.readme-partials.yaml

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  #### Calling Cloud Spanner
  Here is a code snippet showing a simple usage example. Add the following imports
  at the top of your file:

  ```java
  import com.google.cloud.spanner.DatabaseClient;
  import com.google.cloud.spanner.DatabaseId;
  import com.google.cloud.spanner.ResultSet;
  import com.google.cloud.spanner.Spanner;
  import com.google.cloud.spanner.SpannerOptions;
  import com.google.cloud.spanner.Statement;

  ```

  Then, to make a query to Spanner, use the following code:
  ```java
  // Instantiates a client
  SpannerOptions options = SpannerOptions.newBuilder().build();
  Spanner spanner = options.getService();
  String instance = "my-instance";
  String database = "my-database";
  try {
    // Creates a database client
    DatabaseClient dbClient = spanner.getDatabaseClient(
      DatabaseId.of(options.getProjectId(), instance, database));
    // Queries the database
    try (ResultSet resultSet = dbClient.singleUse().executeQuery(Statement.of("SELECT 1"))) {
      // Prints the results
      while (resultSet.next()) {
        System.out.printf("%d\n", resultSet.getLong(0));
      }
    }
  } finally {
    // Closes the client which will free up the resources used
    spanner.close();
  }
  ```

  #### Complete source code

  In [DatabaseSelect.java](https://github.com/googleapis/google-cloud-java/tree/master/google-cloud-examples/src/main/java/com/google/cloud/examples/spanner/snippets/DatabaseSelect.java) we put together all the code shown above in a single program.

  ## Session Pool

  The Cloud Spanner client maintains a session pool, as sessions are expensive to create and are
  intended to be long-lived. The client automatically takes a session from the pool and uses this
  executing queries and transactions.
  See [Session Pool and Channel Pool Configuration](session-and-channel-pool-configuration.md)
  for in-depth background information about sessions and gRPC channels and how these are handled in
  the Cloud Spanner Java client.

  ## OpenCensus Metrics

  Cloud Spanner client supports [Opencensus Metrics](https://opencensus.io/stats/),
  which gives insight into the client internals and aids in debugging/troubleshooting
  production issues. OpenCensus metrics will provide you with enough data to enable you to
  spot, and investigate the cause of any unusual deviations from normal behavior.

  All Cloud Spanner Metrics are prefixed with `cloud.google.com/java/spanner/`. The
  metrics will be tagged with:
  * `database`: the target database name.
  * `instance_id`: the instance id of the target Spanner instance.
  * `client_id`: the user defined database client id.
  * `library_version`: the version of the library that you're using.

  > Note: RPC level metrics can be gleaned from gRPC’s metrics, which are prefixed
  with `grpc.io/client/`.
  ### Available client-side metrics:

  * `cloud.google.com/java/spanner/max_in_use_sessions`: This returns the maximum
    number of sessions that have been in use during the last maintenance window
    interval, so as to provide an indication of the amount of activity currently
    in the database.

  * `cloud.google.com/java/spanner/max_allowed_sessions`: This shows the maximum
    number of sessions allowed.

  * `cloud.google.com/java/spanner/num_sessions_in_pool`: This metric allows users to
     see instance-level and database-level data for the total number of sessions in
     the pool at this very moment.

  * `cloud.google.com/java/spanner/num_acquired_sessions`: This metric allows
    users to see the total number of acquired sessions.

  * `cloud.google.com/java/spanner/num_released_sessions`: This metric allows
    users to see the total number of released (destroyed) sessions.

  * `cloud.google.com/java/spanner/get_session_timeouts`: This gives you an
    indication of the total number of get session timed-out instead of being
    granted (the thread that requested the session is placed in a wait queue where
    it waits until a session is released into the pool by another thread) due to
    pool exhaustion since the server process started.

  * `cloud.google.com/java/spanner/gfe_latency`: This metric shows latency between
    Google's network receiving an RPC and reading back the first byte of the response.

  * `cloud.google.com/java/spanner/gfe_header_missing_count`: This metric shows the
    number of RPC responses received without the server-timing header, most likely
    indicating that the RPC never reached Google's network.

  If you are using Maven, add this to your pom.xml file
  ```xml
  <dependency>
    <groupId>io.opencensus</groupId>
    <artifactId>opencensus-impl</artifactId>
    <version>0.30.0</version>
    <scope>runtime</scope>
  </dependency>
  <dependency>
    <groupId>io.opencensus</groupId>
    <artifactId>opencensus-exporter-stats-stackdriver</artifactId>
    <version>0.30.0</version>
  </dependency>
  ```
  If you are using Gradle, add this to your dependencies
  ```Groovy
  compile 'io.opencensus:opencensus-impl:0.30.0'
  compile 'io.opencensus:opencensus-exporter-stats-stackdriver:0.30.0'
  ```

  At the start of your application configure the exporter:

  ```java
  import io.opencensus.exporter.stats.stackdriver.StackdriverStatsExporter;
  // Enable OpenCensus exporters to export metrics to Stackdriver Monitoring.
  // Exporters use Application Default Credentials to authenticate.
  // See https://developers.google.com/identity/protocols/application-default-credentials
  // for more details.
  // The minimum reporting period for Stackdriver is 1 minute.
  StackdriverStatsExporter.createAndRegister();
  ```

  By default, the functionality is disabled. You need to include opencensus-impl
  dependency to collect the data and exporter dependency to export to backend.

  [Click here](https://medium.com/google-cloud/troubleshooting-cloud-spanner-applications-with-opencensus-2cf424c4c590) for more information.