# Basics, Terms and Concepts

## Basics

### What is EPICS?

[https://docs.epics-controls.org/en/latest/guides/EPICS_Intro.html][epics-intro]

[https://epics-controls.org/][epics-web]

### What is OPC UA?

OPC Unified Architecture (OPC UA) is a cross-platform,
open-source,
IEC62541 standard for data exchange from sensors to cloud applications
developed by the [OPC Foundation][opc-foundation].

The OPC UA protocol has become increasingly popular
with vendors of controllers (like PLCs) and sensors
as a very powerful integration tool.
It allows connecting almost any device to almost any SCADA client,
across vendors, with a certification mechanism that ensures compatibility.

OPC UA integrates all functionality of the individual specifications
of the former OPC Classic protocol into one extensible framework,
providing:

*   _Functional equivalence_: 
    all COM OPC Classic specifications are mapped to UA

*   _Platform independence_:
    from embedded to cloud-based infrastructure

*   _Security_:
    encryption, authentication, and auditing

*   _Extensibility_:
    ability to add new features without affecting existing applications

*   _Information modelling_:
    for defining complex information

OPC UA is designed for exchanging information in an object-oriented manner,
rather than as isolated data points.
This increases the accessibility of plant floor data
by allowing to re-use information stored in a common object.

The [Wikipedia page on OPC UA][opcua-wiki] has a good overview
of its characteristics, history and specification.

The complete [OPC UA specification][opcua-spec] is freely available
on the OPC Foundation web site.

## OPC UA Terms and Concepts

### Node

A _Node_ is a uniquely identified piece of information on the server.
The OPC information model is a mesh network based on nodes.
These nodes are objects holding process data or any kind of meta information,
such as attributes, references, methods, types, ...

### NodeID

A _NodeID_ is an identifier for a node, e.g., for a particular OPC UA item.
A Node ID consists of:
*   a namespace index number
*   an identifier type
*   an identifier

The identifier can be either a string identifier, consisting of a name string,
or a numerical identifier, consisting of an integer.
Whenever a client requests to either read or write data to an OPC UA item,
it identifies the node by means of its NodeID.

### Item

Within the Data Access part of OPC UA,
an _Item_ is any piece of accessible process data on a server.
It can represent a single data point or a block of structured data.
Elements of structured data can be simple data points
or other structured items.

### Data Element

In the context of this Device Support only
(i.e., not part of the OPC UA specification),
a _Data Element_ is a unique data point belonging to a server-side item.
If that item is of basic or array type, the data element corresponds
to the value of the item.
If the item is of a structured type, the data element corresponds
to one element of the item's structure.

### Namespace

A _Namespace_ is one of multiple server-side scopes for node identifiers,
which the server uses to avoid name clashes.

A namespace is uniquely defined by an URI.
To improve performance, the server keeps a table of the URIs
and the index in that table is used between client and server.
Some namespaces (and their index numbers)
are reserved by the OPC UA specification for the protocol and the server.

### Channel

In OPC UA, a _Channel_ is the low-level transport connection
between a client and a server.
(The OPC UA Device Support module only supports the TCP transport.)
A channel can be secured by running it through SSL to be encrypted,
and/or by signing all messages that are sent through it.

### Session

A _Session_ is a named connection
between an OPC UA client and an OPC UA server,
using the transport provided by the channel.
Typically, there is one session between each client and each server.
While it is possible to have multiple sessions
between a single client/server pair,
this increases resource usage on both ends and is generally not recommended
unless there is a particular justifying reason
(such as making use of parallel processing).

### Subscription

A _Subscription_ is a mechanism within a session
that the client uses to retrieve value updates from a server
without the need to poll regularly.
A client establishes a subscription within a session and adds items to it,
requesting that the server notify the client
whenever data of the _Monitored Items_ within that subscription changes.

When defining a subscription,
the client can specify a _publishing interval_ for the server,
telling it how often it would like to receive updated data.
For each monitored item that is part of a subscription,
the client can also specify its _sampling interval_,
which defines the rate at which the server checks the data source for changes.
If the OPC UA server supports server-side queueing of data samples,
the client can also specify for each monitored item the size of that queue.

## The OPC UA Device Support

EPICS applications run processes called IOCs,
which interface device data to the EPICS Process Database,
which provides it to the control system through the EPICS network protocols,
Channel Access and pvAccess.

The Device Support layer enables records in the EPICS Process Database
to communicate with different device types.
The OPC UA Device Support is a client of the OPC UA protocol,
which can read, write and monitor variables residing on an OPC UA server.
(The relevant part of the OPC UA specification is called "Data Access".)

If your IOC is built correctly (see [Building an IOC](building-an-ioc)),
the OPC UA Device Support is ready to be used.
You can set up OPC UA sessions and subscriptions from the IOC startup script
through commands in the IOCshell
and link EPICS database records to remote OPC UA variables
through the configuration of your EPICS records.

To support commissioning and troubleshooting, additional IOCshell commands
allow introspection of the Device Support's internal data structures.
The regular EPICS trace and debug modes can be used to investigate
the OPC UA operations at run time.

### Addressing Basic/Array and Structured Types

If an OPC UA item is of basic or array type,
a single EPICS data record connects to it.
That record is configured with the session or subscription to be used,
the nodeID of the item and options that apply to the item level.

If an OPC UA item is of a structured type, a single instance
of the special opcuaItemRecord type represents the complete structure.
Many EPICS data records each connect to a single structure element.
The opcuaItemRecord is configured with all item level configuration.
The single element data records refer to the opcuaItemRecord
and specify the address of the strcuture element they connect to
as well as data element level options.

% external links

[epics-intro]: https://docs.epics-controls.org/en/latest/guides/EPICS_Intro.html
[epics-web]: https://epics-controls.org/

[opc-foundation]: https://opcfoundation.org/
[opcua-wiki]: https://en.wikipedia.org/wiki/OPC_Unified_Architecture
[opcua-spec]: https://opcfoundation.org/developer-tools/documents/?type=Specification