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Software

Programming, algorithms and systems on openstem — flashcards, notes, quizzes, sketches and flowcharts shared by the community. Study free, or clone anything into your library.

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Flashcards8 cards

What are the 'three pillars' of observability, and what does each answer?1 / 8

Metrics (numeric time series — request rate, error rate, latency) answer 'is something wrong, and roughly how much?'. Logs (discrete timestamped events with detail) answer 'what exactly happened at this point?'. Traces (the causal chain of spans across services for one request) answer 'where in the call chain did the problem happen?'. Real debugging usually moves between all three: a metric alerts, a trace localizes the slow hop, a log explains why.

Software

Observability: Metrics, Logs, and Traces

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Flashcards7 cards

What is CSP, and what are its two core composition operators?1 / 7

Communicating Sequential Processes (Hoare, 1978) is a process algebra where processes interact only through synchronous, rendezvous-style events on named channels — there is no shared memory. The two core operators are prefixing, a → P (perform event a, then behave as P), and choice, P □ Q (external choice between P and Q, resolved by the environment's first event). Parallel composition P ‖ Q synchronises processes on shared alphabet events.

Software

Software · L5 · Process Calculi & Concurrency Theory

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Quiz5 questions

What does `http.get(Uri.parse(url))` return?

AA Future<Response> that completes with a status code and bodyBThe decoded JSON directlyCA Stream of response chunksDNothing — it must be awaited inside a StreamBuilder

Software

Software · L2 · Flutter: Networking & Data

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Flowchart15 objects

Software

Software · L1 · System Design: Choosing a Database Type · flowchart

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Quiz5 questions

Which AWS storage service is object storage accessed over HTTP?

AAmazon S3BAmazon EBSCAmazon EFSDAmazon RDS

Software

Software · L1 · AWS: Storage Options Compared

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Flowchart13 objects

Software

Software · L2 · Next.js: Middleware & Edge Runtime · flowchart

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Flowchart15 objects

Software

Software · L4 · Thread vs event-loop concurrency

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Flashcards8 cards

What is dynamic programming?1 / 8

An optimization technique that solves problems by breaking them into overlapping subproblems and storing results to avoid redundant computation. Requires optimal substructure (optimal solution contains optimal subproblem solutions) and overlapping subproblems.

Software

Dynamic Programming

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Flashcards10 cards

State each ACID property precisely, distinguishing Consistency from Isolation.1 / 10

Atomicity: a transaction's effects are all-or-nothing; a partial failure rolls everything back. Consistency: each committed transaction moves the database from one state satisfying all declared constraints (keys, checks, triggers) to another — it is a property the application+constraints uphold, not the engine alone. Isolation: concurrent transactions produce a result equivalent to some serial schedule (to the degree the chosen level guarantees). Durability: once commit returns, the changes survive a crash, usually via a write-ahead log (WAL) flushed/fsynced before acknowledgment.

Software

Transactions, Isolation & Concurrency

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Flashcards10 cards

Contrast the REQUIRED, REQUIRES_NEW, and NESTED transaction propagation modes.1 / 10

REQUIRED (default) joins the caller's transaction or starts one if none exists. REQUIRES_NEW always suspends any existing transaction and runs in an independent new one that commits/rolls back on its own — useful for audit logs that must persist even if the outer call rolls back. NESTED runs inside the current transaction at a JDBC savepoint, so it can roll back to that savepoint without aborting the outer transaction (requires savepoint support, e.g. the DataSourceTransactionManager on JDBC).

Software

Transactions, Persistence & Reactive

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Flashcards10 cards

What does an integration test verify that a unit test does not?1 / 10

It verifies that multiple components or modules work correctly together, including real interactions across boundaries like a database or service.

Software

Integration & E2E

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Flowchart22 objects

Software

Software · L4 · Consumer Group Partition Assignment and Rebalance

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Flashcards5 cards

Define the complexity classes P and NP.1 / 5

P is the class of decision problems solvable by a deterministic Turing machine in polynomial time. NP is the class of decision problems whose YES-instances have certificates verifiable in polynomial time by a deterministic Turing machine. P ⊆ NP; whether P = NP is the central open question in theoretical computer science.

Software

Software · L5 · Complexity Theory

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Flowchart26 objects

Software

Software · L3 · Message Delivery and Failure Handling

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Quiz5 questions

If both aria-labelledby and aria-label are present, which wins for the accessible name?

Aaria-labelledby takes priorityBaria-label always winsCWhichever comes first in the HTMLDNeither — visible text always wins

Software

Software · L2 · HTML & CSS: Accessibility & Responsive CSS Check

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Flowchart15 objects

Software

Software · L2 · Concurrency: Choosing a Synchronization Primitive · flowchart

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Flashcards10 cards

What is XSS (Cross-Site Scripting) and how do you prevent it?1 / 10

XSS injects malicious scripts into pages viewed by others, running in their browser to steal cookies/tokens or perform actions. Prevent by escaping/encoding output, using a Content-Security-Policy, and never inserting untrusted data into the DOM as HTML (use textContent, not innerHTML).

Software

Web Security

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Note~110 words · 1 min

What Is a Replica Set? A replica set is a group of mongod processes holding the same data set. One member is the primary and accepts all writes; the rest are secondaries that replicate the primary's operations from its oplog (operation log)

Software

Software · L2 · MongoDB: Replica Sets & Failover

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Flashcards10 cards

How do VPC global scope and regional subnets shape a multi-region design?1 / 10

A VPC network is global, so subnets (each tied to one region) share a single private routing domain and can talk over internal IPs without peering or VPNs. This lets you place workloads in multiple regions inside one VPC, but subnet CIDR ranges are regional resources and must be planned to avoid overlap, especially before peering with other VPCs.

Software

Networking, Load Balancing & Reliability

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Collection2 items

01Advanced Structures

02Algorithm Design & Analysis

Software

Software · L4 · Data Structures & Algos

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Flowchart12 objects

Software

Software · L2 · PostgreSQL: Replication & High Availability · flowchart

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Flowchart5 objects

Software

Software · L2 · Dart: Error Handling & Testing · flowchart

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Quiz5 questions

Which service compiles code, runs tests, and builds container images on GCP?

ACloud BuildBArtifact RegistryCCloud DeployDCloud Monitoring

Software

Software · L2 · Google Cloud: CI/CD & Deployment

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Note~988 words · 5 min

Top-Down vs. Bottom-Up Parsing LL(1) parsers work top-down: starting from the grammar's start symbol, they predict which production to expand using the current nonterminal and one token of lookahead, requiring FIRST sets of alternative prod

Software

Software · L5 · Compiler Construction Theory

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Flashcards10 cards

What does MVCC stand for and provide?1 / 10

Multi-Version Concurrency Control: readers see a consistent snapshot without blocking writers, and writers don't block readers.

Software

Transactions & MVCC

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Flashcards10 cards

Why prefer short-lived access tokens with refresh tokens?1 / 10

Short access-token lifetimes limit the damage of a leaked token, while a longer-lived refresh token (stored more securely and revocable server-side) lets clients obtain new access tokens without re-login.

Software

API & Access Control

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Quiz5 questions

Which state is a process in while it waits for a disk read to complete?

ABlockedBRunningCReadyDTerminated

Software

Software · L1 · Operating Systems: The Process Lifecycle

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Flashcards10 cards

What is a recursive type and when is it needed?1 / 10

A type that references itself in its definition, used to model recursive data structures like JSON, trees, or linked lists. e.g. `type JSONValue = string | number | boolean | null | JSONValue[] | { [k: string]: JSONValue }`

Software

Type-Level Programming

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Flashcards8 cards

What is the difference between leader-based (single-master) and leaderless replication?1 / 8

Leader-based replication routes all writes through one designated node, which propagates changes to followers — simple to reason about, but the leader is a bottleneck and failover requires electing a new one. Leaderless replication (e.g. Dynamo-style) lets any replica accept a write and relies on quorum reads/writes plus conflict resolution to stay consistent, trading simplicity for higher write availability.

Software

Distributed Systems Engineering

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Flashcards10 cards

What is the difference between a single-subscription and a broadcast Stream?1 / 10

A single-subscription stream allows only one listener over its lifetime and buffers events until then; a broadcast stream allows many simultaneous listeners and does not buffer for late subscribers.

Software

Streams & Async Generators

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Explore STEM Content

Software

Explore STEM Content

Software

Observability: Metrics, Logs, and Traces

Software · L5 · Process Calculi & Concurrency Theory

Software · L2 · Flutter: Networking & Data

Software · L1 · System Design: Choosing a Database Type · flowchart

Software · L1 · AWS: Storage Options Compared

Software · L2 · Next.js: Middleware & Edge Runtime · flowchart

Software · L4 · Thread vs event-loop concurrency

Dynamic Programming

Transactions, Isolation & Concurrency

Transactions, Persistence & Reactive

Integration & E2E

Software · L4 · Consumer Group Partition Assignment and Rebalance

Software · L5 · Complexity Theory

Software · L3 · Message Delivery and Failure Handling

Software · L2 · HTML & CSS: Accessibility & Responsive CSS Check

Software · L2 · Concurrency: Choosing a Synchronization Primitive · flowchart

Web Security

Software · L2 · MongoDB: Replica Sets & Failover

Networking, Load Balancing & Reliability

Software · L4 · Data Structures & Algos

Software · L2 · PostgreSQL: Replication & High Availability · flowchart

Software · L2 · Dart: Error Handling & Testing · flowchart

Software · L2 · Google Cloud: CI/CD & Deployment

Software · L5 · Compiler Construction Theory

Transactions & MVCC

API & Access Control

Software · L1 · Operating Systems: The Process Lifecycle

Type-Level Programming

Distributed Systems Engineering

Streams & Async Generators