/*
 * Copyright Debezium Authors.
 *
 * Licensed under the Apache Software License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
 */
package io.debezium.embedded;

import java.time.Duration;
import java.util.List;
import java.util.Map;
import java.util.Objects;
import java.util.Optional;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executor;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.AtomicReference;
import java.util.function.Consumer;

import org.apache.kafka.common.config.ConfigDef;
import org.apache.kafka.connect.connector.ConnectorContext;
import org.apache.kafka.connect.connector.Task;
import org.apache.kafka.connect.json.JsonConverter;
import org.apache.kafka.connect.runtime.WorkerConfig;
import org.apache.kafka.connect.runtime.distributed.DistributedConfig;
import org.apache.kafka.connect.runtime.standalone.StandaloneConfig;
import org.apache.kafka.connect.source.SourceConnector;
import org.apache.kafka.connect.source.SourceRecord;
import org.apache.kafka.connect.source.SourceTask;
import org.apache.kafka.connect.source.SourceTaskContext;
import org.apache.kafka.connect.storage.Converter;
import org.apache.kafka.connect.storage.FileOffsetBackingStore;
import org.apache.kafka.connect.storage.KafkaOffsetBackingStore;
import org.apache.kafka.connect.storage.OffsetBackingStore;
import org.apache.kafka.connect.storage.OffsetStorageReader;
import org.apache.kafka.connect.storage.OffsetStorageReaderImpl;
import org.apache.kafka.connect.storage.OffsetStorageWriter;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import io.debezium.annotation.ThreadSafe;
import io.debezium.config.Configuration;
import io.debezium.config.Field;
import io.debezium.embedded.spi.OffsetCommitPolicy;
import io.debezium.util.Clock;
import io.debezium.util.VariableLatch;

/**
 * A mechanism for running a single Kafka Connect {@link SourceConnector} within an application's process. An embedded connector
 * is entirely standalone and only talks with the source system; no Kafka, Kafka Connect, or Zookeeper processes are needed.
 * Applications using an embedded connector simply set one up and supply a {@link Consumer consumer function} to which the
 * connector will pass all {@link SourceRecord}s containing database change events.
 * <p>
 * With an embedded connector, the application that runs the connector assumes all responsibility for fault tolerance,
 * scalability, and durability. Additionally, applications must specify how the connector can store its relational database
 * schema history and offsets. By default, this information will be stored in memory and will thus be lost upon application
 * restart.
 * <p>
 * Embedded connectors are designed to be submitted to an {@link Executor} or {@link ExecutorService} for execution by a single
 * thread, and a running connector can be stopped either by calling {@link #stop()} from another thread or by interrupting
 * the running thread (e.g., as is the case with {@link ExecutorService#shutdownNow()}).
 *
 * @author Randall Hauch
 */
@ThreadSafe
public final class EmbeddedEngine implements Runnable {

    /**
     * A required field for an embedded connector that specifies the unique name for the connector instance.
     */
    public static final Field ENGINE_NAME = Field.create("name")
                                                 .withDescription("Unique name for this connector instance.")
                                                 .withValidation(Field::isRequired);

    /**
     * A required field for an embedded connector that specifies the name of the normal Debezium connector's Java class.
     */
    public static final Field CONNECTOR_CLASS = Field.create("connector.class")
                                                     .withDescription("The Java class for the connector")
                                                     .withValidation(Field::isRequired);

    /**
     * An optional field that specifies the name of the class that implements the {@link OffsetBackingStore} interface,
     * and that will be used to store offsets recorded by the connector.
     */
    public static final Field OFFSET_STORAGE = Field.create("offset.storage")
                                                    .withDescription("The Java class that implements the `OffsetBackingStore` "
                                                            + "interface, used to periodically store offsets so that, upon "
                                                            + "restart, the connector can resume where it last left off.")
                                                    .withDefault(FileOffsetBackingStore.class.getName());

    /**
     * An optional field that specifies the file location for the {@link FileOffsetBackingStore}.
     *
     * @see #OFFSET_STORAGE
     */
    public static final Field OFFSET_STORAGE_FILE_FILENAME = Field.create(StandaloneConfig.OFFSET_STORAGE_FILE_FILENAME_CONFIG)
                                                                  .withDescription("The file where offsets are to be stored. Required when "
                                                                          + "'offset.storage' is set to the " +
                                                                          FileOffsetBackingStore.class.getName() + " class.")
                                                                  .withDefault("");

    /**
     * An optional field that specifies the topic name for the {@link KafkaOffsetBackingStore}.
     *
     * @see #OFFSET_STORAGE
     */
    public static final Field OFFSET_STORAGE_KAFKA_TOPIC = Field.create(DistributedConfig.OFFSET_STORAGE_TOPIC_CONFIG)
                                                                .withDescription("The name of the Kafka topic where offsets are to be stored. "
                                                                        + "Required with other properties when 'offset.storage' is set to the "
                                                                        + KafkaOffsetBackingStore.class.getName() + " class.")
                                                                .withDefault("");

    /**
     * An optional field that specifies the number of partitions for the {@link KafkaOffsetBackingStore}.
     *
     * @see #OFFSET_STORAGE
     */
    public static final Field OFFSET_STORAGE_KAFKA_PARTITIONS = Field.create(DistributedConfig.OFFSET_STORAGE_PARTITIONS_CONFIG)
                                                                     .withType(ConfigDef.Type.INT)
                                                                     .withDescription("The number of partitions used when creating the offset storage topic. "
                                                                             + "Required with other properties when 'offset.storage' is set to the "
                                                                             + KafkaOffsetBackingStore.class.getName() + " class.");

    /**
     * An optional field that specifies the replication factor for the {@link KafkaOffsetBackingStore}.
     *
     * @see #OFFSET_STORAGE
     */
    public static final Field OFFSET_STORAGE_KAFKA_REPLICATION_FACTOR = Field.create(DistributedConfig.OFFSET_STORAGE_REPLICATION_FACTOR_CONFIG)
                                                                             .withType(ConfigDef.Type.SHORT)
                                                                             .withDescription("Replication factor used when creating the offset storage topic. "
                                                                                     + "Required with other properties when 'offset.storage' is set to the "
                                                                                     + KafkaOffsetBackingStore.class.getName() + " class.");

    /**
     * An optional advanced field that specifies the maximum amount of time that the embedded connector should wait
     * for an offset commit to complete.
     */
    public static final Field OFFSET_FLUSH_INTERVAL_MS = Field.create("offset.flush.interval.ms")
                                                              .withDescription("Interval at which to try committing offsets. The default is 1 minute.")
                                                              .withDefault(60000L)
                                                              .withValidation(Field::isNonNegativeInteger);

    /**
     * An optional advanced field that specifies the maximum amount of time that the embedded connector should wait
     * for an offset commit to complete.
     */
    public static final Field OFFSET_COMMIT_TIMEOUT_MS = Field.create("offset.flush.timeout.ms")
                                                              .withDescription("Maximum number of milliseconds to wait for records to flush and partition offset data to be"
                                                                      + " committed to offset storage before cancelling the process and restoring the offset "
                                                                      + "data to be committed in a future attempt.")
                                                              .withDefault(5000L)
                                                              .withValidation(Field::isPositiveInteger);

    public static final Field OFFSET_COMMIT_POLICY = Field.create("offset.commit.policy")
                                                          .withDescription("The fully-qualified class name of the commit policy type. This class must implement the interface "
                                                                      + OffsetCommitPolicy.class.getName()
                                                                      + ". The default is a periodic commit policy based upon time intervals.")
                                                          .withDefault(OffsetCommitPolicy.PeriodicCommitOffsetPolicy.class.getName())
                                                          .withValidation(Field::isClassName);

    protected static final Field INTERNAL_KEY_CONVERTER_CLASS = Field.create("internal.key.converter")
                                                                     .withDescription("The Converter class that should be used to serialize and deserialize key data for offsets.")
                                                                     .withDefault(JsonConverter.class.getName());

    protected static final Field INTERNAL_VALUE_CONVERTER_CLASS = Field.create("internal.value.converter")
                                                                       .withDescription("The Converter class that should be used to serialize and deserialize value data for offsets.")
                                                                       .withDefault(JsonConverter.class.getName());

    /**
     * The array of fields that are required by each connectors.
     */
    public static final Field.Set CONNECTOR_FIELDS = Field.setOf(ENGINE_NAME, CONNECTOR_CLASS);

    /**
     * The array of all exposed fields.
     */
    protected static final Field.Set ALL_FIELDS = CONNECTOR_FIELDS.with(OFFSET_STORAGE, OFFSET_STORAGE_FILE_FILENAME,
                                                                        OFFSET_FLUSH_INTERVAL_MS, OFFSET_COMMIT_TIMEOUT_MS,
                                                                        INTERNAL_KEY_CONVERTER_CLASS, INTERNAL_VALUE_CONVERTER_CLASS);

    private static final Duration WAIT_FOR_COMPLETION_BEFORE_INTERRUPT_DEFAULT = Duration.ofSeconds(2);
    private static final String WAIT_FOR_COMPLETION_BEFORE_INTERRUPT_PROP = "debezium.embedded.shutdown.pause.before.interrupt.ms";

    /**
     * A callback function to be notified when the connector completes.
     */
    public interface CompletionCallback {
        /**
         * Handle the completion of the embedded connector engine.
         *
         * @param success {@code true} if the connector completed normally, or {@code false} if the connector produced an error
         *            that prevented startup or premature termination.
         * @param message the completion message; never null
         * @param error the error, or null if there was no exception
         */
        void handle(boolean success, String message, Throwable error);
    }

    /**
     * Callback function which informs users about the various stages a connector goes through during startup
     */
    public interface ConnectorCallback {

        /**
         * Called after a connector has been successfully started by the engine; i.e. {@link SourceConnector#start(Map)} has
         * completed successfully
         */
        default void connectorStarted() {
            // nothing by default
        }

        /**
         * Called after a connector has been successfully stopped by the engine; i.e. {@link SourceConnector#stop()} has
         * completed successfully
         */
        default void connectorStopped() {
            // nothing by default
        }

        /**
         * Called after a connector task has been successfully started by the engine; i.e. {@link SourceTask#start(Map)} has
         * completed successfully
         */
        default void taskStarted() {
            // nothing by default
        }

        /**
         * Called after a connector task has been successfully stopped by the engine; i.e. {@link SourceTask#stop()} has
         * completed successfully
         */
        default void taskStopped() {
            // nothing by default
        }
    }

    /**
     * A callback function to be notified when the connector completes.
     */
    public static class CompletionResult implements CompletionCallback {
        private final CompletionCallback delegate;
        private final CountDownLatch completed = new CountDownLatch(1);
        private boolean success;
        private String message;
        private Throwable error;

        public CompletionResult() {
            this(null);
        }

        public CompletionResult(CompletionCallback delegate) {
            this.delegate = delegate;
        }

        @Override
        public void handle(boolean success, String message, Throwable error) {
            this.success = success;
            this.message = message;
            this.error = error;
            this.completed.countDown();
            if (delegate != null) {
                delegate.handle(success, message, error);
            }
        }

        /**
         * Causes the current thread to wait until the {@link #handle(boolean, String, Throwable) completion occurs}
         * or until the thread is {@linkplain Thread#interrupt interrupted}.
         * <p>
         * This method returns immediately if the connector has completed already.
         *
         * @throws InterruptedException if the current thread is interrupted while waiting
         */
        public void await() throws InterruptedException {
            this.completed.await();
        }

        /**
         * Causes the current thread to wait until the {@link #handle(boolean, String, Throwable) completion occurs},
         * unless the thread is {@linkplain Thread#interrupt interrupted}, or the specified waiting time elapses.
         * <p>
         * This method returns immediately if the connector has completed already.
         *
         * @param timeout the maximum time to wait
         * @param unit the time unit of the {@code timeout} argument
         * @return {@code true} if the completion was received, or {@code false} if the waiting time elapsed before the completion
         *         was received.
         * @throws InterruptedException if the current thread is interrupted while waiting
         */
        public boolean await(long timeout, TimeUnit unit) throws InterruptedException {
            return this.completed.await(timeout, unit);
        }

        /**
         * Determine if the connector has completed.
         *
         * @return {@code true} if the connector has completed, or {@code false} if the connector is still running and this
         *         callback has not yet been {@link #handle(boolean, String, Throwable) notified}
         */
        public boolean hasCompleted() {
            return completed.getCount() == 0;
        }

        /**
         * Get whether the connector completed normally.
         *
         * @return {@code true} if the connector completed normally, or {@code false} if the connector produced an error that
         *         prevented startup or premature termination (or the connector has not yet {@link #hasCompleted() completed})
         */
        public boolean success() {
            return success;
        }

        /**
         * Get the completion message.
         *
         * @return the completion message, or null if the connector has not yet {@link #hasCompleted() completed}
         */
        public String message() {
            return message;
        }

        /**
         * Get the completion error, if there is one.
         *
         * @return the completion error, or null if there is no error or connector has not yet {@link #hasCompleted() completed}
         */
        public Throwable error() {
            return error;
        }

        /**
         * Determine if there is a completion error.
         *
         * @return {@code true} if there is a {@link #error completion error}, or {@code false} if there is no error or
         *         the connector has not yet {@link #hasCompleted() completed}
         */
        public boolean hasError() {
            return error != null;
        }
    }

    /**
     * Contract passed to {@link ChangeConsumer}s, allowing them to commit single records as they have been processed
     * and to signal that offsets may be flushed eventually.
     */
    @ThreadSafe
    public static interface RecordCommitter {

        /**
         * Marks a single record as processed, must be called for each
         * record.
         *
         * @param record the record to commit
         */
        void markProcessed(SourceRecord record) throws InterruptedException;

        /**
         * Marks a batch as finished, this may result in committing offsets/flushing
         * data.
         * <p>
         * Should be called when a batch of records is finished being processed.
         */
        void markBatchFinished();
    }

    /**
     * A contract invoked by the embedded engine when it has received a batch of change records to be processed. Allows
     * to process multiple records in one go, acknowledging their processing once that's done.
     */
    public static interface ChangeConsumer {

        /**
         * Handles a batch of records, calling the {@link RecordCommitter#markProcessed(SourceRecord)}
         * for each record and {@link RecordCommitter#markBatchFinished()} when this batch is finished.
         * @param records the records to be processed
         * @param committer the committer that indicates to the system that we are finished
         */
        void handleBatch(List<SourceRecord> records, RecordCommitter committer) throws InterruptedException;
    }

    private static ChangeConsumer buildDefaultChangeConsumer(Consumer<SourceRecord> consumer) {
        return new ChangeConsumer() {

            /**
             * the default implementation that is compatible with the old Consumer api.
             *
             * On every record, it calls the consumer, and then only marks the record
             * as processed when accept returns, additionally, it handles StopConnectorExceptions
             * and ensures that we all ways try and mark a batch as finished, even with exceptions
             * @param records the records to be processed
             * @param committer the committer that indicates to the system that we are finished
             *
             * @throws Exception
             */
            @Override
            public void handleBatch(List<SourceRecord> records, RecordCommitter committer) throws InterruptedException {
                try {
                    for (SourceRecord record : records) {
                        try {
                            consumer.accept(record);
                            committer.markProcessed(record);
                        }
                        catch (StopConnectorException ex) {
                            // ensure that we mark the record as finished
                            // in this case
                            committer.markProcessed(record);
                            throw ex;
                        }
                    }
                }
                finally {
                    committer.markBatchFinished();
                }
            }
        };
    }

    /**
     * A builder to set up and create {@link EmbeddedEngine} instances.
     */
    public static interface Builder {

        /**
         * Call the specified function for every {@link SourceRecord data change event} read from the source database.
         * This method must be called with a non-null consumer.
         *
         * @param consumer the consumer function
         * @return this builder object so methods can be chained together; never null
         */
        Builder notifying(Consumer<SourceRecord> consumer);

        /**
         * Pass a custom ChangeConsumer override the default implementation,
         * this allows for more complex handling of records for batch and async handling
         *
         * @param handler the consumer function
         * @return this builder object so methods can be chained together; never null
         */
        Builder notifying(ChangeConsumer handler);

        /**
         * Use the specified configuration for the connector. The configuration is assumed to already be valid.
         *
         * @param config the configuration
         * @return this builder object so methods can be chained together; never null
         */
        Builder using(Configuration config);

        /**
         * Use the specified class loader to find all necessary classes. Passing <code>null</code> or not calling this method
         * results in the connector using this class's class loader.
         *
         * @param classLoader the class loader
         * @return this builder object so methods can be chained together; never null
         */
        Builder using(ClassLoader classLoader);

        /**
         * Use the specified clock when needing to determine the current time. Passing <code>null</code> or not calling this
         * method results in the connector using the {@link Clock#system() system clock}.
         *
         * @param clock the clock
         * @return this builder object so methods can be chained together; never null
         */
        Builder using(Clock clock);

        /**
         * When the engine's {@link EmbeddedEngine#run()} method completes, call the supplied function with the results.
         *
         * @param completionCallback the callback function; may be null if errors should be written to the log
         * @return this builder object so methods can be chained together; never null
         */
        Builder using(CompletionCallback completionCallback);

        /**
         * During the engine's {@link EmbeddedEngine#run()} method, call the supplied the supplied function at different
         * stages according to the completion state of each component running within the engine (connectors, tasks etc)
         *
         * @param connectorCallback the callback function; may be null
         * @return this builder object so methods can be chained together; never null
         */
        Builder using(ConnectorCallback connectorCallback);

        /**
         * During the engine's {@link EmbeddedEngine#run()} method, decide when the offsets
         * should be committed into the {@link OffsetBackingStore}.
         * @param policy
         * @return this builder object so methods can be chained together; never null
         */
        Builder using(OffsetCommitPolicy policy);

        /**
         * Build a new connector with the information previously supplied to this builder.
         *
         * @return the embedded connector; never null
         * @throws IllegalArgumentException if a {@link #using(Configuration) configuration} or {@link #notifying(Consumer)
         *             consumer function} were not supplied before this method is called
         */
        EmbeddedEngine build();
    }

    /**
     * Obtain a new {@link Builder} instance that can be used to construct runnable {@link EmbeddedEngine} instances.
     *
     * @return the new builder; never null
     */
    public static Builder create() {
        return new Builder() {
            private Configuration config;
            private ChangeConsumer handler;
            private ClassLoader classLoader;
            private Clock clock;
            private CompletionCallback completionCallback;
            private ConnectorCallback connectorCallback;
            private OffsetCommitPolicy offsetCommitPolicy = null;

            @Override
            public Builder using(Configuration config) {
                this.config = config;
                return this;
            }

            @Override
            public Builder using(ClassLoader classLoader) {
                this.classLoader = classLoader;
                return this;
            }

            @Override
            public Builder using(Clock clock) {
                this.clock = clock;
                return this;
            }

            @Override
            public Builder using(CompletionCallback completionCallback) {
                this.completionCallback = completionCallback;
                return this;
            }

            @Override
            public Builder using(ConnectorCallback connectorCallback) {
                this.connectorCallback = connectorCallback;
                return this;
            }

            @Override
            public Builder using(OffsetCommitPolicy offsetCommitPolicy) {
                this.offsetCommitPolicy = offsetCommitPolicy;
                return this;
            }

            @Override
            public Builder notifying(Consumer<SourceRecord> consumer) {
                this.handler = buildDefaultChangeConsumer(consumer);
                return this;
            }

            @Override
            public Builder notifying(ChangeConsumer handler) {
                this.handler = handler;
                return this;
            }

            @Override
            public EmbeddedEngine build() {
                if (classLoader == null) {
                    classLoader = getClass().getClassLoader();
                }
                if (clock == null) {
                    clock = Clock.system();
                }
                Objects.requireNonNull(config, "A connector configuration must be specified.");
                Objects.requireNonNull(handler, "A connector consumer or changeHandler must be specified.");
                return new EmbeddedEngine(config, classLoader, clock,
                        handler, completionCallback, connectorCallback, offsetCommitPolicy);
            }

        };
    }

    private final Logger logger = LoggerFactory.getLogger(getClass());
    private final Configuration config;
    private final Clock clock;
    private final ClassLoader classLoader;
    private final ChangeConsumer handler;
    private final CompletionCallback completionCallback;
    private final ConnectorCallback connectorCallback;
    private final AtomicReference<Thread> runningThread = new AtomicReference<>();
    private final VariableLatch latch = new VariableLatch(0);
    private final Converter keyConverter;
    private final Converter valueConverter;
    private final WorkerConfig workerConfig;
    private final CompletionResult completionResult;
    private long recordsSinceLastCommit = 0;
    private long timeOfLastCommitMillis = 0;
    private OffsetCommitPolicy offsetCommitPolicy;

    private SourceTask task;

    private EmbeddedEngine(Configuration config, ClassLoader classLoader, Clock clock, ChangeConsumer handler,
                           CompletionCallback completionCallback, ConnectorCallback connectorCallback,
                           OffsetCommitPolicy offsetCommitPolicy) {
        this.config = config;
        this.handler = handler;
        this.classLoader = classLoader;
        this.clock = clock;
        this.completionCallback = completionCallback != null ? completionCallback : (success, msg, error) -> {
            if (!success) {
                logger.error(msg, error);
            }
        };
        this.connectorCallback = connectorCallback;
        this.completionResult = new CompletionResult();
        this.offsetCommitPolicy = offsetCommitPolicy;

        assert this.config != null;
        assert this.handler != null;
        assert this.classLoader != null;
        assert this.clock != null;
        keyConverter = config.getInstance(INTERNAL_KEY_CONVERTER_CLASS, Converter.class, () -> this.classLoader);
        keyConverter.configure(config.subset(INTERNAL_KEY_CONVERTER_CLASS.name() + ".", true).asMap(), true);
        valueConverter = config.getInstance(INTERNAL_VALUE_CONVERTER_CLASS, Converter.class, () -> this.classLoader);
        Configuration valueConverterConfig = config;
        if (valueConverter instanceof JsonConverter) {
            // Make sure that the JSON converter is configured to NOT enable schemas ...
            valueConverterConfig = config.edit().with(INTERNAL_VALUE_CONVERTER_CLASS + ".schemas.enable", false).build();
        }
        valueConverter.configure(valueConverterConfig.subset(INTERNAL_VALUE_CONVERTER_CLASS.name() + ".", true).asMap(), false);

        // Create the worker config, adding extra fields that are required for validation of a worker config
        // but that are not used within the embedded engine (since the source records are never serialized) ...
        Map<String, String> embeddedConfig = config.asMap(ALL_FIELDS);
        embeddedConfig.put(WorkerConfig.KEY_CONVERTER_CLASS_CONFIG, JsonConverter.class.getName());
        embeddedConfig.put(WorkerConfig.VALUE_CONVERTER_CLASS_CONFIG, JsonConverter.class.getName());
        workerConfig = new EmbeddedConfig(embeddedConfig);
    }

    /**
     * Determine if this embedded connector is currently running.
     *
     * @return {@code true} if running, or {@code false} otherwise
     */
    public boolean isRunning() {
        return this.runningThread.get() != null;
    }

    private void fail(String msg) {
        fail(msg, null);
    }

    private void fail(String msg, Throwable error) {
        if (completionResult.hasError()) {
            // there's already a recorded failure, so keep the original one and simply log this one
            logger.error(msg, error);
            return;
        }
        // don't use the completion callback here because we want to store the error and message only
        completionResult.handle(false, msg, error);
    }

    private void succeed(String msg) {
        // don't use the completion callback here because we want to store the error and message only
        completionResult.handle(true, msg, null);
    }

    /**
     * Run this embedded connector and deliver database changes to the registered {@link Consumer}. This method blocks until
     * the connector is stopped.
     * <p>
     * First, the method checks to see if this instance is currently {@link #run() running}, and if so immediately returns.
     * <p>
     * If the configuration is valid, this method starts the connector and starts polling the connector for change events.
     * All messages are delivered in batches to the {@link Consumer} registered with this embedded connector. The batch size,
     * polling
     * frequency, and other parameters are controlled via configuration settings. This continues until this connector is
     * {@link #stop() stopped}.
     * <p>
     * Note that there are two ways to stop a connector running on a thread: calling {@link #stop()} from another thread, or
     * interrupting the thread (e.g., via {@link ExecutorService#shutdownNow()}).
     * <p>
     * This method can be called repeatedly as needed.
     */
    @Override
    public void run() {
        if (runningThread.compareAndSet(null, Thread.currentThread())) {

            final String engineName = config.getString(ENGINE_NAME);
            final String connectorClassName = config.getString(CONNECTOR_CLASS);
            final Optional<ConnectorCallback> connectorCallback = Optional.ofNullable(this.connectorCallback);
            // Only one thread can be in this part of the method at a time ...
            latch.countUp();
            try {
                if (!config.validateAndRecord(CONNECTOR_FIELDS, logger::error)) {
                    fail("Failed to start connector with invalid configuration (see logs for actual errors)");
                    return;
                }

                // Instantiate the connector ...
                SourceConnector connector = null;
                try {
                    @SuppressWarnings("unchecked")
                    Class<? extends SourceConnector> connectorClass = (Class<SourceConnector>) classLoader.loadClass(connectorClassName);
                    connector = connectorClass.newInstance();
                }
                catch (Throwable t) {
                    fail("Unable to instantiate connector class '" + connectorClassName + "'", t);
                    return;
                }

                // Instantiate the offset store ...
                final String offsetStoreClassName = config.getString(OFFSET_STORAGE);
                OffsetBackingStore offsetStore = null;
                try {
                    @SuppressWarnings("unchecked")
                    Class<? extends OffsetBackingStore> offsetStoreClass = (Class<OffsetBackingStore>) classLoader.loadClass(offsetStoreClassName);
                    offsetStore = offsetStoreClass.newInstance();
                }
                catch (Throwable t) {
                    fail("Unable to instantiate OffsetBackingStore class '" + offsetStoreClassName + "'", t);
                    return;
                }

                // Initialize the offset store ...
                try {
                    offsetStore.configure(workerConfig);
                    offsetStore.start();
                }
                catch (Throwable t) {
                    fail("Unable to configure and start the '" + offsetStoreClassName + "' offset backing store", t);
                    return;
                }

                // Set up the offset commit policy ...
                if (offsetCommitPolicy == null) {
                    offsetCommitPolicy = config.getInstance(EmbeddedEngine.OFFSET_COMMIT_POLICY, OffsetCommitPolicy.class, config);
                }

                // Initialize the connector using a context that does NOT respond to requests to reconfigure tasks ...
                ConnectorContext context = new ConnectorContext() {

                    @Override
                    public void requestTaskReconfiguration() {
                        // Do nothing ...
                    }

                    @Override
                    public void raiseError(Exception e) {
                        fail(e.getMessage(), e);
                    }

                };
                connector.initialize(context);
                OffsetStorageWriter offsetWriter = new OffsetStorageWriter(offsetStore, engineName,
                        keyConverter, valueConverter);
                OffsetStorageReader offsetReader = new OffsetStorageReaderImpl(offsetStore, engineName,
                        keyConverter, valueConverter);
                Duration commitTimeout = Duration.ofMillis(config.getLong(OFFSET_COMMIT_TIMEOUT_MS));

                try {
                    // Start the connector with the given properties and get the task configurations ...
                    connector.start(config.asMap());
                    connectorCallback.ifPresent(ConnectorCallback::connectorStarted);
                    List<Map<String, String>> taskConfigs = connector.taskConfigs(1);
                    Class<? extends Task> taskClass = connector.taskClass();
                    task = null;
                    try {
                        task = (SourceTask) taskClass.newInstance();
                    }
                    catch (IllegalAccessException | InstantiationException t) {
                        fail("Unable to instantiate connector's task class '" + taskClass.getName() + "'", t);
                        return;
                    }
                    try {
                        SourceTaskContext taskContext = new SourceTaskContext() {
                            @Override
                            public OffsetStorageReader offsetStorageReader() {
                                return offsetReader;
                            }

                            public Map<String, String> configs() {
                                // TODO Auto-generated method stub
                                return null;
                            }
                        };
                        task.initialize(taskContext);
                        task.start(taskConfigs.get(0));
                        connectorCallback.ifPresent(ConnectorCallback::taskStarted);
                    }
                    catch (Throwable t) {
                        // Mask the passwords ...
                        Configuration config = Configuration.from(taskConfigs.get(0)).withMaskedPasswords();
                        String msg = "Unable to initialize and start connector's task class '" + taskClass.getName() + "' with config: "
                                + config;
                        fail(msg, t);
                        return;
                    }

                    recordsSinceLastCommit = 0;
                    Throwable handlerError = null;
                    try {
                        timeOfLastCommitMillis = clock.currentTimeInMillis();
                        List<SourceRecord> changeRecords = null;
                        RecordCommitter committer = buildRecordCommitter(offsetWriter, task, commitTimeout);
                        while (runningThread.get() != null) {
                            try {
                                logger.debug("Embedded engine is polling task for records on thread {}", runningThread.get());
                                changeRecords = task.poll(); // blocks until there are values ...
                                logger.debug("Embedded engine returned from polling task for records");
                            }
                            catch (InterruptedException e) {
                                // Interrupted while polling ...
                                logger.debug("Embedded engine interrupted on thread {} while polling the task for records", runningThread.get());
                                if (this.runningThread.get() == Thread.currentThread()) {
                                    // this thread is still set as the running thread -> we were not interrupted
                                    // due the stop() call -> probably someone else called the interrupt on us ->
                                    // -> we should raise the interrupt flag
                                    Thread.currentThread().interrupt();
                                }
                                break;
                            }
                            try {
                                if (changeRecords != null && !changeRecords.isEmpty()) {
                                    logger.debug("Received {} records from the task", changeRecords.size());

                                    try {
                                        handler.handleBatch(changeRecords, committer);
                                    }
                                    catch (StopConnectorException e) {
                                        break;
                                    }
                                }
                                else {
                                    logger.debug("Received no records from the task");
                                }
                            }
                            catch (Throwable t) {
                                // There was some sort of unexpected exception, so we should stop work
                                handlerError = t;
                                break;
                            }
                        }
                    }
                    finally {
                        if (handlerError != null) {
                            // There was an error in the handler so make sure it's always captured...
                            fail("Stopping connector after error in the application's handler method: " + handlerError.getMessage(),
                                 handlerError);
                        }
                        try {
                            // First stop the task ...
                            logger.debug("Stopping the task and engine");
                            task.stop();
                            connectorCallback.ifPresent(ConnectorCallback::taskStopped);
                            // Always commit offsets that were captured from the source records we actually processed ...
                            commitOffsets(offsetWriter, commitTimeout, task);
                            if (handlerError == null) {
                                // We stopped normally ...
                                succeed("Connector '" + connectorClassName + "' completed normally.");
                            }
                        }
                        catch (Throwable t) {
                            fail("Error while trying to stop the task and commit the offsets", t);
                        }
                    }
                }
                catch (Throwable t) {
                    fail("Error while trying to run connector class '" + connectorClassName + "'", t);
                }
                finally {
                    // Close the offset storage and finally the connector ...
                    try {
                        offsetStore.stop();
                    }
                    catch (Throwable t) {
                        fail("Error while trying to stop the offset store", t);
                    }
                    finally {
                        try {
                            connector.stop();
                            connectorCallback.ifPresent(ConnectorCallback::connectorStopped);
                        }
                        catch (Throwable t) {
                            fail("Error while trying to stop connector class '" + connectorClassName + "'", t);
                        }
                    }
                }
            }
            finally {
                latch.countDown();
                runningThread.set(null);
                // after we've "shut down" the engine, fire the completion callback based on the results we collected
                completionCallback.handle(completionResult.success(), completionResult.message(), completionResult.error());
            }
        }
    }

    /**
     * Creates a new RecordCommitter that is responsible for informing the engine
     * about the updates to the given batch
     * @param offsetWriter the offsetWriter current in use
     * @param task the sourcetask
     * @param commitTimeout the time in ms until a commit times out
     * @return the new recordCommitter to be used for a given batch
     */
    protected RecordCommitter buildRecordCommitter(OffsetStorageWriter offsetWriter, SourceTask task, Duration commitTimeout) {
        return new RecordCommitter() {

            @Override
            public synchronized void markProcessed(SourceRecord record) throws InterruptedException {
                task.commitRecord(record);
                recordsSinceLastCommit += 1;
                offsetWriter.offset(record.sourcePartition(), record.sourceOffset());
            }

            @Override
            public synchronized void markBatchFinished() {
                maybeFlush(offsetWriter, offsetCommitPolicy, commitTimeout, task);
            }
        };
    }

    /**
     * Determine if we should flush offsets to storage, and if so then attempt to flush offsets.
     *
     * @param offsetWriter the offset storage writer; may not be null
     * @param policy the offset commit policy; may not be null
     * @param commitTimeout the timeout to wait for commit results
     * @param task the task which produced the records for which the offsets have been committed
     */
    protected void maybeFlush(OffsetStorageWriter offsetWriter, OffsetCommitPolicy policy, Duration commitTimeout,
                              SourceTask task) {
        // Determine if we need to commit to offset storage ...
        long timeSinceLastCommitMillis = clock.currentTimeInMillis() - timeOfLastCommitMillis;
        if (policy.performCommit(recordsSinceLastCommit, Duration.ofMillis(timeSinceLastCommitMillis))) {
            commitOffsets(offsetWriter, commitTimeout, task);
        }
    }

    /**
     * Flush offsets to storage.
     *
     * @param offsetWriter the offset storage writer; may not be null
     * @param commitTimeout the timeout to wait for commit results
     * @param task the task which produced the records for which the offsets have been committed
     */
    protected void commitOffsets(OffsetStorageWriter offsetWriter, Duration commitTimeout, SourceTask task) {
        long started = clock.currentTimeInMillis();
        long timeout = started + commitTimeout.toMillis();
        if (!offsetWriter.beginFlush()) {
            return;
        }
        Future<Void> flush = offsetWriter.doFlush(this::completedFlush);
        if (flush == null) {
            return; // no offsets to commit ...
        }

        // Wait until the offsets are flushed ...
        try {
            flush.get(Math.max(timeout - clock.currentTimeInMillis(), 0), TimeUnit.MILLISECONDS);
            // if we've gotten this far, the offsets have been committed so notify the task
            task.commit();
            recordsSinceLastCommit = 0;
            timeOfLastCommitMillis = clock.currentTimeInMillis();
        }
        catch (InterruptedException e) {
            logger.warn("Flush of {} offsets interrupted, cancelling", this);
            offsetWriter.cancelFlush();
        }
        catch (ExecutionException e) {
            logger.error("Flush of {} offsets threw an unexpected exception: ", this, e);
            offsetWriter.cancelFlush();
        }
        catch (TimeoutException e) {
            logger.error("Timed out waiting to flush {} offsets to storage", this);
            offsetWriter.cancelFlush();
        }
    }

    protected void completedFlush(Throwable error, Void result) {
        if (error != null) {
            logger.error("Failed to flush {} offsets to storage: ", this, error);
        }
        else {
            logger.trace("Finished flushing {} offsets to storage", this);
        }
    }

    /**
     * Stop the execution of this embedded connector. This method does not block until the connector is stopped; use
     * {@link #await(long, TimeUnit)} for this purpose.
     *
     * @return {@code true} if the connector was {@link #run() running} and will eventually stop, or {@code false} if it was not
     *         running when this method is called
     * @see #await(long, TimeUnit)
     */
    public boolean stop() {
        logger.debug("Stopping the embedded engine");
        // Signal that the run() method should stop ...
        Thread thread = this.runningThread.getAndSet(null);
        if (thread != null) {
            try {
                latch.await(Long.valueOf(System.getProperty(WAIT_FOR_COMPLETION_BEFORE_INTERRUPT_PROP, Long.toString(WAIT_FOR_COMPLETION_BEFORE_INTERRUPT_DEFAULT.toMillis()))), TimeUnit.MILLISECONDS);
            }
            catch (InterruptedException e) {
            }
            logger.debug("Interrupting the embedded engine's thread {} (already interrupted: {})", thread, thread.isInterrupted());
            // Interrupt the thread in case it is blocked while polling the task for records ...
            thread.interrupt();
            return true;
        }
        return false;
    }

    /**
     * Wait for the connector to complete processing. If the processor is not running, this method returns immediately; however,
     * if the processor is {@link #stop() stopped} and restarted before this method is called, this method will return only
     * when it completes the second time.
     *
     * @param timeout the maximum amount of time to wait before returning
     * @param unit the unit of time; may not be null
     * @return {@code true} if the connector completed within the timeout (or was not running), or {@code false} if it is still
     *         running when the timeout occurred
     * @throws InterruptedException if this thread is interrupted while waiting for the completion of the connector
     */
    public boolean await(long timeout, TimeUnit unit) throws InterruptedException {
        return latch.await(timeout, unit);
    }

    @Override
    public String toString() {
        return "EmbeddedEngine{id=" + config.getString(ENGINE_NAME) + '}';
    }

    public void runWithTask(Consumer<SourceTask> consumer) {
        consumer.accept(task);
    }

    protected static class EmbeddedConfig extends WorkerConfig {
        private static final ConfigDef CONFIG;

        static {
            ConfigDef config = baseConfigDef();
            Field.group(config, "file", OFFSET_STORAGE_FILE_FILENAME);
            Field.group(config, "kafka", OFFSET_STORAGE_KAFKA_TOPIC);
            Field.group(config, "kafka", OFFSET_STORAGE_KAFKA_PARTITIONS);
            Field.group(config, "kafka", OFFSET_STORAGE_KAFKA_REPLICATION_FACTOR);
            CONFIG = config;
        }

        protected EmbeddedConfig(Map<String, String> props) {
            super(CONFIG, props);
        }
    }

}
