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 setGOOGLE_CLOUD_CPP_ENABLE_CLOGto 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"]],[["Hard to understand","hardToUnderstand","thumb-down"],["Incorrect information or sample code","incorrectInformationOrSampleCode","thumb-down"],["Missing the information/samples I need","missingTheInformationSamplesINeed","thumb-down"],["Other","otherDown","thumb-down"]],["Last updated 2025-09-04 UTC."],[[["\u003cp\u003eThis page details environment variables for configuring the behavior of the Spanner C++ client library, including options for testing, logging, and default settings.\u003c/p\u003e\n"],["\u003cp\u003eThe latest release candidate version available is 2.37.0-rc, with a list of previous versions down to 2.11.0 available for use and documentation.\u003c/p\u003e\n"],["\u003cp\u003eEnvironment variables like \u003ccode\u003eSPANNER_EMULATOR_HOST\u003c/code\u003e and \u003ccode\u003eGOOGLE_CLOUD_CPP_SPANNER_DEFAULT_ENDPOINT\u003c/code\u003e allow for endpoint overrides, mainly for testing purposes.\u003c/p\u003e\n"],["\u003cp\u003eSettings for query optimization, such as \u003ccode\u003eSPANNER_OPTIMIZER_VERSION\u003c/code\u003e and \u003ccode\u003eSPANNER_OPTIMIZER_STATISTICS_PACKAGE\u003c/code\u003e, can be configured to influence query compilation and execution.\u003c/p\u003e\n"],["\u003cp\u003eTracing and logging can be managed 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 to help with troubleshooting and understanding library operations.\u003c/p\u003e\n"]]],[],null,["Version 2.15.1keyboard_arrow_down\n\n- [2.42.0-rc (latest)](/cpp/docs/reference/spanner/latest/spanner-env)\n- [2.41.0](/cpp/docs/reference/spanner/2.41.0/spanner-env)\n- [2.40.0](/cpp/docs/reference/spanner/2.40.0/spanner-env)\n- [2.39.0](/cpp/docs/reference/spanner/2.39.0/spanner-env)\n- [2.38.0](/cpp/docs/reference/spanner/2.38.0/spanner-env)\n- [2.37.0](/cpp/docs/reference/spanner/2.37.0/spanner-env)\n- [2.36.0](/cpp/docs/reference/spanner/2.36.0/spanner-env)\n- [2.35.0](/cpp/docs/reference/spanner/2.35.0/spanner-env)\n- [2.34.0](/cpp/docs/reference/spanner/2.34.0/spanner-env)\n- [2.33.0](/cpp/docs/reference/spanner/2.33.0/spanner-env)\n- [2.32.0](/cpp/docs/reference/spanner/2.32.0/spanner-env)\n- [2.31.0](/cpp/docs/reference/spanner/2.31.0/spanner-env)\n- [2.30.0](/cpp/docs/reference/spanner/2.30.0/spanner-env)\n- [2.29.0](/cpp/docs/reference/spanner/2.29.0/spanner-env)\n- [2.28.0](/cpp/docs/reference/spanner/2.28.0/spanner-env)\n- [2.27.0](/cpp/docs/reference/spanner/2.27.0/spanner-env)\n- [2.26.0](/cpp/docs/reference/spanner/2.26.0/spanner-env)\n- [2.25.1](/cpp/docs/reference/spanner/2.25.1/spanner-env)\n- [2.24.0](/cpp/docs/reference/spanner/2.24.0/spanner-env)\n- [2.23.0](/cpp/docs/reference/spanner/2.23.0/spanner-env)\n- [2.22.1](/cpp/docs/reference/spanner/2.22.1/spanner-env)\n- [2.21.0](/cpp/docs/reference/spanner/2.21.0/spanner-env)\n- [2.20.0](/cpp/docs/reference/spanner/2.20.0/spanner-env)\n- [2.19.0](/cpp/docs/reference/spanner/2.19.0/spanner-env)\n- [2.18.0](/cpp/docs/reference/spanner/2.18.0/spanner-env)\n- [2.17.0](/cpp/docs/reference/spanner/2.17.0/spanner-env)\n- [2.16.0](/cpp/docs/reference/spanner/2.16.0/spanner-env)\n- [2.15.1](/cpp/docs/reference/spanner/2.15.1/spanner-env)\n- [2.14.0](/cpp/docs/reference/spanner/2.14.0/spanner-env)\n- [2.13.0](/cpp/docs/reference/spanner/2.13.0/spanner-env)\n- [2.12.0](/cpp/docs/reference/spanner/2.12.0/spanner-env)\n- [2.11.0](/cpp/docs/reference/spanner/2.11.0/spanner-env) \n\nEnvironment Variables\n=====================\n\nA 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.\n\nEndpoint Overrides\n------------------\n\n`GOOGLE_CLOUD_CPP_SPANNER_DEFAULT_ENDPOINT=...`: changes the default endpoint (spanner.googleapis.com) for the library. Intended for testing only.\n\n`SPANNER_EMULATOR_HOST=host:port`: tells the library to connect to the specified emulator rather than the default endpoint. Intended for testing only.\n\n###### See Also\n\n[`google::cloud::EndpointOption`](https://cloud.google.com/cpp/docs/reference/common/latest/structgoogle_1_1cloud_1_1EndpointOption.html)\n\nDefault behaviors\n-----------------\n\n`SPANNER_OPTIMIZER_VERSION=n`: sets the default query optimizer version to use in `Client::ExecuteQuery()` calls.\n\n###### See Also\n\n[`google::cloud::spanner::QueryOptimizerVersionOption`](/cpp/docs/reference/spanner/2.15.1/structgoogle_1_1cloud_1_1spanner_1_1QueryOptimizerVersionOption)`SPANNER_OPTIMIZER_STATISTICS_PACKAGE=...`: specifies a statistics package for the query optimizer to use when compiling a SQL query.\n\n###### See Also\n\n[`google::cloud::spanner::QueryOptimizerStatisticsPackageOption`](/cpp/docs/reference/spanner/2.15.1/structgoogle_1_1cloud_1_1spanner_1_1QueryOptimizerStatisticsPackageOption)\n\nLogging\n-------\n\n`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.\n\n###### See Also\n\n[`google::cloud::TracingComponentsOption`](https://cloud.google.com/cpp/docs/reference/common/latest/structgoogle_1_1cloud_1_1TracingComponentsOption.html)`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.\n\n###### See Also\n\n[`google::cloud::GrpcTracingOptionsOption`](https://cloud.google.com/cpp/docs/reference/common/latest/structgoogle_1_1cloud_1_1GrpcTracingOptionsOption.html)`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.\n\n###### See Also\n\n[`google::cloud::LogBackend`](https://cloud.google.com/cpp/docs/reference/common/latest/classgoogle_1_1cloud_1_1LogBackend.html)\n\n###### See Also\n\n[`google::cloud::LogSink`](https://cloud.google.com/cpp/docs/reference/common/latest/classgoogle_1_1cloud_1_1LogSink.html)\n\nOther Environment Variables\n---------------------------\n\n`GOOGLE_CLOUD_PROJECT=...`: is used in examples and integration tests to configure the GCP project. This has no effect in the library.\n\n`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.\n\n`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."]]
