# Nakadion
** THIS IS A PREVIEW VERSION, BREAKING CHANGES WITHOUT FURTHER NOTICE **
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` is almost completely configurable from environment variables.
Please have a look at the documentation of [Nakadi](http://nakadi.io)
first to become comfortable with the concepts of Nakadi.
Currently `Nakadion` only works with the `tokio` runtime. Further execution
environments might be added in the future.
## How to use
To run this example the following environment variables need to be set:
* `NAKADION_NAKADI_BASE_URL`
* `NAKADION_SUBSCRIPTION_ID`
* `NAKADION_ACCESS_TOKEN_FIXED` with a valid token or `NAKADION_ACCESS_TOKEN_ALLOW_NONE` set to `true`
```rust
use nakadion::api::ApiClient;
use nakadion::consumer::*;
#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
let client = ApiClient::builder().finish_from_env()?;
let consumer = Consumer::builder_from_env()?.build_with(
client,
handler::MyHandlerFactory,
DevNullLogger,
)?;
let (_handle, consuming) = consumer.start();
// `consuming` will complete once the `Consumer` stopped
// consuming events
consuming.await.into_result().map(|_| ())?;
Ok(())
}
mod handler {
use futures::future::{BoxFuture, FutureExt};
use nakadion::handler::*;
pub struct MyHandler {
count: usize,
}
impl BatchHandler for MyHandler {
fn handle<'a>(
&'a mut self,
_events: Bytes,
meta: BatchMeta<'a>,
) -> BoxFuture<'a, BatchPostAction> {
self.count += 1;
async move {
println!("{}", self.count);
BatchPostAction::commit_no_hint()
}
.boxed()
}
}
pub struct MyHandlerFactory;
impl BatchHandlerFactory for MyHandlerFactory {
type Handler = MyHandler;
fn handler(
&self,
_assignment: &HandlerAssignment,
) -> BoxFuture<Result<Self::Handler, Error>> {
async { Ok(MyHandler { count: 0 }) }.boxed()
}
}
}
```
## 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 `EventsHandler` 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 defaul timeout is 60 seconds.
Furthermore Nakadi has a configuration parameter called `max_uncommitted_events`.
With this parameter 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 problems 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` mostly only logs problems and errors.
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 asynchronous IO.
Nevertheless it is easily possible to consume tens of thousands events per second depending
on the complexity of your processing logic.
## Recent Changes
See CHANGELOG
## 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