Consider batching updates to clients

[aawdls]

For large numbers of rapidly updating subscriptions, batching updates to each client may make the rendering more efficient. This would require changes in both ends.

A basic prototype would allow this hypothesis to be tested.

[coretl]

A possibility based on how we do this for PandA TCP, is keeping a change dict that is sent and cleared on request from the client. That puts the client in charge of polling.

E.g.

• client subscribes to PV1
• client subscribes to PV2
• client sends "what changed?"
• server says {PV1: 45, PV2: 44}
• client sends "what changed?"
• server says {PV2: 42}
• client unsubscribes from PV2
• client sends "what changed?"
• server says {PV1: 42}
• client closes connection

This probably doesn't fit very well with GraphQL subscriptions as the subscription list is modified during the lifetime of the subscription, but you could hack it:

E.g.

• subscribeList(list_id, period=0.1s)
• addPV(pv1, list_id)
• addPV(pv2, list_id)
• server sends batched updates with min period 0.1s to the subscribeList callback