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.
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::TracingComponentsOptionGOOGLE_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::GrpcTracingOptionsOptionGOOGLE_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.
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.
[[["Easy to understand","easyToUnderstand","thumb-up"],["Solved my problem","solvedMyProblem","thumb-up"],["Other","otherUp","thumb-up"]],[["Missing the information I need","missingTheInformationINeed","thumb-down"],["Too complicated / too many steps","tooComplicatedTooManySteps","thumb-down"],["Out of date","outOfDate","thumb-down"],["Samples / code issue","samplesCodeIssue","thumb-down"],["Other","otherDown","thumb-down"]],["Last updated 2025-08-22 UTC."],[[["\u003cp\u003eThis page details environment variables that can configure the behavior of the Spanner C++ client library, alongside corresponding functions for in-code configuration.\u003c/p\u003e\n"],["\u003cp\u003eUsers can override the default endpoint or connect to a Spanner emulator via \u003ccode\u003eGOOGLE_CLOUD_CPP_SPANNER_DEFAULT_ENDPOINT\u003c/code\u003e and \u003ccode\u003eSPANNER_EMULATOR_HOST\u003c/code\u003e environment variables, respectively, primarily for testing purposes.\u003c/p\u003e\n"],["\u003cp\u003eThe environment variables \u003ccode\u003eSPANNER_OPTIMIZER_VERSION\u003c/code\u003e and \u003ccode\u003eSPANNER_OPTIMIZER_STATISTICS_PACKAGE\u003c/code\u003e allow for customization of the query optimizer used in \u003ccode\u003eClient::ExecuteQuery()\u003c/code\u003e calls.\u003c/p\u003e\n"],["\u003cp\u003eLogging and tracing for gRPC calls can be enabled and customized using \u003ccode\u003eGOOGLE_CLOUD_CPP_ENABLE_TRACING\u003c/code\u003e, \u003ccode\u003eGOOGLE_CLOUD_CPP_TRACING_OPTIONS\u003c/code\u003e, and \u003ccode\u003eGOOGLE_CLOUD_CPP_ENABLE_CLOG\u003c/code\u003e environment variables.\u003c/p\u003e\n"],["\u003cp\u003eThe page also provides links to the various reference pages, with the latest version being \u003ccode\u003e2.37.0-rc\u003c/code\u003e, and previous versions going back to \u003ccode\u003e2.11.0\u003c/code\u003e.\u003c/p\u003e\n"]]],[],null,[]]
