support multiple URLs in connection string

the underlying OpenSearch library supports multiple URLs, thus we should
do the same here as well.

since Liquibase has no concept of this on its own we have to use a
workaround and use a character to concatenate multiple URLs together.
`,` is being used for this as it's normally not part of URLs and it's
already used in the OpenSearch client for this purpose.
the `opensearch:` prefix should not be repeated.

i.e. a URL with multiple addresses should be defined like this:
```
opensearch:http://localhost:9200,http://localhost:9201,http://localhost:9203
```
the login information is the same for all, since they must form a
cluster.

src/main/java/liquibase/ext/opensearch/database/OpenSearchConnection.java

@@ -5,6 +5,7 @@
 import lombok.Getter;
 import lombok.NoArgsConstructor;
 import lombok.Setter;
+import lombok.SneakyThrows;
 import org.apache.hc.client5.http.auth.AuthScope;
 import org.apache.hc.client5.http.auth.UsernamePasswordCredentials;
 import org.apache.hc.client5.http.impl.auth.BasicCredentialsProvider;
@@ -28,10 +29,15 @@
 import java.security.KeyStoreException;
 import java.security.NoSuchAlgorithmException;
 import java.sql.Driver;
+import java.util.Arrays;
+import java.util.List;
+import java.util.Objects;
 import java.util.Optional;
 import java.util.Properties;
+import java.util.stream.Collectors;
 
 import static liquibase.ext.opensearch.database.OpenSearchLiquibaseDatabase.OPENSEARCH_PREFIX;
+import static liquibase.ext.opensearch.database.OpenSearchLiquibaseDatabase.OPENSEARCH_URI_SEPARATOR;
 
 @Getter
 @Setter
@@ -41,7 +47,7 @@ public class OpenSearchConnection extends AbstractNoSqlConnection {
     private OpenSearchClient openSearchClient;
     private Optional<OpenSearchVersionInfo> openSearchVersion = Optional.empty();
 
-    private URI uri;
+    private List<URI> uris;
     private Properties connectionProperties;
 
     @Override
@@ -62,18 +68,29 @@ public void open(final String url, final Driver driverObject, final Properties d
         this.connectionProperties = driverProperties;
 
         try {
-            this.uri = new URI(realUrl);
+            this.uris = Arrays.stream(realUrl.split(OPENSEARCH_URI_SEPARATOR))
+                    .map(this::toUri)
+                    .filter(Objects::nonNull)
+                    .toList();
             this.connect();
         } catch (final Exception e) {
             throw new DatabaseException("Could not open connection to database: " + realUrl, e);
         }
     }
 
+    private URI toUri(String uri) {
+        try {
+            return URI.create(uri);
+        } catch (IllegalArgumentException ex) {
+            return null;
+        }
+    }
+
     @Override
     public void close() throws DatabaseException {
         this.openSearchClient = null;
         this.connectionProperties = null;
-        this.uri = null;
+        this.uris = null;
     }
 
     @Override
@@ -88,7 +105,9 @@ public String getDatabaseProductName() throws DatabaseException {
 
     @Override
     public String getURL() {
-        return this.uri.toString();
+        return this.uris.stream()
+                .map(URI::toString)
+                .collect(Collectors.joining(OPENSEARCH_URI_SEPARATOR));
     }
 
     @Override
@@ -101,21 +120,21 @@ public boolean isClosed() throws DatabaseException {
         return this.openSearchClient == null;
     }
 
-    private void connect() throws DatabaseException {
-        final HttpHost host = HttpHost.create(this.uri);
+    private void connect() {
+        final var hosts = this.uris.stream().map(HttpHost::create).toList();
+        final var hostsArray = hosts.toArray(HttpHost[]::new);
 
         final var transport = ApacheHttpClient5TransportBuilder
-                .builder(host)
+                .builder(hostsArray)
                 .setHttpClientConfigCallback(httpClientBuilder -> {
                     // TODO: support other credential providers
                     final var username = Optional.ofNullable(this.connectionProperties.getProperty("user"));
                     final var password = Optional.ofNullable(this.connectionProperties.getProperty("password"));
 
                     if (username.isPresent()) {
-                        final BasicCredentialsProvider credentialsProvider = new BasicCredentialsProvider();
-                        credentialsProvider.setCredentials(new AuthScope(host),
-                                new UsernamePasswordCredentials(username.get(), password.orElse("").toCharArray()));
-
+                        final var credentialsProvider = new BasicCredentialsProvider();
+                        final var credentials = new UsernamePasswordCredentials(username.get(), password.orElse("").toCharArray());
+                        hosts.forEach(host -> credentialsProvider.setCredentials(new AuthScope(host), credentials));
                         httpClientBuilder.setDefaultCredentialsProvider(credentialsProvider);
                     } else if (password.isPresent()) {
                         throw new RuntimeException("password provided but username not set!");

src/main/java/liquibase/ext/opensearch/database/OpenSearchLiquibaseDatabase.java

@@ -10,6 +10,7 @@ public class OpenSearchLiquibaseDatabase extends AbstractNoSqlDatabase {
     public static final String PRODUCT_NAME = "OpenSearch";
     public static final String PRODUCT_SHORT_NAME = "opensearch";
     public static final String OPENSEARCH_PREFIX = PRODUCT_SHORT_NAME + ":";
+    public static final String OPENSEARCH_URI_SEPARATOR = ",";
 
     @Override
     public void dropDatabaseObjects(final CatalogAndSchema schemaToDrop) throws LiquibaseException {

src/test/java/liquibase/ext/opensearch/AbstractOpenSearchLiquibaseIT.java

@@ -25,10 +25,12 @@
 @Testcontainers
 public abstract class AbstractOpenSearchLiquibaseIT {
     protected OpenSearchLiquibaseDatabase database;
-    private OpenSearchConnection connection;
+    protected OpenSearchConnection connection;
+
+    protected static final String OPENSEARCH_DOCKER_IMAGE_NAME = "opensearchproject/opensearch:2.18.0";
 
     @Container
-    public OpensearchContainer<?> container = new OpensearchContainer<>(DockerImageName.parse("opensearchproject/opensearch:2.18.0"));
+    protected OpensearchContainer<?> container = new OpensearchContainer<>(DockerImageName.parse(OPENSEARCH_DOCKER_IMAGE_NAME));
 
 
     @SneakyThrows

src/test/java/liquibase/ext/opensearch/MultipleOpenSearchNodesLiquibaseIT.java

@@ -0,0 +1,34 @@
+package liquibase.ext.opensearch;
+
+import liquibase.database.DatabaseFactory;
+import liquibase.ext.opensearch.database.OpenSearchConnection;
+import liquibase.ext.opensearch.database.OpenSearchLiquibaseDatabase;
+import lombok.SneakyThrows;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class MultipleOpenSearchNodesLiquibaseIT extends AbstractOpenSearchLiquibaseIT {
+    @SneakyThrows
+    @BeforeEach
+    @Override
+    protected void beforeEach() {
+        // if we launch two testcontainers they can't see each other and thus don't form a cluster => just use the same URL twice to show that it's being accepted
+        // note: don't use the constants here to detect if we ever change them (to make sure that we actively decide on doing a breaking change rather than making it by mistake).
+        final String url = "opensearch:" + this.container.getHttpHostAddress() + "," + this.container.getHttpHostAddress();
+        final String username = container.getUsername();
+        final String password = container.getPassword();
+        database = (OpenSearchLiquibaseDatabase) DatabaseFactory.getInstance().openDatabase(url, username, password, null, null);
+        connection = (OpenSearchConnection) this.database.getConnection();
+    }
+
+    @SneakyThrows
+    @Test
+    void itCreatesTheChangelogAndLockIndices() {
+        this.doLiquibaseUpdate("liquibase/ext/changelog.empty.yaml");
+        assertThat(this.indexExists(this.database.getDatabaseChangeLogLockTableName())).isTrue();
+        assertThat(this.indexExists(this.database.getDatabaseChangeLogTableName())).isTrue();
+    }
+
+}