foundationdb/fdbserver/KeyValueStoreMemory.actor.cpp

/*
 * KeyValueStoreMemory.actor.cpp
 *
 * This source file is part of the FoundationDB open source project
 *
 * Copyright 2013-2018 Apple Inc. and the FoundationDB project authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include "fdbserver/IKeyValueStore.h"
#include "fdbserver/IDiskQueue.h"
#include "flow/IndexedSet.h"
#include "flow/ActorCollection.h"
#include "fdbclient/Notified.h"
#include "fdbclient/SystemData.h"
#include "flow/actorcompiler.h"  // This must be the last #include.

#define OP_DISK_OVERHEAD (sizeof(OpHeader) + 1)

//Stored in the IndexedSets that hold the database.
//Each KeyValueMapPair is 32 bytes, excluding arena memory.
//It is stored in an IndexedSet<KeyValueMapPair, uint64_t>::Node, for a total size of 72 bytes.
struct KeyValueMapPair {
	Arena arena; //8 Bytes (excluding arena memory)
	KeyRef key; //12 Bytes
	ValueRef value; //12 Bytes

	void operator= ( KeyValueMapPair const& rhs ) { arena = rhs.arena; key = rhs.key; value = rhs.value; }
	KeyValueMapPair( KeyValueMapPair const& rhs ) : arena(rhs.arena), key(rhs.key), value(rhs.value) {}

	KeyValueMapPair(KeyRef key, ValueRef value) : arena(key.expectedSize() + value.expectedSize()), key(arena, key), value(arena, value) { }

	bool operator<(KeyValueMapPair const& r) const { return key < r.key; }
	bool operator==(KeyValueMapPair const& r) const { return key == r.key; }
	bool operator!=(KeyValueMapPair const& r) const { return key != r.key; }
};

template <class CompatibleWithKey>
bool operator<(KeyValueMapPair const& l, CompatibleWithKey const& r) { return l.key < r; }

template <class CompatibleWithKey>
bool operator<(CompatibleWithKey const& l, KeyValueMapPair const& r) { return l < r.key; }

extern bool noUnseed;

class KeyValueStoreMemory : public IKeyValueStore, NonCopyable {
public:
	KeyValueStoreMemory( IDiskQueue* log, UID id, int64_t memoryLimit, bool disableSnapshot, bool replaceContent, bool exactRecovery );

	// IClosable
	virtual Future<Void> getError() { return log->getError(); }
	virtual Future<Void> onClosed() { return log->onClosed(); }
	virtual void dispose() { recovering.cancel(); log->dispose(); delete this; }
	virtual void close() { recovering.cancel(); log->close(); delete this; }

	// IKeyValueStore
	virtual KeyValueStoreType getType() { return KeyValueStoreType::MEMORY; }

	int64_t getAvailableSize() {
		int64_t residentSize =
			data.sumTo(data.end()) +
			queue.totalSize() +  // doesn't account for overhead in queue
			transactionSize;

		return memoryLimit - residentSize;
	}

	virtual StorageBytes getStorageBytes() {
		StorageBytes diskQueueBytes = log->getStorageBytes();

		// Try to bound how many in-memory bytes we might need to write to disk if we commit() now
		int64_t uncommittedBytes = queue.totalSize() + transactionSize;

		//Check that we have enough space in memory and on disk
		int64_t freeSize = std::min(getAvailableSize(), diskQueueBytes.free / 4 - uncommittedBytes);
		int64_t availableSize = std::min(getAvailableSize(), diskQueueBytes.available / 4 - uncommittedBytes);
		int64_t totalSize = std::min(memoryLimit, diskQueueBytes.total / 4 - uncommittedBytes);

		return StorageBytes(std::max((int64_t)0, freeSize), std::max((int64_t)0, totalSize), diskQueueBytes.used,
		    std::max((int64_t)0, availableSize));
	}

	void semiCommit() {
		transactionSize += queue.totalSize();
		if(transactionSize > 0.5 * committedDataSize) {
			transactionIsLarge = true;
			TraceEvent("KVSMemSwitchingToLargeTransactionMode", id).detail("TransactionSize", transactionSize).detail("DataSize", committedDataSize);
			TEST(true); // KeyValueStoreMemory switching to large transaction mode
			TEST(committedDataSize > 1e3); // KeyValueStoreMemory switching to large transaction mode with committed data
		}

		int64_t bytesWritten = commit_queue(queue, true);
		committedWriteBytes += bytesWritten;
	}

	virtual void set(KeyValueRef keyValue, const Arena* arena) {
		//A commit that occurs with no available space returns Never, so we can throw out all modifications
		if(getAvailableSize() <= 0)
			return;

		if(transactionIsLarge) {
			KeyValueMapPair pair(keyValue.key, keyValue.value);
			data.insert(pair, pair.arena.getSize() + data.getElementBytes());
		}
		else {
			queue.set(keyValue, arena);
			if(recovering.isReady() && !disableSnapshot) {
				semiCommit();
			}
		}
	}

	virtual void clear(KeyRangeRef range, const Arena* arena) {
		//A commit that occurs with no available space returns Never, so we can throw out all modifications
		if(getAvailableSize() <= 0)
			return;

		if(transactionIsLarge) {
			data.erase(data.lower_bound(range.begin), data.lower_bound(range.end));
		}
		else {
			queue.clear(range, arena);
			if(recovering.isReady() && !disableSnapshot) {
				semiCommit();
			}
		}
	}

	virtual Future<Void> commit(bool sequential) {
		if(getAvailableSize() <= 0) {
			TraceEvent(SevError, "KeyValueStoreMemory_OutOfSpace", id);
			return Never();
		}

		if(recovering.isError()) throw recovering.getError();
		if(!recovering.isReady())
			return waitAndCommit(this, sequential);

		if(!disableSnapshot && replaceContent && !firstCommitWithSnapshot) {
			transactionSize += SERVER_KNOBS->REPLACE_CONTENTS_BYTES;
			committedWriteBytes += SERVER_KNOBS->REPLACE_CONTENTS_BYTES;
			semiCommit();
		}

		if(transactionIsLarge) {
			fullSnapshot(data);
			resetSnapshot = true;
			committedWriteBytes = notifiedCommittedWriteBytes.get();
			overheadWriteBytes = 0;

			if(disableSnapshot) {
				return Void();
			}
			log_op(OpCommit, StringRef(), StringRef());
		}
		else {
			int64_t bytesWritten = commit_queue(queue, !disableSnapshot, sequential);

			if(disableSnapshot) {
				return Void();
			}

			if(bytesWritten > 0 || committedWriteBytes > notifiedCommittedWriteBytes.get()) {
				committedWriteBytes += bytesWritten + overheadWriteBytes + OP_DISK_OVERHEAD; //OP_DISK_OVERHEAD is for the following log_op(OpCommit)
				notifiedCommittedWriteBytes.set(committedWriteBytes); //This set will cause snapshot items to be written, so it must happen before the OpCommit
				log_op(OpCommit, StringRef(), StringRef());
				overheadWriteBytes = log->getCommitOverhead();
			}
		}

		auto c = log->commit();

		committedDataSize = data.sumTo(data.end());
		transactionSize = 0;
		transactionIsLarge = false;
		firstCommitWithSnapshot = false;

		addActor.send( commitAndUpdateVersions( this, c, previousSnapshotEnd ) );
		return c;
	}

	virtual Future<Optional<Value>> readValue( KeyRef key, Optional<UID> debugID = Optional<UID>() ) {
		if(recovering.isError()) throw recovering.getError();
		if (!recovering.isReady()) return waitAndReadValue(this, key);

		auto it = data.find(key);
		if (it == data.end()) return Optional<Value>();
		return Optional<Value>(it->value);
	}

	virtual Future<Optional<Value>> readValuePrefix( KeyRef key, int maxLength, Optional<UID> debugID = Optional<UID>() ) {
		if(recovering.isError()) throw recovering.getError();
		if (!recovering.isReady()) return waitAndReadValuePrefix(this, key, maxLength);

		auto it = data.find(key);
		if (it == data.end()) return Optional<Value>();
		auto val = it->value;
		if(maxLength < val.size()) {
			return Optional<Value>(val.substr(0, maxLength));
		}
		else {
			return Optional<Value>(val);
		}
	}

	// If rowLimit>=0, reads first rows sorted ascending, otherwise reads last rows sorted descending
	// The total size of the returned value (less the last entry) will be less than byteLimit
	virtual Future<Standalone<VectorRef<KeyValueRef>>> readRange( KeyRangeRef keys, int rowLimit = 1<<30, int byteLimit = 1<<30 ) {
		if(recovering.isError()) throw recovering.getError();
		if (!recovering.isReady()) return waitAndReadRange(this, keys, rowLimit, byteLimit);

		Standalone<VectorRef<KeyValueRef>> result;
		if (rowLimit >= 0) {
			auto it = data.lower_bound(keys.begin);
			while (it!=data.end() && it->key < keys.end && rowLimit && byteLimit>=0) {
				byteLimit -= sizeof(KeyValueRef) + it->key.size() + it->value.size();
				result.push_back_deep( result.arena(), KeyValueRef(it->key, it->value) );
				++it;
				--rowLimit;
			}
		} else {
			rowLimit = -rowLimit;
			auto it = data.previous( data.lower_bound(keys.end) );
			while (it!=data.end() && it->key >= keys.begin && rowLimit && byteLimit>=0) {
				byteLimit -= sizeof(KeyValueRef) + it->key.size() + it->value.size();
				result.push_back_deep( result.arena(), KeyValueRef(it->key, it->value) );
				it = data.previous(it);
				--rowLimit;
			}
		}
		return result;
	}

	virtual void resyncLog() {
		ASSERT( recovering.isReady() );
		resetSnapshot = true;
		log_op(OpSnapshotAbort, StringRef(), StringRef());
	}

	virtual void enableSnapshot() {
		disableSnapshot = false;
	}

private:
	enum OpType {
		OpSet,
		OpClear,
		OpClearToEnd,
		OpSnapshotItem,
		OpSnapshotEnd,
		OpSnapshotAbort, // terminate an in progress snapshot in order to start a full snapshot
		OpCommit,        // only in log, not in queue
		OpRollback       // only in log, not in queue
	};

	struct OpRef {
		OpType op;
		StringRef p1, p2;
		OpRef() {}
		OpRef(Arena& a, OpRef const& o) : op(o.op), p1(a,o.p1), p2(a,o.p2) {}
		size_t expectedSize() {
			return p1.expectedSize() + p2.expectedSize();
		}
	};
	struct OpHeader {
		int op;
		int len1, len2;
	};

	struct OpQueue {
		OpQueue() : numBytes(0) { }

		int totalSize() const { return numBytes; }

		void clear() {
			numBytes = 0;
			operations = Standalone<VectorRef<OpRef>>();
			arenas.clear();
		}

		void rollback() {
			clear();
		}

		void set( KeyValueRef keyValue, const Arena* arena = NULL ) {
			queue_op(OpSet, keyValue.key, keyValue.value, arena);
		}

		void clear( KeyRangeRef range, const Arena* arena = NULL ) {
			queue_op(OpClear, range.begin, range.end, arena);
		}

		void clear_to_end( StringRef fromKey, const Arena* arena = NULL ) {
			queue_op(OpClearToEnd, fromKey, StringRef(), arena);
		}

		void queue_op( OpType op, StringRef p1, StringRef p2, const Arena* arena ) {
			numBytes += p1.size() + p2.size() + sizeof(OpHeader) + sizeof(OpRef);

			OpRef r; r.op = op; r.p1 = p1; r.p2 = p2;
			if(arena == NULL) {
				operations.push_back_deep( operations.arena(), r );
			} else {
				operations.push_back( operations.arena(), r );
				arenas.push_back(*arena);
			}
		}

		const OpRef* begin() {
			return operations.begin();
		}

		const OpRef* end() {
			return operations.end();
		}

		private:
			Standalone<VectorRef<OpRef>> operations;
			uint64_t numBytes;
			std::vector<Arena> arenas;
	};

	UID id;

	IndexedSet< KeyValueMapPair, uint64_t > data;

	OpQueue queue; // mutations not yet commit()ted
	IDiskQueue *log;
	Future<Void> recovering, snapshotting;
	int64_t committedWriteBytes;
	int64_t overheadWriteBytes;
	NotifiedVersion notifiedCommittedWriteBytes;
	Key recoveredSnapshotKey; // After recovery, the next key in the currently uncompleted snapshot
	IDiskQueue::location currentSnapshotEnd; //The end of the most recently completed snapshot (this snapshot cannot be discarded)
	IDiskQueue::location previousSnapshotEnd; //The end of the second most recently completed snapshot (on commit, this snapshot can be discarded)
	PromiseStream<Future<Void>> addActor;
	Future<Void> commitActors;

	int64_t committedDataSize;
	int64_t transactionSize;
	bool transactionIsLarge;

	bool resetSnapshot; //Set to true after a fullSnapshot is performed.  This causes the regular snapshot mechanism to restart
	bool disableSnapshot;
	bool replaceContent;
	bool firstCommitWithSnapshot;
	int snapshotCount;

	int64_t memoryLimit; //The upper limit on the memory used by the store (excluding, possibly, some clear operations)
	std::vector<std::pair<KeyValueMapPair, uint64_t>> dataSets;

	int64_t commit_queue(OpQueue &ops, bool log, bool sequential = false) {
		int64_t total = 0, count = 0;
		IDiskQueue::location log_location = 0;

		for(auto o = ops.begin(); o != ops.end(); ++o) {
			++count;
			total += o->p1.size() + o->p2.size() + OP_DISK_OVERHEAD;
			if (o->op == OpSet) {
				KeyValueMapPair pair(o->p1, o->p2);
				if(sequential) {
					dataSets.push_back(std::make_pair(pair, pair.arena.getSize() + data.getElementBytes()));
				} else {
					data.insert( pair, pair.arena.getSize() + data.getElementBytes() );
				}
			}
			else if (o->op == OpClear) {
				if(sequential) {
					data.insert(dataSets);
					dataSets.clear();
				}
				data.erase( data.lower_bound(o->p1), data.lower_bound(o->p2) );
			}
			else if (o->op == OpClearToEnd) {
				if(sequential) {
					data.insert(dataSets);
					dataSets.clear();
				}
				data.erase( data.lower_bound(o->p1), data.end() );
			}
			else ASSERT(false);
			if ( log )
				log_location = log_op( o->op, o->p1, o->p2 );
		}
		if(sequential) {
			data.insert(dataSets);
			dataSets.clear();
		}

		bool ok = count < 1e6;
		if( !ok ) {
			TraceEvent(/*ok ? SevInfo : */SevWarnAlways, "KVSMemCommitQueue", id)
				.detail("Bytes", total)
				.detail("Log", log)
				.detail("Ops", count)
				.detail("LastLoggedLocation", log_location)
				.detail("Details", count);
		}

		ops.clear();
		return total;
	}

	IDiskQueue::location log_op(OpType op, StringRef v1, StringRef v2) {
		OpHeader h = {(int)op, v1.size(), v2.size()};
		log->push( StringRef((const uint8_t*)&h, sizeof(h)) );
		log->push( v1 );
		log->push( v2 );
		return log->push( LiteralStringRef("\x01") ); // Changes here should be reflected in OP_DISK_OVERHEAD
	}

	ACTOR static Future<Void> recover( KeyValueStoreMemory* self, bool exactRecovery ) {
		// 'uncommitted' variables track something that might be rolled back by an OpRollback, and are copied into permanent variables
		// (in self) in OpCommit.  OpRollback does the reverse (copying the permanent versions over the uncommitted versions)
		// the uncommitted and committed variables should be equal initially (to whatever makes sense if there are no committed transactions recovered)
		state Key uncommittedNextKey = self->recoveredSnapshotKey;
		state IDiskQueue::location uncommittedPrevSnapshotEnd = self->previousSnapshotEnd = self->log->getNextReadLocation();  // not really, but popping up to here does nothing
		state IDiskQueue::location uncommittedSnapshotEnd = self->currentSnapshotEnd = uncommittedPrevSnapshotEnd;

		state int zeroFillSize = 0;
		state int dbgSnapshotItemCount=0;
		state int dbgSnapshotEndCount=0;
		state int dbgMutationCount=0;
		state int dbgCommitCount=0;
		state double startt = now();
		state UID dbgid = self->id;

		state Future<Void> loggingDelay = delay(1.0);

		state OpQueue recoveryQueue;
		state OpHeader h;

		TraceEvent("KVSMemRecoveryStarted", self->id)
			.detail("SnapshotEndLocation", uncommittedSnapshotEnd);

		try {
			loop {
				{
					Standalone<StringRef> data = wait( self->log->readNext( sizeof(OpHeader) ) );
					if (data.size() != sizeof(OpHeader)) {
						if (data.size()) {
							TEST(true);  // zero fill partial header in KeyValueStoreMemory
							memset(&h, 0, sizeof(OpHeader));
							memcpy(&h, data.begin(), data.size());
							zeroFillSize = sizeof(OpHeader)-data.size() + h.len1 + h.len2 + 1;
						}
						TraceEvent("KVSMemRecoveryComplete", self->id)
							.detail("Reason", "Non-header sized data read")
							.detail("DataSize", data.size())
							.detail("ZeroFillSize", zeroFillSize)
							.detail("SnapshotEndLocation", uncommittedSnapshotEnd)
							.detail("NextReadLoc", self->log->getNextReadLocation());
						break;
					}
					h = *(OpHeader*)data.begin();
				}
				Standalone<StringRef> data = wait( self->log->readNext( h.len1 + h.len2+1 ) );
				if (data.size() != h.len1 + h.len2 + 1) {
					zeroFillSize = h.len1 + h.len2 + 1 - data.size();
					TraceEvent("KVSMemRecoveryComplete", self->id)
						.detail("Reason", "data specified by header does not exist")
						.detail("DataSize", data.size())
						.detail("ZeroFillSize", zeroFillSize)
						.detail("SnapshotEndLocation", uncommittedSnapshotEnd)
						.detail("OpCode", h.op)
						.detail("NextReadLoc", self->log->getNextReadLocation());
					break;
				}

				if (data[data.size()-1]) {
					StringRef p1 = data.substr(0, h.len1);
					StringRef p2 = data.substr(h.len1, h.len2);

					if (h.op == OpSnapshotItem) { // snapshot data item
						/*if (p1 < uncommittedNextKey) {
							TraceEvent(SevError, "RecSnapshotBack", self->id)
								.detail("NextKey", uncommittedNextKey)
								.detail("P1", p1)
								.detail("Nextlocation", self->log->getNextReadLocation());
						}
						ASSERT( p1 >= uncommittedNextKey );*/
						if( p1 >= uncommittedNextKey )
							recoveryQueue.clear( KeyRangeRef(uncommittedNextKey, p1), &uncommittedNextKey.arena() ); //FIXME: Not sure what this line is for, is it necessary?
						recoveryQueue.set( KeyValueRef(p1, p2), &data.arena() );
						uncommittedNextKey = keyAfter(p1);
						++dbgSnapshotItemCount;
					} else if (h.op == OpSnapshotEnd || h.op == OpSnapshotAbort) { // snapshot complete
						TraceEvent("RecSnapshotEnd", self->id)
							.detail("NextKey", uncommittedNextKey)
							.detail("Nextlocation", self->log->getNextReadLocation())
							.detail("IsSnapshotEnd", h.op == OpSnapshotEnd);

						if(h.op == OpSnapshotEnd) {
							uncommittedPrevSnapshotEnd = uncommittedSnapshotEnd;
							uncommittedSnapshotEnd = self->log->getNextReadLocation();
							recoveryQueue.clear_to_end( uncommittedNextKey, &uncommittedNextKey.arena() );
						}

						uncommittedNextKey = Key();
						++dbgSnapshotEndCount;
					} else if (h.op == OpSet) { // set mutation
						recoveryQueue.set( KeyValueRef(p1,p2), &data.arena() );
						++dbgMutationCount;
					} else if (h.op == OpClear) { // clear mutation
						recoveryQueue.clear( KeyRangeRef(p1,p2), &data.arena() );
						++dbgMutationCount;
					} else if (h.op == OpClearToEnd) { //clear all data from begin key to end
						recoveryQueue.clear_to_end( p1, &data.arena() );
					} else if (h.op == OpCommit) { // commit previous transaction
						self->commit_queue(recoveryQueue, false);
						++dbgCommitCount;
						self->recoveredSnapshotKey = uncommittedNextKey;
						self->previousSnapshotEnd = uncommittedPrevSnapshotEnd;
						self->currentSnapshotEnd = uncommittedSnapshotEnd;
					} else if (h.op == OpRollback) { // rollback previous transaction
						recoveryQueue.rollback();
						TraceEvent("KVSMemRecSnapshotRollback", self->id)
							.detail("NextKey", uncommittedNextKey);
						uncommittedNextKey = self->recoveredSnapshotKey;
						uncommittedPrevSnapshotEnd = self->previousSnapshotEnd;
						uncommittedSnapshotEnd = self->currentSnapshotEnd;
					} else
						ASSERT(false);
				} else {
					TraceEvent("KVSMemRecoverySkippedZeroFill", self->id)
						.detail("PayloadSize", data.size())
						.detail("ExpectedSize", h.len1 + h.len2 + 1)
						.detail("OpCode", h.op)
						.detail("EndsAt", self->log->getNextReadLocation());
				}

				if (loggingDelay.isReady()) {
					TraceEvent("KVSMemRecoveryLogSnap", self->id)
						.detail("SnapshotItems", dbgSnapshotItemCount)
						.detail("SnapshotEnd", dbgSnapshotEndCount)
						.detail("Mutations", dbgMutationCount)
						.detail("Commits", dbgCommitCount)
						.detail("EndsAt", self->log->getNextReadLocation());
					loggingDelay = delay(1.0);
				}

				wait( yield() );
			}

			if (zeroFillSize) {
				if( exactRecovery ) {
					TraceEvent(SevError, "KVSMemExpectedExact", self->id);
					ASSERT(false);
				}

				TEST( true );  // Fixing a partial commit at the end of the KeyValueStoreMemory log
				for(int i=0; i<zeroFillSize; i++)
					self->log->push( StringRef((const uint8_t*)"",1) );
			}
			//self->rollback(); not needed, since we are about to discard anything left in the recoveryQueue
			//TraceEvent("KVSMemRecRollback", self->id).detail("QueueEmpty", data.size() == 0);
			// make sure that before any new operations are added to the log that all uncommitted operations are "rolled back"
			self->log_op( OpRollback, StringRef(), StringRef() );  // rollback previous transaction

			self->committedDataSize = self->data.sumTo(self->data.end());

			TraceEvent("KVSMemRecovered", self->id)
				.detail("SnapshotItems", dbgSnapshotItemCount)
				.detail("SnapshotEnd", dbgSnapshotEndCount)
				.detail("Mutations", dbgMutationCount)
				.detail("Commits", dbgCommitCount)
				.detail("TimeTaken", now()-startt);

			self->semiCommit();
			return Void();
		} catch( Error &e ) {
			bool ok = e.code() == error_code_operation_cancelled || e.code() == error_code_file_not_found;
			TraceEvent(ok ? SevInfo : SevError, "ErrorDuringRecovery", dbgid).error(e, true);
			throw e;
		}
	}

	//Snapshots an entire data set
	void fullSnapshot( IndexedSet< KeyValueMapPair, uint64_t> &snapshotData ) {
		previousSnapshotEnd = log_op(OpSnapshotAbort, StringRef(), StringRef());
		replaceContent = false;

		//Clear everything since we are about to write the whole database
		log_op(OpClearToEnd, allKeys.begin, StringRef());

		int count = 0;
		int64_t snapshotSize = 0;
		for(auto kv = snapshotData.begin(); kv != snapshotData.end(); ++kv) {
			log_op(OpSnapshotItem, kv->key, kv->value);
			snapshotSize += kv->key.size() + kv->value.size() + OP_DISK_OVERHEAD;
			++count;
		}

		TraceEvent("FullSnapshotEnd", id)
			.detail("PreviousSnapshotEndLoc", previousSnapshotEnd)
			.detail("SnapshotSize", snapshotSize)
			.detail("SnapshotElements", count);

		currentSnapshotEnd = log_op(OpSnapshotEnd, StringRef(), StringRef());
	}

	ACTOR static Future<Void> snapshot( KeyValueStoreMemory* self ) {
		wait(self->recovering);

		state Key nextKey = self->recoveredSnapshotKey;
		state bool nextKeyAfter = false; //setting this to true is equilvent to setting nextKey = keyAfter(nextKey)
		state uint64_t snapshotTotalWrittenBytes = 0;
		state int lastDiff = 0;
		state int snapItems = 0;
		state uint64_t snapshotBytes = 0;

		TraceEvent("KVSMemStartingSnapshot", self->id).detail("StartKey", nextKey);

		loop {
			wait( self->notifiedCommittedWriteBytes.whenAtLeast( snapshotTotalWrittenBytes + 1 ) );

			if(self->resetSnapshot) {
				nextKey = Key();
				nextKeyAfter = false;
				snapItems = 0;
				snapshotBytes = 0;
				self->resetSnapshot = false;
			}

			auto next = nextKeyAfter ? self->data.upper_bound(nextKey) : self->data.lower_bound(nextKey);
			int diff = self->notifiedCommittedWriteBytes.get() - snapshotTotalWrittenBytes;
			if( diff > lastDiff && diff > 5e7 )
				TraceEvent(SevWarnAlways, "ManyWritesAtOnce", self->id)
					.detail("CommittedWrites", self->notifiedCommittedWriteBytes.get())
					.detail("SnapshotWrites", snapshotTotalWrittenBytes)
					.detail("Diff", diff)
					.detail("LastOperationWasASnapshot", nextKey == Key() && !nextKeyAfter);
			lastDiff = diff;

			if (next == self->data.end()) {
				auto thisSnapshotEnd = self->log_op( OpSnapshotEnd, StringRef(), StringRef() );
				//TraceEvent("SnapshotEnd", self->id)
				//	.detail("LastKey", lastKey.present() ? lastKey.get() : LiteralStringRef("<none>"))
				//	.detail("CurrentSnapshotEndLoc", self->currentSnapshotEnd)
				//	.detail("PreviousSnapshotEndLoc", self->previousSnapshotEnd)
				//	.detail("ThisSnapshotEnd", thisSnapshotEnd)
				//	.detail("Items", snapItems)
				//	.detail("CommittedWrites", self->notifiedCommittedWriteBytes.get())
				//	.detail("SnapshotSize", snapshotBytes);

				ASSERT(thisSnapshotEnd >= self->currentSnapshotEnd);
				self->previousSnapshotEnd = self->currentSnapshotEnd;
				self->currentSnapshotEnd = thisSnapshotEnd;

				if(++self->snapshotCount == 2) {
					self->replaceContent = false;
				}
				nextKey = Key();
				nextKeyAfter = false;
				snapItems = 0;

				snapshotBytes = 0;

				snapshotTotalWrittenBytes += OP_DISK_OVERHEAD;
			} else {
				self->log_op( OpSnapshotItem, next->key, next->value );
				nextKey = next->key;
				nextKeyAfter = true;
				snapItems++;
				uint64_t opBytes = next->key.size() + next->value.size() + OP_DISK_OVERHEAD;
				snapshotBytes += opBytes;
				snapshotTotalWrittenBytes += opBytes;
			}
		}
	}

	ACTOR static Future<Optional<Value>> waitAndReadValue( KeyValueStoreMemory* self, Key key ) {
		wait( self->recovering );
		return self->readValue(key).get();
	}
	ACTOR static Future<Optional<Value>> waitAndReadValuePrefix( KeyValueStoreMemory* self, Key key, int maxLength) {
		wait( self->recovering );
		return self->readValuePrefix(key, maxLength).get();
	}
	ACTOR static Future<Standalone<VectorRef<KeyValueRef>>> waitAndReadRange( KeyValueStoreMemory* self, KeyRange keys, int rowLimit, int byteLimit ) {
		wait( self->recovering );
		return self->readRange(keys, rowLimit, byteLimit).get();
	}
	ACTOR static Future<Void> waitAndCommit(KeyValueStoreMemory* self, bool sequential) {
		wait(self->recovering);
		wait(self->commit(sequential));
		return Void();
	}
	ACTOR static Future<Void> commitAndUpdateVersions( KeyValueStoreMemory* self, Future<Void> commit, IDiskQueue::location location ) {
		wait( commit );
		self->log->pop(location);
		return Void();
	}
};

KeyValueStoreMemory::KeyValueStoreMemory( IDiskQueue* log, UID id, int64_t memoryLimit, bool disableSnapshot, bool replaceContent, bool exactRecovery )
	: log(log), id(id), previousSnapshotEnd(-1), currentSnapshotEnd(-1), resetSnapshot(false), memoryLimit(memoryLimit), committedWriteBytes(0), overheadWriteBytes(0),
	  committedDataSize(0), transactionSize(0), transactionIsLarge(false), disableSnapshot(disableSnapshot), replaceContent(replaceContent), snapshotCount(0), firstCommitWithSnapshot(true)
{
	recovering = recover( this, exactRecovery );
	snapshotting = snapshot( this );
	commitActors = actorCollection( addActor.getFuture() );
}

IKeyValueStore* keyValueStoreMemory( std::string const& basename, UID logID, int64_t memoryLimit, std::string ext ) {
	TraceEvent("KVSMemOpening", logID).detail("Basename", basename).detail("MemoryLimit", memoryLimit);
	IDiskQueue *log = openDiskQueue( basename, ext, logID, DiskQueueVersion::V0 );
	return new KeyValueStoreMemory( log, logID, memoryLimit, false, false, false );
}

IKeyValueStore* keyValueStoreLogSystem( class IDiskQueue* queue, UID logID, int64_t memoryLimit, bool disableSnapshot, bool replaceContent, bool exactRecovery ) {
	return new KeyValueStoreMemory( queue, logID, memoryLimit, disableSnapshot, replaceContent, exactRecovery );
}