@@ -42,6 +42,7 @@
 import java.util.Set;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
+import javax.annotation.Nullable;
 
 /**
 * Internal connection API for Google Cloud Spanner. This class may introduce breaking changes
@@ -152,6 +153,7 @@ public String[] getValidValues() {
 private static final String DEFAULT_NUM_CHANNELS = null;
 private static final String DEFAULT_USER_AGENT = null;
 private static final String DEFAULT_OPTIMIZER_VERSION = "";
+private static final boolean DEFAULT_LENIENT = false;
 
 private static final String PLAIN_TEXT_PROTOCOL = "http:";
 private static final String HOST_PROTOCOL = "https:";
@@ -176,6 +178,8 @@ public String[] getValidValues() {
 private static final String USER_AGENT_PROPERTY_NAME = "userAgent";
 /** Query optimizer version to use for a connection. */
 private static final String OPTIMIZER_VERSION_PROPERTY_NAME = "optimizerVersion";
+/** Name of the 'lenientMode' connection property. */
+public static final String LENIENT_PROPERTY_NAME = "lenient";
 
 /** All valid connection properties. */
 public static final Set<ConnectionProperty> VALID_PROPERTIES =
@@ -212,7 +216,11 @@ public String[] getValidValues() {
 "The custom user-agent property name to use when communicating with Cloud Spanner. This property is intended for internal library usage, and should not be set by applications."),
 ConnectionProperty.createStringProperty(
 OPTIMIZER_VERSION_PROPERTY_NAME,
-"Sets the default query optimizer version to use for this connection."))));
+"Sets the default query optimizer version to use for this connection."),
+ConnectionProperty.createBooleanProperty(
+LENIENT_PROPERTY_NAME,
+"Silently ignore unknown properties in the connection string/properties (true/false)",
+DEFAULT_LENIENT))));
 
 private static final Set<ConnectionProperty> INTERNAL_PROPERTIES =
 Collections.unmodifiableSet(
@@ -416,6 +424,7 @@ public static Builder newBuilder() {
 }
 
 private final String uri;
+private final String warnings;
 private final String credentialsUrl;
 private final String oauthToken;
 private final Credentials fixedCredentials;
@@ -441,7 +450,7 @@ private ConnectionOptions(Builder builder) {
 Matcher matcher = Builder.SPANNER_URI_PATTERN.matcher(builder.uri);
 Preconditions.checkArgument(
 matcher.find(), String.format("Invalid connection URI specified: %s", builder.uri));
-checkValidProperties(builder.uri);
+this.warnings = checkValidProperties(builder.uri);
 
 this.uri = builder.uri;
 this.sessionPoolOptions = builder.sessionPoolOptions;
@@ -574,6 +583,12 @@ static String parseOptimizerVersion(String uri) {
 return value != null ? value : DEFAULT_OPTIMIZER_VERSION;
 }
 
+@VisibleForTesting
+static boolean parseLenient(String uri) {
+String value = parseUriProperty(uri, LENIENT_PROPERTY_NAME);
+return value != null ? Boolean.valueOf(value) : DEFAULT_LENIENT;
+}
+
 @VisibleForTesting
 static String parseUriProperty(String uri, String property) {
 Pattern pattern = Pattern.compile(String.format("(?is)(?:;|\\?)%s=(.*?)(?:;|$)", property));
@@ -586,9 +601,10 @@ static String parseUriProperty(String uri, String property) {
 
 /** Check that only valid properties have been specified. */
 @VisibleForTesting
-static void checkValidProperties(String uri) {
+static String checkValidProperties(String uri) {
 String invalidProperties = "";
 List<String> properties = parseProperties(uri);
+boolean lenient = parseLenient(uri);
 for (String property : properties) {
 if (!INTERNAL_VALID_PROPERTIES.contains(ConnectionProperty.createEmptyProperty(property))) {
 if (invalidProperties.length() > 0) {
@@ -597,9 +613,17 @@ static void checkValidProperties(String uri) {
 invalidProperties = invalidProperties + property;
 }
 }
-Preconditions.checkArgument(
-invalidProperties.isEmpty(),
-"Invalid properties found in connection URI: " + invalidProperties.toString());
+if (lenient) {
+return String.format(
+"Invalid properties found in connection URI: %s", invalidProperties.toString());
+} else {
+Preconditions.checkArgument(
+invalidProperties.isEmpty(),
+String.format(
+"Invalid properties found in connection URI. Add lenient=true to the connection string to ignore unknown properties. Invalid properties: %s",
+invalidProperties.toString()));
+return null;
+}
 }
 
 @VisibleForTesting
@@ -706,6 +730,12 @@ public boolean isRetryAbortsInternally() {
 return retryAbortsInternally;
 }
 
+/** Any warnings that were generated while creating the {@link ConnectionOptions} instance. */
+@Nullable
+public String getWarnings() {
+return warnings;
+}
+
 /** Use http instead of https. Only valid for (local) test servers. */
 boolean isUsePlainText() {
 return usePlainText;

@@ -386,4 +386,39 @@ public void testSetOAuthTokenAndCredentials() {
 assertThat(e.getMessage()).contains("Cannot specify both credentials and an OAuth token");
 }
 }
+
+@Test
+public void testLenient() {
+ConnectionOptions options =
+ConnectionOptions.newBuilder()
+.setUri(
+"cloudspanner:/projects/test-project-123/instances/test-instance/databases/test-database?lenient=true;foo=bar")
+.setCredentialsUrl(FILE_TEST_PATH)
+.build();
+assertThat(options.getWarnings()).isNotNull();
+assertThat(options.getWarnings()).contains("foo");
+assertThat(options.getWarnings()).doesNotContain("lenient");
+
+options =
+ConnectionOptions.newBuilder()
+.setUri(
+"cloudspanner:/projects/test-project-123/instances/test-instance/databases/test-database?bar=foo;lenient=true")
+.setCredentialsUrl(FILE_TEST_PATH)
+.build();
+assertThat(options.getWarnings()).isNotNull();
+assertThat(options.getWarnings()).contains("bar");
+assertThat(options.getWarnings()).doesNotContain("lenient");
+
+try {
+options =
+ConnectionOptions.newBuilder()
+.setUri(
+"cloudspanner:/projects/test-project-123/instances/test-instance/databases/test-database?bar=foo")
+.setCredentialsUrl(FILE_TEST_PATH)
+.build();
+fail("missing expected exception");
+} catch (IllegalArgumentException e) {
+assertThat(e.getMessage()).contains("bar");
+}
+}
 }