Retry, Backoff, and Idempotency Policies.

  namespace spanner = ::google::cloud::spanner;
  using ::google::cloud::StatusOr;
  [](std::string const& project_id, std::string const& instance_id,
     std::string const& database_id) {
    // Use a truncated exponential backoff with jitter to wait between
    // retries:
    //   https://en.wikipedia.org/wiki/Exponential_backoff
    //   https://cloud.google.com/storage/docs/exponential-backoff
    auto client = spanner::Client(spanner::MakeConnection(
        spanner::Database(project_id, instance_id, database_id),
        google::cloud::Options{}
            .set<spanner::SpannerRetryPolicyOption>(
                std::make_shared<spanner::LimitedTimeRetryPolicy>(
                    /*maximum_duration=*/std::chrono::seconds(60)))
            .set<spanner::SpannerBackoffPolicyOption>(
                std::make_shared<spanner::ExponentialBackoffPolicy>(
                    /*initial_delay=*/std::chrono::milliseconds(500),
                    /*maximum_delay=*/std::chrono::seconds(64),
                    /*scaling=*/1.5))));

    std::int64_t rows_inserted;
    auto commit_result = client.Commit(
        [&client, &rows_inserted](
            spanner::Transaction txn) -> StatusOr<spanner::Mutations> {
          auto insert = client.ExecuteDml(
              std::move(txn),
              spanner::SqlStatement(
                  "INSERT INTO Singers (SingerId, FirstName, LastName)"
                  "  VALUES (20, 'George', 'Washington')"));
          if (!insert) return std::move(insert).status();
          rows_inserted = insert->RowsModified();
          return spanner::Mutations{};
        });
    if (!commit_result) throw std::move(commit_result).status();
    std::cout << "Rows inserted: " << rows_inserted;
  }