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本文主要研究一下storm trident的state

StateType

storm-2.0.0/storm-client/src/jvm/org/apache/storm/trident/state/StateType.java

public enum StateType {
    NON_TRANSACTIONAL,
    TRANSACTIONAL,
    OPAQUE
}

• StateType有三种类型，NON_TRANSACTIONAL非事务性，TRANSACTIONAL事务性，OPAQUE不透明事务
• 对应的spout也有三类，non-transactional、transactional以及opaque transactional

State

storm-2.0.0/storm-client/src/jvm/org/apache/storm/trident/state/State.java

/**
 * There's 3 different kinds of state:
 *
 * 1. non-transactional: ignores commits, updates are permanent. no rollback. a cassandra incrementing state would be like this 2.
 * repeat-transactional: idempotent as long as all batches for a txid are identical 3. opaque-transactional: the most general kind of state.
 * updates are always done based on the previous version of the value if the current commit = latest stored commit Idempotent even if the
 * batch for a txid can change.
 *
 * repeat transactional is idempotent for transactional spouts opaque transactional is idempotent for opaque or transactional spouts
 *
 * Trident should log warnings when state is idempotent but updates will not be idempotent because of spout
 */
// retrieving is encapsulated in Retrieval interface
public interface State {
    void beginCommit(Long txid); // can be null for things like partitionPersist occuring off a DRPC stream

    void commit(Long txid);
}

• non-transactional，忽略commits，updates是持久的，没有rollback，cassandra的incrementing state属于这个类型；at-most或者at-least once语义
• repeat-transactional，简称transactional，要求不管是否replayed，同一个batch的txid始终相同，而且里头的tuple也不变，一个tuple只属于一个batch，各个batch之间不会重叠；对于state更新来说，replay遇到相同的txid，即可跳过；在数据库需要较少的state，但是容错性较差，保证exactly once语义
• opaque-transactional，简称opaque，是用的比较多的一类，它的容错性比transactional强，它不要求一个tuple始终在同一个batch/txid，也就是说允许一个tuple在这个batch处理失败，但是在其他batch中处理成功，但是它可以保证每个tuple只在某一个batch中exactly成功处理一次；OpaqueTridentKafkaSpout就是这个类型的实现，它能容忍kafka节点丢失的错误；对于state更新来说，replay遇到相同的txid，则需要基于prevValue使用当前的值覆盖掉；在数据库需要更多空间来存储state，但是容错性好，保证exactly once语义

MapState

storm-2.0.0/storm-client/src/jvm/org/apache/storm/trident/state/map/MapState.java

public interface MapState<T> extends ReadOnlyMapState<T> {
    List<T> multiUpdate(List<List<Object>> keys, List<ValueUpdater> updaters);

    void multiPut(List<List<Object>> keys, List<T> vals);
}

• MapState继承了ReadOnlyMapState接口，而ReadOnlyMapState则继承了State接口
• 这里主要举MapState的几个实现类分析一下

NonTransactionalMap

storm-2.0.0/storm-client/src/jvm/org/apache/storm/trident/state/map/NonTransactionalMap.java

public class NonTransactionalMap<T> implements MapState<T> {
    IBackingMap<T> _backing;

    protected NonTransactionalMap(IBackingMap<T> backing) {
        _backing = backing;
    }

    public static <T> MapState<T> build(IBackingMap<T> backing) {
        return new NonTransactionalMap<T>(backing);
    }

    @Override
    public List<T> multiGet(List<List<Object>> keys) {
        return _backing.multiGet(keys);
    }

    @Override
    public List<T> multiUpdate(List<List<Object>> keys, List<ValueUpdater> updaters) {
        List<T> curr = _backing.multiGet(keys);
        List<T> ret = new ArrayList<T>(curr.size());
        for (int i = 0; i < curr.size(); i++) {
            T currVal = curr.get(i);
            ValueUpdater<T> updater = updaters.get(i);
            ret.add(updater.update(currVal));
        }
        _backing.multiPut(keys, ret);
        return ret;
    }

    @Override
    public void multiPut(List<List<Object>> keys, List<T> vals) {
        _backing.multiPut(keys, vals);
    }

    @Override
    public void beginCommit(Long txid) {
    }

    @Override
    public void commit(Long txid) {
    }
}

• NonTransactionalMap包装了IBackingMap，beginCommit及commit方法都不做任何操作
• multiUpdate方法构造List<T> ret，然后使用IBackingMap的multiPut来实现

TransactionalMap

storm-2.0.0/storm-client/src/jvm/org/apache/storm/trident/state/map/TransactionalMap.java

public class TransactionalMap<T> implements MapState<T> {
    CachedBatchReadsMap<TransactionalValue> _backing;
    Long _currTx;

    protected TransactionalMap(IBackingMap<TransactionalValue> backing) {
        _backing = new CachedBatchReadsMap(backing);
    }

    public static <T> MapState<T> build(IBackingMap<TransactionalValue> backing) {
        return new TransactionalMap<T>(backing);
    }

    @Override
    public List<T> multiGet(List<List<Object>> keys) {
        List<CachedBatchReadsMap.RetVal<TransactionalValue>> vals = _backing.multiGet(keys);
        List<T> ret = new ArrayList<T>(vals.size());
        for (CachedBatchReadsMap.RetVal<TransactionalValue> retval : vals) {
            TransactionalValue v = retval.val;
            if (v != null) {
                ret.add((T) v.getVal());
            } else {
                ret.add(null);
            }
        }
        return ret;
    }

    @Override
    public List<T> multiUpdate(List<List<Object>> keys, List<ValueUpdater> updaters) {
        List<CachedBatchReadsMap.RetVal<TransactionalValue>> curr = _backing.multiGet(keys);
        List<TransactionalValue> newVals = new ArrayList<TransactionalValue>(curr.size());
        List<List<Object>> newKeys = new ArrayList();
        List<T> ret = new ArrayList<T>();
        for (int i = 0; i < curr.size(); i++) {
            CachedBatchReadsMap.RetVal<TransactionalValue> retval = curr.get(i);
            TransactionalValue<T> val = retval.val;
            ValueUpdater<T> updater = updaters.get(i);
            TransactionalValue<T> newVal;
            boolean changed = false;
            if (val == null) {
                newVal = new TransactionalValue<T>(_currTx, updater.update(null));
                changed = true;
            } else {
                if (_currTx != null && _currTx.equals(val.getTxid()) && !retval.cached) {
                    newVal = val;
                } else {
                    newVal = new TransactionalValue<T>(_currTx, updater.update(val.getVal()));
                    changed = true;
                }
            }
            ret.add(newVal.getVal());
            if (changed) {
                newVals.add(newVal);
                newKeys.add(keys.get(i));
            }
        }
        if (!newKeys.isEmpty()) {
            _backing.multiPut(newKeys, newVals);
        }
        return ret;
    }

    @Override
    public void multiPut(List<List<Object>> keys, List<T> vals) {
        List<TransactionalValue> newVals = new ArrayList<TransactionalValue>(vals.size());
        for (T val : vals) {
            newVals.add(new TransactionalValue<T>(_currTx, val));
        }
        _backing.multiPut(keys, newVals);
    }

    @Override
    public void beginCommit(Long txid) {
        _currTx = txid;
        _backing.reset();
    }

    @Override
    public void commit(Long txid) {
        _currTx = null;
        _backing.reset();
    }
}

• TransactionalMap采取的是CachedBatchReadsMap<TransactionalValue>，这里泛型使用的是TransactionalValue，beginCommit会设置当前的txid，重置_backing，commit的时候会重置txid，然后重置_backing
• multiUpdate方法中判断如果_currTx已经存在值，且该值!retval.cached(即不是本次事务中multiPut进去的)，那么不会更新该值(skip the update)，使用newVal = val
• multiPut方法构造批量的TransactionalValue，然后使用CachedBatchReadsMap.multiPut(List<List<Object>> keys, List<T> vals)方法，该方法更新值之后会更新到缓存

OpaqueMap

storm-2.0.0/storm-client/src/jvm/org/apache/storm/trident/state/map/OpaqueMap.java

public class OpaqueMap<T> implements MapState<T> {
    CachedBatchReadsMap<OpaqueValue> _backing;
    Long _currTx;

    protected OpaqueMap(IBackingMap<OpaqueValue> backing) {
        _backing = new CachedBatchReadsMap(backing);
    }

    public static <T> MapState<T> build(IBackingMap<OpaqueValue> backing) {
        return new OpaqueMap<T>(backing);
    }

    @Override
    public List<T> multiGet(List<List<Object>> keys) {
        List<CachedBatchReadsMap.RetVal<OpaqueValue>> curr = _backing.multiGet(keys);
        List<T> ret = new ArrayList<T>(curr.size());
        for (CachedBatchReadsMap.RetVal<OpaqueValue> retval : curr) {
            OpaqueValue val = retval.val;
            if (val != null) {
                if (retval.cached) {
                    ret.add((T) val.getCurr());
                } else {
                    ret.add((T) val.get(_currTx));
                }
            } else {
                ret.add(null);
            }
        }
        return ret;
    }

    @Override
    public List<T> multiUpdate(List<List<Object>> keys, List<ValueUpdater> updaters) {
        List<CachedBatchReadsMap.RetVal<OpaqueValue>> curr = _backing.multiGet(keys);
        List<OpaqueValue> newVals = new ArrayList<OpaqueValue>(curr.size());
        List<T> ret = new ArrayList<T>();
        for (int i = 0; i < curr.size(); i++) {
            CachedBatchReadsMap.RetVal<OpaqueValue> retval = curr.get(i);
            OpaqueValue<T> val = retval.val;
            ValueUpdater<T> updater = updaters.get(i);
            T prev;
            if (val == null) {
                prev = null;
            } else {
                if (retval.cached) {
                    prev = val.getCurr();
                } else {
                    prev = val.get(_currTx);
                }
            }
            T newVal = updater.update(prev);
            ret.add(newVal);
            OpaqueValue<T> newOpaqueVal;
            if (val == null) {
                newOpaqueVal = new OpaqueValue<T>(_currTx, newVal);
            } else {
                newOpaqueVal = val.update(_currTx, newVal);
            }
            newVals.add(newOpaqueVal);
        }
        _backing.multiPut(keys, newVals);
        return ret;
    }

    @Override
    public void multiPut(List<List<Object>> keys, List<T> vals) {
        List<ValueUpdater> updaters = new ArrayList<ValueUpdater>(vals.size());
        for (T val : vals) {
            updaters.add(new ReplaceUpdater<T>(val));
        }
        multiUpdate(keys, updaters);
    }

    @Override
    public void beginCommit(Long txid) {
        _currTx = txid;
        _backing.reset();
    }

    @Override
    public void commit(Long txid) {
        _currTx = null;
        _backing.reset();
    }

    static class ReplaceUpdater<T> implements ValueUpdater<T> {
        T _t;

        public ReplaceUpdater(T t) {
            _t = t;
        }

        @Override
        public T update(Object stored) {
            return _t;
        }
    }
}

• OpaqueMap采取的是CachedBatchReadsMap<OpaqueValue>，这里泛型使用的是OpaqueValue，beginCommit会设置当前的txid，重置_backing，commit的时候会重置txid，然后重置_backing
• 与TransactionalMap的不同，这里在multiPut的时候，使用的是ReplaceUpdater，然后调用multiUpdate强制覆盖
• multiUpdate方法与TransactionalMap的不同，它是基于prev值来进行update的，算出newVal

小结

• trident严格按batch的顺序更新state，比如txid为3的batch必须在txid为2的batch处理完之后才能处理
• state分三种类型，分别是non-transactional、transactional、opaque transactional，对应的spout也是这三种类型
  • non-transactional无法保证exactly once，它可能是at-least once或者at-most once；其state计算参考NonTransactionalMap，对于beginCommit及commit操作都无处理
  • transactional类型能够保证exactly once，但是要求比较严格，要同一个batch的txid及tuple在replayed的时候仍然保持一致，因此容错性差一点，但是它的state计算相对简单，参考TransactionalMap，遇到同一个txid的值，skip掉即可
  • opaque transactional类型也能够保证exactly once，它允许一个tuple处理失败之后，出现在其他batch中处理，因而容错性好，但是state计算要多存储prev值，参考OpaqueMap，遇到同一个txid的值，使用prev值跟当前值进行覆盖
• trident将保证exactly once的state的计算都封装好了，使用的时候，在persistentAggregate传入相应的StateFactory即可，支持多种StateType的factory可以选择使用StateType属性，通过传入不同的参数构造不同transactional的state；也可以通过实现StateFactory自定义实现state factory，另外也可以通过继承BaseQueryFunction来自定义stateQuery查询，自定义更新的话，可以继承BaseStateUpdater，然后通过partitionPersist传入

doc

• Trident Tutorial
• Trident State