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Proxies to Replicated Objects

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A replicated Ice object is hosted by several servers, the replicas, each with a servant for the object's identity. A client reaches any of these replicas with a proxy written in one of three ways:

  • The proxy lists the endpoints of all the replicas.
  • The proxy has a single endpoint that points to a TCP load balancer in front of the replicas.
  • The proxy has a single endpoint with a host name that resolves to the address of each replica.

With IceGrid, the replicas are the object adapters of a replica group, and the client uses an indirect proxy with the replica group ID, such as hello@HelloGroup: the locator supplies the endpoints of the replicas.

A proxy can hold one endpoint for each replica:

hello:tcp -h 10.0.0.1 -p 4061:tcp -h 10.0.0.2 -p 4061

Ice picks one of these endpoints and connects to it, as described in Connection Establishment. By default, the proxy then uses this connection, and therefore this replica, for all its invocations until the connection closes. Proxy-Based Load Balancing shows how to spread the invocations across the replicas.

Adding or removing a replica means updating every proxy that lists the endpoints.

An endpoint can point to a TCP load balancer that forwards each connection to one of the replicas:

hello:tcp -h lb.example.com -p 4061

The load balancer chooses a replica for each connection. Ice keeps a connection open as long as it is in use, so all the invocations sent over a connection reach the same replica. You add or remove a replica in the configuration of the load balancer.

An endpoint can name a host whose DNS record holds the address of each replica:

hello:tcp -h hello.example.com -p 4061

When Ice establishes a connection, it connects to one of the addresses of this host name, as described in Connection Establishment.

With this configuration, all the replicas must use the same port. You add or remove a replica by updating the DNS record. A replica that is down stays in the record until you remove it, and a connection attempt that reaches its address fails or times out before Ice tries the next address.