danube-client 0.14.0

# Danube Client

An async Rust client library for interacting with Danube Messaging platform.

🌊 [Danube Messaging](https://github.com/danube-messaging/danube) is an open-source **distributed** Messaging Broker platform written in Rust. Consult [the documentation](https://danube-docs.dev-state.com/) for supported concepts and the platform architecture.

## Features

### 📤 Producer Capabilities

- **Basic Messaging** - Send messages with byte payloads and optional key-value attributes
- **Key-Based Routing** - Tag messages with routing keys via `send_with_key()` for key-affinity dispatch
- **Partitioned Topics** - Distribute messages across multiple partitions for horizontal scaling
- **Reliable Dispatch** - Guaranteed message delivery with persistence (WAL + cloud storage)
- **Schema Integration** - Type-safe messaging with automatic validation (Bytes, String, Number, Avro, JSON Schema, Protobuf)

### 📥 Consumer Capabilities

- **Flexible Subscriptions** - Four subscription types for different use cases:
  - **Exclusive** - Single active consumer, guaranteed ordering
  - **Shared** - Load balancing across multiple consumers, parallel processing
  - **Failover** - High availability with automatic standby promotion
  - **Key-Shared** - Key-affinity routing with per-key ordering and optional key filtering
- **Message Acknowledgment** - Reliable message processing with at-least-once delivery
- **Partitioned Consumption** - Automatic handling of messages from all partitions
- **Batch Processing** - Efficient batch consumption for high-throughput scenarios
- **Message Attributes** - Access metadata and custom headers

### 🔐 Schema Registry

- **Schema Management** - Register, version, and retrieve schemas (JSON Schema, Avro, Protobuf)
- **Compatibility Checking** - Validate schema evolution (Backward, Forward, Full, None modes)
- **Type Safety** - Automatic validation against registered schemas
- **Schema Evolution** - Safe schema updates with compatibility enforcement
- **Startup Validation** - Validate consumer structs against schemas before processing

### 🏗️ Client Features

- **Async/Await** - Built on Tokio for efficient async I/O
- **Connection Pooling** - Shared connection management across producers/consumers
- **Automatic Reconnection** - Resilient connection handling
- **Topic Namespaces** - Organize topics with namespace structure (`/namespace/topic-name`)

## Example Usage

Check out the [example files](https://github.com/danube-messaging/danube/tree/main/danube-client/examples).

### Producer

```rust
let client = DanubeClient::builder()
    .service_url("http://127.0.0.1:6650")
    .build()
    .await?;

let topic_name = "/default/test_topic";
let producer_name = "test_prod";

let mut producer = client
    .new_producer()
    .with_topic(topic_name)
    .with_name(producer_name)
    .build()?;

producer.create().await?;
println!("The Producer {} was created", producer_name);

let encoded_data = "Hello Danube".as_bytes().to_vec();

let message_id = producer.send(encoded_data, None).await?;
println!("The Message with id {} was sent", message_id);
```

### Consumer

```rust
let client = DanubeClient::builder()
    .service_url("http://127.0.0.1:6650")
    .build()
    .await?;

let topic = "/default/test_topic";
let consumer_name = "test_cons";
let subscription_name = "test_subs";

let mut consumer = client
    .new_consumer()
    .with_topic(topic)
    .with_consumer_name(consumer_name)
    .with_subscription(subscription_name)
    .with_subscription_type(SubType::Exclusive)
    .build()?;

// Subscribe to the topic
consumer.subscribe().await?;
println!("The Consumer {} was created", consumer_name);

// Start receiving messages
let mut message_stream = consumer.receive().await?;

while let Some(message) = message_stream.recv().await {
    let payload = message.payload;

    match String::from_utf8(payload) {
        Ok(message_str) => {
            println!("Received message: {:?}", message_str);

            consumer.ack(&message).await?;
        }
        Err(e) => println!("Failed to convert Payload to String: {}", e),
    }
}
```

### Key-Shared Producer & Consumer

Key-Shared subscriptions route messages by key — all messages with the same routing key are delivered to the same consumer, guaranteeing per-key ordering while distributing load across consumers.

```rust
// Producer: tag each message with a routing key
let mut producer = client
    .new_producer()
    .with_topic("/default/orders")
    .with_name("orders_producer")
    .with_reliable_dispatch()
    .build()?;

producer.create().await?;

// All "payment" messages go to the same consumer
producer.send_with_key(data, None, "payment").await?;
producer.send_with_key(data, None, "shipping").await?;
```

```rust
// Consumer: automatic key distribution (no filters)
let mut consumer = client
    .new_consumer()
    .with_topic("/default/orders")
    .with_consumer_name("worker_1")
    .with_subscription("orders_sub")
    .with_subscription_type(SubType::KeyShared)
    .build()?;

consumer.subscribe().await?;

// Consumer with key filters: only receive matching keys
let mut payments_consumer = client
    .new_consumer()
    .with_topic("/default/orders")
    .with_consumer_name("payments_worker")
    .with_subscription("orders_sub")
    .with_subscription_type(SubType::KeyShared)
    .with_key_filter("payment")    // exact match
    .with_key_filter("invoice-*")  // glob pattern
    .build()?;

payments_consumer.subscribe().await?;
```

## Advanced Features

For detailed guides and examples on advanced capabilities:

- **[Producer Advanced Features](https://danube-docs.dev-state.com/client_libraries/producer-advanced/)** - Partitions, reliable dispatch, and schema integration
- **[Consumer Advanced Features](https://danube-docs.dev-state.com/client_libraries/consumer-advanced/)** - Partitioned consumption, failover patterns, and batch processing
- **[Schema Registry Integration](https://danube-docs.dev-state.com/client_libraries/schema-registry/)** - Type-safe messaging with schema validation and evolution

Browse the [examples directory](https://github.com/danube-messaging/danube/tree/main/danube-client/examples) for complete working code.

## Contribution

Check [the documentation](https://danube-docs.dev-state.com/) on how to setup a Danube Broker.