ARCHITECTURE.md

Architecture

This document describes the high-level architecture of the Google Cloud C++ Client libraries. Its main audience are developers and contributors making changes and additions to these libraries. If you want to familiarize yourself with the code in the google-cloud-cpp project, you are at the right place.

While we expect users of the libraries may find this document useful, this document does not change or define the public API. You can use this document to understand how things work, or maybe to troubleshoot problems. You should not depend on the implementation details described here to write your application. Only the public API is stable, the rest is subject to change without notice.

What these libraries do

The goal of the libraries is to provide idiomatic C++ libraries to access services in Google Cloud Platform. All services are in scope. As of 2024-03 we have over 100 libraries, covering most Google Cloud Platform services.

What do we mean by idiomatic? We mean that C++ developers will find the APIs familiar, or "natural", that these APIs will fit well with the rest of the C++ ecosystem, and that very few new "concepts" are needed to understand how to use these libraries.

More specifically, the functionality offered by these libraries include:

• The libraries automatically retry RPCs that fail with a retryable error, such as network failures.
• The libraries only retry operations when it is safe to do so. For example, "delete the first row" is not a safe operation to retry when the previous result is unknown. In the documentation we refer to operations that are safe to retry as idempotent.
• The libraries also resume large downloads or database reads from the last fully received element, minimizing the amount of duplicate data received by your application.
• The application can always change the retry and resume policies. The libraries have reasonable defaults for these policies, and implement best practices for backing off before retrying an operation. If these defaults do not work in your specific use-case, you can change them.
• The libraries convert long-running operations into a future<T>, an object that will get a value asynchronously. The library takes care of polling the long-running operation until it is completed (or fails).
• The libraries convert pagination APIs into C++ iterators.
• The libraries can be configured to log RPCs and responses, to help application developers troubleshoot their code.
• The libraries return errors using an "outcome" object: StatusOr<T>. Similar to the upcoming std::expected<T, E> class in C++23.
• Application developers can convert outcome objects to exceptions without much effort.

Where is the code?

Each library is found in a subdirectory of /google/cloud/. The name of the subdirectory is chosen to match the most distinctive part of the service name. For example, /google/cloud/storage contains the code for the Google Cloud Storage service.

Within each directory you will find (almost always) the same structure:

• ${library}/*.h are the public headers for the library. The same directory contains the ${library}/*.cc implementation files and *_test.cc for unit tests. More often than not all the unit tests for foo.{h,cc} are found in foo_test.cc, but there may be exceptions when the tests grow too large to fit in a single file (we have lots of tests!).
• mocks/ also contains public APIs, typically this is a header-only library defining mocks so application developers can mock the library in their own tests. Note that some libraries do not have mocks.
• quickstart/ contains a small program showing how to use the library in a larger project. This is intended for application developers wanting to, well, use the libraries quickly.
• samples/ (sometimes examples/) contain code snippets that are extracted both into the Doxygen reference documentation and to the cloud.google.com site, as part of the service documentation. Some libraries have a single program in this directory, others have smaller programs (or more samples).

The remaining directories are implementation details, or at least only intended to be interesting for google-cloud-cpp developers and contributors.

• internal/ contains implementation details for the library. The files in these directories are not intended for public consumption, but contributors and developers need to change them, of course. The relationship between .h, .cc and _test.cc files mirrors the one found in ${library}/.
• benchmarks/ contains "macro benchmarks" for the library, typically programs that interact with the production environment to verify the library meets the latency, throughput, and CPU overhead (as in "low CPU overhead") requirements.
• ci/ may contain helper scripts used in the CI builds, for example, to startup and shutdown any emulators used by the library.
• doc/ contains the landing page for any Doxygen documentation, as well as additional Doxygen docs that are not extracted from the code comments.
• integration_tests/ (or simply tests/ in some libraries) contain any integration tests where we test the library using emulators and/or the production environment.
• testing/ contains testing utilities shared by the unit tests and the integration tests of the library.

The *Client classes

The main interface for each library is the ${library}::*Client class. Roughly speaking each "client" corresponds to a service in the .proto definitions for the service. It is common for services to have separate service definitions for "admin" operations, such as creating a new instance of the service, or changing permissions, as opposed to "normal" operations, such as inserting a new row, or publishing a new message. When "admin" services are defined, there are separate ${library}::*Client objects.

These are some examples:

• Bigtable has a bigtable::DataClient client. It also has two admin clients: bigtable_admin::BigtableInstanceAdminClient and bigtable_admin::BigtableTableAdminClient.
• Storage has a single storage::Client client class.
• Pub/Sub has two non-admin clients, the pubsub::Publisher and the pubsub::Subscriber. It also has two admin clients: pubsub::TopicAdminClient, and pubsub::SubscriptionAdminClient.
• Spanner has one non-admin client, the spanner::Client. It also has two admin clients: spanner_admin::InstanceAdminClient, and spanner_admin::DatabaseAdminClient.

Generally these classes are very "thin"; they take function arguments from the application, package them in lightweight structure, and then forward the request to the ${library}::*Connection class.

It is important to know that almost always there is one RPC generated by each *Client member functions and RPCs. That "almost" is (as the saying goes) "load bearing", the devil is, as usual, in the details.

The *Connection classes

Connections serve two functions:

• They are interfaces, which applications can mock to implement tests against the google-cloud-cpp library.
• The default implementation injects the "retry loop" for each RPC.

Most of the time there is a 1-1 mapping between a FooClient and the corresponding FooConnection.

Because *Connection classes are intended for customers to use (at least as mocks), they are part of the public API.

Typically there are three concrete versions of the *Connection interface:

Name	Description
*Impl	An implementation using the *Stub layer
*Tracing	Instrument each retry loop with an OpenTelemetry trace span
Mock*Connection	An implementation using googlemock

Only Mock*Connection is part of the public API. It is used by application developers that want to test their code using a mocked behavior for the *Client class.

In some cases you may find a fourth implementation, used to implement clients over HTTP and gRPC Transcoding. This class is also not part of the public API.

Name	Description
*Rest*Impl	An implementation using the *Rest*Stub layer

The *Stub classes

The *Stub classes wrap the *Stub generated by gRPC. They provide several functions:

• They are pure interfaces, which allow google-cloud-cpp developers to mock them in our tests.
• They are not part of the public interface, they live in ${library}_internal, and therefore give us some flexibility to change things.
• They return StatusOr<T>, simplifying the usage of these functions.
• They are the classes most amenable for automatic code generation.
• When using asynchronous APIs, they return google::cloud::future<StatusOr<T>>. This is preferred over using a void* to match requests and responses.

The *Stub classes are typically organized as a (small) stack of Decorators, which simplifies their testing.

Layer	Description
*Logging	Optional *Stub decorator that logs each request and response
*Metadata	Injects resource metadata headers for routing
*RoundRobin	Round-robins over several *Stubs, not all libraries have them
*Tracing	Instrument each RPC with an OpenTelemetry trace span

For services where we have enabled REST support, there is a parallel set of *Rest*Stub classes. These implement the same functionality, but make calls using HTTP and gRPC Transcoding.

The Options class(es)

Many functions need a way for a user to specify optional settings. This was traditionally done with distinct classes, like spanner::ConnectionOptions, spanner::QueryOptions, or storage::ClientOptions. These classes often had very different interfaces and semantics (e.g., some included a meaningful default value, others didn't). The new, recommended way to represent options of all varieties is using the google::cloud::Options class.

Any function that needs to accept optional settings should do so by accepting an instance of google::cloud::Options, and by documenting which option classes are expected so that users know how the function can be configured. Functions that accept the old-style option classes can continue to exist and should forward to the new Options-based overload. These old functions need not even be deprecated because they should work just fine. However, to avoid burdening users with unnecessary decisions, functions should clearly document that the Options overload is to be preferred.

Each setting for an Options instance is a unique type. To improve discoverability of available option types, we should minimize the places where users have to look to find them to common_options.h, grpc_options.h, and (preferably) a single <product>/options.h file (e.g., spanner/options.h). It's OK to introduce additional options files, but keep discoverability in mind.

Instances of Options do not contain any default values. Defaults should be computed by a service-specific function, such as spanner_internal::DefaultOptions(). This function (or a related one) is used by our implementations to augment the (optionally) user-provided Options instance with appropriate defaults for the given service. Defaults should be computed in the user-facing function that accepted the Options argument so that all the internal implementation functions lower in the stack can simply accept the Options by const& and can assume it's properly populated. The user-facing function that documented to the user which options it accepts should also call google::cloud::internal::CheckExpectedOptions<...>(...) in order to help users diagnose option-related issues in their code.

Deviations from the "normal" Architecture

Pub/Sub

Pub/Sub generally follow these patterns, but there is substantial code outside the main classes to implement a few features:

• The Publisher needs to buffer messages until there is enough of them to justify a AsyncPublish() call. Moreover, when using ordering keys the library maintains separate buffers per ordering keys, and must respect message ordering. There is a whole hierarchy of Batching* classes to deal with these requirements.
• Likewise, the Subscriber must deliver messages with ordering keys one at a time, and must implement flow control for messages delivered to the application. There is a series of classes dedicate to keeping a "session" working correctly.

Spanner

Spanner implements some key features in the spanner_internal::SessionPool.

Storage

In Storage the *Connection classes are in the storage::internal namespace, which forces our users to reach into the internal namespace to mock things. There is an open bug to fix this. It would involve moving all the *Request and *Response classes out of storage::internal. Some of the member functions in these classes should not be part of the public API. In short, the changes are more involved than a simple git mv.