Adding a New Apache Iggy Connector Transform

A transform implements iggy_connector_sdk::transforms::Transform. It runs after decode (for sinks) or before encode (for sources), in a chain configured per-connector. Config-driven, composable.

Universal connector rules live in connectors-overview. This skill covers transform authoring only.

Contents

• STOP and ask the user before
• File map
• The trait
• Steps to add a new transform
• Rules
• Common pitfalls
• Before declaring done

STOP and ask the user before

• Adding a new transform that mutates payload SHAPE (renaming PK fields, dropping schema-required columns) - breaks downstream sinks silently.
• Introducing non-determinism (system time, RNG without seed) - breaks tests + replay.
• Changing the Transform trait signature (sync vs async, return type) - cascades to every transform.
• Making transforms hold mutable runtime state - they're constructed once and Arc-cloned across tasks. mutation requires interior locking and is rarely what you actually want.

File map

sdk/src/transforms/
├── mod.rs                 Transform trait, TransformType enum, from_config() factory,
│                          FieldValue + ComputedValue helpers
├── add_fields.rs          AddFields - append static or computed fields to JSON
├── delete_fields.rs       DeleteFields - remove fields by key
├── update_fields.rs       UpdateFields - conditional field replacement
├── filter_fields.rs       FilterFields - keep/drop messages by key/value pattern
├── unwrap_envelope.rs     UnwrapEnvelope - promote nested field to top-level payload
├── proto_convert.rs       ProtoConvert - bidirectional Proto ⇄ Json/Text/Raw/etc.
├── flatbuffer_convert.rs  FlatBufferConvert
├── avro_convert.rs        AvroConvert
└── json/                  Shared JSON helpers (path traversal, ops)

The trait

// sdk/src/transforms/mod.rs - Transform
pub trait Transform: Send + Sync {
    fn r#type(&self) -> TransformType;
    fn transform(
        &self,
        metadata: &TopicMetadata,
        message: DecodedMessage,
    ) -> Result<Option<DecodedMessage>, Error>;
}

Semantics:

• &self - transforms are immutable post-construction. Internal state (compiled regex, parsed paths, schema descriptors) is computed in new() and stored as plain struct fields. No locks needed - the whole instance is shared via Arc<dyn Transform>.
• Takes message by value - the transform owns the DecodedMessage and may mutate freely without cloning.
• Returns Ok(Some(msg)) to pass through (possibly modified), Ok(None) to filter (drop silently), Err(e) for actual failures.
• &TopicMetadata is for routing-aware logic. Most transforms ignore it.

Steps to add a new transform

1. Create the file

sdk/src/transforms/my_transform.rs. Apache 2.0 license header. Code reads top to bottom - config struct, then transform struct, then impl, then helpers.

/* Apache 2.0 header */

use crate::transforms::{Transform, TransformType};
use crate::{DecodedMessage, Error, Payload, TopicMetadata};
use serde::{Deserialize, Serialize};

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct MyTransformConfig {
    pub key: String,
    pub option: Option<bool>,
}

pub struct MyTransform {
    config: MyTransformConfig,
}

impl MyTransform {
    pub fn new(config: MyTransformConfig) -> Result<Self, Error> {
        if config.key.is_empty() {
            return Err(Error::InvalidConfigValue(
                "my_transform.key must not be empty".into(),
            ));
        }
        Ok(Self { config })
    }
}

impl Transform for MyTransform {
    fn r#type(&self) -> TransformType {
        TransformType::MyTransform
    }

    fn transform(
        &self,
        _metadata: &TopicMetadata,
        mut message: DecodedMessage,
    ) -> Result<Option<DecodedMessage>, Error> {
        // Most transforms operate only on Payload::Json - non-JSON is a no-op.
        let Payload::Json(ref mut value) = message.payload else {
            return Ok(Some(message));
        };

        // Mutate `value` in place via simd_json::OwnedValue API.
        // Return Ok(None) to filter, Ok(Some(message)) to pass through.

        Ok(Some(message))
    }
}

2. Wire into transforms/mod.rs

Three edits, all required:

(a) Module declaration + re-export at the top:

mod my_transform;
pub use my_transform::{MyTransform, MyTransformConfig};

(b) Discriminant in TransformType:

pub enum TransformType {
    AddFields,
    // ...
    MyTransform,   // snake_case form in TOML: "my_transform"
}

(c) Branch in from_config():

TransformType::MyTransform => {
    // For new transforms, prefer Error::InvalidConfigValue with formatted context -
    // it preserves the underlying serde_json error. Existing code uses bare
    // Error::InvalidConfig for most variants (UnwrapEnvelope is the exception).
    let cfg: MyTransformConfig = serde_json::from_value(raw.clone())
        .map_err(|error| Error::InvalidConfigValue(format!("my_transform: {error}")))?;
    Ok(Arc::new(MyTransform::new(cfg)?))
}

from_config() returns Arc<dyn Transform> so the same instance is shared across the pipeline.

3. Document

Add a TOML usage example to sdk/README.md:

[transforms.my_transform]
enabled = true
key = "some_field"
option = true

4. Tests

Bottom of my_transform.rs:

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn given_matching_field_should_transform() { ... }

    #[test]
    fn given_non_json_payload_should_passthrough() { ... }

    #[test]
    fn given_filter_condition_should_drop_message() { /* assert Ok(None) */ }

    #[test]
    fn given_invalid_config_should_error_in_new() {
        let cfg = MyTransformConfig { key: "".into(), option: None };
        assert!(matches!(MyTransform::new(cfg), Err(Error::InvalidConfigValue(_))));
    }

    fn topic_meta() -> TopicMetadata {
        TopicMetadata { stream: "s".into(), topic: "t".into() }
    }

    fn json_message(json: serde_json::Value) -> DecodedMessage { /* ... */ }
}

Rules

Operate on JSON only (mostly)

Most field-level transforms only make sense for Payload::Json. Convention (see add_fields.rs::AddFields::transform): silently pass through non-JSON:

let Payload::Json(ref mut value) = message.payload else {
    return Ok(Some(message));
};

Format-conversion transforms (proto_convert, avro_convert, flatbuffer_convert) are the exception - they explicitly convert between variants.

Mutate OwnedValue in place, don't clone-then-replace

simd_json::OwnedValue is mutable in place:

if let OwnedValue::Object(ref mut obj) = value {
    obj.insert(key.into(), new_value);
}

Do NOT clone the value, mutate the clone, then message.payload = Payload::Json(new). The whole point of &mut value is in-place mutation.

Filter semantics

Ok(None) drops the message silently - no error log, no metric. By design (filters succeed when they decide to drop). For visibility, log at debug! level.

Config validation

Validate in new() and return Err(Error::InvalidConfigValue(...)). Don't panic!. Don't validate on the hot path.

For transforms with regex or compiled patterns, compile once in new() and store the compiled form.

Stateless except for config

Transforms are constructed once at startup and Arc-cloned across tasks. Must be Send + Sync. No mutable runtime state - if you need accumulator-like behavior, that's not a Transform, that's a sink.

Compiled state goes in the struct, not behind a lock

Things derived from config (compiled regex, parsed paths, schema descriptors) should be computed in new() and stored as plain struct fields. The whole instance is shared via Arc<dyn Transform>, so per-field locking is unnecessary. See sdk/src/transforms/filter_fields.rs::FilterFields::new for the canonical pattern: regexes compile in new(), get stored as struct fields, are then read via &self.

transform() is sync

The trait method is sync (not async). Don't propose making it async - that would require pervasive change across the runtime. Schemas and other external resources should be loaded in new() (potentially via a fallible try_new), not lazily during transform().

Errors

Returning Err from transform() stops the message and is logged. For non-fatal field-level issues (field missing when transform expects it), prefer pass-through (Ok(Some(message))) unless config explicitly says to fail.

Common pitfalls

1. Forgetting one of the three mod.rs edits - module declaration, TransformType variant, from_config() branch.
2. #[serde(rename_all = "snake_case")] mismatch between enum variant name and TOML key.
3. Cloning OwnedValue when in-place mutation would work.
4. Validating config inside transform() instead of new().
5. std::sync::Mutex for internal state on the hot path - precompute in new() and store as plain struct fields. Transforms run on every message. locks are wasted work.
6. Returning Err for a filter - filters use Ok(None).

Before declaring done

cargo fmt --all
cargo sort --no-format --workspace
cargo clippy -p iggy_connector_sdk --all-targets --all-features -- -D warnings
cargo test -p iggy_connector_sdk --all-features

Verify by adding the transform to core/connectors/runtime/example_config/connectors/ and running the runtime - the transform should apply to each message.

If the transform changes payload shape sufficiently to break downstream sinks (e.g., renaming primary keys), call it out in the PR description.

Discussion / help: see AGENTS.md.