package eu.openaire.urls_controller.services;

import eu.openaire.urls_controller.components.BulkImport;
import eu.openaire.urls_controller.configuration.DatabaseConnector;
import eu.openaire.urls_controller.controllers.UrlsController;
import eu.openaire.urls_controller.models.*;
import eu.openaire.urls_controller.payloads.responces.AssignmentsResponse;
import eu.openaire.urls_controller.util.FileUtils;
import eu.openaire.urls_controller.util.ParquetFileUtils;
import io.micrometer.core.annotation.Timed;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.boot.web.client.RestTemplateBuilder;
import org.springframework.dao.EmptyResultDataAccessException;
import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.jdbc.core.JdbcTemplate;
import org.springframework.stereotype.Service;
import org.springframework.web.client.HttpClientErrorException;
import org.springframework.web.client.HttpServerErrorException;
import org.springframework.web.client.RestTemplate;

import java.io.File;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.SQLException;
import java.sql.Timestamp;
import java.time.Duration;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicLong;


@Service
public class UrlsServiceImpl implements UrlsService {

    private static final Logger logger = LoggerFactory.getLogger(UrlsServiceImpl.class);

    @Autowired
    private JdbcTemplate jdbcTemplate;

    @Autowired
    private FileUtils fileUtils;

    @Autowired
    private ParquetFileUtils parquetFileUtils;

    @Value("${services.pdfaggregation.controller.workerReportsDirPath}")
    private String workerReportsDirPath;

    public static final AtomicLong assignmentsBatchCounter = new AtomicLong(0);

    private final AtomicInteger maxAttemptsPerRecordAtomic;

    private static String excludedDatasourceIDsStringList = null;

    public static final ExecutorService insertsExecutor = Executors.newFixedThreadPool(6);
    // TODO - Unify this ExecutorService with the hash-matching executorService. Since one will ALWAYS be called after the other. So why having two ExecServices to handle?


    public UrlsServiceImpl(@Value("${services.pdfaggregation.controller.maxAttemptsPerRecord}") int maxAttemptsPerRecord, BulkImport bulkImport)
    {
        maxAttemptsPerRecordAtomic = new AtomicInteger(maxAttemptsPerRecord);

        HashMap<String, BulkImport.BulkImportSource> bulkImportSources = new HashMap<>(bulkImport.getBulkImportSources());
        // The "bulkImportSources" will not be null, as it will be defined inside the "application.yml" file.
        // In case no bulkImport Datasources are given, then the "bulkImportSources" list will just be empty.
        if ( bulkImportSources.isEmpty() )
            return; // So the "excludedDatasourceIDsStringList" -code should be placed last in this Constructor-method.

        if ( logger.isTraceEnabled() )
            logger.trace("BulkImportSources:\n" + bulkImportSources);

        List<String> excludedIDs = new ArrayList<>();
        for ( BulkImport.BulkImportSource source : bulkImportSources.values() ) {
            String datasourceID = source.getDatasourceID();
            if ( (datasourceID == null) || datasourceID.isEmpty() )
                throw new RuntimeException("One of the bulk-imported datasourceIDs was not found! | source: " + source);
            excludedIDs.add(datasourceID);
        }

        int exclusionListSize = excludedIDs.size(); // This list will not be empty.

        // Prepare the "excludedDatasourceIDsStringList" to be used inside the "findAssignmentsQuery". Create the following string-pattern:
        // ("ID_1", "ID_2", ...)
        final StringBuilder sb = new StringBuilder((exclusionListSize * 46) + (exclusionListSize -1) +2 );
        sb.append("(");
        for ( int i=0; i < exclusionListSize; ++i ) {
            sb.append("\"").append(excludedIDs.get(i)).append("\"");
            if ( i < (exclusionListSize -1) )
                sb.append(", ");
        }
        sb.append(")");

        excludedDatasourceIDsStringList = sb.toString();
        logger.info("The following bulkImport-datasources will be excluded from crawling: " + excludedDatasourceIDsStringList);
    }


    @Timed(value = "getAssignments.time", description = "Time taken to return the assignments.")
    public ResponseEntity<?> getAssignments(String workerId, int assignmentsLimit)
    {
        // Create the Assignments from the id-urls stored in the database up to the < assignmentsLimit >.
        String findAssignmentsQuery =
            "select pubid, url, datasourceid, datasourcename\n" +   // Select the final sorted data with "assignmentsLimit".
            "from (select distinct pubid, url, datasourceid, datasourcename, attempt_count, pub_year\n" +  // Select the distinct id-url data. Beware that this will return duplicate id-url paris, wince one pair may be associated with multiple datasources.
            "   from (select p.id as pubid, pu.url as url, pb.level as level, attempts.counts as attempt_count, p.year as pub_year, d.id as datasourceid, d.name as datasourcename\n" +  // Select all needed columns frm JOINs, order by "boost.level" and limit them to  (assignmentsLimit * 10)
            "       from " + DatabaseConnector.databaseName + ".publication p\n" +
            "           join " + DatabaseConnector.databaseName + ".publication_urls pu on pu.id=p.id\n" +
            "           join " + DatabaseConnector.databaseName + ".datasource d on d.id=p.datasourceid\n" +  // This is needed for the "d.allow_harvest=true" check later on.
            "           left outer join " + DatabaseConnector.databaseName + ".publication_boost pb\n" +
            "                   on p.id=pb.id\n" +
            "           left outer join (select count(a.id) as counts, a.id from " + DatabaseConnector.databaseName + ".attempt a group by a.id) as attempts\n" +
            "                   on attempts.id=p.id\n" +
            "           left outer join (\n" +
            "               select a.id, a.original_url from " + DatabaseConnector.databaseName + ".assignment a\n" +
            "               union all\n" +
            "               select pl.id, pl.original_url from " + DatabaseConnector.databaseName + ".payload pl\n" + // Here we access the payload-VIEW which includes the three payload-tables.
            "               ) as existing\n" +
            "                   on existing.id=p.id and existing.original_url=pu.url\n" +
            "       where d.allow_harvest=true and existing.id is null\n" + // For records not found on existing, the "existing.id" will be null.
                        ((excludedDatasourceIDsStringList != null) ?    // If we have an exclusion-list, use it below.
           ("           and d.id not in " + excludedDatasourceIDsStringList + "\n") : "") +
            "           and coalesce(attempts.counts, 0) <= " + maxAttemptsPerRecordAtomic.get() + "\n" +
            "           and not exists (select 1 from " + DatabaseConnector.databaseName + ".attempt a where a.id=p.id and a.error_class = 'noRetry' limit 1)\n" +
            "           and pu.url != '' and pu.url is not null\n" +   // Some IDs have empty-string urls, there are no "null" urls, but keep the relevant check for future-proofing.
            "       order by coalesce(level, -1000) desc\n" +
            "       limit " + (assignmentsLimit * 10) + "\n" +
            "   ) as non_distinct_results\n" +
            "   order by coalesce(attempt_count, 0), coalesce(pub_year, 0) desc, reverse(pubid), url\n" +  // We also order by reverse "pubid" and "url", in order to get the exactly same records for consecutive runs, all things being equal.
            "   limit " + assignmentsLimit + "\n" +
            ") as findAssignmentsQuery";

        // The datasourceID and datasourceName are currently not used during the processing in the Worker. They may be used by the Worker, in the future to apply a datasource-specific aggregation plugin to take the full-texts quickly, instead of using the general crawling one.
        // However, the "datasourceid" is useful to be able to generate the fileNames for the S3, without needing to perform additional select queries (with JOINs) at that phase.

        // The "order by" in the end makes sure the older attempted records will be re-attempted after a long time.
        //logger.trace("findAssignmentsQuery:\n" + findAssignmentsQuery); // DEBUG!

        final String getAssignmentsQuery = "select * from " + DatabaseConnector.databaseName + ".current_assignment";

        List<Assignment> assignments = new ArrayList<>(assignmentsLimit);

        long curAssignmentsBatchCounter = assignmentsBatchCounter.incrementAndGet();

        DatabaseConnector.databaseLock.lock();

        // For performance reasons, we collect the returned assignments and create a temp-table with them, so that later they can be copied efficiently to the "a-bit-more-permanent" assignment table.
        // This way, we avoid having to manually insert thousands of assignment records there. Instead, we create a new table AS the results from the "findAssignmentsQuery".
        String errorMsg = createAndInitializeCurrentAssignmentsTable(findAssignmentsQuery);
        if ( errorMsg != null ) {
            DatabaseConnector.databaseLock.unlock();
            return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR).body(errorMsg);
        }

        long timestampMillis = System.currentTimeMillis();
        Timestamp timestamp = new Timestamp(timestampMillis); // Store it here, in order to have the same for all current records.

        try {
            jdbcTemplate.query(getAssignmentsQuery, rs -> {
                Assignment assignment = new Assignment();
                assignment.setWorkerId(workerId);
                assignment.setAssignmentsBatchCounter(curAssignmentsBatchCounter);
                assignment.setTimestamp(timestamp);
                Datasource datasource = new Datasource();
                try {   // For each of the 4 columns returned, do the following. The column-indexing starts from  1
                    assignment.setId(rs.getString(1));
                    assignment.setOriginalUrl(rs.getString(2));
                    datasource.setId(rs.getString(3));
                    datasource.setName(rs.getString(4));
                } catch (SQLException sqle) {
                    logger.error("No value was able to be retrieved from one of the columns of row_" + rs.getRow(), sqle);
                }
                assignment.setDatasource(datasource);
                assignments.add(assignment);
            });
        } catch (EmptyResultDataAccessException erdae) {
            errorMsg = "No results were returned for \"getAssignmentsQuery\":\n" + getAssignmentsQuery;
            String tmpErrMsg = dropCurrentAssignmentTable();
            DatabaseConnector.databaseLock.unlock();
            if ( tmpErrMsg != null )
                errorMsg += "\n" + tmpErrMsg;
            logger.warn(errorMsg);
            return ResponseEntity.status(HttpStatus.NO_CONTENT).body(errorMsg);
        } catch (Exception e) {
            errorMsg = DatabaseConnector.handleQueryException("getAssignmentsQuery", getAssignmentsQuery, e);
            String tmpErrMsg = dropCurrentAssignmentTable();
            DatabaseConnector.databaseLock.unlock();
            if ( tmpErrMsg != null )
                errorMsg += "\n" + tmpErrMsg;
            return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR).body(errorMsg);
        }

        int assignmentsSize = assignments.size();
        if ( assignmentsSize == 0 ) {
            errorMsg = "No results retrieved from the \"findAssignmentsQuery\" for worker with id: " + workerId + ". Will increase the \"maxAttempts\" to " + maxAttemptsPerRecordAtomic.incrementAndGet() + " for the next requests.";
            logger.error(errorMsg);
            String tmpErrMsg = dropCurrentAssignmentTable();
            DatabaseConnector.databaseLock.unlock();
            if ( tmpErrMsg != null ) {
                errorMsg += "\n" + tmpErrMsg;   // The additional error-msg is already logged.
                return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR).body(errorMsg);
            } else
                return ResponseEntity.status(HttpStatus.MULTI_STATUS).body(new AssignmentsResponse((long) -1, null));
        } else if ( assignmentsSize < assignmentsLimit )
            logger.warn("The retrieved results were fewer (" + assignmentsSize + ") than the \"assignmentsLimit\" (" + assignmentsLimit + "), for worker with id: " + workerId + ". Will increase the \"maxAttempts\" to " + maxAttemptsPerRecordAtomic.incrementAndGet() + ", for the next requests.");

        logger.debug("Finished gathering " + assignmentsSize + " assignments for worker with id \"" + workerId + "\". Going to insert them into the \"assignment\" table and then return them to the worker.");

        // Write the Assignment details to the assignment-table.
        String insertAssignmentsQuery = "insert into " + DatabaseConnector.databaseName + ".assignment \n select pubid, url, '" + workerId + "', " + curAssignmentsBatchCounter + ", " + timestampMillis
                + "\nfrom " + DatabaseConnector.databaseName + ".current_assignment";

        try {
            jdbcTemplate.execute(insertAssignmentsQuery);
        } catch (Exception e) {
            errorMsg = DatabaseConnector.handleQueryException("insertAssignmentsQuery", insertAssignmentsQuery, e);
            String tmpErrMsg = dropCurrentAssignmentTable();
            if ( tmpErrMsg != null )
                errorMsg += "\n" + tmpErrMsg;
            DatabaseConnector.databaseLock.unlock();
            return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR).body(errorMsg);
        }

        errorMsg = dropCurrentAssignmentTable();
        if ( errorMsg != null ) {
            DatabaseConnector.databaseLock.unlock();
            return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR).body(errorMsg);
        }

        logger.debug("Finished inserting " + assignmentsSize + " assignments into the \"assignment\"-table. Going to merge the parquet files for this table.");

        String mergeErrorMsg = fileUtils.mergeParquetFiles("assignment", "", null);
        if ( mergeErrorMsg != null ) {
            DatabaseConnector.databaseLock.unlock();
            return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR).body(mergeErrorMsg);
        }

        DatabaseConnector.databaseLock.unlock();

        // Due to the fact that one publication with an id-url pair can be connected with multiple datasources, the results returned from the query may be duplicates.
        // So, we apply a post-processing step where we collect only one instance of each id-url pair and send it to the Worker.

        // We would certainly like the query to return assignments with unique id-url pairs, but for now there is no other solution.
        // There is no logic-error though, since the worker requests a "LIMIT" of 5000 not "exactly 5000" nor it matters the actual number it will process.
        // So, it will ask for "up to 5000" and the Controller may return 4700.
        // Since the average number of duplicates will be the same for each worker-request, none of the Workers will have any advantage over the other in the long run,
        // so there will be no conflicting performance data between them.

        final HashMap<String, Assignment> uniquePairsAndAssignments = new HashMap<>((int) (assignmentsLimit * 0.9));

        for ( Assignment assignment : assignments ) {
            uniquePairsAndAssignments.put(assignment.getId() + "_" + assignment.getOriginalUrl(), assignment);
            // This will just update the duplicate record with another "assignment object", containing a different datasource.
        }

        List<Assignment> distinctAssignments = new ArrayList<>(uniquePairsAndAssignments.values());
        int distinctAssignmentsSize = distinctAssignments.size();

        if ( logger.isTraceEnabled() )
            logger.trace("numDuplicates in returned assignments_" + curAssignmentsBatchCounter + " = " + (assignmentsSize - distinctAssignmentsSize));

        logger.info("Sending batch-assignments_" + curAssignmentsBatchCounter + " with " + distinctAssignmentsSize + " assignments to worker with ID: " + workerId + ".");
        return ResponseEntity.status(HttpStatus.OK).body(new AssignmentsResponse(curAssignmentsBatchCounter, distinctAssignments));
    }



    public static final AtomicLong numOfWorkerReportsProcessed = new AtomicLong(0);


    @Timed(value = "addWorkerReport.time", description = "Time taken to add the WorkerReport.")
    public Boolean addWorkerReport(String curWorkerId, long curReportAssignmentsCounter, List<UrlReport> urlReports, int sizeOfUrlReports)
    {
        long currentNumOfWorkerReportsProcessed = numOfWorkerReportsProcessed.incrementAndGet();   // Increment it when it is actually being pressed, not when it arrives at the endpoint.

        logger.info("Initializing the addition of the worker's (" + curWorkerId + ") report for assignments_" + curReportAssignmentsCounter);

        // Before continuing with inserts, take and upload the fullTexts from the Worker. Also, update the file-"location".
        FileUtils.UploadFullTextsResponse uploadFullTextsResponse = fileUtils.getAndUploadFullTexts(urlReports, curReportAssignmentsCounter, curWorkerId);
        if ( uploadFullTextsResponse == FileUtils.UploadFullTextsResponse.databaseError ) {
            postReportResultToWorker(curWorkerId, curReportAssignmentsCounter, "Problem with the Impala-database!");
            return false;
        }
        else if ( uploadFullTextsResponse == FileUtils.UploadFullTextsResponse.unsuccessful ) {
            logger.error("Failed to get and/or upload the fullTexts for batch-assignments_" + curReportAssignmentsCounter);
            // The docUrls were still found! Just update ALL the fileLocations, sizes, hashes and mimetypes, to show that the files are not available.
            fileUtils.updateUrlReportsToHaveNoFullTextFiles(urlReports, false);
            // We write only the payloads which are connected with retrieved full-texts, uploaded to S3-Object-Store.
            // We continue with writing the "attempts", as we want to avoid re-checking the failed-urls later.
            // The urls which give full-text (no matter if we could not get it from the worker), are flagged as "couldRetry" anyway, so they will be picked-up to be checked again later.
        } else
            logger.debug("Finished uploading the full-texts from batch-assignments_" + curReportAssignmentsCounter);

        String localParquetPath = parquetFileUtils.parquetBaseLocalDirectoryPath + "assignments_" + curReportAssignmentsCounter + File.separator;
        try {
            Files.createDirectories(Paths.get(localParquetPath)); // No-op if it already exists. It does not throw a "alreadyExistsException"
        } catch (Exception e) {
            String errorMsg = "Could not create the parquet-directory: " + localParquetPath;
            logger.error(errorMsg, e);
            postReportResultToWorker(curWorkerId, curReportAssignmentsCounter, errorMsg);
            return false;
        }

        logger.debug("Going to write the results in the parquet files, then upload them to HDFS, and then load them into the database's tables. For batch-assignments_" + curReportAssignmentsCounter);

        // Create HDFS subDirs for these assignments. Other background threads handling other assignments will not interfere with loading of parquetFiles to the DB tables.
        String endingMkDirAndParams = curReportAssignmentsCounter + "/" + parquetFileUtils.mkDirsAndParams;
        if ( !parquetFileUtils.applyHDFOperation(parquetFileUtils.webHDFSBaseUrl + parquetFileUtils.parquetHDFSDirectoryPathAttempts + endingMkDirAndParams)
                || !parquetFileUtils.applyHDFOperation(parquetFileUtils.webHDFSBaseUrl + parquetFileUtils.parquetHDFSDirectoryPathPayloadsAggregated + endingMkDirAndParams) )
        {
            postReportResultToWorker(curWorkerId, curReportAssignmentsCounter, "Error when creating the HDFS sub-directories for assignments_" + curReportAssignmentsCounter);
            return false;
        }

        List<Callable<ParquetReport>> callableTasks = parquetFileUtils.getTasksForCreatingAndUploadingParquetFiles(urlReports, sizeOfUrlReports, curReportAssignmentsCounter, localParquetPath, uploadFullTextsResponse);

        boolean hasAttemptParquetFileProblem = false;
        boolean hasPayloadParquetFileProblem = false;

        try {	// Invoke all the tasks and wait for them to finish.
            List<Future<ParquetReport>> futures = insertsExecutor.invokeAll(callableTasks);

            SumParquetSuccess sumParquetSuccess = parquetFileUtils.checkParquetFilesSuccess(futures);
            ResponseEntity<?> errorResponseEntity = sumParquetSuccess.getResponseEntity();
            if ( errorResponseEntity != null ) {    // The related log is already shown.
                postReportResultToWorker(curWorkerId, curReportAssignmentsCounter, "Error when creating or uploading the parquet files!");
                return false;
            }
            hasAttemptParquetFileProblem = sumParquetSuccess.isAttemptParquetFileProblem();
            hasPayloadParquetFileProblem = sumParquetSuccess.isPayloadParquetFileProblem();

            if ( hasAttemptParquetFileProblem && hasPayloadParquetFileProblem )
                throw new RuntimeException("All of the parquet files failed to be created or uploaded! Will avoid to execute load-requests into the database, for batch-assignments_" + curReportAssignmentsCounter);
            else {
                if ( hasAttemptParquetFileProblem )
                    logger.error("All of the attempt-parquet files failed to be created or uploaded! Will avoid to execute load-requests into the database-table \"attempt\", for batch-assignments_" + curReportAssignmentsCounter);
                else if ( hasPayloadParquetFileProblem )
                    logger.error("The single payload-parquet-file failed to be created or uploaded! Will avoid to execute load-requests into the database-table \"payload_aggregated\", for batch-assignments_" + curReportAssignmentsCounter);
                else
                    logger.debug("Going to execute \"load\"-requests on the database, for the uploaded parquet-files.");
            }

            // Load all the parquet files of each type into its table.
            DatabaseConnector.databaseLock.lock();

            if ( ! hasAttemptParquetFileProblem )
                hasAttemptParquetFileProblem = ! parquetFileUtils.loadParquetDataIntoTable(parquetFileUtils.parquetHDFSDirectoryPathAttempts + curReportAssignmentsCounter + "/", "attempt");

            if ( ! hasPayloadParquetFileProblem )
                hasPayloadParquetFileProblem = ! parquetFileUtils.loadParquetDataIntoTable(parquetFileUtils.parquetHDFSDirectoryPathPayloadsAggregated + curReportAssignmentsCounter + "/", "payload_aggregated");

            DatabaseConnector.databaseLock.unlock();

            if ( hasAttemptParquetFileProblem && hasPayloadParquetFileProblem )
                throw new RuntimeException("The data from the HDFS parquet sub-directories COULD NOT be loaded into the \"attempt\" and the \"payload_aggregated\" tables, for batch-assignments_" + curReportAssignmentsCounter);
            else if ( hasAttemptParquetFileProblem || hasPayloadParquetFileProblem )
                logger.error("The data from the HDFS parquet sub-directories COULD NOT be loaded into the \"attempt\" or the \"payload_aggregated\" table, for batch-assignments_" + curReportAssignmentsCounter);
            else
                logger.debug("The data from the HDFS parquet sub-directories was loaded into the \"attempt\" and the \"payload_aggregated\" tables, for batch-assignments_" + curReportAssignmentsCounter);

        } catch (InterruptedException ie) {	// In this case, any unfinished tasks are cancelled.
            logger.warn("The current thread was interrupted when waiting for the worker-threads to finish inserting into the tables: " + ie.getMessage());
            // This is a very rare case. At the moment, we just move on with table-merging.
        } catch (RuntimeException re) {
            DatabaseConnector.databaseLock.lock();
            String assignmentErrorMsg = deleteAssignmentsBatch(curReportAssignmentsCounter);
            DatabaseConnector.databaseLock.unlock();
            String errorMsg = re.getMessage();
            if ( assignmentErrorMsg != null )
                errorMsg += "\n" + assignmentErrorMsg;
            logger.error(errorMsg);
            postReportResultToWorker(curWorkerId, curReportAssignmentsCounter, errorMsg);
            return false;
        } catch (Exception e) {
            String errorMsg = "Unexpected error when inserting into the \"attempt\" and \"payload_aggregated\" tables in parallel! " + e.getMessage();
            logger.error(errorMsg, e);
            postReportResultToWorker(curWorkerId, curReportAssignmentsCounter, errorMsg);
            return false;   // No tables-merging is happening.
        } finally {
            logger.debug("Deleting parquet directory: " + localParquetPath);
            fileUtils.deleteDirectory(new File(localParquetPath));
            // Delete the HDFS subDirs for this Report.
            String endingRmDirAndParams = curReportAssignmentsCounter + "/" + parquetFileUtils.rmDirsAndParams;
            if ( !parquetFileUtils.applyHDFOperation(parquetFileUtils.webHDFSBaseUrl + parquetFileUtils.parquetHDFSDirectoryPathAttempts + endingRmDirAndParams)
                || !parquetFileUtils.applyHDFOperation(parquetFileUtils.webHDFSBaseUrl + parquetFileUtils.parquetHDFSDirectoryPathPayloadsAggregated + endingRmDirAndParams) )
            {
                logger.error("Error when deleting the HDFS sub-directories for assignments_" + curReportAssignmentsCounter);   // A directory-specific log has already appeared.
                // The failure to delete the assignments_subDirs is not that of a problem and should not erase the whole process. So all goes as planned (the worker deletes any remaining files).
                // The worst case is that a few subDirs will be left back in the HDFS, although, without their parquetFiles, since they have already moved inside the DB tables.
            }
        }

        // For every "numOfWorkers" assignment-batches that go to workers, we merge the tables, once a workerReport comes in.
        // After the first few increases of "assignmentsBatchCounter" until all workers get assignment-batches,
        // there will always be a time when the counter will be just before the "golden-value" and then one workerReport has to be processed here and the counter will be incremented by one and signal the merging-time.
        if ( (currentNumOfWorkerReportsProcessed % UrlsController.numOfWorkers.get()) == 0 )
            if ( ! mergeWorkerRelatedTables(curWorkerId, curReportAssignmentsCounter, hasAttemptParquetFileProblem, hasPayloadParquetFileProblem) )
                return false;

        if ( uploadFullTextsResponse == FileUtils.UploadFullTextsResponse.unsuccessful ) {
            postReportResultToWorker(curWorkerId, curReportAssignmentsCounter, "The full-text files failed to be acquired from the worker!");
            return false;
        }

        // Notify the Worker that the processing of this report was successful, so that the Worker can delete the files.
        postReportResultToWorker(curWorkerId, curReportAssignmentsCounter, null);
        return true;
    }


    private String createAndInitializeCurrentAssignmentsTable(String findAssignmentsQuery)
    {
        final String createCurrentAssignmentsQuery = "create table " + DatabaseConnector.databaseName + ".current_assignment as \n" + findAssignmentsQuery;
        final String computeCurrentAssignmentsStatsQuery = "COMPUTE STATS " + DatabaseConnector.databaseName + ".current_assignment";

        try {
            jdbcTemplate.execute(createCurrentAssignmentsQuery);
        } catch (Exception e) {
            String errorMsg = DatabaseConnector.handleQueryException("createCurrentAssignmentsQuery", createCurrentAssignmentsQuery, e);
            String tmpErrMsg = dropCurrentAssignmentTable();    // The table may be partially created, e.g. in case of an "out of memory" error in the database-server, during the creation, resulting in an empty table (yes it has happened).
            if ( tmpErrMsg != null )
                errorMsg += "\n" + tmpErrMsg;
            return errorMsg;
        }

        try {
            jdbcTemplate.execute(computeCurrentAssignmentsStatsQuery);
        } catch (Exception e) {
            String errorMsg = DatabaseConnector.handleQueryException("computeCurrentAssignmentsStatsQuery", computeCurrentAssignmentsStatsQuery, e);
            String tmpErrMsg = dropCurrentAssignmentTable();
            if ( tmpErrMsg != null )
                errorMsg += "\n" + tmpErrMsg;
            return errorMsg;
        }

        return null;    // All good.
    }


    private String dropCurrentAssignmentTable() {
        String dropCurrentAssignmentsQuery = "DROP TABLE IF EXISTS " + DatabaseConnector.databaseName + ".current_assignment PURGE";
        try {
            jdbcTemplate.execute(dropCurrentAssignmentsQuery);
            return null;    // All good. No error-message.
        } catch (Exception e) {
            return DatabaseConnector.handleQueryException("dropCurrentAssignmentsQuery", dropCurrentAssignmentsQuery, e); // The error is already logged inside.
        }
    }


    private boolean mergeWorkerRelatedTables(String curWorkerId, long curReportAssignmentsCounter, boolean hasAttemptParquetFileProblem, boolean hasPayloadParquetFileProblem)
    {
        logger.debug("Going to merge the parquet files for the tables which were altered.");
        // When the uploaded parquet files are "loaded" into the tables, they are actually moved into the directory which contains the data of the table.
        // This means that over time a table may have thousand of parquet files and the search through them will be very slow. Thus, we merge them every now and then.

        String mergeErrorMsg;

        DatabaseConnector.databaseLock.lock();

        if ( ! hasAttemptParquetFileProblem ) {
            mergeErrorMsg = fileUtils.mergeParquetFiles("attempt", "", null);
            if ( mergeErrorMsg != null ) {
                DatabaseConnector.databaseLock.unlock();
                postReportResultToWorker(curWorkerId, curReportAssignmentsCounter, mergeErrorMsg);
                return false;
            }
        }

        if ( ! hasPayloadParquetFileProblem ) {
            mergeErrorMsg = fileUtils.mergeParquetFiles("payload_aggregated", "", null);
            if ( mergeErrorMsg != null ) {
                DatabaseConnector.databaseLock.unlock();
                postReportResultToWorker(curWorkerId, curReportAssignmentsCounter, mergeErrorMsg);
                return false;
            }
        }

        mergeErrorMsg = deleteAssignmentsBatch(curReportAssignmentsCounter);
        if ( mergeErrorMsg != null ) {
            DatabaseConnector.databaseLock.unlock();
            postReportResultToWorker(curWorkerId, curReportAssignmentsCounter, mergeErrorMsg);
            return false;
        }

        DatabaseConnector.databaseLock.unlock();
        logger.debug("Finished merging the database tables.");
        return true;
    }


    private String deleteAssignmentsBatch(long givenAssignmentsBatchCounter)
    {
        // This will delete the rows of the "assignment" table which refer to the "curWorkerId". As we have non-KUDU Impala tables, the Delete operation can only succeed through a "merge" operation of the rest of the data.
        // Only the rows referring to OTHER workerIDs get stored in a temp-table, while the "assignment" table gets deleted. Then, the temp_table becomes the "assignment" table.
        // We don't need to keep the assignment-info anymore, the "findAssignmentsQuery" checks the "payload_aggregated" table for previously handled tasks.
        return fileUtils.mergeParquetFiles("assignment", " WHERE assignments_batch_counter != ", givenAssignmentsBatchCounter);
    }
    

    private static final RestTemplate restTemplate = new RestTemplate();

    private boolean postReportResultToWorker(String workerId, long assignmentRequestCounter, String errorMsg)
    {
        // Get the IP of this worker.
        WorkerInfo workerInfo = UrlsController.workersInfoMap.get(workerId);
        if ( workerInfo == null ) {
            logger.error("Could not find any info for worker with id: \"" + workerId +"\".");
            return false;
        }
        String url = "http://" + workerInfo.getWorkerIP() + ":1881/api/addReportResultToWorker/" + assignmentRequestCounter;    // This workerIP will not be null.

        if ( logger.isTraceEnabled() )
            logger.trace("Going to \"postReportResultToWorker\": \"" + workerId + "\", for assignments_" + assignmentRequestCounter + ((errorMsg != null) ? "\nError: " + errorMsg : ""));

        try {
            ResponseEntity<String> responseEntity = restTemplate.postForEntity(url, errorMsg, String.class);  // We may pass a "null" entity.
            int responseCode = responseEntity.getStatusCodeValue();
            if ( responseCode != HttpStatus.OK.value() ) {
                logger.error("HTTP-Connection problem with the submission of the \"postReportResultToWorker\" of worker \"" + workerId + "\" and assignments_" + assignmentRequestCounter + "! Error-code was: " + responseCode);
                return false;
            } else {
                fileUtils.deleteFile(workerReportsDirPath + "/" + workerId + "/" + workerId + "_assignments_" + assignmentRequestCounter + "_report.json");
                return true;
            }
        } catch (HttpServerErrorException hsee) {
            logger.error("The Worker \"" + workerId + "\" failed to handle the \"postReportResultToWorker\", of assignments_" + assignmentRequestCounter + ": " + hsee.getMessage());
            return false;
        } catch (Exception e) {
            errorMsg = "Error for \"postReportResultToWorker\", of assignments_" + assignmentRequestCounter + " to the Worker: " + workerId;
            Throwable cause = e.getCause();
            String exMsg;
            if ( (cause != null) && ((exMsg = cause.getMessage()) != null) && exMsg.contains("Connection refused") ) {
                logger.error(errorMsg + " | The worker has probably crashed, since we received a \"Connection refused\"!");
                workerInfo.setHasShutdown(true);    // Avoid sending possible shutdown-Requests later on. Also show a specific message if this Worker requests new assignments in the future.
            } else
                logger.error(errorMsg, e);
            return false;
        }
    }


    // The "batchExecute" does not work in this Impala-Database, so this is a "giant-query" solution.
    // Note: this causes an "Out of memory"-ERROR in the current version of the Impala JDBC driver. If a later version is provided, then this code should be tested.
    private static PreparedStatement constructLargeInsertQuery(Connection con, String baseInsertQuery, int dataSize, int numParamsPerRow) throws RuntimeException {
        StringBuilder sb = new StringBuilder(baseInsertQuery.length() + (dataSize * 6 * numParamsPerRow));    // TODO - If this is ever used, make it a global Thread-Local var. And then "clear" (reset) it after each use.
        sb.append(baseInsertQuery);
        for ( int i=1; i <= dataSize; ++i ) {
            sb.append("(");
            for ( int j=1; j <= numParamsPerRow; ++j ) {
                sb.append("?");
                if ( j < numParamsPerRow )
                    sb.append(",");
            }
            sb.append(")");
            if ( i < dataSize )
                sb.append(",");
        }

        PreparedStatement preparedInsertStatement;
        try {   // We use a "PreparedStatement" to do insertions, for security reasons.
            preparedInsertStatement = con.prepareStatement(sb.toString());
        } catch (SQLException sqle) {
            String errorMsg = "Problem when creating the prepared statement for the insertQuery: \"" + baseInsertQuery + "\"...!\n";
            logger.error(errorMsg + sqle.getMessage());
            throw new RuntimeException(errorMsg);
        }
        return preparedInsertStatement;
    }

}