Package io.grpc

Class LoadBalancer.Subchannel

  • Direct Known Subclasses:
    ForwardingSubchannel
    Enclosing class:
    LoadBalancer
    @ExperimentalApi("https://github.com/grpc/grpc-java/issues/1771")
public abstract static class LoadBalancer.Subchannel
extends Object
    A logical connection to a server, or a group of equivalent servers represented by an EquivalentAddressGroup.

    It maintains at most one physical connection (aka transport) for sending new RPCs, while also keeps track of previous transports that has been shut down but not terminated yet.

    If there isn't an active transport yet, and an RPC is assigned to the Subchannel, it will create a new transport. It won't actively create transports otherwise. requestConnection() can be used to ask Subchannel to create a transport if there isn't any.

    start(io.grpc.LoadBalancer.SubchannelStateListener) must be called prior to calling any other methods, with the exception of shutdown(), which can be called at any time.

    Since:
    1.2.0

    • Constructor Detail

      • Subchannel

        public Subchannel()

    • Method Detail

      • start

        public void start​(LoadBalancer.SubchannelStateListener listener)
        Starts the Subchannel. Can only be called once.

        Must be called prior to any other method on this class, except for shutdown() which may be called at any time.

        Must be called from the Synchronization Context, otherwise it may throw. See #5015 for more discussions.

        Parameters:
        listener - receives state updates for this Subchannel.

      • shutdown

        public abstract void shutdown()
        Shuts down the Subchannel. After this method is called, this Subchannel should no longer be returned by the latest picker, and can be safely discarded.

        Calling it on an already shut-down Subchannel has no effect.

        It should be called from the Synchronization Context. Currently will log a warning if violated. It will become an exception eventually. See #5015 for the background.

        Since:
        1.2.0

      • requestConnection

        public abstract void requestConnection()
        Asks the Subchannel to create a connection (aka transport), if there isn't an active one.

        It should be called from the Synchronization Context. Currently will log a warning if violated. It will become an exception eventually. See #5015 for the background.

        Since:
        1.2.0

      • getAddresses

        public final EquivalentAddressGroup getAddresses()
        Returns the addresses that this Subchannel is bound to. This can be called only if the Subchannel has only one EquivalentAddressGroup. Under the hood it calls getAllAddresses().

        It should be called from the Synchronization Context. Currently will log a warning if violated. It will become an exception eventually. See #5015 for the background.

        Throws:
        IllegalStateException - if this subchannel has more than one EquivalentAddressGroup. Use getAllAddresses() instead
        Since:
        1.2.0

      • getAllAddresses

        public List<EquivalentAddressGroup> getAllAddresses()
        Returns the addresses that this Subchannel is bound to. The returned list will not be empty.

        It should be called from the Synchronization Context. Currently will log a warning if violated. It will become an exception eventually. See #5015 for the background.

        Since:
        1.14.0

      • getAttributes

        public abstract Attributes getAttributes()
        The same attributes passed to Helper.createSubchannel(). LoadBalancer can use it to attach additional information here, e.g., the shard this Subchannel belongs to.
        Since:
        1.2.0

      • asChannel

        @Internal
public Channel asChannel()
        (Internal use only) returns a Channel that is backed by this Subchannel. This allows a LoadBalancer to issue its own RPCs for auxiliary purposes, such as health-checking, on already-established connections. This channel has certain restrictions:
        1. It can issue RPCs only if the Subchannel is READY. If Channel.newCall(io.grpc.MethodDescriptor<RequestT, ResponseT>, io.grpc.CallOptions) is called when the Subchannel is not READY, the RPC will fail immediately.
        2. It doesn't support wait-for-ready RPCs. Such RPCs will fail immediately.

        RPCs made on this Channel is not counted when determining ManagedChannel's idle mode. In other words, they won't prevent ManagedChannel from entering idle mode.

        Warning: RPCs made on this channel will prevent a shut-down transport from terminating. If you make long-running RPCs, you need to make sure they will finish in time after the Subchannel has transitioned away from READY state (notified through LoadBalancer.handleSubchannelState(io.grpc.LoadBalancer.Subchannel, io.grpc.ConnectivityStateInfo)).

        Warning: this is INTERNAL API, is not supposed to be used by external users, and may change without notice. If you think you must use it, please file an issue.

      • updateAddresses

        public void updateAddresses​(List<EquivalentAddressGroup> addrs)
        Replaces the existing addresses used with this Subchannel. If the new and old addresses overlap, the Subchannel can continue using an existing connection.

        It must be called from the Synchronization Context or will throw.

        Throws:
        IllegalArgumentException - if addrs is empty
        Since:
        1.22.0

      • getInternalSubchannel

        @Internal
public Object getInternalSubchannel()
        (Internal use only) returns an object that represents the underlying subchannel that is used by the Channel for sending RPCs when this LoadBalancer.Subchannel is picked. This is an opaque object that is both provided and consumed by the Channel. Its type is not Subchannel.

        Warning: this is INTERNAL API, is not supposed to be used by external users, and may change without notice. If you think you must use it, please file an issue and we can consider removing its "internal" status.