io.grpc.LoadBalancer Maven / Gradle / Ivy
/*
* Copyright 2016, gRPC Authors All rights reserved.
*
* 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 io.grpc;
import com.google.common.base.MoreObjects;
import com.google.common.base.Objects;
import com.google.common.base.Preconditions;
import java.util.List;
import javax.annotation.Nonnull;
import javax.annotation.Nullable;
import javax.annotation.concurrent.Immutable;
import javax.annotation.concurrent.NotThreadSafe;
import javax.annotation.concurrent.ThreadSafe;
/**
* A pluggable component that receives resolved addresses from {@link NameResolver} and provides the
* channel a usable subchannel when asked.
*
* Overview
*
* A LoadBalancer typically implements three interfaces:
*
* - {@link LoadBalancer} is the main interface. All methods on it are invoked sequentially
* from the Channel Executor. It receives the results from the {@link NameResolver}, updates
* of subchannels' connectivity states, and the channel's request for the LoadBalancer to
* shutdown.
* - {@link SubchannelPicker SubchannelPicker} does the actual load-balancing work. It selects
* a {@link Subchannel Subchannel} for each new RPC.
* - {@link Factory Factory} creates a new {@link LoadBalancer} instance.
*
*
* {@link Helper Helper} is implemented by gRPC library and provided to {@link Factory
* Factory}. It provides functionalities that a {@code LoadBalancer} implementation would typically
* need.
*
*
Channel Executor
*
* Channel Executor is an internal executor of the channel, which is used to serialize all the
* callback methods on the {@link LoadBalancer} interface, thus the balancer implementation doesn't
* need to worry about synchronization among them. However, the actual thread of the Channel
* Executor is typically the network thread, thus the following rules must be followed to prevent
* blocking or even dead-locking in a network
*
*
*
* - Never block in Channel Executor. The callback methods must return
* quickly. Examples or work that must be avoided: CPU-intensive calculation, waiting on
* synchronization primitives, blocking I/O, blocking RPCs, etc.
*
* - Avoid calling into other components with lock held. Channel Executor may
* run callbacks under a lock, e.g., the transport lock of OkHttp. If your LoadBalancer has a
* lock, holds the lock in a callback method (e.g., {@link #handleSubchannelState
* handleSubchannelState()}) while calling into another class that may involve locks, be cautious
* of deadlock. Generally you wouldn't need any locking in the LoadBalancer.
*
*
*
* {@link Helper#runSerialized Helper.runSerialized()} allows you to schedule a task to be run in
* the Channel Executor.
*
*
The canonical implementation pattern
*
* A {@link LoadBalancer} keeps states like the latest addresses from NameResolver, the
* Subchannel(s) and their latest connectivity states. These states are mutated within the Channel
* Executor.
*
*
A typical {@link SubchannelPicker SubchannelPicker} holds a snapshot of these states. It may
* have its own states, e.g., a picker from a round-robin load-balancer may keep a pointer to the
* next Subchannel, which are typically mutated by multiple threads. The picker should only mutate
* its own state, and should not mutate or re-acquire the states of the LoadBalancer. This way the
* picker only needs to synchronize its own states, which is typically trivial to implement.
*
*
When the LoadBalancer states changes, e.g., Subchannels has become or stopped being READY, and
* we want subsequent RPCs to use the latest list of READY Subchannels, LoadBalancer would create
* a new picker, which holds a snapshot of the latest Subchannel list. Refer to the javadoc of
* {@link #handleSubchannelState handleSubchannelState()} how to do this properly.
*
*
No synchronization should be necessary between LoadBalancer and its pickers if you follow
* the pattern above. It may be possible to implement in a different way, but that would usually
* result in more complicated threading.
*
* @since 1.2.0
*/
@ExperimentalApi("https://github.com/grpc/grpc-java/issues/1771")
@NotThreadSafe
public abstract class LoadBalancer {
/**
* Handles newly resolved server groups and metadata attributes from name resolution system.
* {@code servers} contained in {@link EquivalentAddressGroup} should be considered equivalent
* but may be flattened into a single list if needed.
*
*
Implementations should not modify the given {@code servers}.
*
* @param servers the resolved server addresses, never empty.
* @param attributes extra metadata from naming system.
* @since 1.2.0
*/
public abstract void handleResolvedAddressGroups(
List servers, Attributes attributes);
/**
* Handles an error from the name resolution system.
*
* @param error a non-OK status
* @since 1.2.0
*/
public abstract void handleNameResolutionError(Status error);
/**
* Handles a state change on a Subchannel.
*
* The initial state of a Subchannel is IDLE. You won't get a notification for the initial IDLE
* state.
*
*
If the new state is not SHUTDOWN, this method should create a new picker and call {@link
* Helper#updateBalancingState Helper.updateBalancingState()}. Failing to do so may result in
* unnecessary delays of RPCs. Please refer to {@link PickResult#withSubchannel
* PickResult.withSubchannel()}'s javadoc for more information.
*
*
SHUTDOWN can only happen in two cases. One is that LoadBalancer called {@link
* Subchannel#shutdown} earlier, thus it should have already discarded this Subchannel. The other
* is that Channel is doing a {@link ManagedChannel#shutdownNow forced shutdown} or has already
* terminated, thus there won't be further requests to LoadBalancer. Therefore, SHUTDOWN can be
* safely ignored.
*
* @param subchannel the involved Subchannel
* @param stateInfo the new state
* @since 1.2.0
*/
public abstract void handleSubchannelState(
Subchannel subchannel, ConnectivityStateInfo stateInfo);
/**
* The channel asks the load-balancer to shutdown. No more callbacks will be called after this
* method. The implementation should shutdown all Subchannels and OOB channels, and do any other
* cleanup as necessary.
*
* @since 1.2.0
*/
public abstract void shutdown();
/**
* The main balancing logic. It must be thread-safe. Typically it should only
* synchronize on its own state, and avoid synchronizing with the LoadBalancer's state.
*
* @since 1.2.0
*/
@ThreadSafe
public abstract static class SubchannelPicker {
/**
* Make a balancing decision for a new RPC.
*
* @param args the pick arguments
* @since 1.3.0
*/
public abstract PickResult pickSubchannel(PickSubchannelArgs args);
/**
* Tries to establish connections now so that the upcoming RPC may then just pick a ready
* connection without having to connect first.
*
*
No-op if unsupported.
*
* @since 1.11.0
*/
public void requestConnection() {}
}
/**
* Provides arguments for a {@link SubchannelPicker#pickSubchannel(
* LoadBalancer.PickSubchannelArgs)}.
*
* @since 1.2.0
*/
public abstract static class PickSubchannelArgs {
/**
* Call options.
*
* @since 1.2.0
*/
public abstract CallOptions getCallOptions();
/**
* Headers of the call. {@link SubchannelPicker#pickSubchannel} may mutate it before before
* returning.
*
* @since 1.2.0
*/
public abstract Metadata getHeaders();
/**
* Call method.
*
* @since 1.2.0
*/
public abstract MethodDescriptor getMethodDescriptor();
}
/**
* A balancing decision made by {@link SubchannelPicker SubchannelPicker} for an RPC.
*
*
The outcome of the decision will be one of the following:
*
* - Proceed: if a Subchannel is provided via {@link #withSubchannel withSubchannel()}, and is
* in READY state when the RPC tries to start on it, the RPC will proceed on that
* Subchannel.
* - Error: if an error is provided via {@link #withError withError()}, and the RPC is not
* wait-for-ready (i.e., {@link CallOptions#withWaitForReady} was not called), the RPC will
* fail immediately with the given error.
* - Buffer: in all other cases, the RPC will be buffered in the Channel, until the next
* picker is provided via {@link Helper#updateBalancingState Helper.updateBalancingState()},
* when the RPC will go through the same picking process again.
*
*
* @since 1.2.0
*/
@Immutable
public static final class PickResult {
private static final PickResult NO_RESULT = new PickResult(null, null, Status.OK, false);
@Nullable private final Subchannel subchannel;
@Nullable private final ClientStreamTracer.Factory streamTracerFactory;
// An error to be propagated to the application if subchannel == null
// Or OK if there is no error.
// subchannel being null and error being OK means RPC needs to wait
private final Status status;
// True if the result is created by withDrop()
private final boolean drop;
private PickResult(
@Nullable Subchannel subchannel, @Nullable ClientStreamTracer.Factory streamTracerFactory,
Status status, boolean drop) {
this.subchannel = subchannel;
this.streamTracerFactory = streamTracerFactory;
this.status = Preconditions.checkNotNull(status, "status");
this.drop = drop;
}
/**
* A decision to proceed the RPC on a Subchannel.
*
* Only Subchannels returned by {@link Helper#createSubchannel Helper.createSubchannel()}
* will work. DO NOT try to use your own implementations of Subchannels, as they won't work.
*
*
When the RPC tries to use the return Subchannel, which is briefly after this method
* returns, the state of the Subchannel will decide where the RPC would go:
*
*
* - READY: the RPC will proceed on this Subchannel.
* - IDLE: the RPC will be buffered. Subchannel will attempt to create connection.
* - All other states: the RPC will be buffered.
*
*
* All buffered RPCs will stay buffered until the next call of {@link
* Helper#updateBalancingState Helper.updateBalancingState()}, which will trigger a new picking
* process.
*
*
Note that Subchannel's state may change at the same time the picker is making the
* decision, which means the decision may be made with (to-be) outdated information. For
* example, a picker may return a Subchannel known to be READY, but it has become IDLE when is
* about to be used by the RPC, which makes the RPC to be buffered. The LoadBalancer will soon
* learn about the Subchannels' transition from READY to IDLE, create a new picker and allow the
* RPC to use another READY transport if there is any.
*
*
You will want to avoid running into a situation where there are READY Subchannels out
* there but some RPCs are still buffered for longer than a brief time.
*
* - This can happen if you return Subchannels with states other than READY and IDLE. For
* example, suppose you round-robin on 2 Subchannels, in READY and CONNECTING states
* respectively. If the picker ignores the state and pick them equally, 50% of RPCs will
* be stuck in buffered state until both Subchannels are READY.
* - This can also happen if you don't create a new picker at key state changes of
* Subchannels. Take the above round-robin example again. Suppose you do pick only READY
* and IDLE Subchannels, and initially both Subchannels are READY. Now one becomes IDLE,
* then CONNECTING and stays CONNECTING for a long time. If you don't create a new picker
* in response to the CONNECTING state to exclude that Subchannel, 50% of RPCs will hit it
* and be buffered even though the other Subchannel is READY.
*
*
* In order to prevent unnecessary delay of RPCs, the rules of thumb are:
*
* - The picker should only pick Subchannels that are known as READY or IDLE. Whether to
* pick IDLE Subchannels depends on whether you want Subchannels to connect on-demand or
* actively:
*
* - If you want connect-on-demand, include IDLE Subchannels in your pick results,
* because when an RPC tries to use an IDLE Subchannel, the Subchannel will try to
* connect.
* - If you want Subchannels to be always connected even when there is no RPC, you
* would call {@link Subchannel#requestConnection Subchannel.requestConnection()}
* whenever the Subchannel has transitioned to IDLE, then you don't need to include
* IDLE Subchannels in your pick results.
*
* - Always create a new picker and call {@link Helper#updateBalancingState
* Helper.updateBalancingState()} whenever {@link #handleSubchannelState
* handleSubchannelState()} is called, unless the new state is SHUTDOWN. See
* {@code handleSubchannelState}'s javadoc for more details.
*
*
* @param subchannel the picked Subchannel
* @param streamTracerFactory if not null, will be used to trace the activities of the stream
* created as a result of this pick. Note it's possible that no
* stream is created at all in some cases.
* @since 1.3.0
*/
public static PickResult withSubchannel(
Subchannel subchannel, @Nullable ClientStreamTracer.Factory streamTracerFactory) {
return new PickResult(
Preconditions.checkNotNull(subchannel, "subchannel"), streamTracerFactory, Status.OK,
false);
}
/**
* Equivalent to {@code withSubchannel(subchannel, null)}.
*
* @since 1.2.0
*/
public static PickResult withSubchannel(Subchannel subchannel) {
return withSubchannel(subchannel, null);
}
/**
* A decision to report a connectivity error to the RPC. If the RPC is {@link
* CallOptions#withWaitForReady wait-for-ready}, it will stay buffered. Otherwise, it will fail
* with the given error.
*
* @param error the error status. Must not be OK.
* @since 1.2.0
*/
public static PickResult withError(Status error) {
Preconditions.checkArgument(!error.isOk(), "error status shouldn't be OK");
return new PickResult(null, null, error, false);
}
/**
* A decision to fail an RPC immediately. This is a final decision and will ignore retry
* policy.
*
* @param status the status with which the RPC will fail. Must not be OK.
* @since 1.8.0
*/
public static PickResult withDrop(Status status) {
Preconditions.checkArgument(!status.isOk(), "drop status shouldn't be OK");
return new PickResult(null, null, status, true);
}
/**
* No decision could be made. The RPC will stay buffered.
*
* @since 1.2.0
*/
public static PickResult withNoResult() {
return NO_RESULT;
}
/**
* The Subchannel if this result was created by {@link #withSubchannel withSubchannel()}, or
* null otherwise.
*
* @since 1.2.0
*/
@Nullable
public Subchannel getSubchannel() {
return subchannel;
}
/**
* The stream tracer factory this result was created with.
*
* @since 1.3.0
*/
@Nullable
public ClientStreamTracer.Factory getStreamTracerFactory() {
return streamTracerFactory;
}
/**
* The status associated with this result. Non-{@code OK} if created with {@link #withError
* withError}, or {@code OK} otherwise.
*
* @since 1.2.0
*/
public Status getStatus() {
return status;
}
/**
* Returns {@code true} if this result was created by {@link #withDrop withDrop()}.
*
* @since 1.8.0
*/
public boolean isDrop() {
return drop;
}
@Override
public String toString() {
return MoreObjects.toStringHelper(this)
.add("subchannel", subchannel)
.add("streamTracerFactory", streamTracerFactory)
.add("status", status)
.add("drop", drop)
.toString();
}
@Override
public int hashCode() {
return Objects.hashCode(subchannel, status, streamTracerFactory, drop);
}
/**
* Returns true if the {@link Subchannel}, {@link Status}, and
* {@link ClientStreamTracer.Factory} all match.
*/
@Override
public boolean equals(Object other) {
if (!(other instanceof PickResult)) {
return false;
}
PickResult that = (PickResult) other;
return Objects.equal(subchannel, that.subchannel) && Objects.equal(status, that.status)
&& Objects.equal(streamTracerFactory, that.streamTracerFactory)
&& drop == that.drop;
}
}
/**
* Provides essentials for LoadBalancer implementations.
*
* @since 1.2.0
*/
@ThreadSafe
public abstract static class Helper {
/**
* Creates a Subchannel, which is a logical connection to the given group of addresses which are
* considered equivalent. The {@code attrs} are custom attributes associated with this
* Subchannel, and can be accessed later through {@link Subchannel#getAttributes
* Subchannel.getAttributes()}.
*
* The LoadBalancer is responsible for closing unused Subchannels, and closing all
* Subchannels within {@link #shutdown}.
*
* @since 1.2.0
*/
public abstract Subchannel createSubchannel(EquivalentAddressGroup addrs, Attributes attrs);
/**
* Replaces the existing addresses used with {@code subchannel}. This method is superior to
* {@link #createSubchannel} when the new and old addresses overlap, since the subchannel can
* continue using an existing connection.
*
* @throws IllegalArgumentException if {@code subchannel} was not returned from {@link
* #createSubchannel}
* @since 1.4.0
*/
public void updateSubchannelAddresses(
Subchannel subchannel, EquivalentAddressGroup addrs) {
throw new UnsupportedOperationException();
}
/**
* Out-of-band channel for LoadBalancer’s own RPC needs, e.g., talking to an external
* load-balancer service.
*
*
The LoadBalancer is responsible for closing unused OOB channels, and closing all OOB
* channels within {@link #shutdown}.
*
* @since 1.4.0
*/
public abstract ManagedChannel createOobChannel(EquivalentAddressGroup eag, String authority);
/**
* Updates the addresses used for connections in the {@code Channel}. This is supperior to
* {@link #createOobChannel} when the old and new addresses overlap, since the channel can
* continue using an existing connection.
*
* @throws IllegalArgumentException if {@code channel} was not returned from {@link
* #createOobChannel}
* @since 1.4.0
*/
public void updateOobChannelAddresses(ManagedChannel channel, EquivalentAddressGroup eag) {
throw new UnsupportedOperationException();
}
/**
* Set a new state with a new picker to the channel.
*
*
When a new picker is provided via {@code updateBalancingState()}, the channel will apply
* the picker on all buffered RPCs, by calling {@link SubchannelPicker#pickSubchannel(
* LoadBalancer.PickSubchannelArgs)}.
*
*
The channel will hold the picker and use it for all RPCs, until {@code
* updateBalancingState()} is called again and a new picker replaces the old one. If {@code
* updateBalancingState()} has never been called, the channel will buffer all RPCs until a
* picker is provided.
*
*
The passed state will be the channel's new state. The SHUTDOWN state should not be passed
* and its behavior is undefined.
*
* @since 1.6.0
*/
public abstract void updateBalancingState(
@Nonnull ConnectivityState newState, @Nonnull SubchannelPicker newPicker);
/**
* Schedule a task to be run in the Channel Executor, which serializes the task with the
* callback methods on the {@link LoadBalancer} interface.
*
* @since 1.2.0
*/
public abstract void runSerialized(Runnable task);
/**
* Returns the NameResolver of the channel.
*
* @since 1.2.0
*/
public abstract NameResolver.Factory getNameResolverFactory();
/**
* Returns the authority string of the channel, which is derived from the DNS-style target name.
*
* @since 1.2.0
*/
public abstract String getAuthority();
}
/**
* A logical connection to a server, or a group of equivalent servers represented by an {@link
* 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. {@link
* #requestConnection requestConnection()} can be used to ask Subchannel to create a transport if
* there isn't any.
*
* @since 1.2.0
*/
@ThreadSafe
public abstract static class Subchannel {
/**
* Shuts down the Subchannel. After this method is called, this Subchannel should no longer
* be returned by the latest {@link SubchannelPicker picker}, and can be safely discarded.
*
* @since 1.2.0
*/
public abstract void shutdown();
/**
* Asks the Subchannel to create a connection (aka transport), if there isn't an active one.
*
* @since 1.2.0
*/
public abstract void requestConnection();
/**
* Returns the addresses that this Subchannel is bound to.
*
* @since 1.2.0
*/
public abstract EquivalentAddressGroup getAddresses();
/**
* The same attributes passed to {@link Helper#createSubchannel Helper.createSubchannel()}.
* LoadBalancer can use it to attach additional information here, e.g., the shard this
* Subchannel belongs to.
*
* @since 1.2.0
*/
public abstract Attributes getAttributes();
}
/**
* Factory to create {@link LoadBalancer} instance.
*
* @since 1.2.0
*/
@ThreadSafe
public abstract static class Factory {
/**
* Creates a {@link LoadBalancer} that will be used inside a channel.
*
* @since 1.2.0
*/
public abstract LoadBalancer newLoadBalancer(Helper helper);
}
}