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IntermediateDistributed Systems

By Twilio engineers · Twilio · Added 2026-10-12

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About 37 min left. Your work is saved as you go.

What you are about to study

Twilio wrote this to explain how they thought about a problem. You should know the basics: caching, queues and replication.

Ideas you will meet

Distributed Systems
Consensus, time, failure and the fundamentals of scale-out.

What we already know about this

Our own lessons on these ideas. Read them here before you start, so it makes sense.

  • Core Concepts: How Things Actually Work · 7 minHTTP/1.1, HTTP/2 and HTTP/3What changed between HTTP versions and why it makes pages faster.Read here

    HTTP/1.1

    • One request at a time per connection (responses come back in order).
    • To fetch many resources, browsers open about six connections per host.
    • Each request repeats large text headers.
    • A slow response blocks those behind it: head-of-line blocking.
    • Keep-alive lets a connection be reused, saving handshakes.

    HTTP/2

    • Multiplexing: many requests and responses share one connection at the same time, interleaved as separate streams.
    • Header compression removes the repeated cost.
    • Binary framing is more efficient to parse than text.
    • Server push existed but is rarely used and is being phased out.
    • Still runs over TCP, so if one packet is lost, TCP holds back all streams until it is resent (head-of-line blocking at the transport layer).

    Before: HTTP/1.1: many connections

    After: HTTP/2: one shared connection

    What changed

    • One connection carries many requests at once
    • Headers are compressed
    • Old tricks like domain sharding stop helping

    HTTP/3

    • Runs over QUIC, built on UDP, with encryption (TLS 1.3) included.
    • Independent streams: a lost packet only delays its own stream.
    • Faster connection setup: fewer round trips, and resuming a previous connection can send data immediately.
    • Connection migration: a phone moving from Wi-Fi to mobile data can keep the connection, because it is identified by an ID, not an IP address.

    Figure SDA-CORE-04-1 · HTTP/3 · © forcsguy.com

    Figure SDA-CORE-04-2 · What one lost packet does in each version · © forcsguy.com

    What this means for you

    • Use HTTPS everywhere. Browsers only use HTTP/2 and 3 over TLS.
    • Serve through a CDN or server that supports HTTP/2 and 3; clients negotiate automatically and fall back if needed.
    • Do not over-optimise for HTTP/1.1 any more. Keep requests cacheable and compressed.

    More on this site

    Related reading

    Show 6 more

    Open this lesson with its quiz and exercise

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Go deeper on this

A path through videos, articles, other case studies and code on the same ideas as Twilio Blog.

  1. Warm upGet the vocabulary first.

  2. WatchHow others solve the same problem, on video.

    Videos belong to their creators and play from YouTube. More on YouTube (opens in a new tab)

  3. ReadEngineers explain it in their own words.

  4. See it in another companyThe same ideas, a different system.

  5. Read the codeSee how it looks when it is real.

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