The Aeron Context is used to configure various settings related to Aeron, including clustering. In order to use Aeron clustering, you need to set up a cluster by configuring a Cluster Node and a Consensus Module, and then connecting to the cluster from your Aeron client applications. ## How do I achieve clustering? ### Configure the Cluster Node 1. Each node in the Aeron Cluster needs to be configured with the same set of properties in order to properly communicate with each other. These properties include things like the cluster members, the consensus module endpoint, the cluster node ID, and more. You can configure these properties using the **aeron.cluster** system properties or via an **Aeron.Context** instance. 2. You can configure the Cluster Node by setting the **aeron.cluster.** properties in the Aeron Context. These properties include the cluster members, cluster node role, cluster node ID, and other settings related to the cluster. You can also set the **aeron.cluster.client.heartbeatTimeoutNs** property to define the heartbeat timeout for clients connecting to the cluster ### Configure the Consensus Module 1. The Consensus Module is a special node in the Aeron Cluster that manages the consensus algorithm used to determine the cluster leader. It is important to start the Consensus Module before starting any other nodes in the cluster. You can start the Consensus Module by running the **io.aeron.cluster.ConsensusModule** class. 2. You can configure the Consensus Module by setting the **aeron.cluster.consensus.** properties in the Aeron Context. These properties include the consensus module's communication settings, including the multicast endpoint, the consensus module's ID, and the maximum number of active sessions. ### Connect to the Cluster 1. To connect to the cluster from your Aeron client applications, you need to create an instance of **AeronCluster** using the **AeronCluster.connect()** method. This method takes an instance of **Aeron.Context** as a parameter, which allows you to configure the Aeron Context with the cluster-specific settings described above. 2. Once the nodes have started up, you can verify that they have successfully joined the cluster by checking the logs for any errors or warnings. You can also use the **AeronCluster.isLeader()** method to determine which node is currently the leader of the cluster. Once you have connected to the cluster, you can use the **AeronCluster** API to send messages and perform other operations within the context of the cluster. For example, you can use the **offer()** method to publish a message to the cluster, or use the **pollEgress()** method to receive messages from the cluster. ### How can I use the Aeron Media Driver via Scala, `cats`, and `fs2` ? ```scala import fs2._ import fs2.concurrent.Queue import io.aeron.Aeron import io.aeron.Publication import io.aeron.Subscription import io.aeron.driver.MediaDriver object AeronMessagingExample { def main(args: Array[String]): Unit = { // Set up a media driver and Aeron context for the first node val driver1 = MediaDriver.launchEmbedded() val ctx1 = new Aeron.Context() // Set up a media driver and Aeron context for the second node val driver2 = MediaDriver.launchEmbedded() val ctx2 = new Aeron.Context() // Create a publisher and subscriber for a channel on the first node val channel = "aeron:udp?endpoint=localhost:40123" val publication1: Publication = Aeron.connect(ctx1).addPublication(channel, 10) val subscription1: Subscription = Aeron.connect(ctx1).addSubscription(channel, 10) // Create a publisher and subscriber for a channel on the second node val publication2: Publication = Aeron.connect(ctx2).addPublication(channel, 10) val subscription2: Subscription = Aeron.connect(ctx2).addSubscription(channel, 10) // Join the first node to the cluster ctx1.aeronDirectoryName(driver1.aeronDirectoryName()) ctx1.clusterMemberEndpoints("localhost:20121,localhost:20122") ctx1.clusterMemberId(0) // Join the second node to the cluster ctx2.aeronDirectoryName(driver2.aeronDirectoryName()) ctx2.clusterMemberEndpoints("localhost:20121,localhost:20122") ctx2.clusterMemberId(1) // Create a queue to handle incoming messages val queue: Queue[Task, String] = Queue.unbounded[Task, String] // Define the message handling logic val messageHandler: FragmentAssembler = new FragmentAssembler((buffer, offset, length, header) => { val message = new String(buffer.byteArray(), offset, length) queue.enqueue1(message).unsafeRunSync() }) // Start the subscriber Stream .repeatEval(Task(subscription1.poll(messageHandler, 10))) .merge(Stream.repeatEval(Task(subscription2.poll(messageHandler, 10)))) .repeat .compile .drain .unsafeRunAsync(_ => ()) // Start the publisher Stream("Hello, World!", "How are you today?", "Goodbye!") .evalMap(msg => Task(if (msg.hashCode % 2 == 0) publication1 else publication2).flatMap(_.offer(msg))) .compile .drain .unsafeRunSync() // Print out the messages received by the subscriber queue.dequeue.take(3).compile.toList.unsafeRunSync().foreach(println) // Clean up resources subscription1.close() publication1.close() subscription2.close() publication2.close() Aeron.connect(ctx1).close() Aeron.connect(ctx2).close() driver1.close() driver2.close() } } ``` ### What is the Aeron Media Driver? The Aeron Media Driver is a component of the Aeron messaging system that provides the underlying transport layer for message comminication. It is responsible for handling the low-level details of communication such as network protocol processing, data framing, flow control, and congestion control The media Driver is designed to be highly effiecient and scalable and is intended to be used in high-performance, low-latency applications that require hight-throughput and low message-delivery latency. It achieves this by minimizing the overhead associated with message transmission and by leeraging advanced networking thechniques such as kernel bypass and zero-copy messaging. The Aeron Media Driver is typically run as a separate process or as and embedded component within an application. It provided hughly configurable and customizable environment for message communication. It supports a variety of network protocols including UDP and IPC, and provides advanced features such as message fragmentation, message batchming, and congestion control. #### Memory allocation: The media driver is responsible for allocating memory on the system, which is used to store messages that are sent between nodes. The media driver uses a technique called "zero copy" to minimize the amount of data copying that occurs when messages are transmitted. #### Network communication: The media driver is responsible for managing the communication between different nodes in the system. It handles low-level networking protocols, such as TCP and UDP, to ensure that messages are transmitted quickly and reliably. #### Flow control: The media driver is also responsible for managing the flow of messages between nodes in the system. It ensures that nodes are not overwhelmed with messages and can process them at a reasonable rate. #### Data serialization: The media driver can also handle serialization of data, which is the process of converting data from one format to another for transmission over the network. This can be useful for ensuring compatibility between different systems and programming languages. ### How does the Aeron Cluster work in a compute cluster like Kubernetes? When running Aeron in a Kubernetes cluster, the first step is to set up the cluster with the necessary infrastructure, including the required resources for the media drivers, which are responsible for handling the low-level networking and transport protocols. To add nodes to the Aeron cluster, you will need to deploy the media drivers as Kubernetes pods, either as standalone pods or as part of a Kubernetes deployment. Once the media driver pods are up and running, you can configure your Aeron application to connect to them. To ensure that your Aeron cluster is highly available and fault-tolerant, it's important to ensure that the media driver pods are distributed across multiple nodes in the Kubernetes cluster. Kubernetes provides various tools and features to manage the distribution of pods across nodes, such as node affinity and anti-affinity rules. Additionally, you may want to consider using Kubernetes StatefulSets to manage your Aeron nodes, which provide a stable network identity for the pods and ensure that they are scheduled onto the same nodes if they are ever restarted. When you scale up your Aeron cluster by adding new nodes, you need to register the new media drivers with the existing Aeron cluster. This process is done automatically by Aeron using a discovery mechanism that allows nodes to discover and connect to each other. The Aeron cluster discovery mechanism works by using multicast messaging to advertise the location of the media drivers to other nodes in the cluster. When a new media driver pod is scheduled, it sends out a multicast message to advertise its presence, and other nodes in the cluster receive this message and connect to the new pod. To make sure that all nodes in the Aeron cluster can discover each other, you need to ensure that the media driver pods are all configured to use the same multicast address and port. This allows the multicast messages to be sent and received across all nodes in the cluster. Multicast traffic is a type of network traffic where a single packet is sent from a source device to multiple destination devices simultaneously. This is in contrast to unicast traffic, where a packet is sent from a source device to a single destination device, and broadcast traffic, where a packet is sent from a source device to all devices on the network. By default, Kubernetes doesn't support multicast traffic, so you need to configure your network topology to allow it. One way to do this is to configure your Kubernetes nodes and pods to use a special multicast-enabled network interface called a multicast-enabled overlay network. A multicast-enabled overlay network is a type of network overlay that supports multicast traffic. It provides a logical network that runs on top of the underlying physical network, and allows pods on different nodes to communicate with each other using multicast traffic. There are several options for implementing a multicast-enabled overlay network in Kubernetes, such as using a third-party network plugin like Calico, Flannel, or Weave Net. These plugins provide built-in support for multicast traffic, and can be used to configure the network topology of your Kubernetes cluster to allow Aeron's discovery mechanism to work. Another option is to configure multicast routing directly on your Kubernetes nodes using tools like iptables or iproute2. This requires more manual configuration and is generally more complex, but can provide more fine-grained control over the multicast traffic in your cluster. Additionally, you can configure the Aeron cluster to use unicast messaging instead of multicast messaging, which can be more efficient in some cases. With unicast messaging, each node maintains a list of the IP addresses and ports of the other nodes in the cluster, and sends messages directly to each other rather than broadcasting them to all nodes. To sum up, the process of registering new media drivers in a scaled-up Aeron cluster is automatic, and relies on the Aeron cluster discovery mechanism, which uses either multicast or unicast messaging to allow nodes to discover and connect to each other. ### How can I achieve sharding in Aeron and Kubernetes? The general recommendation is to [avoid sharding where possible](https://aeroncookbook.com/aeron-cluster/on-sharding/), but if we must, we might be able to follow these general steps: 1. Set up a Kubernetes cluster: Deploy Aeron nodes as Kubernetes pods in a cluster to form a distributed messaging system. This provides the underlying infrastructure for message exchange between nodes. 2. Define topics or channels: Define the topics or channels you want to use for sharding your data streams. For example, you might have different channels for different types of messages, or different partitions of a large stream. 3. Assign partitions to nodes: Use Kubernetes labels to assign specific partitions or channels to specific nodes or groups of nodes. You can use tools like Kubernetes StatefulSets or DaemonSets to manage the distribution of nodes. 4. Configure Aeron nodes: Configure each Aeron node to subscribe to the appropriate channels or partitions. You can use the Aeron Configuration file to specify the subscriptions for each node. 5. Publish messages: Publish messages to the appropriate channels or partitions, and let Aeron handle the distribution of messages to the appropriate nodes. ### Summary Overall, the Aeron Context is a powerful tool that allows you to configure Aeron to work within a clustered environment, and provides a high-performance messaging infrastructure that can scale to meet the demands of large-scale distributed systems. Deploying Aeron in a Kubernetes cluster requires careful planning and configuration to ensure that the messaging system operates efficiently and reliably in a distributed environment. To support Aeron's multicast-based discovery mechanism in a Kubernetes cluster, you need to configure your network topology to allow multicast traffic. This can be done using a multicast-enabled overlay network, a third-party network plugin, or by configuring multicast routing directly on your Kubernetes nodes.