Tango event subscription modes

Taurus supports the event subscription modes introduced in Tango 10.1.0. Two aspects are relevant to Taurus clients:

1. Selecting the event subscription mode (EventSubMode).

2. A lightweight first read to initialize attribute values when needed.

For background on Tango events and PyTango API see the official docs: Tango Events and PyTango DeviceProxy API. PyTango exposes the EventSubMode enum and adds EventReason in the EventData structure (see documentation in PyTango client API).

EventSubMode in Taurus

Taurus supports all event subscription modes that Tango >= 10.1.0 provides. The Taurus functionality is independent of the selected mode, but application startup performance may vary depending on the attributes and devices involved.

When available, Taurus uses AsyncRead as the default EventSubMode. In most cases it should not be changed, since AsyncRead provides a good compromise between compatibility, simplicity, and performance. The mode can be changed when needed; see Change the Tango event subscription mode.

If you experience problems when starting applications, especially with value initialization, the mode can be changed to SyncRead to recover the previous Taurus (< 5.4) default behaviour.

The following table (adapted from PyTango EventSubMode) summarizes the available event subscription modes:

EventSubMode	Tries subscription before returning	Raises on subscription failure	Retries automatically	Reads entity	First callback
SyncRead	Yes	Yes	No	Yes, during subscription	Immediately, with data
AsyncRead	No	No	Yes	Yes, after subscription	After read, with data
Sync	Yes	Yes	No	No	Not on subscription. Only on next event
Async	No	No	Yes	No	After subscription, no data
Stateless	Yes	No	Yes	Yes, during subscription	Immediately, with data

First Read (client-side initialization)

Taurus now provides a one-shot read used when the chosen mode does not perform an initial read by itself (Sync and Async). This first read is performed asynchronously in a dedicated thread that finishes when all attributes have been initialized.

This feature ensures that the attribute has a value cached right after subscription setup, delegating to Taurus the responsibility for providing the initial attribute value.

API involved in Taurus:

• Factory helper to register a first-time read for a given attribute.

• Attribute-side helper that delegates to the factory during initialization.