Synopsis

This chapter explains how changes to routing, configuration, and content are tested and deployed in a multi-CDN environment. It covers pre-production validation, cohort design, canary strategies, progressive exposure, measurement, guard rails, rollback, and record keeping. The goal is to make changes reversible, observable, and limited in blast radius.

Purpose and scope

Multi-CDN adds variables at several layers. DNS answers, proxy behavior, client logic, cache identity, and origin controls can all change outcomes. Testing must isolate causes and confirm improvements using indicators that reflect user experience. Rollouts must keep a safe default path and apply controls that prevent fast spread of harm.

Change types

Typical changes include routing policy updates, provider configuration updates, TLS and certificate changes, cache key or TTL changes, security policy updates, content publishing and purge behavior changes, and client code that influences endpoint selection. Each change type has a preferred test method and a standard rollback.

Pre-production validation

Static checks catch many faults before any traffic moves. Configuration should pass schema validation, referential integrity checks, and unit tests for policy logic. Simulation against recorded requests verifies that decisions are stable and predictable. For cache identity changes, key calculators should be run against real paths to detect unintended multiplicity. Small synthetic probes can confirm reachability and correctness from representative regions.

Cohort design

A cohort is a defined subset of traffic used for experiments. Selection must be stable so the same session does not switch routes during comparison. Common keys include cookie values, user or device identifiers where policy allows, IP and user agent hashes, or resolver hints. The cohort size starts small and grows only after results meet objectives. Cohorts should reflect diversity in region, ASN, device class, and protocol so that results generalize.

Canary strategies

Canaries compare a candidate path to a control. Several strategies are used. Header based splits route a fraction of requests to the candidate using an injected header that edges and proxies honor. Cookie flags pin sessions to a candidate for stickiness. DNS split horizon serves distinct answers to a named subset of recursive resolvers in a region. Client feature flags select endpoints inside applications. Selection should minimize interference with caching and avoid protocol or path differences that confound attribution.

flowchart TD All[All traffic] --> Sel[Stable cohort selection] Sel --> Ctrl[Control route] Sel --> Cand[Candidate route] Ctrl --> Compare[Compare outcomes] Cand --> Compare Compare --> Decide{Meets objective} Decide -- yes --> Increase[Increase cohort size] Decide -- no --> Revert[Rollback to control]

Signals and objectives

Decisions require clear objectives. User facing indicators include success ratio, time to first byte, page or API latency, and for media, startup delay and stall time. Provider level indicators include cache hit rate, upstream error rate, and purge success. Objectives specify acceptable deltas between candidate and control over defined windows. Error budgets cap allowable failure during exposure.

Guard rails and safety

Guard rails stop exposure growth or trigger rollback. Examples include hard limits on error ratio deltas, latency deltas, unexpected cache hit rate drops, or origin load increases. A minimum dwell time prevents rapid toggling. Maximum exposure caps limit the fraction of traffic that can run on an unproven path. Manual overrides exist for incidents and are recorded with context.

Progressive exposure

Exposure follows a sequence that begins at a very small fraction and increases in steps. Each step requires measurement that meets objectives with confidence. Exposure can be regional at first to reduce blast radius. Expansion proceeds by geography and ASN based on observed variance. When a change affects cache identity or origin load, waiting periods should allow caches to warm and origin metrics to settle before evaluation.

flowchart LR Start[Start at 1 percent] --> Step2[Hold and measure] Step2 --> Gate{Objectives met} Gate -- yes --> Ten[Increase to 10 percent] Gate -- no --> Rollback[Rollback] Ten --> Hold[Hold and measure] Hold --> Gate2{Objectives met} Gate2 -- yes --> Fifty[Increase to 50 percent] Gate2 -- no --> Rollback Fifty --> Final{Objectives met} Final -- yes --> Full[Full rollout] Final -- no --> Rollback

Interaction with caching

Experiments can distort cache metrics. Candidate and control should share cache identity where possible to avoid bias from cold caches. When identity must change, evaluation windows should separate cold and warm phases. Soft purge with revalidation reduces origin spikes during exposure. Shield hit rate and edge hit rate should be observed separately.

Routing layer specifics

DNS based canaries rely on resolver selection and TTL behavior. Short TTLs increase responsiveness but add network load and variance. Proxy based canaries can enforce per request splits and use richer signals but add another hop. Client based canaries reflect last mile variance but require versioning control and simple logic. Hybrid deployments should define precedence so layers do not fight each other.

Rollback

Rollback must be immediate, well tested, and limited in scope. Control plane changes revert to the last known good version. DNS reverts to a prior answer set. Proxy rules disable candidate branches. Client flags return to control. Cache and origin effects are monitored after rollback to confirm recovery. Documentation records the trigger, scope, and timings.

Records and reproducibility

Every change produces a record that includes configuration diffs, cohort definition, exposure steps, measurement outcomes, guard rail states, and the final decision. Records enable audits and future tuning of thresholds. Dashboards should render annotations for each step so correlation with RUM, synthetic, and origin metrics is clear.

Tooling

Useful components include a cohort selector that generates stable keys, a traffic router that honors headers or flags, an exposure manager that schedules steps and enforces guard rails, a result evaluator that computes deltas with confidence bounds, and a recorder that writes structured logs and annotations. Tools should be vendor neutral and translate intent to each provider API.

Operations

Rollouts should run during staffed hours, with pre-defined abort criteria and contacts at providers. Quiet period drills should test selection logic, guard rails, and rollback without affecting user traffic. Post-change reviews evaluate objective fit and adjust thresholds for future work. Large or risky changes should run in shadow mode first by duplicating requests for analysis without affecting responses when privacy policy allows.

sequenceDiagram participant C as Control plane participant R as Router or DNS participant E as Exposure manager participant M as Metrics C->>R: Apply candidate configuration E->>R: Set 1 percent split with stable key M-->>E: Report deltas vs control E->>R: Increase or rollback C->>R: Promote to default or revert E->>M: Write annotations and results

For routing logic and policy see /multicdn/traffic-steering/. For measurement and windows see /multicdn/signals-telemetry/. For cache behavior during exposure see /multicdn/cache-consistency/.

Further reading

RFC 9110 defines HTTP semantics that influence cache and validation during tests. Material on service level objectives and error budgets provides background for guard rails and exposure gates. Provider documentation should be reviewed for rate limits and stickiness behavior that affect cohort stability.