Ably

Ably positions itself above Pusher on infrastructure capability: message ordering guarantees, message history for replay, presence, push notifications, and MQTT support create a platform suitable for systems that need reliable real-time messaging rather than just best-effort event broadcasting. For agents that need to broadcast state updates with delivery guarantees, or that need clients to replay missed messages after reconnection, Ably's additional capabilities matter. The trade-off is more complexity than Pusher for simple broadcast use cases.

The channel and message model extends Pusher's concepts with message history, rewind (replay from a point in time), and delivery confirmation. Those additions are significant for agents that need to ensure downstream consumers receive critical state updates even if they were briefly disconnected. The REST API and Realtime SDK both support all core features, giving agents flexibility in how they produce and consume messages.

Authentication uses API keys and token-based auth with fine-grained capability controls per channel. The capability model lets teams define precisely which channels an agent or client can publish to or subscribe from, which is important for multi-tenant real-time architectures where channel isolation matters. Key management and capability definition deserve careful attention upfront.

Reliability is a primary Ably differentiator — the platform's architecture is designed for exactly-once delivery semantics and guaranteed message ordering within channels. That is meaningful for agents orchestrating distributed state where event ordering affects correctness. Teams should understand that these guarantees hold within Ably's delivery window; extremely long client disconnections may still miss messages beyond the history retention period.

Documentation is comprehensive and technically precise. The conceptual guides explain the reliability model clearly, which matters because teams need to understand what guarantees they're getting and under what conditions. The API reference covers all features with enough detail for production integration. Ably's docs are among the better real-time platform documentation sets available.