Connectors Overview

Apache Iggy Connectors - Overview

Repo-wide rules (Apache headers, fmt/sort/clippy order, idiomatic Rust traits, imports at top, no cargo install, LLM-slop tells) live in AGENTS.md. This file owns connector-specific rules and routes to the per-area skills.

Contents

• STOP and ask the user before
• What sinks and sources are
• Which skill to load
• Stick to conventions
• Connector-wide rules
• Benchmark mode (per-batch timing observability)
• Logging format
• Exemplars
• Concrete efficiency patterns
• Drop accounting
• Common review smells
• File map
• Cite by symbol, not line number

STOP and ask the user before

• Bumping iggy_connector_sdk MAJOR version or changing any FFI signature in sdk/src/{sink,source}.rs - breaks every pre-built plugin .so.
• Changing the runtime's wire conventions (postcard FFI payload structs, default consumer group naming, plugin path resolution).
• Modifying runtime/src/state.rs save protocol (atomic rename + fsync ordering) - corruption risk.
• Renaming or repurposing a Schema variant - decoders/encoders pinned to wire bytes.
• Promoting a transient Error variant to Permanent* or vice versa - affects retry behavior across every plugin.

What sinks and sources are

A sink is a plugin that consumes messages from Apache Iggy streams and writes them to an external system (Postgres, Elasticsearch, Iceberg, HTTP endpoint, Mongo, object store, stdout, ...). Implements iggy_connector_sdk::Sink.

A source is a plugin that produces messages from an external system into Apache Iggy streams (poll a DB table, scroll an ES index, generate random data, ...). Implements iggy_connector_sdk::Source.

Both compile as cdylib shared libraries (.so/.dylib/.dll) loaded by the connectors runtime at startup via dlopen. The runtime drives lifecycle (open/handle/consume/close), bridges Apache Iggy ⇄ plugin via a small FFI, applies optional transforms, and persists source state.

                      ┌─ optional transforms ─┐
External  ──poll──▶  SOURCE  ──FFI──▶  RUNTIME  ──encode──▶  Apache Iggy stream
   system    plugin            ▲                    ▲
                               │                    │
                       state save (msgpack)         │
                                                    │
                       ┌─ optional transforms ─┐    │
Apache Iggy stream  ──decode──▶  RUNTIME  ──FFI──▶  SINK  ──write──▶ External
                                              plugin                   system

Headers set on the source side ride through transforms (which may modify, drop, or pass them) and arrive at the sink with BTreeMap<HeaderKey, HeaderValue> preserved deterministically.

Which skill to load

Stick to conventions

The connectors codebase is intentionally repetitive across plugins. Cross-plugin consistency makes review, LLM-assisted contribution, and onboarding tractable.

1. Find the closest existing plugin by shape (DB-write -> postgres_sink, HTTP -> http_sink, polling source -> postgres_source).
2. Copy its file layout, naming, log format, error mapping, test structure verbatim. Only the backend-specific code (the client call) should differ.
3. Don't reinvent existing patterns. The SDK's retry.rs covers HTTP retry + circuit breaker. Existing plugins cover batching, idempotency, header encoding, secret handling.
4. Don't refactor unrelated code in the same PR. Match conventions. propose convention changes separately.

Connector-wide rules

Hot loops

• Zero clone. Use &self, &str, &[T], &Payload. For Payload::Json, try_to_bytes(&self) serializes single-pass without cloning the OwnedValue tree.
• Move-out without cloning via std::mem::replace / std::mem::take.
• Vec::with_capacity(n) when building per-batch buffers. reallocation in hot path is wasted work.

Async + concurrency

• async fn only. Never .block_on() inside a Sink/Source impl - the runtime drives the executor.
• Hold locks briefly. Pattern: lock, clone/read what's needed, drop guard, then do I/O. Never hold a Mutex across an external .await.
• Atomic counters (AtomicU64) preferred over Mutex<u64> for hot-path metrics.
• No tokio::spawn from plugin code. Runtime owns lifecycle. orphans survive close().

Logging

• tracing only. info! for lifecycle, debug! per-batch detail, warn! recoverable, error! failures. Include connector ID: {self.id} and the connector name on every line.
• verbose: bool on SinkConfig/SourceConfig (TOML top-level). Runtime upgrades its own per-batch debug! to info! when set. Plugins should mirror with a verbose_logging: Option<bool> field inside plugin_config (convention: postgres_sink::PostgresSinkConfig::verbose_logging). Default both to false.
• Never log secrets (connection strings, API keys, tokens). Redact URLs at the log site. Credential fields must use SecretString (see below).

Secrets

Any credential-bearing field (connection strings, API keys, bearer tokens, AWS keys) must be SecretString from the secrecy crate. Plain String for a credential is a review-blocker: SecretString redacts on Debug, so it is what keeps a credential out of a log line that formats the whole config.

serde_secret::serialize_secret EXPOSES the secret. It does not redact. It calls expose_secret() and writes the plaintext. SecretString deliberately has no Serialize impl, and that absence is the protection - so adding serialize_with is what unblocks the derive and turns a compile-time guarantee into plaintext output. Use it only where the plaintext is the point: a wire payload, a persisted config, an API response that exposes credentials by design.

So the default for a plugin config struct is derive Deserialize, but not Serialize. Deserialize is required: the SDK glue deserializes the config into the plugin's own struct (sdk/src/{sink,source}.rs call serde_json::from_str::<C> under a DeserializeOwned bound).

What never happens is the return trip. The runtime holds plugin configuration as a serde_json::Value - parsed from TOML, posted as JSON to the control API, or injected by env var - and hands that across the FFI, so nothing re-serializes the plugin's struct. Leaving Serialize off makes that compiler-enforced instead of convention-enforced (sources/http_source/src/lib.rs::HttpSourceConfig does this, and comments the omission so nobody adds it back).

Pattern:

use secrecy::{ExposeSecret, SecretString};

// `Deserialize` only. Nothing re-serializes a plugin config, and leaving
// `Serialize` off is what makes the credential unserializable rather than
// merely un-serialized.
#[derive(Debug, Clone, Deserialize)]
pub struct MyConfig {
    pub connection_string: SecretString,
}

// At call site:
let pool = PgPoolOptions::new()
    .connect(self.config.connection_string.expose_secret())
    .await?;

If a config struct genuinely needs Serialize, serde_secret::serialize_redacted (and serialize_optional_redacted) write [REDACTED] in place of the value. Reach for serialize_secret only when the caller must get the real thing back. The sinks and sources listed below predate that helper and use the exposing one; the annotation is inert today, but it is not the protection it looks like.

Note that none of this protects the credential from the runtime's own control API, which returns plugin configuration verbatim - see #3802. Plugin-side annotations are inert there because the runtime never routes through them.

Plugin-side uses of the exposing helpers: sinks/{postgres,mongodb,elasticsearch,influxdb,s3,surrealdb}_sink, sources/{postgres,elasticsearch,influxdb}_source.

That list is plugin-side only, not an inventory of every caller in the tree, and the others are not all mistakes: runtime/src/api/config.rs puts serialize_secret on HttpConfig::api_key (inert for the same reason), and several core/common wire-payload types (login, create-user, change-password, PAT) use these helpers by design, because there the credential is the payload.

Errors

• Use Error::PermanentHttpError / Error::SchemaMismatch / Error::CatalogCommitError for non-retryable failures. Returning a transient variant for bad data trips circuit breakers and hammers the backend.
• No unwrap() / expect() on Results from external I/O outside tests.

Forward-compat config

New TOML fields use #[serde(default)] or Option<T>. Adding a field must not break existing configs.

Benchmark mode (per-batch timing observability)

Every sink and source supports an opt-in benchmark: bool flag (SinkConfig/SourceConfig, also env IGGY_CONNECTORS_<TYPE>_<KEY>_BENCHMARK=true). Two independent surfaces:

1. Tracing events (only when flag is on) - one structured info! per batch under target iggy_connectors::benchmark with per-stage microsecond timings:

   • Sink: connector_type, connector_key, stream, topic, partition_id, current_offset, batch_size, processed_count, decode_us, prepare_us, ffi_us, total_us
   • Source: connector_type, connector_key, stream, topic, batch_size, sent_count, decode_us, prepare_us, iggy_send_us, state_saved, state_save_us, total_us (state_save_us is 0 when state_saved is false - emitted as flat number + bool, not an Option, so the JSON layer keeps it numeric)
   • Filter live: RUST_LOG=iggy_connectors::benchmark=info
   • Also emitted on startup: a one-shot "Benchmark mode enabled for ..." info log per connector.

2. Prometheus histograms (always on, regardless of flag):

   • Metric: iggy_connector_stage_duration_seconds{connector_key, connector_type, stage} (both labels snake_case)
   • stage (snake_case): decode, prepare, total on both sides; ffi (sink only); iggy_send, state_save (source only). decode and prepare mean the same thing on both sides - decode is schema decoding, prepare is transform + encode/serialize.
   • Buckets: see runtime/src/metrics.rs::STAGE_BUCKETS_SECONDS
   • Example: histogram_quantile(0.95, sum(rate(iggy_connector_stage_duration_seconds_bucket{stage="ffi"}[5m])) by (le, connector_key))

The flag only gates the verbose text event. the histogram observation is always on. Per-stage call path: Instant::now() + SipHash over StageLabels + parking_lot RwLock::read + HashMap probe + linear scan inside Histogram::observe. Benchmark on target hardware if you suspect the overhead matters at your batch rate.

iggy_connector_messages_filtered_total{connector_key, connector_type} counts intentional drops by transforms returning Ok(None), distinct from errors_total for unexpected drops. Both also exposed in /stats JSON (ConnectorStats.messages_filtered, .errors).

Implementation: runtime/src/benchmark.rs (text-event emitters + as_micros), runtime/src/metrics.rs::{Stage, SinkLabels, SourceLabels, observe_stage_with_labels} (pre-built label cache avoids per-batch String clones. &str-based wrappers are #[cfg(test)] only). Tests: runtime/src/metrics.rs::tests::given_stage_histogram_* (unit), core/integration/tests/connectors/runtime/benchmark.rs (integration: text emission, env override, disabled-flag, histograms via /metrics, JSON log format, parser unit tests).

Logging format

runtime/src/configs/runtime.rs::LoggingConfig exposes format = "text" | "json" (text default), env IGGY_CONNECTORS_LOGGING_FORMAT=json. runtime/src/log.rs::init_logging matches on (telemetry.enabled, format) to install the right fmt::layer()/fmt::layer().json() combined with the OpenTelemetry layer when telemetry is on. JSON applies to the stdout layer only - OTel export pipeline unaffected by this flag.

Exemplars

Read the relevant exemplar end-to-end before writing or modifying a connector.

Concrete efficiency patterns

Each implemented in at least one in-tree plugin or runtime path.

Drop accounting

• Filter drops (transform returning Ok(None)) bump messages_filtered.
• Every non-filter drop bumps errors. Inside the per-batch loops the count is accumulated in a local and flushed once after the loop via inc_errors_by_with_labels (filters likewise via inc_messages_filtered_with_labels) - one Family lookup per batch, not per message. One-shot drops outside a loop call inc_errors_with_labels directly. Hot-path calls always use the pre-built SinkLabels/SourceLabels cache. &str-based wrappers on Metrics are #[cfg(test)]-only.

Common review smells

• .clone() on Payload::Json or OwnedValue - use try_to_bytes(&self).
• &mut self on Sink::consume or Source::poll impls - won't compile, flag any creative workaround.
• std::sync::Mutex held across .await - swap for tokio::sync::Mutex.
• Missing [lib] crate-type = ["cdylib", "lib"] in plugin Cargo.toml.
• Source plugin without the four canonical state tests (see connector-testing).
• New silent message drop without a metric increment.
• Wrapping format!() around args passed to error!/warn!/info!/debug! - eager format! allocates even when level filters the line out. Pass args directly: error!("foo: {x}") or error!(error = %x, "foo").
• Logging a connection string, API key, or token.
• Plain String for a credential field - use SecretString.
• tokio::spawn inside plugin code - runtime owns lifecycle.
• std::time::SystemTime::now() in transforms - non-deterministic, breaks tests.

File map

core/connectors/
├── README.md
├── runtime/
│   ├── src/
│   │   ├── main.rs              FFI structs (SinkApi, SourceApi), plugin path resolution
│   │   ├── manager/{sink,source}.rs  Lifecycle, restart_guard, status transitions
│   │   ├── configs/             Local + HTTP config providers, ConfigEnv derive
│   │   ├── {sink,source,stream,transform,state,context}.rs
│   │   ├── benchmark.rs, log.rs, metrics.rs
│   │   └── api/                 HTTP control plane (/sinks, /sources, /stats, /metrics)
│   └── example_config/          Reference TOML
├── sdk/
│   └── src/
│       ├── lib.rs               Sink/Source traits, Payload, Schema, Error
│       ├── {sink,source}.rs     FFI containers + sink_connector!/source_connector! macros
│       ├── decoders/encoders/   Per-schema (json/raw/text/proto/flatbuffer/avro)
│       ├── transforms/          add/delete/update/filter fields, format conversions
│       └── retry.rs             CircuitBreaker, HttpRetryMiddleware
├── sinks/<name>_sink/
└── sources/<name>_source/

core/integration/tests/connectors/    # Real-infra integration tests
├── fixtures/                          # testcontainers-modules wrappers per backend
├── postgres/, elasticsearch/, mongodb/, iceberg/, influxdb/, quickwit/, http/, delta/
├── runtime/                           # error_isolation, benchmark
└── api/                               # HTTP API endpoint tests

Cite by symbol, not line number

Skills cite files by path + symbol (PostgresContainer::start in core/integration/tests/connectors/fixtures/postgres/container.rs). Line numbers go stale on the first refactor. symbol names survive. Grep the symbol if you need the exact spot.

Discussion / help: see AGENTS.md.