# Nakadion
A client for the [Nakadi](http://nakadi.io) Event Broker.
## Summary
`Nakadion` is client that connects to the Nakadi Subscription API. It
does all the cursor management so that users can concentrate on
implementing their logic for processing events. The code implemented
to process events by a user does not get in touch with the internals of Nakadi.
`Nakadion` processes batches of events from Nakadi on a worker per partition basis.
A worker new is spawned for each new partion discovered. These workers are guaranteed
to be run ona single thread at a time. To process batches of events a
handler factory has to be implemented which is creates handlers that are executed by the
workers.
`Nakadion` is almost completely configurable with environment variables.
Please have a look at the documentation of [Nakadi](http://nakadi.io)
first to become comfortable with the concepts of Nakadi.
## How to use
1. Do some imports
```rust
use nakadion::*;
use nakadion::auth::*;
```
2. Implement a `BatchHandler` that contains all your batch processing logic
```rust
// Use a struct to maintain state
struct MyHandler {
pub count: i32,
}
// Implement the processing logic by implementing `BatchHandler`
// Keep in mind that there is also `TypedBatchHandler` which can
// deserialize the events.
impl BatchHandler for MyHandler {
fn handle(&mut self, _event_type: EventType, _events: &[u8]) -> ProcessingStatus {
self.count += 1;
ProcessingStatus::processed_no_hint()
}
}
```
3. Implement a `HandlerFactory` that creates handlers for the workers.
```rust
// You could also maintain shared state in the `HandlerFactory`
struct MyHandlerFactory;
// Now we implement the trait `HandlerFactory` to control how
// our `BatchHandler`s are created
impl HandlerFactory for MyHandlerFactory {
type Handler = MyHandler;
fn create_handler(&self, _partition: &PartitionId)
-> Result<Self::Handler, CreateHandlerError> {
Ok(MyHandler{ count: 0 })
}
}
let handler_factory = MyHandlerFactory;
```
4. Configure `Nakadion` and the access token provider
```rust
// You only need this if you do not want to
// create this from the environment
use nakadion::api::SubscriptionRequest;
// This can be configured via environment variables
let subscription_discovery = SubscriptionDiscovery::Application(
SubscriptionRequest {
owning_application : "my_app".to_string(),
event_types: vec!["my_event_type".to_string()],
read_from: None,
}
);
// Create a builder and configure it
let builder = NakadionBuilder::default()
.nakadi_host("https://my.nakadi.net")
.subscription_discovery(subscription_discovery);
// We also need to tell Nakadion how to get tokens
let token_provider = NoAuthAccessTokenProvider;
```
5. Start Nakadion
```rust
// Start Nakadion
let nakadion = builder.build_and_start(handler_factory, token_provider).unwrap();
// Nakadion will stop once the binding `nakadion` runs out of scope.
// Nakadion can be cloned and also be stopped it manually
// You can also let Nakadion block the current thread until it stops.
nakadion.block_until_stopped();
```
## How Nakadion works
### Load balancing
A started instance connects to the Nakadi Event Broker with one active connection. Due to
Nakadi`s capability of automatically distributing partitions among clients Nakadion does
not need to track concurrently consuming clients. In most use cases it does not make
any sense to have more clients running than the number partitions assigned
to an event type.
### Consuming events
Nakadi delivers events in batches. Each batch contains the events of a single partition
along with a cursor that is used for reporting progress to Nakadi.
To consume events with `Nakadion` one has to implement a `BatchHandler`. This `BatchHandler`
provides the processing logic and is passed the bytes containing the events of a batch.
`Nakadion` itself does not do any deserialization of events. The `BatchHandler` is responsible
for deserializing events. Nevertheless there is a `TypedBatchHandler` for convinience
that does the deserialization of events using `serde`.
When `Nakadion` receives a batch it just extract the necessary data from
the bytes received over the network and then delagates the batch
to a dispatcher which spawns workers that are then passed the batch.
This means `Nakadion` itself does not have any knowledge of the events contained in a batch.
### Buffering batches and maximizing throughput
`Nakadion` has an unbounded buffer for events. When looking at how Nakadi works it turns
out that a bounded buffer is not necessary.
Nakadi has a timeout for committing the cursors of batches. This tiemout is 60 seconds.
Furthermore Nakadi has a configuration parameter called `max_uncommitted_events`.
With this paramteter which can be configured for `Nakadion` one can steer how many
events can be at most in `Nakadion`s buffers. In conjunction with a
`CommitStrategy` one can optimize for maximum throughput and keep the amount
of buffered events under control.
### Logging
`Nakadion` does verbose logging when connecting to a stream and when a stream is closed. The
reason is that this information can be quite important when probles arise. A reconnect
happens roughly every full hour unless configured otherwise on Nakadi's side.
`Nakadion` also logs a message each time a new worker is created and each time a worker is
shut down.
Otherwise `Nakadion` only logs problems and errors.
So in the end your log files will not be flodded with messages from `Nakadion`.
### Metrics
`Nakadion` provides an interface for attaching metrics libraries. Metrics are especially
useful when optimizing for maximum throughput since one can see what
effect (especially on cursors) the different possible settings have.
### Performance
Nakadion is not meant to be used in a high performance scenario. It uses synchronous IO.
Nevertheless it is easily possible to consume tens of thousands events per second depending
on the complexity of your processing logic.
## Recent Changes
* 0.11.2 more logging on workers
* 0.11.0 use reqwest 0.9
* 0.10.2
* update crate uuid to 0.7
* 0.10.1
* Event types must be an optional vec in the incoming metadata
* 0.10.0
* Improved typed `TypedHandler` to handle deserialization failures on individual events
* Updated metrix to 0.8
* 0.9.0
* Updated metrix to 0.7
## License
Nakadion is distributed under the terms of both the MIT license and the Apache License (Version
2.0).
See LICENSE-APACHE and LICENSE-MIT for details.
License: Apache-2.0/MIT