A number of environment variables can be used to configure the behavior of the library. There are also functions to configure this behavior in code. The environment variables are convenient when troubleshooting problems.

Endpoint Overrides

GOOGLE_CLOUD_CPP_SPANNER_DEFAULT_ENDPOINT=...: changes the default endpoint (spanner.googleapis.com) for the library. Intended for testing only.

SPANNER_EMULATOR_HOST=host:port: tells the library to connect to the specified emulator rather than the default endpoint. Intended for testing only.

See also: google::cloud::EndpointOption

Default behaviors

SPANNER_OPTIMIZER_VERSION=n: sets the default query optimizer version to use in Client::ExecuteQuery() calls.

See also: google::cloud::spanner::QueryOptimizerVersionOption

SPANNER_OPTIMIZER_STATISTICS_PACKAGE=...: specifies a statistics package for the query optimizer to use when compiling a SQL query.

See also: google::cloud::spanner::QueryOptimizerStatisticsPackageOption

Logging

GOOGLE_CLOUD_CPP_ENABLE_TRACING=rpc: turns on tracing for most gRPC calls. The library injects an additional Stub decorator that prints each gRPC request and response. Unless you have configured your own logging backend, you should also set GOOGLE_CLOUD_CPP_ENABLE_CLOG to produce any output on the program's console.

See also: google::cloud::TracingComponentsOption

GOOGLE_CLOUD_CPP_TRACING_OPTIONS=...: modifies the behavior of gRPC tracing, including whether messages will be output on multiple lines, or whether string/bytes fields will be truncated.

See also: google::cloud::GrpcTracingOptionsOption

GOOGLE_CLOUD_CPP_ENABLE_CLOG=yes: turns on logging in the library, basically the library always "logs" but the logging infrastructure has no backend to actually print anything until the application sets a backend or they set this environment variable.

See also: google::cloud::LogBackend; google::cloud::LogSink

Other Environment Variables

GOOGLE_CLOUD_PROJECT=...: is used in examples and integration tests to configure the GCP project. This has no effect in the library.

GOOGLE_CLOUD_CPP_SPANNER_TEST_INSTANCE_ID=... is used in examples and integration tests to, well, configure the spanner instance. This has no effect in the library.

GOOGLE_CLOUD_CPP_SPANNER_TEST_SERVICE_ACCOUNT=...: is used in examples and integration tests to set the service account for testing IAM operations. This has no effect in the library.