add queue length operation

jdbc-mariadb/src/main/java/org/xbib/jdbc/mariadb/MariaDB.java

@@ -187,6 +187,12 @@ public class MariaDB implements Flavor {
         return false;
     }
 
+    @Override
+    public String queueLength(String table) {
+        return "select count(*) from " + table
+                + " where channel = ? and state is NULL";
+    }
+
     @Override
     public String writeNextIntoQueue(String table) {
         return "insert into " + table

jdbc-oracle/src/main/java/org/xbib/jdbc/oracle/Oracle.java

@@ -198,6 +198,12 @@ public class Oracle implements Flavor {
         return false;
     }
 
+    @Override
+    public String queueLength(String table) {
+        return "select count(*) from " + table
+                + " where channel = ? and state is NULL";
+    }
+
     @Override
     public String writeNextIntoQueue(String table) {
         return "insert into " + table

jdbc-postgresql/src/main/java/org/xbib/jdbc/postgresql/Postgresql.java

@@ -184,6 +184,12 @@ public class Postgresql implements Flavor {
         return false;
     }
 
+    @Override
+    public String queueLength(String table) {
+        return "select count(*) from " + table
+            + " where channel = ? and state is NULL";
+    }
+
     @Override
     public String writeNextIntoQueue(String table) {
         return "insert into " + table
@@ -195,7 +201,7 @@ public class Postgresql implements Flavor {
     public String readNextFromQueue(String table) {
         return "select id, data from " + table +
                 " where channel = ? and state is NULL and pushed_at <= (CAST (? as TIMESTAMP) - (INTERVAL '1 minute' * delay))" +
-                " order by id asc limit 1 for update skip lock";
+                " order by id asc limit ? for update skip lock";
     }
 
     @Override

jdbc-query/src/main/java/org/xbib/jdbc/query/Database.java

@@ -245,7 +245,16 @@ public interface Database extends Supplier<Database> {
     void writeQueue(String table, String channel, String data, Consumer<Long> consumer) throws SQLException;
 
     /**
-     * Read from a queue table and lock rows in a transaction until processing by a consumer
+     * Proble queue for length.
+     * @param table the queue table
+     * @param channel the queue table channel. A queue table can have many channels
+     * @return the queue length
+     * @throws SQLException if statement fails
+     */
+    long probeQueue(String table, String channel) throws SQLException;
+
+    /**
+     * Read from a queue table until limit is reached and lock rows in a transaction until processing by a consumer
      * has succeeded or failed.
      * Commit only if processing was successful, otherwise roll back the transaction.
      * The connection is switched into manual commit and afterwards, auto-commit is enabled again.
@@ -254,5 +263,5 @@ public interface Database extends Supplier<Database> {
      * @param consumer a consumer for the queue table data column or null
      * @throws SQLException if rollback fails
      */
-    public void readQueue(String table, String channel, Consumer<String> consumer) throws SQLException;
+    public void consumeQueue(String table, String channel, int limit, Consumer<String> consumer) throws SQLException;
 }

jdbc-query/src/main/java/org/xbib/jdbc/query/DatabaseImpl.java

@@ -533,19 +533,32 @@ public class DatabaseImpl implements Database {
     }
 
     @Override
-    public void readQueue(String table, String channel, Consumer<String> consumer) throws SQLException {
+    public long probeQueue(String table, String channel) throws SQLException {
+        try (ResultSet resultSet = queueLength(connection, table, channel)) {
+            if (resultSet.next()) {
+                return resultSet.getLong(0);
+            } else {
+                return -1L;
+            }
+        }
+    }
+
+    @Override
+    public void consumeQueue(String table, String channel, int limit, Consumer<String> consumer) throws SQLException {
+        List<Long> consumedKeys = new ArrayList<>();
         try {
             connection.setAutoCommit(false);
-            while (true) {
-                try (ResultSet resultSet = readNextFromQueue(connection, table, channel)) {
-                    if (resultSet.next()) {
+            try (ResultSet resultSet = readNextFromQueue(connection, table, channel, limit)) {
+                while (resultSet.next()) {
                     Long key = resultSet.getLong("key");
+                    consumedKeys.add(key);
                     try {
                         if (consumer != null) {
                             consumer.accept(resultSet.getString("data"));
                         }
                         succeedInQueue(connection, table, key);
-                        } catch (Exception e) {
+                    } catch (QueueException e) {
+                        connection.rollback();
                         failInQueue(connection, table, key);
                         throw e;
                     }
@@ -553,9 +566,11 @@ public class DatabaseImpl implements Database {
             } finally {
                 connection.commit();
             }
-            }
         } catch (Exception e) {
             connection.rollback();
+            for (Long key : consumedKeys) {
+                failInQueue(connection, table, key);
+            }
             throw e;
         } finally {
             connection.setAutoCommit(true);
@@ -585,10 +600,17 @@ public class DatabaseImpl implements Database {
         }
     }
 
-    private ResultSet readNextFromQueue(Connection connection, String table, String channel) throws SQLException {
+    private ResultSet queueLength(Connection connection, String table, String channel) throws SQLException {
+        PreparedStatement preparedStatement = connection.prepareStatement(flavor().queueLength(table));
+        preparedStatement.setString(1, channel);
+        return preparedStatement.executeQuery();
+    }
+
+    private ResultSet readNextFromQueue(Connection connection, String table, String channel, int limit) throws SQLException {
         PreparedStatement preparedStatement = connection.prepareStatement(flavor().readNextFromQueue(table));
         preparedStatement.setString(1, channel);
         preparedStatement.setTimestamp(2, Timestamp.valueOf(LocalDateTime.now()));
+        preparedStatement.setInt(3, limit);
         return preparedStatement.executeQuery();
     }
 

jdbc-query/src/main/java/org/xbib/jdbc/query/Flavor.java

@@ -77,11 +77,12 @@ public interface Flavor {
 
     /**
      * In JDBC, all drivers are set to autocommit by default.
-     * Returns true if we let the JDBC driver exclusively decide about commit(). If false, it allows explicit commit() calls.
-     * @return
+     * @return Returns true if we let the JDBC driver exclusively decide about commit(). If false, it allows explicit commit() calls.
      */
     boolean isAutoCommitOnly();
 
+    String queueLength(String table);
+
     String writeNextIntoQueue(String table);
 
     String readNextFromQueue(String table);

jdbc-query/src/main/java/org/xbib/jdbc/query/QueueException.java

@@ -0,0 +1,8 @@
+package org.xbib.jdbc.query;
+
+public class QueueException extends DatabaseException {
+
+    public QueueException(String message, Throwable cause) {
+        super(message, cause);
+    }
+}

jdbc-query/src/main/java/org/xbib/jdbc/query/flavor/Derby.java

@@ -180,6 +180,11 @@ public class Derby implements Flavor {
         return false;
     }
 
+    @Override
+    public String queueLength(String table) {
+        throw new UnsupportedOperationException();
+    }
+
     @Override
     public String writeNextIntoQueue(String table) {
         throw new UnsupportedOperationException();

jdbc-query/src/main/java/org/xbib/jdbc/query/flavor/H2.java

@@ -180,6 +180,11 @@ public class H2 implements Flavor {
         return false;
     }
 
+    @Override
+    public String queueLength(String table) {
+        throw new UnsupportedOperationException();
+    }
+
     @Override
     public String writeNextIntoQueue(String table) {
         throw new UnsupportedOperationException();

jdbc-query/src/main/java/org/xbib/jdbc/query/flavor/Hsql.java

@@ -180,6 +180,11 @@ public class Hsql implements Flavor {
         return false;
     }
 
+    @Override
+    public String queueLength(String table) {
+        throw new UnsupportedOperationException();
+    }
+
     @Override
     public String writeNextIntoQueue(String table) {
         throw new UnsupportedOperationException();

jdbc-sqlserver/src/main/java/org/xbib/jdbc/sqlserver/SqlServer.java

@@ -187,6 +187,11 @@ public class SqlServer implements Flavor {
         return false;
     }
 
+    @Override
+    public String queueLength(String table) {
+        throw new UnsupportedOperationException();
+    }
+
     @Override
     public String writeNextIntoQueue(String table) {
         throw new UnsupportedOperationException();