ACID

Atomicity, Consistency, Isolation, Durability. Transaction guarantees suited to strongly consistent metadata and relational workflows.

API

Application Programming Interface. The contract between clients and a service, including operations, payloads, errors, and auth.

APNS / FCM

Apple and Firebase push gateways. Your system sends device tokens and payloads to these providers.

Availability

The fraction of time a service can answer requests. Replication, failover, and isolation raise it.

BLOB / Object storage

Storage for large opaque byte objects. Keep videos and file blocks here; keep searchable metadata in a database.

Bloom filter

Space-efficient probabilistic membership test. A negative result is definitive; a positive result may be a false positive.

Cache

Fast temporary copies of hot data. Design around expiry, invalidation, eviction, stampedes, and cold starts.

CAP

During a network partition, a distributed system chooses consistency or availability for affected operations.

CDN

Content Delivery Network. Edge servers deliver cacheable assets near users and reduce origin load.

Codec / Container

A codec compresses audio or video. A container packages streams and metadata, such as MP4.

Consistency

Defines which write a read may observe. Strong reads see the latest committed value; eventual replicas converge over time.

Consistent hashing

Ring-based partitioning that moves a small key range when nodes change. Virtual nodes improve balance.

DAG

Directed Acyclic Graph. Models dependent jobs and exposes independent branches for parallel execution.

DAU / MAU

Daily or Monthly Active Users. Useful starting numbers for deriving actions per day and QPS.

Deduplication

Discard repeated work or data through an event ID, idempotency key, URL set, content hash, or block hash.

DNS / GeoDNS

DNS maps names to IPs. GeoDNS returns region-aware endpoints for latency and failover.

Fanout

Copy one event to many recipients. Push performs work on write; pull aggregates sources on read.

FIFO

First In, First Out ordering. Useful for simple queues and per-host crawler scheduling.

GOP

Group of Pictures. An independently playable video chunk boundary used for segmentation and parallel transcoding.

Heartbeat

Periodic liveness signal. Missing heartbeats trigger suspicion, failover, or an offline presence state.

HLS / MPEG-DASH

HTTP adaptive-streaming protocols. A manifest lists media segments and bitrate renditions.

Horizontal scaling

Add machines and partition work. Stateless services, sharding, and shared state make this practical.

Idempotency

Repeated processing produces one logical result. Store a request or event key with its outcome.

Long polling

The server holds an HTTP request until an event or timeout. Good for infrequent server-to-client updates.

Message queue

Durable asynchronous buffer between producers and consumers. It absorbs bursts and lets each side scale independently.

NoSQL

Key-value, document, column, and graph databases with access-pattern-focused models and horizontal scale.

Partition / Shard

A subset of data assigned by key or range. The partition key controls balance, locality, and query routing.

Pub/Sub

Publishers emit events to topics; subscribers receive matching events. Useful for notifications and presence.

QPS

Queries or requests per second. State average and peak separately, plus read/write mix.

Quorum

A minimum replica count for success. With N replicas, W + R > N gives a read/write overlap.

Replication

Copies data across nodes or regions for availability and durability. Lag defines staleness exposure.

Retry / Backoff / Jitter

Retry transient failures, increase delay after each attempt, and randomize timing to prevent synchronized storms.

SLA / SLO

An SLA is a customer commitment. An SLO is an internal reliability target measured by indicators.

SPOF

Single Point of Failure. One component whose loss stops the service; replication or graceful degradation removes it.

SSTable

Sorted String Table. Immutable sorted key-value file used by log-structured stores.

TTL

Time To Live. Expiry attached to cache entries, rate-limit state, DNS records, or temporary objects.

Vector clock

Per-participant counters that reveal causal ancestry and concurrent sibling versions.

WebSocket

Persistent bidirectional client-server connection. Useful for chat, presence, and frequent live events.