Add docs for Abseil

third_party/ABSEIL_MANUAL.md

@@ -0,0 +1,30 @@
+# Abseil in gRPC
+
+This document explains how to use Abseil throughout gRPC. Note that this isn't
+supposed to explain general usage of Abseil.
+
+## The version of Abseil
+
+gRPC is inteded to use the LTS versions of Abseil only because it simplfies
+dependency management. Abseil is being distributed via package distribution
+systems such as vcpkg and cocoapods. If gRPC depends on the certain version
+that aren't registered, gRPC in that system wouldn't build or work.
+Therefore, gRPC will use the LTS version only, preferably the latest one.
+
+## Libraries that are not ready to use
+
+Most of Abseil libraries are okay to use but there are some exceptions
+because they're not going well yet on some of our test machinaries or
+platforms it supports. Following is targets that are NOT ready to use.
+
+- `absl/synchronization:*`: This will be ready from the LTS version in 2021.
+- `absl/random`: [WIP](https://github.com/grpc/grpc/pull/23346).
+- `absl/types:variant`: [WIP](https://github.com/grpc/grpc/pull/22961).
+
+## Implemetation only
+
+You can use Abseil in gRPC Core and gRPC C++. But you cannot use it in
+the public interface of gRPC C++ because i) it doesn't gurantee no breaking
+API changes like gRPC C++ does and ii) it may make users change their build
+system to address Abseil.  
+ 

third_party/README.md

@@ -48,6 +48,14 @@ Usually the process is
 
 Updating some dependencies requires extra care.
 
+### Updating third_party/abseil-cpp
+
+- Two additional steps should be done before running `generate_projects.sh` above.
+  - Running `src/abseil-cpp/preprocessed_builds.yaml.gen.py`.
+  - Updating `abseil_version =` scripts in `templates/gRPC-C++.podspec.template` and 
+    `templates/gRPC-Core.podspec.template`.
+- You can see an example of previous [upgrade](https://github.com/grpc/grpc/pull/24270).
+
 ### Updating third_party/boringssl-with-bazel
 
 - Update the `third_party/boringssl-with-bazel` submodule to the latest [`master-with-bazel`](https://github.com/google/boringssl/tree/master-with-bazel) branch