Logging Components

Introduction

The client libraries never use logging to report errors, but logging can be enabled to help troubleshoot problems when the last error message does not provide a good enough indication of the root cause.

In general, we abide by the following principles:

• Logging should be controlled by the application developer. Unless explicitly instructed, the libraries produce no output to the console, except to emit a message to std::clog immediately before a GCP_LOG(FATAL) terminates the process.
• Logging should have very low cost:
  • It should be possible to disable logs at compile time. They should disappear as-if there were #ifdef / #endif directives around them.
  • A log line at a disabled log level should be about as expensive as an extra if() statement. At the very least it should not incur additional memory allocations or locks.
• It should be easy to log complex objects.
• The logging framework should play well with the C++ iostream classes.
• The application should be able to intercept log records and re-direct them to their own logging framework.

Enabling logs

The application needs to do two things to enable logging:

• First, to configure the destination of the logs you must add a backend (see AddBackend ) to the default LogSink .
• Second, you must configure what gets logged. Typically, you initialize the *Connection object with a TracingComponentsOption . Consult the documentation for each *Client class to find what tracing components are available.

At run-time, setting the GOOGLE_CLOUD_CPP_ENABLE_CLOG to a non-empty value configures a LogBackend that uses std::clog . Likewise, setting the GOOGLE_CLOUD_CPP_ENABLE_TRACING=a,b will enable tracing for components a and b across allclient objects. The most common components are auth , rpc , and rpc-streams .

Note that while std::clog is buffered, the framework will flush any log message at severity WARNING or higher.

Example: Logging From Library

Use the GCP_LOG() macro to log from a Google Cloud Platform C++ library:

 void LibraryCode(ComplexThing const& thing) {
  GCP_LOG(INFO) << "I am here";
  if (thing.is_bad()) {
    GCP_LOG(ERROR) << "Poor thing is bad: " << thing;
  }
} 

Example: Enable Logs to std::clog

To enable logs to std::clog the application can call:

 void AppCode() {
  google::cloud::LogSink::EnableStdClog();
} 

As previously noted, this can be switched at run-time using the GOOGLE_CLOUD_CPP_ENABLE_CLOG environment variable.