Load Balancer — Detailed#

flowchart TB
  subgraph Client[Clients]
    C1([Browser])
    C2([Mobile App])
    C3[Service-to-service]
  end

  subgraph DNS_Tier[DNS / GSLB]
    DNS[Authoritative DNS]
    GSLB[Geo / Latency<br/>Routing - Route53 / Akamai]
  end

  subgraph Edge[Edge L4/L7 Tier]
    Anycast[Anycast IP / BGP]
    L4[L4 LB<br/>ECMP, Maglev,<br/>LVS / IPVS]
    L7A[L7 LB A<br/>Envoy / NGINX / HAProxy]
    L7B[L7 LB B<br/>Active]
    VRRP[(VRRP / Keepalived<br/>HA pair)]
  end

  subgraph Algos[Selection Algorithms]
    RR[Round Robin]
    WRR[Weighted RR]
    LC[Least Connections]
    EWMA[EWMA Latency]
    PEAK[Power-of-2-Choices]
    HASH[Consistent Hash<br/>session/sticky]
  end

  subgraph Health[Health & Service Discovery]
    HC[Active Health Checks<br/>HTTP/TCP/gRPC]
    PC[Passive checks<br/>outlier detection]
    SD[Service Registry<br/>Consul / etcd / xDS]
  end

  subgraph Pool[Backend Pool]
    direction LR
    B1[Backend 1]
    B2[Backend 2]
    B3[Backend 3]
    BN[Backend N]
  end

  subgraph Observability
    M[(Metrics<br/>p50/p95/p99, RPS)]
    L[(Access Logs)]
    T[(Traces)]
  end

  C1 --> DNS
  C2 --> DNS
  C3 --> DNS
  DNS --> GSLB
  GSLB --> Anycast
  Anycast --> L4
  L4 --> L7A
  L4 --> L7B
  L7A <-.VRRP failover.-> L7B
  L7A --> Algos
  Algos --> Pool
  SD -.config push.-> L7A
  SD -.config push.-> L7B
  HC --> B1
  HC --> B2
  HC --> B3
  HC --> BN
  B1 -.status.-> SD
  B2 -.status.-> SD
  B3 -.status.-> SD
  BN -.status.-> SD
  L7A --> M
  L7A --> L
  L7A --> T
  PC -. eject on 5xx .-> L7A

    classDef client fill:#dbeafe,stroke:#1e40af,stroke-width:1px,color:#0f172a;
    classDef edge fill:#cffafe,stroke:#0e7490,stroke-width:1px,color:#0f172a;
    classDef service fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
    classDef datastore fill:#fee2e2,stroke:#991b1b,stroke-width:1px,color:#0f172a;
    classDef cache fill:#fed7aa,stroke:#9a3412,stroke-width:1px,color:#0f172a;
    classDef queue fill:#ede9fe,stroke:#5b21b6,stroke-width:1px,color:#0f172a;
    classDef compute fill:#d1fae5,stroke:#065f46,stroke-width:1px,color:#0f172a;
    classDef storage fill:#e5e7eb,stroke:#374151,stroke-width:1px,color:#0f172a;
    classDef external fill:#fce7f3,stroke:#9d174d,stroke-width:1px,color:#0f172a;
    classDef obs fill:#f3e8ff,stroke:#6b21a8,stroke-width:1px,color:#0f172a;
    class C1,C2 client;
    class DNS,Anycast,L4,L7A,L7B edge;
    class C3,GSLB,RR,WRR,LC,EWMA,PEAK,HASH,HC,PC,SD,B1,B2,B3,BN service;
    class VRRP,M,L,T datastore;

Notes#

L4 vs L7: L4 forwards TCP/UDP (fast, opaque); L7 understands HTTP/gRPC, can do path routing, retries, header manipulation, TLS termination, mTLS.
TLS: terminate at L7; re-encrypt to backend if zero-trust required.
HA: keepalived/VRRP for active-passive, or anycast + ECMP for active-active (Google Maglev, Cloudflare Unimog).
Sticky sessions: cookie-based (SERVERID) or source-IP hash; prefer stateless tokens.
Outlier detection: eject hosts on consecutive 5xx; gradually re-admit.
Rate limiting & circuit breaking are commonly co-located at L7.

Glossary & fundamentals#

Concepts referenced in this design. Each row links to its canonical page; the tag column shows whether it is a high-level (HLD) or low-level (LLD) concept.

Tag	Concept	What it is	Page
`HLD`	Load balancer / GSLB	L4/L7 traffic distribution and failover	load-balancer
`HLD`	CDN	edge caching for static assets	cdn
`HLD`	Consistent hashing	key placement with minimal remap	consistent-hashing
`HLD`	Idempotency & retries	safe re-execution, backoff + jitter	idempotency-retries
`HLD`	Observability	metrics, logs, traces, SLOs	observability
`HLD`	Service mesh	sidecar mesh, mTLS, traffic policy	service-mesh
`HLD`	Multi-region & DR	RTO / RPO, active-active, failover	multi-region-dr