Messages and connections in Azure SignalR Service

The billing model for Azure SignalR Service is based on the number of connections and the number of messages. This article explains how messages and connections are defined and counted for billing.

Message formats

Azure SignalR Service supports the same formats as ASP.NET Core SignalR: JSON and MessagePack.

Message size

Azure SignalR Service has no size limit for messages.

Large messages are split into smaller messages that are no more than 2 KB each and transmitted separately. SDKs handle message splitting and assembling. No developer efforts are needed.

Large messages do negatively affect messaging performance. Use smaller messages whenever possible, and test to determine the optimal message size for each use-case scenario.

How messages are counted for billing

For billing, only outbound messages from Azure SignalR Service are counted. Ping messages between clients and servers are ignored.

Messages larger than 2 KB are counted as multiple messages of 2 KB each. The message count chart in the Azure portal is updated every 100 messages per hub.

For example, imagine you have one application server, and three clients:

App server broadcasts a 1-KB message to all connected clients, the message from app server to the service is considered free inbound message. Only the three messages sending from service to each of the client are billed as outbound messages.

Client A sends a 1-KB message to another client B, without going through app server. The message from client A to service is free inbound message. The message from service to client B is billed as outbound message.

If you have three clients and one application server. One client sends a 4-KB message to let the server broadcast to all clients. The billed message count is eight: one message from the service to the application server and three messages from the service to the clients. Each message is counted as two 2-KB messages.

How connections are counted

There are server connections and client connections with Azure SignalR Service. By default, each application server starts with five initial connections per hub, and each client has one client connection.

The connection count shown in the Azure portal includes both server connections and client connections.

For example, assume that you have two application servers and you define five hubs in code. The server connection count will be 50: 2 app servers * 5 hubs * 5 connections per hub.

ASP.NET SignalR calculates server connections in a different way. It includes one default hub in addition to hubs that you define. By default, each application server needs five more initial server connections. The initial connection count for the default hub stays consistent with other hubs.

The service and the application server keep syncing connection status and making adjustment to server connections to get better performance and service stability. So you might see server connection number changes from time to time.

How inbound/outbound traffic is counted

Message sent into the service is inbound message. Message sent out of the service is outbound message. Traffic is calculated in bytes.