Webhooks vs. Server-Sent Events vs. Polling: A Framework for API Teams

Three ways to move data from your API to your customer's system: they poll you, you push to their endpoint, or you hold open a stream. Every API team makes this choice at least once. Most default to webhooks because Stripe does and Stripe is the reference implementation for developer platforms. That default is usually correct, but "usually" is not "always," and getting it wrong costs a year of engineering.

This is the framework for making the choice deliberately.

What are the three real options?

Not five, not ten. Three, because everything else is a variant.

• Polling. Consumer sends a GET to your API on an interval. You return current state or a delta since last poll. Consumer decides how fresh they want to be by tuning the interval.
• Webhooks. You send a POST to a URL the consumer registered with you when state changes. Consumer receives asynchronously, acks with a 2xx, and processes.
• Server-Sent Events (SSE) or WebSockets. Consumer opens a long-lived connection to you. You stream events over that connection as they happen. Consumer processes inline.

Long-poll and HTTP/2 push exist but are essentially polling with a different transport. GraphQL subscriptions run on top of WebSockets. Message brokers like Kafka are a different category, used between services not across customer boundaries.

How do the three compare on the criteria that matter?

Score each option across the dimensions that actually drive the decision. Do not weight equally: pick the two or three that matter most for your product and check where each option stands.

The scoring is not universal. A payments company weighs latency and ordering more heavily than a batch analytics API. Weight before you sum.

When should you actually choose polling?

Polling is the right answer more often than API design blogs admit. Pick it when at least three of these are true.

• Event rate is low. Fewer than one event per customer per hour on average.
• Freshness tolerance is high. Consumers are okay with minute-scale latency, not second-scale.
• Consumer environment is constrained. Serverless functions, batch jobs, or firewalled corporate networks where receiving inbound requests is hard.
• Data has natural checkpoints. Your API can return "everything since cursor X" cleanly, so the consumer maintains state without missing events.

The classic fit is B2B enterprise APIs where every consumer is a large customer running a batch job on a schedule. The cost curve is favorable, the operational load is minimal, and you avoid webhook infrastructure entirely.

When should you choose webhooks?

Choose webhooks when you have all of these:

• Consumer base is 100 or more. Below that, polling is cheaper to operate and simpler for everyone.
• Consumers can host a public endpoint. Startups and API-first companies can. Enterprise IT often cannot, or not without a proxy.
• Freshness matters. Under one second latency is expected.
• Event rate is high enough that polling would be wasteful. If a customer would need to poll every 10 seconds to be current, they will hate polling and you will pay for the noise.

The webhook cost model works because you pay per event delivered, not per customer polling. At scale that reverses the polling cost curve. A single customer generating 100,000 events per day is cheap to notify with webhooks and expensive to service with polling.

When is SSE the right call?

SSE is underused because it feels exotic, but for the right shape of product it is dramatically better than webhooks.

• Consumer is already connected to you. SaaS dashboards, developer tools, and interactive applications typically hold a session.
• Firewall constraints prevent inbound webhooks. SSE goes out from the consumer, so no inbound URL is needed.
• High-volume, unidirectional streaming. Log streams, price feeds, notification streams to a browser.
• Sub-second latency requirement. Webhooks have retry and delivery variance that SSE avoids.

SSE is not right when the consumer is a serverless function, a scheduled batch job, or anything short-lived. It also is not right when you need to broadcast to a very large fanout, because holding tens of thousands of connections open per server is expensive.

What are the hybrid patterns worth using?

The most reliable APIs run more than one delivery mechanism in parallel.

• Webhooks plus daily reconciliation endpoint. Webhooks handle real-time, polling handles reconciliation. If a webhook is lost or arrives out of order, the daily poll catches it. Every mature payments API does this.
• SSE for interactive dashboards, webhooks for backend integrations. The same event flows to two channels, chosen by consumer type.
• Polling for enterprise customers, webhooks for self-serve. Split by tier because enterprise IT often cannot accept webhooks without a security review that takes a quarter.

Hybrid does not mean doubling the engineering. It means designing the event model once, then exposing it through multiple channels. The delivery infrastructure is the shared substrate.

How do you migrate from polling to webhooks without breaking customers?

Ship webhooks alongside polling for at least six months before deprecating.

1. Emit webhooks for every event that also updates a pollable resource. Do not turn off polling.
2. Publish a migration guide with the exact schema mapping. Cursor semantics are usually the tricky part.
3. Instrument which customers are on which mechanism.
4. Contact the biggest polling customers directly. The long tail migrates on their own timeline.
5. Deprecate polling only when usage falls below a threshold, and give six months notice regardless.

Never force migration on a fixed date. You will lose customers who cannot make the timeline for reasons unrelated to your API.

What actually matters

The mistake to avoid is choosing based on which option is trendier or on what the incumbent in your space does. Polling is fine for a lot of API products and cheaper than webhooks below a certain scale. Webhooks are the right default above that scale, and SSE beats both when your consumers hold a session with you. The decision is not about elegance; it is about matching the delivery mechanism to how your customers actually run their systems. Get the framework right on paper, score your options against real weights, and the answer usually falls out clean. If it does not, you are missing information about your consumers, not about your infrastructure.