videoCDNmobile

Designing Cache Policies for Short-Form Episodic Video (Vertical + Mobile)

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2026-02-03

10 min read

Optimize cache TTLs, segment sizes, CDN tiering, and client-side recipes to cut startup latency for mobile-first vertical episodic video.

Hook: Your mobile viewers quit in the first 3 seconds — caching is usually why

For mobile-first, vertical episodic platforms (think Holywater-style microdramas), the biggest friction is startup latency and unpredictable stalls — not raw bitrate. You can engineer dramatic improvements by designing cache policies that treat manifests, init segments, and media segments differently, tier CDN behavior, and combine client-side prefetching with edge caching and origin coordination. This article gives you battle-tested TTLs, segment-size guidance, CDN-tiering patterns, and worked recipes (service worker, headers, Redis, Varnish) to reduce startup times and cost while keeping correctness intact.

Executive summary — what to apply first

Segment size: target 1–4s for short, vertical episodes. Use 1–2s for aggressive startup and LL-HLS/CMAF; 3–4s for better throughput on poor networks.
TTL by asset: manifest: 1–10s (stale-while-revalidate); variant playlist: 3–15s; media segments: long TTL + versioned URLs (days→years) or immutable header.
CDN tiering: short TTLs at the edge (fast revalidation), longer TTLs at regional caches and origin-shield for cost control.
Cache-Control strategy: use s-maxage, stale-while-revalidate, and immutable for segments; use surrogate-keys for fast purges.
Client-side: service worker prefetch for the next 1–3 segments; keep cache-limits tuned for mobile memory.
Infrastructure: coordinate invalidation with Redis pub/sub and use Varnish or your CDN’s edge logic to normalize requests and apply TTLs.

Why mobile-first vertical episodic video changes caching in 2026

Short-form, vertical episodic content flips traditional streaming assumptions. Episodes are short (30s–3min), viewer sessions are mobile-first, and retention depends on immediate responsiveness. In late 2025 and into 2026 the industry accelerated adoption of LL-HLS/CMAF, HTTP/3/QUIC for lower handshake time, and edge compute. Platforms like Holywater (which raised new funding in Jan 2026 to scale mobile-first vertical streaming) emphasize rapid episode-to-episode transitions and data-driven recommendations — both place a premium on predictable, low-latency cache behavior.

What changes relative to traditional OTT?

More frequent manifest updates (for new episodes, ad insertion, personalization).
Higher request rates per viewer because of smaller segments.
Greater sensitivity to TLS and DNS latency on mobile networks; HTTP/3 adoption reduces this.
Desire for aggressive edge caching but precise invalidation when creative or ad slots change.

Design principles

Asset-differentiated caching: treat manifests, init segments, media segments, subtitles, and thumbnails differently.
Immutability-first: version URLs for media segments so the CDN can cache them long-term.
Short manifest TTL + stale revalidation: manifests should be fresh to reflect new episodes and ABR ladders.
Edge-friendly prefetching: push small next-segment prefetches from client to edge when possible.
Cost-aware tiering: keep high churn assets near the edge and shield origin traffic with a regional cache.

Segment sizes and trade-offs

Segment size is the single most impactful decision for startup latency and request load.

1s segments (LL-HLS/CMAF): fastest startup and lowest tail latency. Ideal for microdramas where dropping into a scene matters. But it increases HTTP requests and CPU for packaging and CDN request overhead.
2–3s segments: Sweet spot for many mobile networks — balances startup with fewer requests.
4–6s segments: Better throughput and lower request rates; acceptable for slightly longer episodes where initial start is less critical.

Practical rule: for episodes ≤ 3min, aim for 1–3s segments; for >3min, 3–4s. Monitor connection RTT and use adaptive encoding switch to slightly larger segments on high-latency networks.

TTL strategy — detailed

Design TTLs by asset type. Use versioned URLs for immutable assets and short TTLs with revalidation for frequently updated resources.

Manifest / master playlist (.m3u8)

Set Cache-Control: public, max-age=5, s-maxage=10, stale-while-revalidate=30. This keeps startup fresh but lets the edge serve slightly stale content while revalidating in the background.
If personalization is per-user, deliver a signed master manifest via edge compute or API gateway with no-store or very short TTL and use signed URLs to enable caching inside edge workers.

Variant / variant playlists

Cache-Control: public, max-age=10-30, s-maxage=30, stale-while-revalidate=60.
Update frequency usually low; still keep short TTLs to reflect bitrate ladder changes or mid-episode ads.

Init segments and media segments

Use versioned filenames (content-addressable or version token). Then set Cache-Control: public, max-age=31536000, immutable.
If using non-versioned URLs, use s-maxage: 3600 with surrogate-key for invalidation — but versioned URLs are strongly preferred.

Subtitles / thumbnails / static assets

Long TTLs and immutable headers. Use CDN edge rules to serve them from the regional cache.

Header recipes — practical examples

Use these as starting points; adapt to your CDN and origin capabilities.

Master manifest headers (m3u8)

Cache-Control: public, max-age=5, s-maxage=10, stale-while-revalidate=30
Surrogate-Key: episode:1234 master
Content-Type: application/vnd.apple.mpegurl; charset=utf-8

Variant playlist

Cache-Control: public, max-age=15, s-maxage=30, stale-while-revalidate=60
Surrogate-Key: episode:1234 variant:720p
Content-Type: application/vnd.apple.mpegurl; charset=utf-8

Media segment (versioned URL)

Cache-Control: public, max-age=31536000, immutable
Content-Type: video/iso.segment

Key idea: version your media segments and make them immutable. The CDN becomes a read-only archive — cheap and fast.

CDN tiering and origin shielding

Design a three-tier caching topology: Edge -> Regional/POP -> Origin-shield. Use short edge TTLs and longer regional TTLs.

Edge: Serve manifests with short TTL + stale revalidate. Serve versioned segments from edge until expiry (long TTL).
Regional (origin shield): Acts as long-living cache for segments to reduce origin egress and absorb traffic spikes.
Origin: Keep origin stateless and durable; prefer object storage (S3/Blob) for segment storage and a small API layer for manifests and personalization.

Enable request coalescing at the CDN (many CDNs support this) to avoid origin stampedes when multiple clients request the same new manifest simultaneously.

Service worker recipe — prefetching and offline play

Service workers can improve startup by prefetching the next 1–3 segments and serving from cache quickly, but be mindful of mobile memory and CPU.

self.addEventListener('install', (e) => self.skipWaiting());
self.addEventListener('activate', (e) => self.clients.claim());

self.addEventListener('fetch', (event) => {
  const url = new URL(event.request.url);
  // Quick path: let the browser handle range requests and video streaming
  if (url.pathname.endsWith('.m3u8')) {
    event.respondWith(fetch(event.request));
    return;
  }

  // For small segments, try cache-first then network
  if (url.pathname.match(/\.ts$|\.cmf$/)) {
    event.respondWith(caches.open('segments-v1').then(async (cache) => {
      const cached = await cache.match(event.request);
      if (cached) return cached;
      const res = await fetch(event.request);
      // Keep a shallow cache to avoid filling device storage
      cache.put(event.request, res.clone());
      // Optionally prune cache here
      return res;
    }));
  }
});

// Prefetch next segments (call from player) - be conservative on mobile
async function prefetchNext(urls){
  const cache = await caches.open('segments-v1');
  for(const u of urls.slice(0,3)){
    try{ await cache.add(u); }catch(e){ /* ignore */ }
  }
}

Notes: Work with the player to prefetch only next segments. On mobile, prefer 1–2 segments. Always provide a way to clear the cache and limit total cached items.

Redis: manifest coordination and purge signaling

Use Redis as a fast control plane: store manifest versions, publish purge events, and coordinate edge invalidation.

# Key patterns
episode:1234:manifest = v20260116-abc123
episode:1234:segments = [list of URLs]

# When you publish a new manifest
SET episode:1234:manifest v20260116-abc124
PUBLISH purge:episode:1234 v20260116-abc124

# Edge/pop subscribes to purge channel and calls CDN purge API or invalidation via surrogate-key

Use Redis TTLs to auto-expire temporary manifests or staging variants. For atomic updates, write new version first, then switch the manifest pointer. For massive purges use surrogate-key tags rather than object-by-object purges.

Varnish VCL recipe

If you use Varnish or an edge compute with VCL-like capability, apply conditional caching for HLS/DASH resources.

sub vcl_recv {
  if (req.url ~ "\.m3u8$") {
    set req.http.X-Cache-Group = "manifest";
  } elsif (req.url ~ "\.(ts|cmf|m4s)$") {
    set req.http.X-Cache-Group = "segment";
  }
}

sub vcl_backend_response {
  if (beresp.http.Content-Type ~ "application/vnd.apple.mpegurl") {
    set beresp.ttl = 5s;
    set beresp.http.Cache-Control = "public, max-age=5, s-maxage=10, stale-while-revalidate=30";
  } elsif (bere resp.http.Content-Type ~ "video/") {
    # long-term cache assuming versioned URL
    set beresp.ttl = 365d;
    set beresp.http.Cache-Control = "public, max-age=31536000, immutable";
  }
}

Operational monitoring and KPIs

Track these metrics continuously:

Startup latency (time-to-first-frame) — primary UX KPI.
Segment request rate per session — helps size edge limits.
Cache hit ratio per asset class (manifests vs segments).
Origin egress (bytes and requests) and cost per 1M plays.
Error rate and 4xx/5xx on manifests and segments.

Implement synthetic tests that emulate mobile networks (3G/4G/5G emulation) and LL-HLS streams to measure startup across CDN tiers. See practical mobile test patterns in mobile creator kits and low-latency streaming playbooks.

Cost-control patterns

Use long TTLs for versioned segments to leverage CDN caching economy.
Use origin-shield/regional cache to reduce origin egress on spikes.
Implement request coalescing and head-of-line limiting on edge to avoid origin storms for new manifests; tie this into your edge registry and filing rules.
Use conditional requests (If-None-Match / ETag) and stale-while-revalidate to balance freshness and cost.

Case study: Applying the pattern to a Holywater-style service

Scenario: vertical episodic app with 60–180s episodes, heavy mobile traffic, rapid content drops (new episodes daily) and personalized promos. Implementation roadmap:

Switch to versioned media segment filenames during packaging (CMAF chunking).
Set manifest TTLs low: master playlist max-age=5s + stale-while-revalidate=30s.
Make segments immutable with max-age=31536000 and immutable header. Store segments in object storage and let edge cache long-term.
Implement a Redis control plane: mapping episode → manifest version + pub/sub to trigger surrogate-key invalidation for non-versioned collateral (promos, ad manifests).
Implement a lightweight service worker that prefetches the next 1–2 segments after the first successful segment fetch, and prunes cache on background sync.
Configure CDN tiering: edge short TTLs, regional shield long TTL. Enable HTTP/3 and request coalescing for faster start on mobile.
Run synthetic tests pre/post-deploy comparing time-to-first-frame and tail latency. Tune segment size between 1–3s based on observed median startup and CDN request rates.

Future trends to prepare for (2026+)

HTTP/3 as default: lower handshake/tail latency; favor QUIC-optimized edge deployments.
Edge compute manifests: personalized manifests generated at the edge reduce origin round trips but require precise TTL and invalidation strategy.
AI-driven predictive caching: using recommendation signals to pre-warm edges for likely next-episodes or scene clips.
Codec shifts: more AV1/next-gen codecs reduce bitrates but increase CPU; caching immutable encoded chunks remains best practice.

Actionable takeaways (copy-paste checklist)

Version your segments and set them to Cache-Control: public, max-age=31536000, immutable.
Set master manifest to max-age=5s and use stale-while-revalidate.
Target 1–3s segments for short vertical episodes and measure startup vs request overhead.
Use Redis pub/sub + surrogate-keys to coordinate invalidation and avoid mass purges.
Implement a conservative service worker prefetch of next 1–3 segments on mobile devices.
Enable CDN tiering: short edge TTLs, long regional TTLs, origin shielding to reduce egress costs.

Conclusion & call to action

Mobile-first, vertical episodic video demands a cache policy that is granular, version-aware, and integrated across client and CDN layers. By differentiating TTLs, choosing the right segment size (1–3s for most short episodes), and coordinating invalidation with Redis and surrogate-keys, you can cut startup latency, reduce origin cost, and make your episodes feel instant. Start with manifest TTL tuning and segment versioning in staging — run A/B tests on startup latency and CDN egress before rolling to production.

Try these recipes: implement the header rules and Varnish/Redis flows in a staging environment, then run a synthetic mobile workload. If you want a checklist or example VCL/Redis scripts exported for your team, reach out or clone the starter repo in your infra. Measure startup before/after and aim for single-digit-second startup on 4G devices as your first milestone.

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