from outbox.” Then test it against “AML hold races withdrawal”. State the retry identity, durable checkpoint, recovery signal, and cost of the alternative.

How do you reconcile a bank rail?

Transfer idempotency belongs at the command boundary, not the HTTP retry layer alone, because clients and workers can both repeat work. Walk the concrete lifecycle step “Reconcile provider settlement files and repair projection mismatches from ledger truth.” Then test it against “Balance projection lags recent credit”. 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	no scarce resource or monetary effect is created twice, lost after acknowledgment, or committed without an audit trail.
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 no scarce resource or monetary effect is created twice, lost after acknowledgment, or committed without an audit trail. The online path stays bounded, authoritative state is versioned and partitioned, and asynchronous work is replayable and observable.”

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Wallet Client

Wallet API

Identity / KYC

Limits Engine

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