| commit | 7582049a95ba9befaf975956c0da00d4e1d52fb6 | [log] [tgz] |
|---|---|---|
| author | Kun Zhang <zhangkun83@users.noreply.github.com> | Tue Oct 23 15:25:15 2018 -0700 |
| committer | GitHub <noreply@github.com> | Tue Oct 23 15:25:15 2018 -0700 |
| tree | 34bef57e955360fc1f91f7ea429cfcd9d0414788 | |
| parent | 41c8d8020f787a1f898a08177e1cd428a60512c2 [diff] |
core: SynchronizationContext exposed by LoadBalancer.Helper (#4971) Provides a `SynchronizationContext` for scheduling tasks, with and without delay, from LoadBalancer implementations. This absorbs and extends the internal utility `ChannelExecutor`. It supersedes `Helper.runSerialized()`, which is now deprecated. # Motivation I see multiple cases that schedule tasks with a delay while requiring the task to run in the "Channel Executor". There have been repeated work to wrap scheduled tasks and handle races between cancellation and task run (see the diff in `GrpclbState.java` for example). The LoadBalancer implementation (e.g., GrpclbLoadBalancer) also has to acquire the `ScheduledExecutorService` from somewhere and release it upon shutdown. The upcoming HealthCheckLoadBalancer (#4932), which would use back-off policy to retry health-checking streams, would have to do all the things above. At this point I think we need to provide something that combines `runSerialized()` with a scheduled executor with the same synchronization guarantees. # Design details `SynchronizationContext` is a similar to `ScheduledExecutorService` but tailored for use in `LoadBalancer` and potentially other cases outside of `LoadBalancer`. It offers task queuing and serialization and delayed scheduling. It guarantees non-reentrancy and happens-before among tasks. It owns no thread, but run tasks on caller's or caller-provided threads. All channel-level state mutations and callback methods on `LoadBalancer` are done in a SynchronizationContext, which was previously referred to as "Channel Executor". `SynchronizationContext.schedule()` returns a `ScheduledHandle` for status checking and cancellation. `ScheduedFuture` from `SchedulingExecutorService.schedule()` is too broad for our use cases (e.g., the blocking `get()` should never be used). `SynchronizationContext.schedule()` requires a `ScheduledExecutorService`, which is now available through `Helper.getScheduledExecutorService()`. LoadBalancers don't need to worry about where to get `SchedulingExecutorService` any more. # Alternatives Alternatively, we could keep `Helper.runSerialized()` and add something like `Helper.runSerialiezdWithDelay()`, but having them on their own interface allows clean fake implementation by `FakeClock` for test, and allows other components (potentially `InternalSubchannel` for reconnection backoff) to use it too. Instead of asking caller of `schedule()` to provide the `ScheduledExecutorService`, we considered having SynchronizationContext take a `ScheduledExecutorService` at construction. It would be inconvenient for LoadBalancer implementations that don't use `schedule()`, as they would be forced to provide a fake `ScheduledExecutorService` (which is cumbersome). Instead of making `SynchronizationContext` a (semi-)concrete class, we considered making it an pure abstract class. However, we found it nontrivial to implement `execute()` correctly with the non-reentrancy guarantee.
gRPC-Java works with JDK 7. gRPC-Java clients are supported on Android API levels 14 and up (Ice Cream Sandwich and later). Deploying gRPC servers on an Android device is not supported.
TLS usage typically requires using Java 8, or Play Services Dynamic Security Provider on Android. Please see the Security Readme.
For a guided tour, take a look at the quick start guide or the more explanatory gRPC basics.
The examples and the Android example are standalone projects that showcase the usage of gRPC.
Download the JARs. Or for Maven with non-Android, add to your pom.xml:
<dependency> <groupId>io.grpc</groupId> <artifactId>grpc-netty-shaded</artifactId> <version>1.15.1</version> </dependency> <dependency> <groupId>io.grpc</groupId> <artifactId>grpc-protobuf</artifactId> <version>1.15.1</version> </dependency> <dependency> <groupId>io.grpc</groupId> <artifactId>grpc-stub</artifactId> <version>1.15.1</version> </dependency>
Or for Gradle with non-Android, add to your dependencies:
compile 'io.grpc:grpc-netty-shaded:1.15.1' compile 'io.grpc:grpc-protobuf:1.15.1' compile 'io.grpc:grpc-stub:1.15.1'
For Android client, use grpc-okhttp instead of grpc-netty-shaded and grpc-protobuf-lite instead of grpc-protobuf:
compile 'io.grpc:grpc-okhttp:1.15.1' compile 'io.grpc:grpc-protobuf-lite:1.15.1' compile 'io.grpc:grpc-stub:1.15.1'
Development snapshots are available in Sonatypes's snapshot repository.
For protobuf-based codegen, you can put your proto files in the src/main/proto and src/test/proto directories along with an appropriate plugin.
For protobuf-based codegen integrated with the Maven build system, you can use protobuf-maven-plugin (Eclipse and NetBeans users should also look at os-maven-plugin's IDE documentation):
<build> <extensions> <extension> <groupId>kr.motd.maven</groupId> <artifactId>os-maven-plugin</artifactId> <version>1.5.0.Final</version> </extension> </extensions> <plugins> <plugin> <groupId>org.xolstice.maven.plugins</groupId> <artifactId>protobuf-maven-plugin</artifactId> <version>0.5.1</version> <configuration> <protocArtifact>com.google.protobuf:protoc:3.5.1-1:exe:${os.detected.classifier}</protocArtifact> <pluginId>grpc-java</pluginId> <pluginArtifact>io.grpc:protoc-gen-grpc-java:1.15.1:exe:${os.detected.classifier}</pluginArtifact> </configuration> <executions> <execution> <goals> <goal>compile</goal> <goal>compile-custom</goal> </goals> </execution> </executions> </plugin> </plugins> </build>
For protobuf-based codegen integrated with the Gradle build system, you can use protobuf-gradle-plugin:
apply plugin: 'com.google.protobuf' buildscript { repositories { mavenCentral() } dependencies { classpath 'com.google.protobuf:protobuf-gradle-plugin:0.8.5' } } protobuf { protoc { artifact = "com.google.protobuf:protoc:3.5.1-1" } plugins { grpc { artifact = 'io.grpc:protoc-gen-grpc-java:1.15.1' } } generateProtoTasks { all()*.plugins { grpc {} } } }
The prebuilt protoc-gen-grpc-java binary uses glibc on Linux. If you are compiling on Alpine Linux, you may want to use the Alpine grpc-java package which uses musl instead.
APIs annotated with @Internal are for internal use by the gRPC library and should not be used by gRPC users. APIs annotated with @ExperimentalApi are subject to change in future releases, and library code that other projects may depend on should not use these APIs.
We recommend using the grpc-java-api-checker (an Error Prone plugin) to check for usages of @ExperimentalApi and @Internal in any library code that depends on gRPC. It may also be used to check for @Internal usage or unintended @ExperimentalApi consumption in non-library code.
If you are making changes to gRPC-Java, see the compiling instructions.
At a high level there are three distinct layers to the library: Stub, Channel, and Transport.
The Stub layer is what is exposed to most developers and provides type-safe bindings to whatever datamodel/IDL/interface you are adapting. gRPC comes with a plugin to the protocol-buffers compiler that generates Stub interfaces out of .proto files, but bindings to other datamodel/IDL are easy and encouraged.
The Channel layer is an abstraction over Transport handling that is suitable for interception/decoration and exposes more behavior to the application than the Stub layer. It is intended to be easy for application frameworks to use this layer to address cross-cutting concerns such as logging, monitoring, auth, etc.
The Transport layer does the heavy lifting of putting and taking bytes off the wire. The interfaces to it are abstract just enough to allow plugging in of different implementations. Note the transport layer API is considered internal to gRPC and has weaker API guarantees than the core API under package io.grpc.
gRPC comes with three Transport implementations: