Merge pull request #239 from kennycud/master

Restructuring database connections for better garbage collection - resolves long-standing memory leak in multiple places that was discovered more specifically after the thread crashes were made to restart if crashed. Thanks so much to @kennycud  for this improvement!

src/main/java/org/qortal/controller/BlockMinter.java

@@ -97,21 +97,27 @@ public class BlockMinter extends Thread {
 
 		final boolean isSingleNodeTestnet = Settings.getInstance().isSingleNodeTestnet();
 
-		try (final Repository repository = RepositoryManager.getRepository()) {
-			// Going to need this a lot...
-			BlockRepository blockRepository = repository.getBlockRepository();
-
 		// Flags for tracking change in whether minting is possible,
 		// so we can notify Controller, and further update SysTray, etc.
 		boolean isMintingPossible = false;
 		boolean wasMintingPossible = isMintingPossible;
+		try {
 			while (running) {
+				// recreate repository for new loop iteration
+				try (final Repository repository = RepositoryManager.getRepository()) {
+
+					// Going to need this a lot...
+					BlockRepository blockRepository = repository.getBlockRepository();
+
 					if (isMintingPossible != wasMintingPossible)
 						Controller.getInstance().onMintingPossibleChange(isMintingPossible);
 
 					wasMintingPossible = isMintingPossible;
 
 					try {
+						// reset the repository, to the repository recreated for this loop iteration
+						for( Block newBlock : newBlocks ) newBlock.setRepository(repository);
+
 						// Free up any repository locks
 						repository.discardChanges();
 
@@ -383,7 +389,7 @@ public class BlockMinter extends Thread {
 							// Add unconfirmed transactions
 							addUnconfirmedTransactions(repository, newBlock);
 
-						LOGGER.info(String.format("Adding %d unconfirmed transactions took %d ms", newBlock.getTransactions().size(), (NTP.getTime()-unconfirmedStartTime)));
+							LOGGER.info(String.format("Adding %d unconfirmed transactions took %d ms", newBlock.getTransactions().size(), (NTP.getTime() - unconfirmedStartTime)));
 
 							// Sign to create block's signature
 							newBlock.sign();
@@ -452,9 +458,14 @@ public class BlockMinter extends Thread {
 						// We've been interrupted - time to exit
 						return;
 					}
-			}
 				} catch (DataException e) {
 					LOGGER.warn("Repository issue while running block minter - NO LONGER MINTING", e);
+				} catch (Exception e) {
+					LOGGER.error(e.getMessage(), e);
+				}
+			}
+		} catch (Exception e) {
+			LOGGER.error(e.getMessage(), e);
 		}
 	}
 

src/main/java/org/qortal/controller/Controller.java

@@ -13,6 +13,7 @@ import org.qortal.block.Block;
 import org.qortal.block.BlockChain;
 import org.qortal.block.BlockChain.BlockTimingByHeight;
 import org.qortal.controller.arbitrary.*;
+import org.qortal.controller.hsqldb.HSQLDBBalanceRecorder;
 import org.qortal.controller.hsqldb.HSQLDBDataCacheManager;
 import org.qortal.controller.repository.NamesDatabaseIntegrityCheck;
 import org.qortal.controller.repository.PruneManager;
@@ -36,7 +37,6 @@ import org.qortal.network.Peer;
 import org.qortal.network.PeerAddress;
 import org.qortal.network.message.*;
 import org.qortal.repository.*;
-import org.qortal.repository.hsqldb.HSQLDBRepository;
 import org.qortal.repository.hsqldb.HSQLDBRepositoryFactory;
 import org.qortal.settings.Settings;
 import org.qortal.transaction.Transaction;
@@ -73,6 +73,8 @@ import java.util.stream.Collectors;
 
 public class Controller extends Thread {
 
+	public static HSQLDBRepositoryFactory REPOSITORY_FACTORY;
+
 	static {
 		// This must go before any calls to LogManager/Logger
 		System.setProperty("log4j2.formatMsgNoLookups", "true");
@@ -403,22 +405,37 @@ public class Controller extends Thread {
 
 		LOGGER.info("Starting repository");
 		try {
-			RepositoryFactory repositoryFactory = new HSQLDBRepositoryFactory(getRepositoryUrl());
+			REPOSITORY_FACTORY = new HSQLDBRepositoryFactory(getRepositoryUrl());
-			RepositoryManager.setRepositoryFactory(repositoryFactory);
+			RepositoryManager.setRepositoryFactory(REPOSITORY_FACTORY);
 			RepositoryManager.setRequestedCheckpoint(Boolean.TRUE);
 
 			try (final Repository repository = RepositoryManager.getRepository()) {
 				// RepositoryManager.rebuildTransactionSequences(repository);
 				ArbitraryDataCacheManager.getInstance().buildArbitraryResourcesCache(repository, false);
+			}
 
 			if( Settings.getInstance().isDbCacheEnabled() ) {
 				LOGGER.info("Db Cache Starting ...");
-					HSQLDBDataCacheManager hsqldbDataCacheManager = new HSQLDBDataCacheManager((HSQLDBRepository) repositoryFactory.getRepository());
+				HSQLDBDataCacheManager hsqldbDataCacheManager = new HSQLDBDataCacheManager();
 				hsqldbDataCacheManager.start();
 			}
 			else {
 				LOGGER.info("Db Cache Disabled");
 			}
+
+			if( Settings.getInstance().isBalanceRecorderEnabled() ) {
+				Optional<HSQLDBBalanceRecorder> recorder = HSQLDBBalanceRecorder.getInstance();
+
+				if( recorder.isPresent() ) {
+					LOGGER.info("Balance Recorder Starting ...");
+					recorder.get().start();
+				}
+				else {
+					LOGGER.info("Balance Recorder won't start.");
+				}
+			}
+			else {
+				LOGGER.info("Balance Recorder Disabled");
 			}
 		} catch (DataException e) {
 			// If exception has no cause or message then repository is in use by some other process.
@@ -639,10 +656,8 @@ public class Controller extends Thread {
 				boolean canBootstrap = Settings.getInstance().getBootstrap();
 				boolean needsArchiveRebuild = false;
 				int checkHeight = 0;
-				Repository repository = null;
 
-				try {
+				try (final Repository repository = RepositoryManager.getRepository()){
-					repository = RepositoryManager.getRepository();
 					needsArchiveRebuild = (repository.getBlockArchiveRepository().fromHeight(2) == null);
 					checkHeight = repository.getBlockRepository().getBlockchainHeight();
 				} catch (DataException e) {

src/main/java/org/qortal/controller/hsqldb/HSQLDBBalanceRecorder.java

@@ -0,0 +1,117 @@
+package org.qortal.controller.hsqldb;
+
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+import org.qortal.data.account.AccountBalanceData;
+import org.qortal.repository.hsqldb.HSQLDBCacheUtils;
+import org.qortal.settings.Settings;
+
+import java.util.ArrayList;
+import java.util.Comparator;
+import java.util.List;
+import java.util.Optional;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.stream.Collectors;
+
+public class HSQLDBBalanceRecorder extends Thread{
+
+    private static final Logger LOGGER = LogManager.getLogger(HSQLDBBalanceRecorder.class);
+
+    private static HSQLDBBalanceRecorder SINGLETON = null;
+
+    private ConcurrentHashMap<Integer, List<AccountBalanceData>> balancesByHeight = new ConcurrentHashMap<>();
+
+    private ConcurrentHashMap<String, List<AccountBalanceData>> balancesByAddress = new ConcurrentHashMap<>();
+
+    private int priorityRequested;
+    private int frequency;
+    private int capacity;
+
+    private HSQLDBBalanceRecorder( int priorityRequested, int frequency, int capacity) {
+
+        super("Balance Recorder");
+
+        this.priorityRequested = priorityRequested;
+        this.frequency = frequency;
+        this.capacity = capacity;
+    }
+
+    public static Optional<HSQLDBBalanceRecorder> getInstance() {
+
+        if( SINGLETON == null ) {
+
+            SINGLETON
+                = new HSQLDBBalanceRecorder(
+                    Settings.getInstance().getBalanceRecorderPriority(),
+                    Settings.getInstance().getBalanceRecorderFrequency(),
+                    Settings.getInstance().getBalanceRecorderCapacity()
+            );
+
+        }
+        else if( SINGLETON == null ) {
+
+            return Optional.empty();
+        }
+
+        return Optional.of(SINGLETON);
+    }
+
+    @Override
+    public void run() {
+
+        Thread.currentThread().setName("Balance Recorder");
+
+        HSQLDBCacheUtils.startRecordingBalances(this.balancesByHeight, this.balancesByAddress, this.priorityRequested, this.frequency, this.capacity);
+    }
+
+    public List<AccountBalanceData> getLatestRecordings(int limit, long offset) {
+        ArrayList<AccountBalanceData> data;
+
+        Optional<Integer> lastHeight = getLastHeight();
+
+        if(lastHeight.isPresent() ) {
+            List<AccountBalanceData> latest = this.balancesByHeight.get(lastHeight.get());
+
+            if( latest != null ) {
+                data = new ArrayList<>(latest.size());
+                data.addAll(
+                    latest.stream()
+                        .sorted(Comparator.comparingDouble(AccountBalanceData::getBalance).reversed())
+                        .skip(offset)
+                        .limit(limit)
+                        .collect(Collectors.toList())
+                );
+            }
+            else {
+                data = new ArrayList<>(0);
+            }
+        }
+        else {
+            data = new ArrayList<>(0);
+        }
+
+        return data;
+    }
+
+    private Optional<Integer> getLastHeight() {
+        return this.balancesByHeight.keySet().stream().sorted(Comparator.reverseOrder()).findFirst();
+    }
+
+    public List<Integer> getBlocksRecorded() {
+
+        return this.balancesByHeight.keySet().stream().collect(Collectors.toList());
+    }
+
+    public List<AccountBalanceData> getAccountBalanceRecordings(String address) {
+        return this.balancesByAddress.get(address);
+    }
+
+    @Override
+    public String toString() {
+        return "HSQLDBBalanceRecorder{" +
+                "priorityRequested=" + priorityRequested +
+                ", frequency=" + frequency +
+                ", capacity=" + capacity +
+                '}';
+    }
+}

src/main/java/org/qortal/controller/hsqldb/HSQLDBDataCacheManager.java

@@ -8,11 +8,7 @@ import org.qortal.settings.Settings;
 
 public class HSQLDBDataCacheManager extends Thread{
 
-    private HSQLDBRepository respository;
+    public HSQLDBDataCacheManager() {}
-
-    public HSQLDBDataCacheManager(HSQLDBRepository respository) {
-        this.respository = respository;
-    }
 
     @Override
     public void run() {
@@ -20,8 +16,7 @@ public class HSQLDBDataCacheManager extends Thread{
 
         HSQLDBCacheUtils.startCaching(
             Settings.getInstance().getDbCacheThreadPriority(),
-            Settings.getInstance().getDbCacheFrequency(),
+            Settings.getInstance().getDbCacheFrequency()
-            this.respository
         );
     }
 }

src/main/java/org/qortal/controller/repository/AtStatesPruner.java

@@ -39,15 +39,24 @@ public class AtStatesPruner implements Runnable {
 			}
 		}
 
+		int pruneStartHeight;
+		int maxLatestAtStatesHeight;
+
 		try (final Repository repository = RepositoryManager.getRepository()) {
-			int pruneStartHeight = repository.getATRepository().getAtPruneHeight();
+			pruneStartHeight = repository.getATRepository().getAtPruneHeight();
-			int maxLatestAtStatesHeight = PruneManager.getMaxHeightForLatestAtStates(repository);
+			maxLatestAtStatesHeight = PruneManager.getMaxHeightForLatestAtStates(repository);
 
 			repository.discardChanges();
 			repository.getATRepository().rebuildLatestAtStates(maxLatestAtStatesHeight);
 			repository.saveChanges();
+		} catch (Exception e) {
+			LOGGER.error("AT States Pruning is not working! Not trying again. Restart ASAP. Report this error immediately to the developers.", e);
+			return;
+		}
 
 		while (!Controller.isStopping()) {
+			try (final Repository repository = RepositoryManager.getRepository()) {
+
 				try {
 					repository.discardChanges();
 
@@ -102,28 +111,25 @@ public class AtStatesPruner implements Runnable {
 
 							final int finalPruneStartHeight = pruneStartHeight;
 							LOGGER.info(() -> String.format("Bumping AT state base prune height to %d", finalPruneStartHeight));
-						}
+						} else {
-						else {
 							// We've pruned up to the upper prunable height
 							// Back off for a while to save CPU for syncing
 							repository.discardChanges();
-							Thread.sleep(5*60*1000L);
+							Thread.sleep(5 * 60 * 1000L);
 						}
 					}
 				} catch (InterruptedException e) {
-					if(Controller.isStopping()) {
+					if (Controller.isStopping()) {
 						LOGGER.info("AT States Pruning Shutting Down");
-					}
+					} else {
-					else {
 						LOGGER.warn("AT States Pruning interrupted. Trying again. Report this error immediately to the developers.", e);
 					}
 				} catch (Exception e) {
 					LOGGER.warn("AT States Pruning stopped working. Trying again. Report this error immediately to the developers.", e);
 				}
-			}
+			} catch(Exception e){
-		} catch (Exception e) {
 				LOGGER.error("AT States Pruning is not working! Not trying again. Restart ASAP. Report this error immediately to the developers.", e);
 			}
 		}
-
+	}
 }

src/main/java/org/qortal/controller/repository/AtStatesTrimmer.java

@@ -26,15 +26,23 @@ public class AtStatesTrimmer implements Runnable {
 			return;
 		}
 
+		int trimStartHeight;
+		int maxLatestAtStatesHeight;
+
 		try (final Repository repository = RepositoryManager.getRepository()) {
-			int trimStartHeight = repository.getATRepository().getAtTrimHeight();
+			trimStartHeight = repository.getATRepository().getAtTrimHeight();
-			int maxLatestAtStatesHeight = PruneManager.getMaxHeightForLatestAtStates(repository);
+			maxLatestAtStatesHeight = PruneManager.getMaxHeightForLatestAtStates(repository);
 
 			repository.discardChanges();
 			repository.getATRepository().rebuildLatestAtStates(maxLatestAtStatesHeight);
 			repository.saveChanges();
+		} catch (Exception e) {
+			LOGGER.error("AT States Trimming is not working! Not trying again. Restart ASAP. Report this error immediately to the developers.", e);
+			return;
+		}
 
 		while (!Controller.isStopping()) {
+			try (final Repository repository = RepositoryManager.getRepository()) {
 				try {
 					repository.discardChanges();
 
@@ -92,10 +100,10 @@ public class AtStatesTrimmer implements Runnable {
 				} catch (Exception e) {
 					LOGGER.warn("AT States Trimming stopped working. Trying again. Report this error immediately to the developers.", e);
 				}
-			}
 			} catch (Exception e) {
 				LOGGER.error("AT States Trimming is not working! Not trying again. Restart ASAP. Report this error immediately to the developers.", e);
 			}
 		}
+	}
 
 }

src/main/java/org/qortal/controller/repository/BlockArchiver.java

@@ -30,11 +30,13 @@ public class BlockArchiver implements Runnable {
 			return;
 		}
 
+		int startHeight;
+
 		try (final Repository repository = RepositoryManager.getRepository()) {
 			// Don't even start building until initial rush has ended
 			Thread.sleep(INITIAL_SLEEP_PERIOD);
 
-			int startHeight = repository.getBlockArchiveRepository().getBlockArchiveHeight();
+			startHeight = repository.getBlockArchiveRepository().getBlockArchiveHeight();
 
 			// Don't attempt to archive if we have no ATStatesHeightIndex, as it will be too slow
 			boolean hasAtStatesHeightIndex = repository.getATRepository().hasAtStatesHeightIndex();
@@ -43,10 +45,16 @@ public class BlockArchiver implements Runnable {
 				repository.discardChanges();
 				return;
 			}
+		} catch (Exception e) {
+			LOGGER.error("Block Archiving is not working! Not trying again. Restart ASAP. Report this error immediately to the developers.", e);
+			return;
+		}
 
 		LOGGER.info("Starting block archiver from height {}...", startHeight);
 
 		while (!Controller.isStopping()) {
+			try (final Repository repository = RepositoryManager.getRepository()) {
+
 				try {
 					repository.discardChanges();
 
@@ -107,20 +115,17 @@ public class BlockArchiver implements Runnable {
 						LOGGER.info("Caught exception when creating block cache", e);
 					}
 				} catch (InterruptedException e) {
-					if(Controller.isStopping()) {
+					if (Controller.isStopping()) {
 						LOGGER.info("Block Archiving Shutting Down");
-					}
+					} else {
-					else {
 						LOGGER.warn("Block Archiving interrupted. Trying again. Report this error immediately to the developers.", e);
 					}
 				} catch (Exception e) {
 					LOGGER.warn("Block Archiving stopped working. Trying again. Report this error immediately to the developers.", e);
 				}
-			}
+			} catch(Exception e){
-		} catch (Exception e) {
 				LOGGER.error("Block Archiving is not working! Not trying again. Restart ASAP. Report this error immediately to the developers.", e);
 			}
-
 		}
-
+	}
 }

src/main/java/org/qortal/controller/repository/BlockPruner.java

@@ -39,8 +39,10 @@ public class BlockPruner implements Runnable {
 			}
 		}
 
+		int pruneStartHeight;
+
 		try (final Repository repository = RepositoryManager.getRepository()) {
-			int pruneStartHeight = repository.getBlockRepository().getBlockPruneHeight();
+			pruneStartHeight = repository.getBlockRepository().getBlockPruneHeight();
 
 			// Don't attempt to prune if we have no ATStatesHeightIndex, as it will be too slow
 			boolean hasAtStatesHeightIndex = repository.getATRepository().hasAtStatesHeightIndex();
@@ -48,8 +50,15 @@ public class BlockPruner implements Runnable {
 				LOGGER.info("Unable to start block pruner due to missing ATStatesHeightIndex. Bootstrapping is recommended.");
 				return;
 			}
+		} catch (Exception e) {
+			LOGGER.error("Block Pruning is not working! Not trying again. Restart ASAP. Report this error immediately to the developers.", e);
+			return;
+		}
 
 		while (!Controller.isStopping()) {
+
+			try (final Repository repository = RepositoryManager.getRepository()) {
+
 				try {
 					repository.discardChanges();
 
@@ -122,10 +131,9 @@ public class BlockPruner implements Runnable {
 				} catch (Exception e) {
 					LOGGER.warn("Block Pruning stopped working. Trying again. Report this error immediately to the developers.", e);
 				}
-			}
+			} catch(Exception e){
-		} catch (Exception e) {
 				LOGGER.error("Block Pruning is not working! Not trying again. Restart ASAP. Report this error immediately to the developers.", e);
 			}
 		}
-
+	}
 }

src/main/java/org/qortal/controller/repository/OnlineAccountsSignaturesTrimmer.java

@@ -28,13 +28,21 @@ public class OnlineAccountsSignaturesTrimmer implements Runnable {
 			return;
 		}
 
+		int trimStartHeight;
+
 		try (final Repository repository = RepositoryManager.getRepository()) {
 			// Don't even start trimming until initial rush has ended
 			Thread.sleep(INITIAL_SLEEP_PERIOD);
 
-			int trimStartHeight = repository.getBlockRepository().getOnlineAccountsSignaturesTrimHeight();
+			trimStartHeight = repository.getBlockRepository().getOnlineAccountsSignaturesTrimHeight();
+		} catch (Exception e) {
+			LOGGER.error("Online Accounts Signatures Trimming is not working! Not trying again. Restart ASAP. Report this error immediately to the developers.", e);
+			return;
+		}
 
 		while (!Controller.isStopping()) {
+			try (final Repository repository = RepositoryManager.getRepository()) {
+
 				try {
 					repository.discardChanges();
 
@@ -88,10 +96,9 @@ public class OnlineAccountsSignaturesTrimmer implements Runnable {
 				} catch (Exception e) {
 					LOGGER.warn("Online Accounts Signatures Trimming stopped working. Trying again. Report this error immediately to the developers.", e);
 				}
-			}
 			} catch (Exception e) {
 				LOGGER.error("Online Accounts Signatures Trimming is not working! Not trying again. Restart ASAP. Report this error immediately to the developers.", e);
 			}
 		}
-
+	}
 }

src/main/java/org/qortal/repository/hsqldb/HSQLDBCacheUtils.java

@@ -5,10 +5,13 @@ import org.apache.logging.log4j.Logger;
 import org.qortal.api.SearchMode;
 import org.qortal.arbitrary.misc.Category;
 import org.qortal.arbitrary.misc.Service;
+import org.qortal.controller.Controller;
+import org.qortal.data.account.AccountBalanceData;
 import org.qortal.data.arbitrary.ArbitraryResourceCache;
 import org.qortal.data.arbitrary.ArbitraryResourceData;
 import org.qortal.data.arbitrary.ArbitraryResourceMetadata;
 import org.qortal.data.arbitrary.ArbitraryResourceStatus;
+import org.qortal.repository.DataException;
 
 import java.sql.ResultSet;
 import java.sql.SQLException;
@@ -48,6 +51,11 @@ public class HSQLDBCacheUtils {
         }
     };
     private static final String DEFAULT_IDENTIFIER = "default";
+    private static final int ZERO = 0;
+    public static final String DB_CACHE_TIMER = "DB Cache Timer";
+    public static final String DB_CACHE_TIMER_TASK = "DB Cache Timer Task";
+    public static final String BALANCE_RECORDER_TIMER = "Balance Recorder Timer";
+    public static final String BALANCE_RECORDER_TIMER_TASK = "Balance Recorder Timer Task";
 
     /**
      *
@@ -352,12 +360,123 @@ public class HSQLDBCacheUtils {
      *
      * @param priorityRequested the thread priority to fill cache in
      * @param frequency         the frequency to fill the cache (in seconds)
-     * @param respository the data source
      *
      * @return the data cache
      */
-    public static void startCaching(int priorityRequested, int frequency, HSQLDBRepository respository) {
+    public static void startCaching(int priorityRequested, int frequency) {
 
+        Timer timer = buildTimer(DB_CACHE_TIMER, priorityRequested);
+
+        TimerTask task = new TimerTask() {
+            @Override
+            public void run() {
+
+                Thread.currentThread().setName(DB_CACHE_TIMER_TASK);
+
+                try (final HSQLDBRepository respository = (HSQLDBRepository) Controller.REPOSITORY_FACTORY.getRepository()) {
+                    fillCache(ArbitraryResourceCache.getInstance(), respository);
+                }
+                catch( DataException e ) {
+                    LOGGER.error(e.getMessage(), e);
+                }
+            }
+        };
+
+        // delay 1 second
+        timer.scheduleAtFixedRate(task, 1000, frequency * 1000);
+    }
+
+    /**
+     * Start Recording Balances
+     *
+     * @param queue             the queue to add to, remove oldest data if necssary
+     * @param repository        the db repsoitory
+     * @param priorityRequested the requested thread priority
+     * @param frequency         the recording frequencies, in minutes
+     */
+    public static void startRecordingBalances(
+            final ConcurrentHashMap<Integer, List<AccountBalanceData>> balancesByHeight,
+            final ConcurrentHashMap<String, List<AccountBalanceData>> balancesByAddress,
+            int priorityRequested,
+            int frequency,
+            int capacity) {
+
+        Timer timer = buildTimer(BALANCE_RECORDER_TIMER, priorityRequested);
+
+        TimerTask task = new TimerTask() {
+            @Override
+            public void run() {
+
+                Thread.currentThread().setName(BALANCE_RECORDER_TIMER_TASK);
+
+                try (final HSQLDBRepository repository = (HSQLDBRepository) Controller.REPOSITORY_FACTORY.getRepository()) {
+                    while (balancesByHeight.size() > capacity + 1) {
+                        Optional<Integer> firstHeight = balancesByHeight.keySet().stream().sorted().findFirst();
+
+                        if (firstHeight.isPresent()) balancesByHeight.remove(firstHeight.get());
+                    }
+
+                    // get current balances
+                    List<AccountBalanceData> accountBalances = getAccountBalances(repository);
+
+                    // get anyone of the balances
+                    Optional<AccountBalanceData> data = accountBalances.stream().findAny();
+
+                    // if there are any balances, then record them
+                    if (data.isPresent()) {
+                        // map all new balances to the current height
+                        balancesByHeight.put(data.get().getHeight(), accountBalances);
+
+                        // for each new balance, map to address
+                        for (AccountBalanceData accountBalance : accountBalances) {
+
+                            // get recorded balances for this address
+                            List<AccountBalanceData> establishedBalances
+                                    = balancesByAddress.getOrDefault(accountBalance.getAddress(), new ArrayList<>(0));
+
+                            // start a new list of recordings for this address, add the new balance and add the established
+                            // balances
+                            List<AccountBalanceData> balances = new ArrayList<>(establishedBalances.size() + 1);
+                            balances.add(accountBalance);
+                            balances.addAll(establishedBalances);
+
+                            // reset tha balances for this address
+                            balancesByAddress.put(accountBalance.getAddress(), balances);
+
+                            // TODO: reduce account balances to capacity
+                        }
+
+                        // reduce height balances to capacity
+                        while( balancesByHeight.size() > capacity ) {
+                            Optional<Integer> lowestHeight
+                                = balancesByHeight.entrySet().stream()
+                                    .min(Comparator.comparingInt(Map.Entry::getKey))
+                                    .map(Map.Entry::getKey);
+
+                            if (lowestHeight.isPresent()) balancesByHeight.entrySet().remove(lowestHeight);
+                        }
+                    }
+                } catch (DataException e) {
+                    LOGGER.error(e.getMessage(), e);
+                }
+            }
+        };
+
+        // wait 5 minutes
+        timer.scheduleAtFixedRate(task, 300_000, frequency * 60_000);
+    }
+
+    /**
+     * Build Timer
+     *
+     * Build a timer for scheduling a timer task.
+     *
+     * @param name the name for the thread running the timer task
+     * @param priorityRequested the priority for the thread running the timer task
+     *
+     * @return a timer for scheduling a timer task
+     */
+    private static Timer buildTimer( final String name, int priorityRequested) {
         // ensure priority is in between 1-10
         final int priority = Math.max(0, Math.min(10, priorityRequested));
 
@@ -365,7 +484,7 @@ public class HSQLDBCacheUtils {
         Timer timer = new Timer(true) { // 'true' to make the Timer daemon
             @Override
             public void schedule(TimerTask task, long delay) {
-                Thread thread = new Thread(task) {
+                Thread thread = new Thread(task, name) {
                     @Override
                     public void run() {
                         this.setPriority(priority);
@@ -376,17 +495,7 @@ public class HSQLDBCacheUtils {
                 thread.start();
             }
         };
-
+        return timer;
-        TimerTask task = new TimerTask() {
-            @Override
-            public void run() {
-
-                fillCache(ArbitraryResourceCache.getInstance(), respository);
-            }
-        };
-
-        // delay 1 second
-        timer.scheduleAtFixedRate(task, 1000, frequency * 1000);
     }
 
     /**
@@ -541,4 +650,43 @@ public class HSQLDBCacheUtils {
 
         return resources;
     }
+
+    public static List<AccountBalanceData> getAccountBalances(HSQLDBRepository repository) {
+
+        StringBuilder sql = new StringBuilder();
+
+        sql.append("SELECT account, balance, height ");
+        sql.append("FROM ACCOUNTBALANCES as balances ");
+        sql.append("JOIN (SELECT height FROM BLOCKS ORDER BY height DESC LIMIT 1) AS max_height ON true ");
+        sql.append("WHERE asset_id=0");
+
+        List<AccountBalanceData> data = new ArrayList<>();
+
+        LOGGER.info( "Getting account balances ...");
+
+        try {
+            Statement statement = repository.connection.createStatement();
+
+            ResultSet resultSet = statement.executeQuery(sql.toString());
+
+            if (resultSet == null || !resultSet.next())
+                return new ArrayList<>(0);
+
+            do {
+                String account = resultSet.getString(1);
+                long balance = resultSet.getLong(2);
+                int height = resultSet.getInt(3);
+
+                data.add(new AccountBalanceData(account, ZERO, balance, height));
+            } while (resultSet.next());
+        } catch (SQLException e) {
+            LOGGER.warn(e.getMessage());
+        } catch (Exception e) {
+            LOGGER.error(e.getMessage(), e);
+        }
+
+        LOGGER.info("Retrieved account balances: count = " + data.size());
+
+        return data;
+    }
 }

src/main/java/org/qortal/settings/Settings.java

@@ -441,6 +441,14 @@ public class Settings {
 	 */
 	private long archivingPause = 3000;
 
+	private boolean balanceRecorderEnabled = false;
+
+	private int balanceRecorderPriority = 1;
+
+	private int balanceRecorderFrequency = 2*60*1000;
+
+	private int balanceRecorderCapacity = 1000;
+
 	// Domain mapping
 	public static class ThreadLimit {
 		private String messageType;
@@ -1230,4 +1238,20 @@ public class Settings {
 	public long getArchivingPause() {
 		return archivingPause;
 	}
+
+	public int getBalanceRecorderPriority() {
+		return balanceRecorderPriority;
+	}
+
+	public int getBalanceRecorderFrequency() {
+		return balanceRecorderFrequency;
+	}
+
+	public int getBalanceRecorderCapacity() {
+		return balanceRecorderCapacity;
+	}
+
+	public boolean isBalanceRecorderEnabled() {
+		return balanceRecorderEnabled;
+	}
 }