How to update the C++ sources of gRPC:

1. Update the gRPC definitions in WORKSPACE file, currently we use https://github.com/grpc/grpc/archive/v1.31.1.tar.gz
2. Update the gRPC patch file if necessary, it mostly helps avoid unnecessary dependencies.
3. Update third_party/grpc/BUILD to redirect targets to @com_github_grpc_grpc if necessary.

How to update the BUILD/bzl sources of gRPC:

1. git clone http://github.com/grpc/grpc.git in a convenient directory
2. git checkout <tag> (current is v1.31.1, commithash 7d7e456762)
3. mkdir -p third_party/grpc/bazel
4. cp <gRPC git tree>/bazel/{BUILD,cc_grpc_library.bzl,generate_cc.bzl,protobuf.bzl} third_party/grpc/bazel
5. In the third_party/grpc directory, apply local patches: patch -p3 < bazel_1.31.1.patch

How to update the Java plugin:

1. Checkout tag v1.26.0 from https://github.com/grpc/grpc-java
2. cp -R <grpc-java git tree>/compiler/src/java_plugin third_party/grpc/compiler/src

How to update the Java code:

Download the necessary jars at version 1.26.0 from maven central.