In #293, `balance` was refactored to manage dispatching requests over a
set of equivalent inner services that may or may not be ready.
This change extracts the core logic of managing a cache of ready
services into a dedicated crate, leaving the balance crate to deal with
node selection.
This makes all tower subcrates have the following lints as warn (rather
than allow): `missing_docs`, `rust_2018_idioms`, `unreachable_pub`, and
`missing_debug_implementations`. In addition, it consistently applies
`deny(warning)` *only* under CI so that deprecations and macro changes in minor
version bumps in dependencies will never cause `tower` crates to stop
compiling, and so that tests can be run even if not all warnings have been
dealt with. See also https://github.com/rust-unofficial/patterns/blob/master/anti_patterns/deny-warnings.md
Note that `tower-reconnect` has the `missing_docs` lint disabled for now
since it contained _no_ documentation previously. Also note that this
patch does not add documentation to the various `new` methods, as they
are considered self-explanatory. They are instead marked as
`#[allow(missing_docs)]`.
* fix up broken dependencies and deprecated methods
* use released version of tracing-subscriber
Co-Authored-By: Lucio Franco <luciofranco14@gmail.com>
Rather than consuming `self` and returning `(Self, _)`. This did mean
that a few crates that depended on `Ready` to own the `Service` and
provide it once it was ready had to change to call `poll_ready`
directly. Which in turn meant adding in some PhantomData<Request> so
that the impl blocks wouldn't be under-constrainted. Take, for example:
```
impl<K, S: Service<Req>, Req> Future for UnreadyService<K, S>
```
would fail to compile with
```
error[E0207]: the type parameter `Req` is not constrained by the impl trait, self type, or predicates
```
Of particular note is that this change lets spans trace requests through `tower::Buffer` by internally carrying the `Span` at the time of `call` along with the request to the worker.
In 03ec4aa, the balancer was changed to make a quick endpoint decision.
This, however, means that the balancer can return NotReady when it does
in fact have a ready endpoint.
This changes the balancer to separate unready endpoints, only
performing p2c over ready endpoints. Unready endpoints are tracked with
a FuturesUnordered that supports eviction via oneshots.
The main downside of this change is that the Balancer must become
generic over the Request type.
As described in #286, `Balance` had a few problems:
- it is responsible for driving all inner services to readiness, making
its `poll_ready` O(n) and not O(1);
- the `choose` abstraction was a hinderance. If a round-robin balancer
is needed it can be implemented separately without much duplicate
code; and
- endpoint errors were considered fatal to the balancer.
This changes replaces `Balance` with `p2c::Balance` and removes the
`choose` module.
Endpoint service failures now cause the service to be removed from the
balancer gracefully.
Endpoint selection is now effectively constant time, though it biases
for availability in the case when random selection does not yield an
available endpoint.
`tower-test` had to be updated so that a mocked service could fail after
advertising readiness.
Lucio Franco
Oliver Gould
Taiki Endo
Jon Gjengset
Luke Steensen