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Contribute a built-in adapter

How-to guide
For:
Developers

This guide shows how to contribute email adapter Go code to ReputeCast: a built-in adapter for a sending provider, compiled into the binary, tested against the shared contract suite and released with every version. If you only need your own instance to reach a provider, a generic HTTP mapping or a community adapter is quicker; provider adapters compares the three.

Before you begin

Section titled “Before you begin”
  • A checkout of the ReputeCast repository and Go 1.27 (the toolchain downloads automatically). Run ReputeCast from source sets up make dev.
  • golangci-lint v2, for make lint.
  • The provider’s API documentation, its current acceptable-use policy, and sample webhook payloads from its documentation.
  • docs/dev/CONVENTIONS.md in the repository, which every change follows.

Write the adapter

Section titled “Write the adapter”

  1. Read the contract in internal/providers/provider.go. It is read-only for adapter authors. Every adapter implements providers.Provider:

    Method Does
    Adapter() string The registry key, like "mailgun".
    Capabilities() Capabilities The badges: bulk policy, webhooks, domain API, suppression sync, batch size, dedicated IPs, sandbox, quota and rate discovery, pricing, tracking.
    Send(ctx, msgs) ([]SendResult, error) Sends at most BatchSize messages and returns one result per message, in input order.
    ParseWebhook(ctx, r, body) ([]Event, error) Verifies the request and maps its payload to normalized events.

    Then implement every optional interface the provider’s API really supports. Core code type-asserts before using them, and the screens explain the missing ones:

    Interface Used for
    Verifier, QuotaReporter The connect step and the health job (every 5 minutes, every minute while the provider is down). Quota wins when both exist.
    ReputationReporter The provider’s own bounce and complaint rates, which its review thresholds apply to.
    DomainManager DNS records with live status for the domain step and Deliverability.
    WebhookRegistrar, WebhookUnregistrar Registering ReputeCast’s webhook, and removing it when the provider is removed.
    WebhookInspector, WebhookTester The “webhooks silent” diagnosis and “Send test event”.
    WebhookVerifier, WebhookHandshaker Verifying before ingest stores a request; answering handshakes (SNS subscriptions, Event Grid validation) without storing them.
    WebhookSecrets Naming headers that carry a shared webhook secret as it is, so ingest leaves them out of the stored raw event.
    SuppressionSyncer Two-way sync with the shared suppression list. Return providers.ErrNotSupported from AddSuppressions for pull only.
    TrackingManager Reading and turning off provider-side tracking, so opens and clicks aren’t counted twice.
    EventBackfiller Fetching message status when webhooks went silent.
    StatusPage, StatusReporter The status button and incident text during an outage.
    BroadcastStreamCreator Providers whose terms want a separate stream for bulk mail (Postmark).
    ExtrasSender Sending copies and attachments; a message with extras only goes to a provider that has it.

  2. Create the package. internal/providers/resend is a compact example to follow:

    • Directoryinternal/providers/example/
      • example.go package comment with the provider’s terms, Descriptor, settings, credentials, New, Send
      • webhook.go VerifyWebhook, ParseWebhook, normalization
      • account.go Verify or Quota, domains, webhook registration, suppressions, tracking, backfill
      • lang.go the message catalog and its messages (text for people)
      • Directorymessages/
        • en.json the English people read
        • vi.json its translation
      • example_test.go fake API, contracttest.Run, adapter-specific tests
      • Directorytestdata/ recorded webhook payloads
        • …

  3. Write the descriptor, register it from init, and fill its words from the adapter’s message catalog:

    internal/providers/example/example.go
    //go:embed messages/*.json
    var messagesFS embed.FS
    

    var msgs = i18n.MustLoad(messagesFS, "providers.example")
    

    func init() { providers.Register(Descriptor()) }
    

    func Descriptor() providers.Descriptor {
      return providers.Descriptor{
        Adapter: "example",
        Name:    "Example ESP",
        Kind:    "api", // api, smtp, self_hosted, http
        Capabilities: providers.Capabilities{
          Bulk:            providers.BulkAllowed, // cite the AUP in the package comment
          Webhooks:        "Registered",
          DomainAPI:       "full",
          SuppressionSync: true,
          BatchSize:       1,
          Sandbox:         "mode",
          QuotaDiscovery:  true,
          // Pricing and DedicatedIPs are in messages/en.json.
        },
        Version:    version.Version,
        Maintainer: "HostRepute",
        Terms: []providers.Terms{{Bulk: providers.BulkAllowed,
          Streams: []providers.Stream{providers.StreamBroadcast, providers.StreamAutomation, providers.StreamSystem}}},
        Fields: []providers.Field{
          {Key: "api_key", Secret: true, Required: true},
          {Key: "region", Options: []string{"us", "eu"}},
        },
        New: New,
      }.WithMessages(msgs)
    }

    The words people read go in messages/en.json, with a note where a translator needs one, and their translation in messages/vi.json, with the terms of docs/i18n/glossary-vi.md:

    internal/providers/example/messages/en.json
    {
      "pricing": { "message": "plan, from $10 per month", "note": "on a capability chip: Pricing · …" },
      "terms.0.note": { "message": "Opted-in lists only…", "note": "what the terms allow, on Routing" },
      "events": "Webhooks",
      "domainSetup": "API",
      "suppressionMode": "Two-way",
      "field.api_key.label": "API key",
      "field.api_key.help": { "message": "Example › Settings › API keys.", "note": "Example's own menus" },
      "field.region.label": "Region"
    }

    The badges must be true for the provider today:

    • Bulk: BulkAllowed, BulkBroadcastStreams (only on a dedicated stream), BulkForbidden or BulkYourPolicy (self-hosted). Quote the provider’s acceptable-use policy in the package comment with its URL and the date you checked it, and put the conditions that matter in Terms.Note (the terms.0.note message): the routing screen shows it.
    • Sandbox: detection, test token, mode or test addresses; empty when there is none.
    • Quota and rate discovery: set them only when an API call reports them.
    • Pricing and dedicated IPs: short summaries in the pricing and dedicatedIps messages; prefer a plan name to a price that will go stale.
    • Tracking: set RewritesLinks or AddsOpenPixel when the provider tracks by default and the adapter can’t turn it off per message.
    • The events, domainSetup and suppressionMode messages are the words the Adapters tab shows, like “SNS webhooks”, “API” and “Two-way”.

  4. Write the text people read. What an adapter tells people reads in each reader’s language: server text follows the reader’s app language (i18n.LangFrom(ctx)), and API clients that send no Accept-Language read English. Catalogs are global, so core code reads a kept message without importing the adapter.

    • The descriptor’s words. WithMessages(msgs) fills the descriptor from well-known keys: field.<key>.label and field.<key>.help, terms.<n>.variant and terms.<n>.note (the nth Terms, from 0), pricing, dedicatedIps, and the Adapters tab’s events, domainSetup and suppressionMode. Logic keeps comparing the English.
    • What the adapter writes at runtime keeps its message beside the English: Quota.AccountText (the account’s standing), ConnectionReport.SummaryText, ProvisionStep.LabelText, ProvisionResult.DoneText, Tracking.DetailText and ScopeText, ServiceStatus.SummaryText, and CallError.Text for an error the adapter words itself. Return the English as before and the message with msg.Keep(args); most adapters have a say(m, args) helper in lang.go that returns both. Pass numbers as numbers and times as time.Time, so {n, number} and {at, time} follow the reader’s region format.
    • Names stay as they are: the provider’s name and its own words for its plans, keys, menus and stream types, its answers (error codes, API messages), event names, logs and internal errors.
    • Stored text. Code that keeps an adapter’s text for later readers (a provider’s health, a setup’s state, an incident) stores the English and the message. A row written before its message was kept reads in English.

    SMS adapters (internal/sms/<name>) work the same way: their catalog’s namespace is sms.<name>, Manifest.WithMessages reads the keys summary, badge.<capability>.note, field.<key>.label and field.<key>.help, and internal/sms/contracttest checks the texts.

  5. Handle settings and credentials. providers.Config.Settings holds the non-secret fields; Config.Credentials holds the decrypted secrets as a flat JSON object keyed by the Secret fields’ keys. Credentials are write-only: sealed at rest, never returned by the API, replaced one key at a time. Mark every field whose key looks secret (password, token, secret, or ending in api_key) as Secret, or the contract suite fails.

    New must not do network I/O and must not fail because an optional value is missing; report missing credentials from the first call. The factory builds an adapter per request, so keep anything worth reusing in a package-level cache keyed by the account. Keep signing material read from the provider’s API in a webhookauth.Material, which reads it again when a signature fails (at most once a minute) and every hour.

  6. Implement Send:

    • Return a per-message SendError for a refusal that concerns one message (an invalid address, an unverified sender). Return an error for an account or service failure, wrapping providers.ErrUnauthorized (rejected credentials) or providers.ErrUnavailable (outage). apiclient does this mapping: Do for JSON calls, Exec for form or multipart bodies, signed requests and response headers. 401 and 403 become rejected credentials, 429 throttling, 408 and 5xx outages; apiclient.MessageError turns the other 4xx answers into a permanent SendError.
    • For one call per message, use apiclient.Fanout: it keeps results in order, starts no new call after an account-level failure, and gives unsent messages a temporary error so the engine retries them.
    • Echo the ReputeCast message id in whatever the provider returns on events (custom args, tags, metadata), and send the X-ReputeCast-Message header.
    • Where the provider supports idempotency, derive the key from the message id, so a retry after a timeout never sends twice.
    • Turn the provider’s open and click tracking off on every message: ReputeCast tracks itself.
    • Never mark an accepted message as failed. If the provider accepts it but returns no id, use the ReputeCast id as the provider id.

  7. Verify and normalize webhooks:

    • VerifyWebhook must return an error wrapping providers.ErrBadSignature for every failure, including “couldn’t fetch the key”. Ingest stores a request with any other error as verified.
    • ParseWebhook verifies unless providers.SignatureVerified(ctx) is set. The normalization job sets it when it re-parses a stored payload, which no longer has the original URL or the Authorization header.
    • Use the scheme the provider signs with; internal/providers/webhookauth has Standard Webhooks, HMAC helpers and constant-time comparisons. Where the provider doesn’t sign, register the webhook with basic auth or a bearer token carrying Config.WebhookSecret, which travels in Authorization and is never stored.
    • Bound the signed timestamp: five minutes with webhookauth.Fresh, or a window that spans the provider’s retries with webhookauth.Within.
    • Map every delivery-relevant event to delivered, deferred, hard_bounce, soft_bounce, complaint, rejected or unsubscribe. Keep the provider’s type in OriginalType, its code in OriginalCode (split it with bounce.Split), the rest in Diagnostic, and set BounceClass with bounce.Classify. Lowercase the recipient, keep the raw JSON in Raw, and return no event for things that aren’t outcomes (sent, processed, auto-replies, test pings).
    • Don’t suppress on a guess: a permanent failure caused by the sender (a spam or reputation block, retries running out) is a soft bounce. Only an address problem is a hard bounce, because hard bounces and complaints go on the shared suppression list.

  8. Test against the contract suite. Every adapter passes internal/providers/contracttest, which checks registration, capabilities, a send, a permanent rejection, an outage and rejected credentials against your fake API, and every recorded webhook: verified, normalized, re-parsed with a verified context, and refused once tampered with. Its Texts subtest checks the text people read: a built-in adapter of kind api, smtp, self_hosted or http keeps a catalog, every text it declares has a message whose English is that text, and every message is translated into each app language. contracttest.CheckText checks a text the adapter writes at runtime.

    internal/providers/example/example_test.go
    func TestContract(t *testing.T) {
      contracttest.Run(t, contracttest.Suite{
        Adapter: example.Adapter,
        API:     fakeAPI, // reads env.Mode(): "", contracttest.ModeOutage, contracttest.ModeAuth
        New:     newProvider,
        Delivered: func(t *testing.T, env *contracttest.Env, msgs []providers.Message, res []providers.SendResult) {
          // check env.Requests(): headers, the echoed message id, tracking off
        },
        Webhooks: []contracttest.Webhook{{
          Name:    "HardBounce",
          Request: signed("testdata/bounce.json"), // signed at test time with a test key
          Want:    []providers.Event{{Type: providers.EventHardBounce, Recipient: "[email protected]", OriginalCode: "550 5.1.1", BounceClass: "mailbox_unknown"}},
        }},
        // Tamper defaults to flipping a body byte; override it when the
        // signature doesn't cover the body or auth is a header.
      })
    }

    Tests use no network: run a fake of the provider’s API, record webhook payloads from the provider’s documentation as fixtures, and sign them in the test with a test key. Where the provider publishes a signature test vector, test against it. Test the optional interfaces, the error mapping and every quirk you code around (ids above 2^53, a field that is a string or an array, a 400 that carries per-message results).

  9. Register the adapter in the build:

    • Add a blank import of the package to internal/app/wire_adapters.go. Registering the same key twice panics at startup.
    • Add the key to adapterOrder in internal/providers/manage/adapters.go (the Adapters tab order) and to catalogOrder in internal/connect/catalog.go (the setup catalog order).
    • Optionally, give the connect wizard its wording in copyTable in internal/connect/copy.go, with the words as adapter.<key>.* messages in internal/connect/messages/en.json and vi.json; without an entry, the wizard uses the descriptor’s name and fields.
    • Built-in adapters carry the ReputeCast version (version.Version) and the maintainer “HostRepute”.

Check that it worked

Section titled “Check that it worked”

  1. Run the adapter’s tests and the linter:

    Terminal
    go test ./internal/providers/example/
    golangci-lint run ./internal/providers/...

    make test runs every Go and web test, against the development Postgres when it is running; make lint runs every linter.

  2. Start the app with make dev, sign in, and open Sending providers, then Adapters. The adapter is listed under built-in adapters with the version dev and the maintainer HostRepute, and Sending providers, then Add provider offers it in the catalog.

Troubleshooting

Section titled “Troubleshooting”
  • The server panics with providers: duplicate adapter <key>: another adapter already registers that key. Choose a unique Adapter key.
  • The contract suite fails a connection field: its key looks like a secret (password, token, secret, …api_key). Set Secret: true.
  • The contract suite’s Texts test says the adapter “keeps no catalog”: the descriptor doesn’t load its messages. Embed messages/*.json, load them with i18n.MustLoad, and end Descriptor() with .WithMessages(msgs).
  • Texts says a text “has no message” or that the catalog “isn’t translated into Tiếng Việt”: add the key to messages/en.json with the English the adapter uses, or its translation to messages/vi.json.
  • A tampered webhook passes, or a refused one is stored as verified: VerifyWebhook returned an error that doesn’t wrap providers.ErrBadSignature.
  • Stored webhooks fail when they are normalized: ParseWebhook verifies again although providers.SignatureVerified(ctx) is set; the stored request has no Authorization header.
  • Events don’t match their messages: the provider doesn’t echo the ReputeCast message id. Pass it in the provider’s metadata, tags or custom args, and send the X-ReputeCast-Message header.
  • Opens or clicks are counted twice: the provider still tracks. Turn its tracking off on every message, or implement TrackingManager to turn it off for the account.
Section titled “Related”
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