12 files changed, 770 insertions, 0 deletions
diff --git a/src/main/java/io/trygvis/queue/JdbcQueueService.java b/src/main/java/io/trygvis/queue/JdbcQueueService.java
new file mode 100644
index 0000000..ef0b5bb
--- /dev/null
+++ b/src/main/java/io/trygvis/queue/JdbcQueueService.java
@@ -0,0 +1,55 @@
+package io.trygvis.queue;
+
+import java.sql.Connection;
+import java.sql.SQLException;
+import java.util.HashMap;
+import java.util.Map;
+
+public class JdbcQueueService {
+
+    private final QueueSystem queueSystem;
+
+    private final SqlEffectExecutor sqlEffectExecutor;
+
+    private final Map<String, QueueExecutor> queues = new HashMap<>();
+
+    JdbcQueueService(QueueSystem queueSystem) {
+        this.queueSystem = queueSystem;
+        this.sqlEffectExecutor = queueSystem.sqlEffectExecutor;
+    }
+
+    public synchronized QueueExecutor getQueue(String name) {
+        QueueExecutor queueExecutor = queues.get(name);
+
+        if (queueExecutor != null) {
+            return queueExecutor;
+        }
+
+        throw new IllegalArgumentException("No such queue: " + name);
+    }
+
+    public synchronized QueueExecutor lookupQueue(Connection c, String name, long interval, boolean autoCreate) throws SQLException {
+        QueueExecutor queueExecutor = queues.get(name);
+
+        if (queueExecutor != null) {
+            return queueExecutor;
+        }
+
+        QueueDao queueDao = queueSystem.createQueueDao(c);
+
+        Queue q = queueDao.findByName(name);
+
+        if (q == null) {
+            if (!autoCreate) {
+                throw new SQLException("No such queue: '" + name + "'.");
+            }
+
+            q = new Queue(name, interval);
+            queueDao.insert(q);
+        }
+
+        queueExecutor = new QueueExecutor(queueSystem, sqlEffectExecutor, q);
+        queues.put(name, queueExecutor);
+        return queueExecutor;
+    }
+}
diff --git a/src/main/java/io/trygvis/queue/Queue.java b/src/main/java/io/trygvis/queue/Queue.java
new file mode 100755
index 0000000..15003f7
--- /dev/null
+++ b/src/main/java/io/trygvis/queue/Queue.java
@@ -0,0 +1,24 @@
+package io.trygvis.queue;
+
+public class Queue {
+
+    public final String name;
+
+    public final long interval;
+
+    public Queue(String name, long interval) {
+        this.name = name;
+        this.interval = interval;
+    }
+
+    public Queue withInterval(int interval) {
+        return new Queue(name, interval);
+    }
+
+    public String toString() {
+        return "Queue{" +
+                "name='" + name + '\'' +
+                ", interval=" + interval +
+                '}';
+    }
+}
diff --git a/src/main/java/io/trygvis/queue/QueueDao.java b/src/main/java/io/trygvis/queue/QueueDao.java
new file mode 100644
index 0000000..2f69e11
--- /dev/null
+++ b/src/main/java/io/trygvis/queue/QueueDao.java
@@ -0,0 +1,45 @@
+package io.trygvis.queue;
+
+import java.sql.Connection;
+import java.sql.PreparedStatement;
+import java.sql.ResultSet;
+import java.sql.SQLException;
+
+public class QueueDao {
+
+    private final Connection connection;
+
+    public QueueDao(Connection connection) {
+        this.connection = connection;
+    }
+
+    public Queue findByName(String name) throws SQLException {
+        try (PreparedStatement stmt = connection.prepareStatement("SELECT name, interval FROM queue WHERE name=?")) {
+            stmt.setString(1, name);
+            ResultSet rs = stmt.executeQuery();
+            return rs.next() ? mapRow(rs) : null;
+        }
+    }
+
+    public void insert(Queue q) throws SQLException {
+        try (PreparedStatement stmt = connection.prepareStatement("INSERT INTO queue(name, interval) VALUES(?, ?)")) {
+            int i = 1;
+            stmt.setString(i++, q.name);
+            stmt.setLong(i, q.interval);
+            stmt.executeUpdate();
+        }
+    }
+
+    public void update(Queue q) throws SQLException {
+        try (PreparedStatement stmt = connection.prepareStatement("UPDATE queue SET interval=? WHERE name=?")) {
+            int i = 1;
+            stmt.setLong(i++, q.interval);
+            stmt.setString(i, q.name);
+            stmt.executeUpdate();
+        }
+    }
+
+    public Queue mapRow(ResultSet rs) throws SQLException {
+        return new Queue(rs.getString(1), rs.getLong(2));
+    }
+}
diff --git a/src/main/java/io/trygvis/queue/QueueExecutor.java b/src/main/java/io/trygvis/queue/QueueExecutor.java
new file mode 100644
index 0000000..88e5b46
--- /dev/null
+++ b/src/main/java/io/trygvis/queue/QueueExecutor.java
@@ -0,0 +1,177 @@
+package io.trygvis.queue;
+
+import io.trygvis.async.QueueStats;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.sql.Connection;
+import java.sql.SQLException;
+import java.util.Date;
+import java.util.List;
+
+import static io.trygvis.queue.QueueExecutor.TaskExecutionResult.*;
+import static io.trygvis.queue.Task.TaskState.NEW;
+
+public class QueueExecutor {
+    private final Logger log = LoggerFactory.getLogger(getClass());
+
+    private final QueueSystem queueSystem;
+
+    private final SqlEffectExecutor sqlEffectExecutor;
+
+    public final Queue queue;
+
+    private final Stats stats = new Stats();
+
+    public enum TaskExecutionResult {
+        OK,
+        FAILED,
+        MISSED
+    }
+
+    public QueueExecutor(QueueSystem queueSystem, SqlEffectExecutor sqlEffectExecutor, Queue queue) {
+        this.queueSystem = queueSystem;
+        this.sqlEffectExecutor = sqlEffectExecutor;
+        this.queue = queue;
+    }
+
+    private static class Stats {
+        public int total;
+        public int ok;
+        public int failed;
+        public int scheduled;
+        public int missed;
+
+        public synchronized QueueStats toStats() {
+            return new QueueStats(total, ok, failed, missed, scheduled);
+        }
+    }
+
+    public QueueStats getStats() {
+        return stats.toStats();
+    }
+
+    public void consumeAll(final QueueService.TaskExecutionRequest req, final TaskEffect effect) throws SQLException {
+        log.info("Consuming tasks: request={}", req);
+
+        List<Task> tasks;
+        do {
+            tasks = sqlEffectExecutor.transaction(new SqlEffect<List<Task>>() {
+                @Override
+                public List<Task> doInConnection(Connection c) throws SQLException {
+                    return queueSystem.createTaskDao(c).findByQueueAndState(queue.name, NEW, req.chunkSize);
+                }
+            });
+
+            log.info("Consuming chunk with {} tasks", tasks.size());
+
+            applyTasks(req, effect, tasks);
+        } while (tasks.size() > 0);
+    }
+
+    public void applyTasks(QueueService.TaskExecutionRequest req, TaskEffect effect, List<Task> tasks) {
+        for (Task task : tasks) {
+            TaskExecutionResult result = applyTask(effect, task);
+
+            if (result == FAILED && req.stopOnError) {
+                throw new RuntimeException("Error while executing task, id=" + task.id());
+            }
+        }
+    }
+
+    /**
+     * Executed each task in its own transaction.
+     * <p/>
+     * If the task fails, the status is set to error in a separate transaction.
+     */
+    public TaskExecutionResult applyTask(TaskEffect effect, final Task task) {
+        try {
+            Integer count = sqlEffectExecutor.transaction(new SqlEffect<Integer>() {
+                @Override
+                public Integer doInConnection(Connection c) throws SQLException {
+                    return queueSystem.createTaskDao(c).update(task.markProcessing(), NEW);
+                }
+            });
+
+            if (count == 0) {
+                log.warn("Missed task {}", task.id());
+                synchronized (stats) {
+                    stats.missed++;
+                }
+                return MISSED;
+            }
+
+            log.info("Executing task {}", task.id());
+
+            final List<Task> newTasks = effect.apply(task);
+
+            final Date now = new Date();
+
+            log.info("Executed task {} at {}, newTasks: {}", task.id(), now, newTasks.size());
+
+            sqlEffectExecutor.transaction(new SqlEffect.Void() {
+                @Override
+                public void doInConnection(Connection c) throws SQLException {
+                    for (Task newTask : newTasks) {
+                        schedule(c, newTask);
+                    }
+
+                    queueSystem.createTaskDao(c).update(task.markOk(now));
+                }
+            });
+
+            synchronized (stats) {
+                stats.total++;
+                stats.ok++;
+            }
+
+            return OK;
+        } catch (Exception e) {
+            final Date now = new Date();
+            log.error("Unable to execute task, id=" + task.id(), e);
+
+            synchronized (stats) {
+                stats.total++;
+                stats.failed++;
+            }
+
+            try {
+                sqlEffectExecutor.transaction(new SqlEffect.Void() {
+                    @Override
+                    public void doInConnection(Connection c) throws SQLException {
+                        TaskDao taskDao = queueSystem.createTaskDao(c);
+                        taskDao.update(task.markFailed(now));
+                    }
+                });
+            } catch (SQLException e1) {
+                log.error("Error while marking task as failed.", e1);
+            }
+
+            return FAILED;
+        }
+    }
+
+    public void schedule(Connection c, Task task) throws SQLException {
+        schedule(c, task.queue, task.parent, task.scheduled, task.arguments);
+    }
+
+    public Task schedule(Connection c, Date scheduled, List<String> arguments) throws SQLException {
+        return schedule(c, queue.name, null, scheduled, arguments);
+    }
+
+    public Task schedule(Connection c, long parent, Date scheduled, List<String> arguments) throws SQLException {
+        return schedule(c, queue.name, parent, scheduled, arguments);
+    }
+
+    private Task schedule(Connection c, String queue, Long parent, Date scheduled, List<String> arguments) throws SQLException {
+        TaskDao taskDao = queueSystem.createTaskDao(c);
+
+        long id = taskDao.insert(parent, queue, NEW, scheduled, arguments);
+
+        synchronized (stats) {
+            stats.scheduled++;
+        }
+
+        return new Task(id, parent, queue, NEW, scheduled, null, 0, null, arguments);
+    }
+}
diff --git a/src/main/java/io/trygvis/queue/QueueService.java b/src/main/java/io/trygvis/queue/QueueService.java
new file mode 100644
index 0000000..1c38f1f
--- /dev/null
+++ b/src/main/java/io/trygvis/queue/QueueService.java
@@ -0,0 +1,54 @@
+package io.trygvis.queue;
+
+import java.sql.SQLException;
+import java.util.Date;
+import java.util.List;
+
+public interface QueueService {
+    QueueExecutor getQueue(String name, int interval, boolean autoCreate) throws SQLException;
+
+    void schedule(Queue queue, Date scheduled, List<String> arguments) throws SQLException;
+
+    public static class TaskExecutionRequest {
+        public final long chunkSize;
+        public final boolean stopOnError;
+        public final Long interval;
+        public final boolean continueOnFullChunk;
+        // TODO: saveExceptions
+
+        public TaskExecutionRequest(long chunkSize, boolean stopOnError) {
+            this(chunkSize, stopOnError, null, true);
+        }
+
+        private TaskExecutionRequest(long chunkSize, boolean stopOnError, Long interval, boolean continueOnFullChunk) {
+            this.chunkSize = chunkSize;
+            this.stopOnError = stopOnError;
+            this.interval = interval;
+            this.continueOnFullChunk = continueOnFullChunk;
+        }
+
+        public TaskExecutionRequest interval(long interval) {
+            return new TaskExecutionRequest(chunkSize, stopOnError, interval, continueOnFullChunk);
+        }
+
+        public TaskExecutionRequest continueOnFullChunk(boolean continueOnFullChunk) {
+            return new TaskExecutionRequest(chunkSize, stopOnError, interval, continueOnFullChunk);
+        }
+
+        @Override
+        public String toString() {
+            return "TaskExecutionRequest{" +
+                    "chunkSize=" + chunkSize +
+                    ", stopOnError=" + stopOnError +
+                    '}';
+        }
+
+        public long interval(Queue queue) {
+            if (interval != null) {
+                return interval;
+            }
+
+            return queue.interval;
+        }
+    }
+}
diff --git a/src/main/java/io/trygvis/queue/QueueStats.java b/src/main/java/io/trygvis/queue/QueueStats.java
new file mode 100644
index 0000000..5b048b3
--- /dev/null
+++ b/src/main/java/io/trygvis/queue/QueueStats.java
@@ -0,0 +1,20 @@
+package io.trygvis.queue;
+
+import java.util.EnumMap;
+
+public class QueueStats {
+    public final String name;
+    public final long totalTaskCount;
+    public final EnumMap<Task.TaskState, Long> states;
+
+    public QueueStats(String name, EnumMap<Task.TaskState, Long> states) {
+        this.name = name;
+        this.states = states;
+
+        long c = 0;
+        for (Long l : states.values()) {
+            c += l;
+        }
+        this.totalTaskCount = c;
+    }
+}
diff --git a/src/main/java/io/trygvis/queue/QueueSystem.java b/src/main/java/io/trygvis/queue/QueueSystem.java
new file mode 100644
index 0000000..5954526
--- /dev/null
+++ b/src/main/java/io/trygvis/queue/QueueSystem.java
@@ -0,0 +1,61 @@
+package io.trygvis.queue;
+
+import io.trygvis.async.JdbcAsyncService;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.sql.Connection;
+import java.sql.DatabaseMetaData;
+import java.sql.SQLException;
+
+public class QueueSystem {
+    private final Logger log = LoggerFactory.getLogger(getClass());
+
+    public final SqlEffectExecutor sqlEffectExecutor;
+
+    private final JdbcQueueService queueService;
+
+    private QueueSystem(SqlEffectExecutor sqlEffectExecutor) throws SQLException {
+        sqlEffectExecutor.transaction(new SqlEffect.Void() {
+            @Override
+            public void doInConnection(Connection c) throws SQLException {
+                if (c.getAutoCommit()) {
+                    throw new SQLException("The connection cannot be in auto-commit mode.");
+                }
+
+                DatabaseMetaData metaData = c.getMetaData();
+                String productName = metaData.getDatabaseProductName();
+                String productVersion = metaData.getDatabaseProductVersion();
+
+                log.info("productName = " + productName);
+                log.info("productVersion = " + productVersion);
+            }
+        });
+
+        this.sqlEffectExecutor = sqlEffectExecutor;
+        queueService = new JdbcQueueService(this);
+    }
+
+    /**
+     * Initializes the queue system. Use this as the first thing do as it will validate the database.
+     */
+    public static QueueSystem initialize(SqlEffectExecutor sqlEffectExecutor) throws SQLException {
+        return new QueueSystem(sqlEffectExecutor);
+    }
+
+    public JdbcQueueService createQueueService() {
+        return queueService;
+    }
+
+    public QueueDao createQueueDao(Connection c) {
+        return new QueueDao(c);
+    }
+
+    public TaskDao createTaskDao(Connection c) {
+        return new TaskDao(c);
+    }
+
+    public JdbcAsyncService createAsyncService() {
+        return new JdbcAsyncService(this);
+    }
+}
diff --git a/src/main/java/io/trygvis/queue/SqlEffect.java b/src/main/java/io/trygvis/queue/SqlEffect.java
new file mode 100644
index 0000000..7864bcd
--- /dev/null
+++ b/src/main/java/io/trygvis/queue/SqlEffect.java
@@ -0,0 +1,12 @@
+package io.trygvis.queue;
+
+import java.sql.Connection;
+import java.sql.SQLException;
+
+public interface SqlEffect<A> {
+    A doInConnection(Connection c) throws SQLException;
+
+    interface Void {
+        void doInConnection(Connection c) throws SQLException;
+    }
+}
diff --git a/src/main/java/io/trygvis/queue/SqlEffectExecutor.java b/src/main/java/io/trygvis/queue/SqlEffectExecutor.java
new file mode 100644
index 0000000..92802da
--- /dev/null
+++ b/src/main/java/io/trygvis/queue/SqlEffectExecutor.java
@@ -0,0 +1,50 @@
+package io.trygvis.queue;
+
+import javax.sql.DataSource;
+import java.sql.Connection;
+import java.sql.SQLException;
+
+public class SqlEffectExecutor {
+
+    private final DataSource dataSource;
+
+    public SqlEffectExecutor(DataSource dataSource) {
+        this.dataSource = dataSource;
+    }
+
+    public <A> A transaction(SqlEffect<A> effect) throws SQLException {
+//        int pid;
+
+        try (Connection c = dataSource.getConnection()) {
+//            pid = getPid(c);
+//            System.out.println("pid = " + pid);
+
+            boolean ok = false;
+            try {
+                A a = effect.doInConnection(c);
+                c.commit();
+                ok = true;
+                return a;
+            } finally {
+//                System.out.println("Closing, pid = " + pid);
+                if (!ok) {
+                    try {
+                        c.rollback();
+                    } catch (SQLException e) {
+                        // ignore
+                    }
+                }
+            }
+        }
+    }
+
+    public void transaction(final SqlEffect.Void effect) throws SQLException {
+        transaction(new SqlEffect<Object>() {
+            @Override
+            public Object doInConnection(Connection c) throws SQLException {
+                effect.doInConnection(c);
+                return null;
+            }
+        });
+    }
+}
diff --git a/src/main/java/io/trygvis/queue/Task.java b/src/main/java/io/trygvis/queue/Task.java
new file mode 100755
index 0000000..8038050
--- /dev/null
+++ b/src/main/java/io/trygvis/queue/Task.java
@@ -0,0 +1,117 @@
+package io.trygvis.queue;
+
+import java.util.Date;
+import java.util.List;
+
+import static io.trygvis.queue.Task.TaskState.*;
+import static java.util.Arrays.asList;
+
+public class Task {
+
+    public enum TaskState {
+        NEW,
+        PROCESSING,
+        OK,
+        FAILED
+    }
+
+    private final long id;
+
+    public final Long parent;
+
+    public final String queue;
+
+    public final TaskState state;
+
+    public final Date scheduled;
+
+    public final Date lastRun;
+
+    public final int runCount;
+
+    public final Date completed;
+
+    public final List<String> arguments;
+
+    public Task(long id, Long parent, String queue, TaskState state, Date scheduled, Date lastRun, int runCount, Date completed, List<String> arguments) {
+        this.id = id;
+        this.parent = parent;
+        this.queue = queue;
+        this.state = state;
+        this.scheduled = scheduled;
+        this.lastRun = lastRun;
+        this.runCount = runCount;
+        this.completed = completed;
+
+        this.arguments = arguments;
+    }
+
+    public Task markProcessing() {
+        return new Task(id, parent, queue, PROCESSING, scheduled, new Date(), runCount + 1, completed, arguments);
+    }
+
+    public Task markOk(Date completed) {
+        return new Task(id, parent, queue, OK, scheduled, lastRun, runCount, completed, arguments);
+    }
+
+    public Task markFailed(Date now) {
+        return new Task(id, parent, queue, FAILED, scheduled, lastRun, runCount, completed, arguments);
+    }
+
+    public String toString() {
+        return "Task{" +
+                "id=" + id +
+                ", parent=" + parent +
+                ", queue=" + queue +
+                ", state=" + state +
+                ", scheduled=" + scheduled +
+                ", lastRun=" + lastRun +
+                ", runCount=" + runCount +
+                ", completed=" + completed +
+                ", arguments='" + arguments + '\'' +
+                '}';
+    }
+
+    public long id() {
+        if (id == 0) {
+            throw new RuntimeException("This task has not been persisted yet.");
+        }
+
+        return id;
+    }
+
+    public boolean isDone() {
+        return completed != null;
+    }
+
+    public Task childTask(String queue, Date scheduled, String... arguments) {
+        return new Task(0, id(), queue, NEW, scheduled, null, 0, null, asList(arguments));
+    }
+
+    public static Task newTask(String queue, Date scheduled, String... arguments) {
+        return new Task(0, null, queue, NEW, scheduled, null, 0, null, asList(arguments));
+    }
+
+    public static Task newTask(String queue, Date scheduled, List<String> arguments) {
+        return new Task(0, null, queue, NEW, scheduled, null, 0, null, arguments);
+    }
+
+    public static List<String> stringToArguments(String arguments) {
+        return asList(arguments.split(","));
+    }
+
+    public static String argumentsToString(List<String> arguments) {
+        StringBuilder builder = new StringBuilder();
+        for (int i = 0, argumentsLength = arguments.size(); i < argumentsLength; i++) {
+            String argument = arguments.get(i);
+            if (argument.contains(",")) {
+                throw new RuntimeException("The argument string can't contain a comma.");
+            }
+            if (i > 0) {
+                builder.append(',');
+            }
+            builder.append(argument);
+        }
+        return builder.toString();
+    }
+}
diff --git a/src/main/java/io/trygvis/queue/TaskDao.java b/src/main/java/io/trygvis/queue/TaskDao.java
new file mode 100644
index 0000000..365b44b
--- /dev/null
+++ b/src/main/java/io/trygvis/queue/TaskDao.java
@@ -0,0 +1,148 @@
+package io.trygvis.queue;
+
+import java.sql.Connection;
+import java.sql.PreparedStatement;
+import java.sql.ResultSet;
+import java.sql.SQLException;
+import java.sql.Timestamp;
+import java.sql.Types;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.Date;
+import java.util.EnumMap;
+import java.util.List;
+
+import static io.trygvis.queue.Task.*;
+
+public class TaskDao {
+
+    private final Connection c;
+
+    public static final String fields = "id, parent, queue, state, scheduled, last_run, run_count, completed, arguments";
+
+    TaskDao(Connection c) {
+        this.c = c;
+    }
+
+    public long insert(Long parent, String queue, TaskState state, Date scheduled, List<String> arguments) throws SQLException {
+        String sql = "INSERT INTO task(id, parent, run_count, queue, state, scheduled, arguments) " +
+                "VALUES(nextval('task_seq'), ?, 0, ?, ?, ?, ?)";
+        try (PreparedStatement stmt = c.prepareStatement(sql)) {
+            int i = 1;
+            if (parent == null) {
+                stmt.setNull(i++, Types.BIGINT);
+            } else {
+                stmt.setLong(i++, parent);
+            }
+            stmt.setString(i++, queue);
+            stmt.setString(i++, state.name());
+            stmt.setTimestamp(i++, new Timestamp(scheduled.getTime()));
+            stmt.setString(i, argumentsToString(arguments));
+            stmt.executeUpdate();
+        }
+        try (PreparedStatement stmt = c.prepareStatement("SELECT currval('task_seq')")) {
+            ResultSet rs = stmt.executeQuery();
+            rs.next();
+            return rs.getLong(1);
+        }
+    }
+
+    public Task findById(long id) throws SQLException {
+        try (PreparedStatement stmt = c.prepareStatement("SELECT " + fields + " FROM task WHERE id=?")) {
+            stmt.setLong(1, id);
+            ResultSet rs = stmt.executeQuery();
+            return rs.next() ? mapRow(rs) : null;
+        }
+    }
+
+    public List<Task> findByQueueAndState(String queue, TaskState state, long limit) throws SQLException {
+        try (PreparedStatement stmt = c.prepareStatement("SELECT " + fields + " FROM task WHERE queue=? AND state=? LIMIT ?")) {
+            int i = 1;
+            stmt.setString(i++, queue);
+            stmt.setString(i++, state.name());
+            stmt.setLong(i, limit);
+            ResultSet rs = stmt.executeQuery();
+            List<Task> list = new ArrayList<>();
+            while (rs.next()) {
+                list.add(mapRow(rs));
+            }
+            return list;
+        }
+    }
+
+    public QueueStats findQueueStatsByName(String queue) throws SQLException {
+        try (PreparedStatement stmt = c.prepareStatement("SELECT state, COUNT(id) FROM task WHERE queue=? GROUP BY state")) {
+            int i = 1;
+            stmt.setString(i, queue);
+            ResultSet rs = stmt.executeQuery();
+            EnumMap<TaskState, Long> states = new EnumMap<>(TaskState.class);
+            while (rs.next()) {
+                states.put(TaskState.valueOf(rs.getString(1)), rs.getLong(2));
+            }
+            return new QueueStats(queue, states);
+        }
+    }
+
+    public int update(Task task) throws SQLException {
+        return update(task, null);
+    }
+
+    public int update(Task task, TaskState state) throws SQLException {
+        String sql = "UPDATE task SET state=?, scheduled=?, last_run=?, run_count=?, completed=? WHERE id=?";
+
+        if (state != null) {
+            sql += " AND state=?";
+        }
+
+        try (PreparedStatement stmt = c.prepareStatement(sql)) {
+            int i = 1;
+            stmt.setString(i++, task.state.name());
+            stmt.setTimestamp(i++, new Timestamp(task.scheduled.getTime()));
+            setTimestamp(stmt, i++, task.lastRun);
+            stmt.setInt(i++, task.runCount);
+            setTimestamp(stmt, i++, task.completed);
+            stmt.setLong(i++, task.id());
+            if (state != null) {
+                stmt.setString(i, state.name());
+            }
+            return stmt.executeUpdate();
+        }
+    }
+
+    public void setState(Task task, TaskState state) throws SQLException {
+        try (PreparedStatement stmt = c.prepareStatement("UPDATE task SET state=? WHERE id = ?")) {
+            int i = 1;
+            stmt.setString(i++, state.name());
+            stmt.setLong(i, task.id());
+            stmt.executeUpdate();
+        }
+    }
+
+    private static void setTimestamp(PreparedStatement stmt, int parameterIndex, Date date) throws SQLException {
+        if (date == null) {
+            stmt.setNull(parameterIndex, Types.TIMESTAMP);
+        } else {
+            stmt.setTimestamp(parameterIndex, new Timestamp(date.getTime()));
+        }
+    }
+
+    public Task mapRow(ResultSet rs) throws SQLException {
+        String arguments = rs.getString(9);
+        int i = 1;
+        return new Task(
+                rs.getLong(i++),
+                rs.getLong(i++),
+                rs.getString(i++),
+                TaskState.valueOf(rs.getString(i++)),
+                rs.getTimestamp(i++),
+                rs.getTimestamp(i++),
+                rs.getInt(i++),
+                rs.getTimestamp(i),
+                arguments != null ? stringToArguments(arguments) : Collections.<String>emptyList());
+    }
+
+    public void rollback() throws SQLException {
+        c.rollback();
+        c.close();
+    }
+}
diff --git a/src/main/java/io/trygvis/queue/TaskEffect.java b/src/main/java/io/trygvis/queue/TaskEffect.java
new file mode 100644
index 0000000..186797f
--- /dev/null
+++ b/src/main/java/io/trygvis/queue/TaskEffect.java
@@ -0,0 +1,7 @@
+package io.trygvis.queue;
+
+import java.util.List;
+
+public interface TaskEffect {
+    List<Task> apply(Task task) throws Exception;
+}