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# 13. Example Reference Systems

The OpenSplice middleware can be deployed for different kinds of systems. This section identifies several different systems that will be used as reference systems throughout the rest of this manual. Each needs to be configured differently in order to fit its requirements. The intention of this section is to give the reader an impression of the possible differences in system requirements and the configuration aspects induced.

## 13.1. Zero Configuration System

The OpenSplice middleware comes with a default configuration file that is intended to give a satisfactory out-of-the-box experience. It suits the standard situation of a system containing a handful of nodes with only a few applications per node (enabling standalone ‘single-process’ deployment) and where requirements on data distribution latencies, volumes and determinism are not too demanding (enabling use of the standard DDSI networking service).

Starting and running any systems that satisfy these conditions should not be a problem. Nodes can be started and shut down without any extra configuration because the default discovery mechanism will keep track of the networking topology.

## 13.2. Single Node System

Systems that have to run as a federation on a single node can be down-scaled considerably by not starting the networking and durability daemons. The networking daemon is obviously not needed because its responsibility is forwarding data to and from the network, which is not present. The durability daemon is not needed because the OpenSplice libraries themselves are capable of handling durable data on a single node.

Note that this is not the case for single process deployments. Multiple single process applications that are running on the same machine node can only communicate when there is a networking service running within each process. This is because there is no shared administration between the applications, unlike for the shared memory deployments when a networking service is not required for a single node system.

With a single node system, the OpenSplice services do not have much influence on application behaviour. The application has full control over its own thread priorities and all OpenSplice activities will be executed in the scope of the application threads.

One exception to this is the listener thread. This thread is responsible for calling listener functions as described in the DDS specification.

## 13.3. Medium Size Static (Near) Real-time System

Many medium size systems have highly demanding requirements with respect to data distribution latencies, volumes and predictability. Such systems require configuration and tuning at many levels. The OpenSplice middleware will be an important player in the system and therefore is highly configurable in order to meet these requirements. Every section reflects on an aspect of the configuration.

### 13.3.1. High Volumes

The OpenSplice middleware architecture is designed for efficiently transporting many small messages. The networking service is capable of packing messages from different writing applications into one network packet. For this, the latency budget quality of service should be applied. A latency budget allows the middleware to optimise on throughput. Messages will be collected and combined during an interval allowed by the latency budget. This concerns networking traffic only.

A network channel that has to support high volumes should be configured to do so. By default, the `Resolution` parameter is set to `50 ms`. This means that latency budget values will be truncated to multiples of 50 ms, which is a suitable value. For efficient packing, the `FragmentSize` should be set to a large value, for example `8000`. This means that data will be sent to the network in chunks of 8 kilobytes. A good value for `MaxBurstSize` depends on the speed of the attached network and on the networking load. If several writers start writing simultaneously at full speed during a longer period, receiving nodes could start losing packets. Therefore, the writers might need to be slowed down to a suitable speed.

Note that message with a large latency budget might be overtaken by messages with a smaller latency budget, especially if they are transported via different networking channels.

### 13.3.2. Low Latencies

If messages are to be transported with requirements on their end to end times, a zero latency budget quality of service should be attached. This results in an immediate wake-up of the networking service at the moment that the message arrives in a networking queue. For optimal results with respect to end-to-end latencies, the thread priority of the corresponding networkChannel should be higher than the thread priority of the writing application. with the current implementation, a context switch between the writing application and the networking channel is always required. With the correct priorities, the induced latency is minimized.

The value of the `Resolution` parameter has its consequences for using latency budgets. The networking service will ignore any latency budgets that have a value smaller than Resolution.

The effect of sending many messages with a zero latency budget is an increase of CPU load. The increasing number of context switches require extra processing. This quality of service should therefore be consciously used.

### 13.3.3. Responsiveness

Especially with respect to reliable transport over the network, responsiveness is an important aspect. Whenever a reliably sent message is lost on the network, the sending node has to initiate a resend. Since OpenSplice networking uses an acknowledgement protocol, it is the up to the sending side to decide when to resend a message. This behaviour can be tuned.

First of all, the `Resolution` parameter is important. This parameter gives the interval at which is checked if any messages have to be resent. The `RecoveryFactor` parameter indicates how many of these checks have to be executed before actually resending a message. If Resolution is scaled down, messages will be resent earlier. If Recovery factor is scaled down, messages will be resent earlier as well.

### 13.3.4. Topology Discovery

OpenSplice RT-Networking Service implements a discovery protocol for discovering other nodes in the system. As long as only one node is present, nothing has to be sent to the network. As soon as at least two nodes are present, networking starts sending data to the network. The node-topology detection also allows for quick reaction to topology changes, such as when a publishing node disappears (due to a disconnection or a node crash); a ‘tightly-configured’ discovery allows for swift detection of such topology changes, and related updating of DDS-level liveliness administration.

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