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+/*
+ *
+ * Copyright 2017 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+// Package balancer defines APIs for load balancing in gRPC.
+// All APIs in this package are experimental.
+package balancer
+
+import (
+ "errors"
+ "net"
+
+ "golang.org/x/net/context"
+ "google.golang.org/grpc/connectivity"
+ "google.golang.org/grpc/credentials"
+ "google.golang.org/grpc/resolver"
+)
+
+var (
+ // m is a map from name to balancer builder.
+ m = make(map[string]Builder)
+)
+
+// Register registers the balancer builder to the balancer map.
+// b.Name will be used as the name registered with this builder.
+func Register(b Builder) {
+ m[b.Name()] = b
+}
+
+// Get returns the resolver builder registered with the given name.
+// If no builder is register with the name, nil will be returned.
+func Get(name string) Builder {
+ if b, ok := m[name]; ok {
+ return b
+ }
+ return nil
+}
+
+// SubConn represents a gRPC sub connection.
+// Each sub connection contains a list of addresses. gRPC will
+// try to connect to them (in sequence), and stop trying the
+// remainder once one connection is successful.
+//
+// The reconnect backoff will be applied on the list, not a single address.
+// For example, try_on_all_addresses -> backoff -> try_on_all_addresses.
+//
+// All SubConns start in IDLE, and will not try to connect. To trigger
+// the connecting, Balancers must call Connect.
+// When the connection encounters an error, it will reconnect immediately.
+// When the connection becomes IDLE, it will not reconnect unless Connect is
+// called.
+type SubConn interface {
+ // UpdateAddresses updates the addresses used in this SubConn.
+ // gRPC checks if currently-connected address is still in the new list.
+ // If it's in the list, the connection will be kept.
+ // If it's not in the list, the connection will gracefully closed, and
+ // a new connection will be created.
+ //
+ // This will trigger a state transition for the SubConn.
+ UpdateAddresses([]resolver.Address)
+ // Connect starts the connecting for this SubConn.
+ Connect()
+}
+
+// NewSubConnOptions contains options to create new SubConn.
+type NewSubConnOptions struct{}
+
+// ClientConn represents a gRPC ClientConn.
+type ClientConn interface {
+ // NewSubConn is called by balancer to create a new SubConn.
+ // It doesn't block and wait for the connections to be established.
+ // Behaviors of the SubConn can be controlled by options.
+ NewSubConn([]resolver.Address, NewSubConnOptions) (SubConn, error)
+ // RemoveSubConn removes the SubConn from ClientConn.
+ // The SubConn will be shutdown.
+ RemoveSubConn(SubConn)
+
+ // UpdateBalancerState is called by balancer to nofity gRPC that some internal
+ // state in balancer has changed.
+ //
+ // gRPC will update the connectivity state of the ClientConn, and will call pick
+ // on the new picker to pick new SubConn.
+ UpdateBalancerState(s connectivity.State, p Picker)
+
+ // Target returns the dial target for this ClientConn.
+ Target() string
+}
+
+// BuildOptions contains additional information for Build.
+type BuildOptions struct {
+ // DialCreds is the transport credential the Balancer implementation can
+ // use to dial to a remote load balancer server. The Balancer implementations
+ // can ignore this if it does not need to talk to another party securely.
+ DialCreds credentials.TransportCredentials
+ // Dialer is the custom dialer the Balancer implementation can use to dial
+ // to a remote load balancer server. The Balancer implementations
+ // can ignore this if it doesn't need to talk to remote balancer.
+ Dialer func(context.Context, string) (net.Conn, error)
+}
+
+// Builder creates a balancer.
+type Builder interface {
+ // Build creates a new balancer with the ClientConn.
+ Build(cc ClientConn, opts BuildOptions) Balancer
+ // Name returns the name of balancers built by this builder.
+ // It will be used to pick balancers (for example in service config).
+ Name() string
+}
+
+// PickOptions contains addition information for the Pick operation.
+type PickOptions struct{}
+
+// DoneInfo contains additional information for done.
+type DoneInfo struct {
+ // Err is the rpc error the RPC finished with. It could be nil.
+ Err error
+}
+
+var (
+ // ErrNoSubConnAvailable indicates no SubConn is available for pick().
+ // gRPC will block the RPC until a new picker is available via UpdateBalancerState().
+ ErrNoSubConnAvailable = errors.New("no SubConn is available")
+ // ErrTransientFailure indicates all SubConns are in TransientFailure.
+ // WaitForReady RPCs will block, non-WaitForReady RPCs will fail.
+ ErrTransientFailure = errors.New("all SubConns are in TransientFailure")
+)
+
+// Picker is used by gRPC to pick a SubConn to send an RPC.
+// Balancer is expected to generate a new picker from its snapshot everytime its
+// internal state has changed.
+//
+// The pickers used by gRPC can be updated by ClientConn.UpdateBalancerState().
+type Picker interface {
+ // Pick returns the SubConn to be used to send the RPC.
+ // The returned SubConn must be one returned by NewSubConn().
+ //
+ // This functions is expected to return:
+ // - a SubConn that is known to be READY;
+ // - ErrNoSubConnAvailable if no SubConn is available, but progress is being
+ // made (for example, some SubConn is in CONNECTING mode);
+ // - other errors if no active connecting is happening (for example, all SubConn
+ // are in TRANSIENT_FAILURE mode).
+ //
+ // If a SubConn is returned:
+ // - If it is READY, gRPC will send the RPC on it;
+ // - If it is not ready, or becomes not ready after it's returned, gRPC will block
+ // this call until a new picker is updated and will call pick on the new picker.
+ //
+ // If the returned error is not nil:
+ // - If the error is ErrNoSubConnAvailable, gRPC will block until UpdateBalancerState()
+ // - If the error is ErrTransientFailure:
+ // - If the RPC is wait-for-ready, gRPC will block until UpdateBalancerState()
+ // is called to pick again;
+ // - Otherwise, RPC will fail with unavailable error.
+ // - Else (error is other non-nil error):
+ // - The RPC will fail with unavailable error.
+ //
+ // The returned done() function will be called once the rpc has finished, with the
+ // final status of that RPC.
+ // done may be nil if balancer doesn't care about the RPC status.
+ Pick(ctx context.Context, opts PickOptions) (conn SubConn, done func(DoneInfo), err error)
+}
+
+// Balancer takes input from gRPC, manages SubConns, and collects and aggregates
+// the connectivity states.
+//
+// It also generates and updates the Picker used by gRPC to pick SubConns for RPCs.
+//
+// HandleSubConnectionStateChange, HandleResolvedAddrs and Close are guaranteed
+// to be called synchronously from the same goroutine.
+// There's no guarantee on picker.Pick, it may be called anytime.
+type Balancer interface {
+ // HandleSubConnStateChange is called by gRPC when the connectivity state
+ // of sc has changed.
+ // Balancer is expected to aggregate all the state of SubConn and report
+ // that back to gRPC.
+ // Balancer should also generate and update Pickers when its internal state has
+ // been changed by the new state.
+ HandleSubConnStateChange(sc SubConn, state connectivity.State)
+ // HandleResolvedAddrs is called by gRPC to send updated resolved addresses to
+ // balancers.
+ // Balancer can create new SubConn or remove SubConn with the addresses.
+ // An empty address slice and a non-nil error will be passed if the resolver returns
+ // non-nil error to gRPC.
+ HandleResolvedAddrs([]resolver.Address, error)
+ // Close closes the balancer. The balancer is not required to call
+ // ClientConn.RemoveSubConn for its existing SubConns.
+ Close()
+}
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