external_service

# Changelog

All notable changes to this project, from version 1.0.0 onward, will be documented in this file.

The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).

## [Unreleased]

## [2.3.0] - 2026-07-31

### Added
- **A public control API for the circuit breaker and rate limiter**
  ([issue #26](https://github.com/jvoegele/external_service/issues/26)), for the
  cases that fall outside a guarded `call/3`. 2.2.0 made
  `ExternalService.CircuitBreaker` and `ExternalService.RateLimiter` public as
  *behaviours*; this makes them usable directly.
  - `ExternalService.CircuitBreaker.melt/1` records a failure the library never
    saw — a dropped streaming connection, a webhook that never arrived, a call
    made through a different client. It counts toward the service's `:tolerate`
    exactly as an in-call failure does, so enough melts open the breaker (and,
    with the cluster backend, open it across the cluster). The mirror image of
    `reset/1`.
  - `ExternalService.CircuitBreaker.ask/1` reports `:ok`, `:blown`, or
    `:not_started` — the three-valued form of `available?/1` and `blown?/1`.
  - `ExternalService.CircuitBreaker.reset/1`, the same operation
    `ExternalService.reset/1` performs.
  - `ExternalService.RateLimiter.request/1` spends one call's worth of budget
    without running anything, for traffic that reaches the service by some path
    other than `call/3`. It honors the service's `:wait` setting and returns
    `ExternalService.RateLimited` if that budget runs out.
- **Rate limit introspection.** `ExternalService.rate_limited?/1` reports whether
  a call would currently be throttled, and `ExternalService.RateLimiter.peek/1`
  reports how long the wait would be. Both are reads that **consume nothing**, so
  they are safe to ask speculatively before committing to expensive work —
  symmetric with `available?/1` for the circuit breaker.

### Changed
- **The `ExternalService.RateLimiter` behaviour gained a `peek/2` callback.**
  Backends must answer the same way as `check/2` without consuming anything.
  Both shipped backends implement it; `Local` reads its atomics slot without the
  compare-and-exchange, and `Hammer` assembles the answer from `get/2` and
  `expires_at/2`, since Hammer's `hit/3` both checks and consumes.

  This is a breaking change for anyone who wrote a rate limiter backend against
  2.2.0. It is being made immediately after that release, while no third-party
  backends exist, precisely so that it does not have to be made later.

## [2.2.0] - 2026-07-30

This line makes `ExternalService` work correctly on more than one node. See the
new [Distributed Elixir](guides/distributed.md) guide for the full picture.

The two halves of that problem are not the same kind of problem, and are not
solved the same way. A node-local **rate limit** is a correctness bug — four
nodes configured for 100 calls per second send up to 400, violating the quota you
configured — so the fix is shared counters. A node-local **circuit breaker** is a
defensible design rather than a bug, since a node with a bad network path should
stop calling a service without taking the cluster down with it, so cross-node
tripping is offered as an opt-in choice.

### Added
- **Pluggable circuit breaker and rate limiter backends**
  ([issue #12](https://github.com/jvoegele/external_service/issues/12),
  [issue #13](https://github.com/jvoegele/external_service/issues/13)).
  Both `:circuit_breaker` and `:rate_limit` accept a `:backend` option, given as
  a module or a `{module, options}` tuple whose options are passed through to
  that backend:

  ```elixir
  use ExternalService,
    circuit_breaker: [backend: ExternalService.CircuitBreaker.Cluster],
    rate_limit: [limit: 100, per: 1_000, backend: {MyApp.Limiter, some: :option}]
  ```

  `ExternalService.CircuitBreaker` and `ExternalService.RateLimiter` are now
  documented behaviours you can implement — five callbacks for a breaker, two for
  a limiter. Backends are stateless modules: the `install`/`init` callback returns
  an opaque config term that is stored with the rest of the service state and
  handed back to every other callback, so a backend needs no process, supervisor,
  or registry of its own.

  Note that this exposes the breaker and limiter *as behaviours*, not as
  user-facing control APIs; the operations themselves remain internal
  ([issue #26](https://github.com/jvoegele/external_service/issues/26)).
- **`ExternalService.CircuitBreaker.Cluster`**, an opt-in circuit breaker that
  trips the whole cluster when any one node trips
  ([issue #13](https://github.com/jvoegele/external_service/issues/13)). Each node
  keeps its own ordinary breaker; when one transitions from closed to open it
  sends a fire-and-forget `:erpc.multicast/4` to the other nodes, each of which
  trips its own breaker and then recovers on its own reset timer. There is no
  shared store, no distributed state, and no process or supervision tree for this
  library to run. A `:nodes` option (a list, or a zero-arity function returning
  one; default `&Node.list/0`) narrows the broadcast. Read the module docs before
  enabling it: it trades isolation for convergence, and one bad node can trip the
  whole cluster.
- **`ExternalService.RateLimiter.Hammer`**, a rate limiter backend that meters
  against a [Hammer](https://hexdocs.pm/hammer) module
  ([issue #12](https://github.com/jvoegele/external_service/issues/12)). With a
  shared Hammer backend such as
  [`hammer_backend_redis`](https://hexdocs.pm/hammer_backend_redis) every node
  draws from the same counters, so the service sees the limit you configured
  rather than that limit multiplied by your node count. Hammer is **not** a
  dependency of this library — the backend calls `hit/3` on the module you supply.
- **`rate_limit: [wait: ...]`** to bound how long a throttled call may block:
  `:infinity` (the default, and the previous behavior), a millisecond budget for
  the whole call, or `false` to never wait. Previously a throttled call waited as
  long as the limiter required with no upper bound.
- **`ExternalService.RateLimited`**, returned by `call/3` and raised by `call!/3`
  when the `:wait` budget runs out. The wrapped function is not called. It carries
  `:context.retry_after` (milliseconds until the call would have been admitted)
  and reports `http_status/1` of `429`. Being throttled is this library's own
  back-pressure rather than a failure of the external service, so it does **not**
  melt the circuit breaker and is **not** retried.
- A [Distributed Elixir](guides/distributed.md) guide, plus rate limiting and
  circuit breaker guide sections and cheatsheet entries covering the above.

### Changed
- **The default rate limiter is now a token bucket, and paces calls differently.**
  `ExternalService.RateLimiter.Local` replaces the `ex_rated` fixed window. It
  admits a burst of exactly `:limit` and then paces the rest at one call per
  `:per / :limit`, refilling one call at a time.

  What you will notice: waiting out a full window no longer hands you a fresh
  full burst. The fixed window allowed `:limit` calls at the end of one window and
  another `:limit` at the start of the next, briefly sending **twice** your
  configured rate at the service — which could trip the provider's own limiter
  even though you had configured yours correctly. Smoothing that out is the point
  of the change, but it does mean bursty workloads are now paced where they
  previously were not.

  No configuration changes: `:limit` and `:per` mean what they did before. The
  new limiter keeps its counters in a single `:atomics` slot per service, so it
  needs no owning process, and it is correct under concurrent access (a
  compare-and-exchange loop, rather than a lock or a best-effort counter).
- **Rate limit sleeps are now as long as they need to be, and no longer.** Backends
  report a real time-to-next-window, where `ex_rated` could only be given the
  `window / limit` estimate this library computed for it. Expect the
  `[:external_service, :rate_limit, :sleep]` telemetry to report different (and
  more accurate) durations.

### Removed
- **The `ex_rated` dependency**, which has had no release since December 2021.
  Rate limiting is now handled by the built-in `ExternalService.RateLimiter.Local`
  or a backend of your choosing.

  If your own code called `ExRated` directly — it was previously reaching you as a
  transitive dependency — add `{:ex_rated, "~> 2.1"}` to your `deps`. Nothing in
  the `ExternalService` API changes.

## [2.1.0] - 2026-07-30

### Added
- `ExternalService.Decorator`: decorator-based annotations for marking a function
  as an external call ([issue #28](https://github.com/jvoegele/external_service/issues/28)).
  `use ExternalService.Decorator` brings `@decorate external_call(service)` (and a
  raising `external_call!`) into scope, wrapping the function body in
  `ExternalService.call/2` (or `call/3` when passed per-call retry options) instead
  of writing `call fn -> ... end` by hand. Built on the
  [`decorator`](https://hex.pm/packages/decorator) library.
- `ExternalService.Flow`: process an enumerable (or an existing `Flow`) through
  guarded `ExternalService` calls as a stage of a [`Flow`](https://hexdocs.pm/flow)
  pipeline ([issue #27](https://github.com/jvoegele/external_service/issues/27)).
  `ExternalService.Flow.map/3,4,5` returns a `Flow`, reusing `call/3` per element
  so retries, the circuit breaker, rate limiting, telemetry, and the
  structured-error returns all apply (errors arrive as `{:error, ...}` elements;
  results are unordered). `:flow` is an **optional** dependency — the module is
  only compiled when you add it. For simple ordered parallel maps,
  `call_async_stream/5` remains the right tool.

## [2.0.0] - 2026-06-23

The 2.0 line modernizes the project and introduces breaking changes. See the
[migration guide](guides/migrating-to-2.0.md) for a step-by-step upgrade from
1.x.

### Added
- Documentation overhaul: a set of guides (Getting Started, the module front
  door, circuit breakers, retries, rate limiting, error handling, telemetry), a
  cheatsheet, and a step-by-step [migration guide](guides/migrating-to-2.0.md),
  all published on HexDocs.
- Introspection for circuit breaker state ([issue #5](https://github.com/jvoegele/external_service/issues/5)):
  `ExternalService.available?/1`, `ExternalService.blown?/1`, and
  `ExternalService.all_available?/1`, plus `available?/0` and `blown?/0` on
  modules using `ExternalService.Gateway`.
- `:telemetry` events for guarded calls: `[:external_service, :call, :start | :stop | :exception]`
  (a span around each call), `[:external_service, :call, :retry]`,
  `[:external_service, :circuit_breaker, :blown]`, and
  `[:external_service, :rate_limit, :sleep]`. See the `ExternalService` module
  docs for measurements and metadata.
- `RetryOptions.max_attempts` to bound the total number of attempts (initial plus
  retries), complementing the existing time-based `:expiry`.
- `RetryOptions.jitter` to control random jitter on retry delays (`true` for
  +/- 10%, or a float proportion such as `0.25`).
- `RetryOptions.retry_on` accepts a **predicate over the return value** (an
  arity-1 function), so retries can be driven from a function that does not itself
  return `:retry` / `{:retry, reason}` (the common case when adapting an existing
  client function). When the predicate returns a truthy value the call is retried
  — the result becomes the retry reason and the circuit breaker melts — exactly
  like an explicit `:retry` return, which still takes precedence
  ([issue #29](https://github.com/jvoegele/external_service/issues/29)).
- **Declarative module front door**: `use ExternalService` generates a small
  wrapper (`call/1,2`, `call!/1,2`, async/stream variants, `available?/0`,
  `blown?/0`, `reset/0`, `child_spec/1`, `start_link/1`) around a service
  configured with validated `:circuit_breaker`/`:rate_limit`/`:retry` options.
- A service now remembers the default retry options given to `start/2`; the
  two-argument `call/2` (and `call!/2`, `call_async/2`) use that default.
- Option validation via NimbleOptions for `start/2` and `RetryOptions`, with the
  accepted options rendered into the docs.
- Structured error types (built on [Errata](https://hexdocs.pm/errata)):
  `ExternalService.RetriesExhausted`, `ExternalService.CircuitBreakerOpen`, and
  `ExternalService.ServiceNotStarted`. Each is an exception struct carrying a
  `:context` (always including the `:service`), an `http_status/1`, and JSON
  encoding, so the same value can be returned from `call/3` or raised by
  `call!/3`.

### Changed (breaking)
- **Error representation overhauled.** `call/3` now returns structured error
  structs instead of nested tuples, and `call!/3` raises the same structs:

  | Before (1.x) | After (2.0) |
  | --- | --- |
  | `{:error, {:retries_exhausted, reason}}` | `{:error, %ExternalService.RetriesExhausted{context: %{service: name, reason: reason}}}` |
  | `{:error, {:fuse_blown, name}}` | `{:error, %ExternalService.CircuitBreakerOpen{context: %{service: name}}}` |
  | `{:error, {:fuse_not_found, name}}` | `{:error, %ExternalService.ServiceNotStarted{context: %{service: name}}}` |
  | raise `ExternalService.RetriesExhaustedError` | raise `ExternalService.RetriesExhausted` |
  | raise `ExternalService.FuseBlownError` | raise `ExternalService.CircuitBreakerOpen` |
  | raise `ExternalService.FuseNotFoundError` | raise `ExternalService.ServiceNotStarted` |

  Results returned directly by the wrapped function (including its own
  `{:error, reason}` values) are unchanged. See the
  [migration guide](guides/migrating-to-2.0.md) for the full mapping.
- **Configuration and terminology overhauled** to drop the leaked "fuse" wording:
  - `start/2` now takes `circuit_breaker: [tolerate:, within:, reset:, fault_injection:]`
    and `rate_limit: [limit:, per:]` (and an optional `retry:`) instead of
    `fuse_strategy: {:standard, max, window}` / `fuse_refresh:` and the
    `rate_limit: {limit, window}` tuple. Options are validated by NimbleOptions.
  - The `fuse_name` argument/type is now `service`.
  - `reset_fuse/1` is now `reset/1`.
- **Retry options reshaped** (`ExternalService.RetryOptions`):
  - `backoff` is now `:exponential` / `:linear` with separate `:base` and
    `:factor`, instead of `{:exponential, delay}` / `{:linear, delay, factor}`.
  - `randomize` is now `jitter`.
  - `rescue_only` is now `retry_exceptions`, and **defaults to `[]`** — raised
    exceptions are no longer retried by default ([issue #7](https://github.com/jvoegele/external_service/issues/7)).
    List exception modules in `:retry_exceptions` to retry on them.
    `:retry_exceptions` now also governs the circuit breaker: an exception that is
    not retried no longer melts the breaker (it propagates untouched), so a raised
    exception counts as a circuit-breaker failure only when its type is in
    `:retry_exceptions`. Explicit `:retry` / `{:retry, reason}` return values
    always melt the breaker.
  - `call/3` and `call!/3` now also accept a keyword list of retry options. A
    keyword list is treated as per-call *overrides*: it is merged onto the
    service's configured `:retry` defaults (overriding only the keys it lists and
    inheriting the rest). A `%RetryOptions{}` struct still replaces the defaults
    entirely.
- `use ExternalService.Gateway` is **deprecated** in favor of `use ExternalService`.
  It still works (emitting a deprecation warning) and keeps the `external_call/*`
  and `reset_fuse/0` names as aliases, but uses the same new option shape as
  `use ExternalService` — the old `fuse: [...]` options are no longer supported.

### Removed (breaking)
- The `ExternalService.RetriesExhaustedError`, `ExternalService.FuseBlownError`,
  and `ExternalService.FuseNotFoundError` exception modules, replaced by the
  structured error types above.

### Fixed
- `ExternalService.Gateway` now applies the `fuse: [strategy:, refresh:]` options
  it was configured with. Previously these keys did not match the
  `:fuse_strategy`/`:fuse_refresh` keys that `ExternalService.start/2` reads, so
  every gateway silently ran on the default circuit-breaker configuration.
- Added a regression test for the `:fault_injection` strategy (issue #4); the
  `:fuse_monitor` crash no longer reproduces on fuse 2.5.
- Rate limiting now works for a service whose name is any term, not only an atom
  or binary. The rate-limit bucket name is now derived with `inspect/1`;
  previously it used `Module.concat/2`, which raised for names such as tuples
  (circuit breaker and retries already accepted any term).

### Changed
- Raise the minimum Elixir requirement to `~> 1.15`.
- Modernize the build: refreshed dependency versions, added `nimble_options` and
  `telemetry`, ExDoc/Dialyxir bumps, GitHub Actions CI (test matrix, quality, and
  Dialyzer jobs), and Hex package/docs metadata cleanup.
- Store per-service state in `:persistent_term` instead of an unsupervised
  `Agent`, removing a process that could crash and was never linked to a
  supervisor. `ExternalService.stop/1` now accepts any term as a fuse name
  (matching `start/2`), not only atoms, and is idempotent — it is safe to call
  on a service that was never started or has already been stopped.

## 1.1.4 - 2024-01-04
### Fixed
- Replace use of deprecated `System.stacktrace/0` with `__STACKTRACE__/0` ([PR #17 from @iperks](https://github.com/jvoegele/external_service/pull/17))

## [1.1.3] - 2023-05-12
### Changed
- Update to retry 0.18.0
- Update ex_rated to 2.1

## [1.1.2] - 2021-09-30

### Changed
- Make sleep function configurable ([PR #11 from @doorgan](https://github.com/jvoegele/external_service/pull/11))

## [1.1.1] - 2021-09-17
### Changed
- Update to fuse 2.5
- Update ex_rated to 2.0

## [1.1.0] - 2021-09-17
### Added
- Add `ExternalService.stop/1` ([PR #9 from @doorgan](https://github.com/jvoegele/external_service/pull/9))

### Changed
- Allow any term as fuse name ([PR #10 from @doorgan](https://github.com/jvoegele/external_service/pull/10))


## [1.0.1] - 2020-06-08
### Added
- Add ability to reset fuses
- Add documentation for initialization and configuration of gateway modules

## [1.0.0] - 2020-06-05
### Added
- Add new ExternalService.Gateway module for module-based service gateways.
- Add this changelog...better late than never!

[Unreleased]: https://github.com/jvoegele/external_service/compare/2.3.0...HEAD
[2.3.0]: https://github.com/jvoegele/external_service/compare/2.2.0...2.3.0
[2.2.0]: https://github.com/jvoegele/external_service/compare/2.1.0...2.2.0
[2.1.0]: https://github.com/jvoegele/external_service/compare/2.0.0...2.1.0
[2.0.0]: https://github.com/jvoegele/external_service/compare/1.1.4...2.0.0
[1.1.2]: https://github.com/jvoegele/external_service/compare/1.1.1...1.1.2
[1.1.1]: https://github.com/jvoegele/external_service/compare/1.1.0...1.1.1
[1.1.0]: https://github.com/jvoegele/external_service/compare/1.0.1...1.1.0
[1.0.1]: https://github.com/jvoegele/external_service/compare/1.0.0...1.0.1
[1.0.0]: https://github.com/jvoegele/external_service/compare/0.9.3...1.0.0