A /health endpoint or similar mock might be available and responding fast, while a real request might fail or time out.
For example, if you have a CRUD app fronting a database, your CRUD app’s /health endpoint should attempt to make a simple database query. If you have an ETL daemon that pulls from a third-party API and pushes to a message queue, it should probe both the readiness of the API and the message-queue before reporting its own readiness to work. (Of course, it is exactly in the case where the service has its own circuit-breaking logic with back-up paths “around” these dependencies, that it gets to say it’s healthy when its dependencies aren’t.)
A test of the /health endpoint is, after all, what determines if a service is considered viable for routing by load-balancers; and, vice-versa, if the service is considered to be “stuck” and in need of a restart by platform auto-scaling logic. As such, your /health endpoints really should be configured in a way where they generate false positives—reporting being unhealthy when they’re really healthy—rather than false negatives.
If you’ve got a pool of instances, better to have them paranoid of their own readiness in a way where your system will be constantly draining traffic from + restarting them, than to have them lackadaisical about their readiness in a way where they’re receiving traffic they can’t handle.
Our environment now supports distinct answers to those two questions but most service authors don't know about it.
For the latter, someone related a real-world example of this to me the other day. Say you have a bunch of people managing customers. Every employee has 4 customers, and those take up all of their time.
You get a new customer. Instead of hiring a new rep, you give someone a 5th customer to manage. They struggle, and eventually they quit. Now, all of your employees have 5 customers. Sooner or later one of those will also quit, and then it's a race to see who can get out the door fastest.
The moral of that story is that all the load balancing in the world is for naught if you haven't done your capacity planning properly. And once the system starts to buckle it may be too late to bring new capacity online (since startup usually consumes more resources).
You cannot make such a "true" health endpoint, it's super easy to make a service that contains a paradox about what such an endpoint should do.
1 service with 2 endpoints A and B. A relies on an external service and the database, A and B both rely on the database. What to do if the external service is down bringing A down? Either your health endpoint is useless because A is down and it reports the service is fine or you just cascaded the downtime to B while there is no reason to do so. Same situation for 1 endpoint but any branch dependent on the request etc. etc.
Of course you can use a health endpoint for determining restarts, load balancers etc, but its not replacement for circuit breakers on your calls.
Make one /health/a endpoint and one /health/b endpoint. Client-service A uses /health/a to check if the service is "healthy in terms of A's ability to use it." Client-service B likewise pings /health/b.
In a scenario with many different dependent services (a crawler that can hit arbitrary third-party sites, say, or something like Hadoop that can load arbitrary not-necessarily-existent extensions per job) these endpoints should be something clients can create/register themselves, ala SQL stored procedures; or the service can offer a connection-oriented health-state-change streaming endpoint, where the client can hold open a connection and be told when about readiness-state-change events.
But to be clear, these are edge-case considerations: in most cases, a service has only critical-path dependencies (which it needs to bootstrap itself, or to "do its job" in the most general SLA sense); and optional dependencies (which it doesn't actually need, and can offer degraded functionality when the service isn't available via circuit-breaking.)
It's a rare—and IMHO not-well-factored—service that has dependencies that are on the critical path for some use-cases but not others. Such a service should probably be split into two or more services: a core service that all use-cases depend on; and then one or more services that each just do the things unique to a particular use-case, with all their dependencies being on the critical path to achieve their functionality of serving that specific use-case. Then, those use-case-specific services can be healthy or unhealthy.
An example of doing this factoring right: CouchDB. It has a core "database" service, but also a higher-level "view-document querying" service, that can go down if its critical-path dependency (a connection to a Javascript sandbox process) isn't met. Both "services" are contained in one binary and one process, but are presented as two separate collections of endpoints, each with their own health endpoint.
An example of doing this factoring wrong: Wordpress. It's got image thumbnailing! Comment spam filtering! CMS publication! All in one! And yet it's either monolithically "healthy" or "unhealthy"; "ready" or "not ready" to run. That is clearly ridiculous, right?
I've done exactly this, and it worked well in my case were the # of related services was pretty small. Each endpoint would return an HTTP status code indicating overall health with additional details stating exactly which checks succeeded or not.
That's my point. If your editor cannot create new articles that's a problem that needs to be resolved.
I probably wouldn't want all my editors smashing the submit button until it worked, leading to additional overload problems. So I open the "circuit breaker" for submission by sending an email to all the editors to please stop submitting while wordpress is broken.
But shutting down the complete website would make the problem much worse because readers wouldn't see anything.
Conclusion: It's neither 100% healthy nor 100% unhealthy. Healthiness cannot be captured by a single boolean.
Well, you can make such an endpoint, you already have. It's called...
Your endpoint.
The answer to the top level question is, "because it's easier, more accurate, and more maintainable to call a real endpoint than to try to maintain an endpoint whose sole purpose is to predict whether your other endpoints are actually working."
Aka: Just Ask.
Not matter how complex a health endpoint is, the circuit breaker is more so. The health endpoint might make a legit request, checking all downstream services, etc etc, to confirm that everything is working...but the actual traffic is many different requests, with different code flows.
As an example, imagine service Bs health check actually makes a request. Great. Only, service B is misconfigured to talk to staging, whereas the upstream, service A, is in prod. The health check may always pass; after all, the staging request's data all gets resolved and handled appropriately. The upstream one, however, leads to constant failure, as that data doesn't exist.
Health check passes 100% of the time. Actual upstream calls pass 0% of the time. Don't have your upstream calls circuit break on health checks; have them circuit break on actual responses.
(And yes, you should have network isolation between staging and prod so that particular example can't happen. It is intentionally a contrived example)
Instead, the circuit breaker will first get an upper-bound on healthiness from the /health endpoint—and that'll be enough to tell it when even the upper bound says the service is unhealthy. Then, when the upper bound says "healthy", it'll seek a tighter bound on healthiness by letting through actual requests in the "half-open" state.
In this way, depending on how you've written your /health endpoint, you can ensure that far fewer users get their request abused as the scope for the service's health-checking reconaissance-in-force mission; and so decrease your 99th-percentile latency. (Since, of course, a background poller hitting the /health endpoint is outside the request flow, and so doesn't count toward 99th-percentile latency.)
If the health route says everything's up and running, then you can try a normal request, and if that fails, you back off anew (and you log an error entry that can be used to update the health endpoint's data generator, because something, somewhere, is misreporting)
Or you're overshooting. Say you have a service with 2 endpoints, both depend on the DB and 1 depends on an external API. External API goes down, you don't have a correct choice if you have 1 health endpoint for circuit breakers. If you open the breaker you needlessly disable requests to the healthy endpoint. If you leave it closed your circuit breaker is not working for the broken endpoint.
You can of course use a health check in addition to the normal endpoint in your circuit breaker, but then its just a fail-faster for the scenarios that the health endpoint covers.
Er, yes, that was the thing I was trying to suggest. You want to fail fast (or, more precisely, keep the system in the "circuit open, degraded functionality code-path" state) so your users don't experience mysterious latency spikes when their request becomes the guinea-pig for the circuit-breaker. That'll still happen, but not as often if a health check can tell the service there's not even a chance the service is worth checking via user-request.