SERVER-16951: option to limit # open wt transactions

(cherry picked from commit bbd95ca6a8b538b4cffece0b9d9c3ed811a455a7)

23 files changed, 222 insertions, 75 deletions

diff --git a/src/mongo/db/concurrency/lock_state.cpp b/src/mongo/db/concurrency/lock_state.cpp
index a76e75ff0d1..dc9bbdfd024 100644
--- a/src/mongo/db/concurrency/lock_state.cpp
+++ b/src/mongo/db/concurrency/lock_state.cpp
@@ -222,7 +222,7 @@ namespace {
         _lock.lock();
         LockRequestsMap::ConstIterator it = _requests.begin();
         while (!it.finished()) {
-            ss << " " << it.key().toString();
+            ss << " " << it.key().toString() << " held in " << modeName(it->mode);
             it.next();
         }
         _lock.unlock();
@@ -786,6 +786,23 @@ namespace {
         }
     }
 
+    template<bool IsForMMAPV1>
+    bool LockerImpl<IsForMMAPV1>::hasStrongLocks() const {
+        if (!isLocked()) return false;
+
+        boost::lock_guard<SpinLock> lk(_lock);
+        LockRequestsMap::ConstIterator it = _requests.begin();
+        while (!it.finished()) {
+            if (it->mode == MODE_X || it->mode == MODE_S) {
+                return true;
+            }
+
+            it.next();
+        }
+
+        return false;
+    }
+
 
     //
     // Auto classes
diff --git a/src/mongo/db/concurrency/lock_state.h b/src/mongo/db/concurrency/lock_state.h
index 2022199c6d3..86847e7a475 100644
--- a/src/mongo/db/concurrency/lock_state.h
+++ b/src/mongo/db/concurrency/lock_state.h
@@ -244,6 +244,8 @@ namespace mongo {
             _lockPendingParallelWriter = newValue;
         }
 
+        virtual bool hasStrongLocks() const;
+
     private:
 
         bool _batchWriter;
diff --git a/src/mongo/db/concurrency/locker.h b/src/mongo/db/concurrency/locker.h
index 81b1ccdddd1..f19edfe4d3f 100644
--- a/src/mongo/db/concurrency/locker.h
+++ b/src/mongo/db/concurrency/locker.h
@@ -277,6 +277,12 @@ namespace mongo {
         virtual bool isBatchWriter() const = 0;
         virtual void setLockPendingParallelWriter(bool newValue) = 0;
 
+        /**
+         * A string lock is MODE_X or MODE_S.
+         * These are incompatible with other locks and therefore are strong.
+         */
+        virtual bool hasStrongLocks() const = 0;
+
     protected:
         Locker() { }
     };
diff --git a/src/mongo/db/concurrency/locker_noop.h b/src/mongo/db/concurrency/locker_noop.h
index 44510c49f9d..eaa17b102d7 100644
--- a/src/mongo/db/concurrency/locker_noop.h
+++ b/src/mongo/db/concurrency/locker_noop.h
@@ -143,7 +143,7 @@ namespace mongo {
         }
 
         virtual bool isWriteLocked() const {
-            invariant(false);
+            return false;
         }
 
         virtual bool isReadLocked() const {
@@ -170,6 +170,10 @@ namespace mongo {
             invariant(false);
         }
 
+        virtual bool hasStrongLocks() const {
+            return false;
+        }
+
     };
 
 } // namespace mongo
diff --git a/src/mongo/db/operation_context.h b/src/mongo/db/operation_context.h
index 0a72c557fef..f12bdcea6b6 100644
--- a/src/mongo/db/operation_context.h
+++ b/src/mongo/db/operation_context.h
@@ -149,7 +149,7 @@ namespace mongo {
                    _ended(false) {
 
             _txn->lockState()->beginWriteUnitOfWork();
-            _txn->recoveryUnit()->beginUnitOfWork();
+            _txn->recoveryUnit()->beginUnitOfWork(_txn);
         }
 
         ~WriteUnitOfWork() {
diff --git a/src/mongo/db/storage/in_memory/in_memory_recovery_unit.cpp b/src/mongo/db/storage/in_memory/in_memory_recovery_unit.cpp
index 1690baea054..5ade0603a96 100644
--- a/src/mongo/db/storage/in_memory/in_memory_recovery_unit.cpp
+++ b/src/mongo/db/storage/in_memory/in_memory_recovery_unit.cpp
@@ -37,7 +37,7 @@ namespace mongo {
         invariant(_depth == 0);
     }
 
-    void InMemoryRecoveryUnit::beginUnitOfWork() {
+    void InMemoryRecoveryUnit::beginUnitOfWork(OperationContext* opCtx) {
         _depth++;
     }
 
diff --git a/src/mongo/db/storage/in_memory/in_memory_recovery_unit.h b/src/mongo/db/storage/in_memory/in_memory_recovery_unit.h
index ddf38dff73e..78de9b71d68 100644
--- a/src/mongo/db/storage/in_memory/in_memory_recovery_unit.h
+++ b/src/mongo/db/storage/in_memory/in_memory_recovery_unit.h
@@ -45,7 +45,7 @@ namespace mongo {
         InMemoryRecoveryUnit() : _depth(0) {}
         virtual ~InMemoryRecoveryUnit();
 
-        virtual void beginUnitOfWork();
+        virtual void beginUnitOfWork(OperationContext* opCtx);
         virtual void commitUnitOfWork();
         virtual void endUnitOfWork();
 
diff --git a/src/mongo/db/storage/mmap_v1/dur_recovery_unit.cpp b/src/mongo/db/storage/mmap_v1/dur_recovery_unit.cpp
index 5947cae00bd..85ff9e8e4f9 100644
--- a/src/mongo/db/storage/mmap_v1/dur_recovery_unit.cpp
+++ b/src/mongo/db/storage/mmap_v1/dur_recovery_unit.cpp
@@ -46,7 +46,7 @@ namespace mongo {
 
     }
 
-    void DurRecoveryUnit::beginUnitOfWork() {
+    void DurRecoveryUnit::beginUnitOfWork(OperationContext* opCtx) {
         _startOfUncommittedChangesForLevel.push_back(Indexes(_changes.size(), _writes.size()));
     }
 
diff --git a/src/mongo/db/storage/mmap_v1/dur_recovery_unit.h b/src/mongo/db/storage/mmap_v1/dur_recovery_unit.h
index 6307bf66b31..322dd170dcd 100644
--- a/src/mongo/db/storage/mmap_v1/dur_recovery_unit.h
+++ b/src/mongo/db/storage/mmap_v1/dur_recovery_unit.h
@@ -45,7 +45,7 @@ namespace mongo {
 
         virtual ~DurRecoveryUnit() { }
 
-        virtual void beginUnitOfWork();
+        virtual void beginUnitOfWork(OperationContext* opCtx);
         virtual void commitUnitOfWork();
         virtual void endUnitOfWork();
 
diff --git a/src/mongo/db/storage/mmap_v1/heap_record_store_btree.cpp b/src/mongo/db/storage/mmap_v1/heap_record_store_btree.cpp
index 3d8323d3682..9dfd6628a4a 100644
--- a/src/mongo/db/storage/mmap_v1/heap_record_store_btree.cpp
+++ b/src/mongo/db/storage/mmap_v1/heap_record_store_btree.cpp
@@ -112,7 +112,7 @@ namespace mongo {
         invariant( _depth == 0 );
     }
 
-    void HeapRecordStoreBtreeRecoveryUnit::beginUnitOfWork() {
+    void HeapRecordStoreBtreeRecoveryUnit::beginUnitOfWork(OperationContext* opCtx) {
         _depth++;
     }
 
diff --git a/src/mongo/db/storage/mmap_v1/heap_record_store_btree.h b/src/mongo/db/storage/mmap_v1/heap_record_store_btree.h
index 710a34a8eec..a3596d88851 100644
--- a/src/mongo/db/storage/mmap_v1/heap_record_store_btree.h
+++ b/src/mongo/db/storage/mmap_v1/heap_record_store_btree.h
@@ -189,7 +189,7 @@ namespace mongo {
 
         virtual ~HeapRecordStoreBtreeRecoveryUnit();
 
-        virtual void beginUnitOfWork();
+        virtual void beginUnitOfWork(OperationContext* opCtx);
         virtual void commitUnitOfWork();
         virtual void endUnitOfWork();
 
diff --git a/src/mongo/db/storage/recovery_unit.h b/src/mongo/db/storage/recovery_unit.h
index ac456e02b7c..15d2bae4466 100644
--- a/src/mongo/db/storage/recovery_unit.h
+++ b/src/mongo/db/storage/recovery_unit.h
@@ -37,6 +37,7 @@
 namespace mongo {
 
     class BSONObjBuilder;
+    class OperationContext;
 
     /**
      * A RecoveryUnit is responsible for ensuring that data is persisted.
@@ -72,7 +73,7 @@ namespace mongo {
          *
          * TODO see if we can get rid of nested UnitsOfWork.
          */
-        virtual void beginUnitOfWork() = 0;
+        virtual void beginUnitOfWork(OperationContext* opCtx) = 0;
         virtual void commitUnitOfWork() = 0;
         virtual void endUnitOfWork() = 0;
 
diff --git a/src/mongo/db/storage/recovery_unit_noop.h b/src/mongo/db/storage/recovery_unit_noop.h
index e6f560a7871..58c341fb76b 100644
--- a/src/mongo/db/storage/recovery_unit_noop.h
+++ b/src/mongo/db/storage/recovery_unit_noop.h
@@ -32,10 +32,12 @@
 
 namespace mongo {
 
+    class OperationContext;
+
     class RecoveryUnitNoop : public RecoveryUnit {
     public:
         // TODO implement rollback
-        virtual void beginUnitOfWork() {}
+        virtual void beginUnitOfWork(OperationContext* opCtx) {}
         virtual void commitUnitOfWork() {}
         virtual void endUnitOfWork() {}
 
diff --git a/src/mongo/db/storage/wiredtiger/SConscript b/src/mongo/db/storage/wiredtiger/SConscript
index 1c5353fdd6d..dc0ccc3be30 100644
--- a/src/mongo/db/storage/wiredtiger/SConscript
+++ b/src/mongo/db/storage/wiredtiger/SConscript
@@ -30,6 +30,7 @@ if wiredtiger:
             '$BUILD_DIR/mongo/elapsed_tracker',
             '$BUILD_DIR/mongo/foundation',
             '$BUILD_DIR/mongo/processinfo',
+            '$BUILD_DIR/mongo/util/concurrency/ticketholder',
             '$BUILD_DIR/third_party/shim_wiredtiger',
             '$BUILD_DIR/third_party/shim_snappy',
             '$BUILD_DIR/third_party/shim_zlib',
diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_index.cpp b/src/mongo/db/storage/wiredtiger/wiredtiger_index.cpp
index 19a1e6d0817..0f854078e07 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_index.cpp
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_index.cpp
@@ -193,7 +193,7 @@ namespace {
     int WiredTigerIndex::Create(OperationContext* txn,
                                 const std::string& uri,
                                 const std::string& config) {
-        WT_SESSION* s = WiredTigerRecoveryUnit::get( txn )->getSession()->getSession();
+        WT_SESSION* s = WiredTigerRecoveryUnit::get( txn )->getSession(txn)->getSession();
         LOG(1) << "create uri: " << uri << " config: " << config;
         return s->create(s, uri.c_str(), config.c_str());
     }
@@ -301,7 +301,7 @@ namespace {
             output->append("type", type);
         }
 
-        WiredTigerSession* session = WiredTigerRecoveryUnit::get(txn)->getSession();
+        WiredTigerSession* session = WiredTigerRecoveryUnit::get(txn)->getSession(txn);
         WT_SESSION* s = session->getSession();
         Status status = WiredTigerUtil::exportTableToBSON(s, "statistics:" + uri(),
                                                           "statistics=(fast)", output);
@@ -340,7 +340,7 @@ namespace {
     }
 
     long long WiredTigerIndex::getSpaceUsedBytes( OperationContext* txn ) const {
-        WiredTigerSession* session = WiredTigerRecoveryUnit::get(txn)->getSession();
+        WiredTigerSession* session = WiredTigerRecoveryUnit::get(txn)->getSession(txn);
         return static_cast<long long>( WiredTigerUtil::getIdentSize( session->getSession(),
                                                                      _uri ) );
     }
@@ -399,7 +399,7 @@ namespace {
         WT_CURSOR* openBulkCursor(WiredTigerIndex* idx) {
             // Open cursors can cause bulk open_cursor to fail with EBUSY.
             // TODO any other cases that could cause EBUSY?
-            WiredTigerSession* outerSession = WiredTigerRecoveryUnit::get(_txn)->getSession();
+            WiredTigerSession* outerSession = WiredTigerRecoveryUnit::get(_txn)->getSession(_txn);
             outerSession->closeAllCursors();
 
             // Not using cursor cache since we need to set "bulk".
@@ -662,7 +662,7 @@ namespace {
 
             if ( !wt_keeptxnopen() && !_eof ) {
                 // Ensure an active session exists, so any restored cursors will bind to it
-                WiredTigerRecoveryUnit::get(txn)->getSession();
+                WiredTigerRecoveryUnit::get(txn)->getSession(txn);
 
                 _locate(_savedLoc);
             }
diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp b/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp
index 84122dfa2b2..fbe0a8da1ab 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp
@@ -224,13 +224,13 @@ namespace mongo {
 
     int64_t WiredTigerKVEngine::getIdentSize( OperationContext* opCtx,
                                               const StringData& ident ) {
-        WiredTigerSession* session = WiredTigerRecoveryUnit::get(opCtx)->getSession();
+        WiredTigerSession* session = WiredTigerRecoveryUnit::get(opCtx)->getSession(opCtx);
         return WiredTigerUtil::getIdentSize(session->getSession(), _uri(ident) );
     }
 
     Status WiredTigerKVEngine::repairIdent( OperationContext* opCtx,
                                             const StringData& ident ) {
-        WiredTigerSession* session = WiredTigerRecoveryUnit::get(opCtx)->getSession();
+        WiredTigerSession* session = WiredTigerRecoveryUnit::get(opCtx)->getSession(opCtx);
         session->closeAllCursors();
         string uri = _uri(ident);
         return _salvageIfNeeded(uri.c_str());
@@ -455,7 +455,8 @@ namespace mongo {
     }
 
     bool WiredTigerKVEngine::hasIdent(OperationContext* opCtx, const StringData& ident) const {
-        return _hasUri(WiredTigerRecoveryUnit::get(opCtx)->getSession()->getSession(), _uri(ident));
+        return _hasUri(WiredTigerRecoveryUnit::get(opCtx)->getSession(opCtx)->getSession(),
+                       _uri(ident));
     }
 
     bool WiredTigerKVEngine::_hasUri(WT_SESSION* session, const std::string& uri) const {
diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp b/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp
index 2e77a729397..29b5346705b 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp
@@ -303,7 +303,7 @@ namespace {
     int64_t WiredTigerRecordStore::storageSize( OperationContext* txn,
                                                 BSONObjBuilder* extraInfo,
                                                 int infoLevel ) const {
-        WiredTigerSession* session = WiredTigerRecoveryUnit::get(txn)->getSession();
+        WiredTigerSession* session = WiredTigerRecoveryUnit::get(txn)->getSession(txn);
         StatusWith<int64_t> result = WiredTigerUtil::getStatisticsValueAs<int64_t>(
             session->getSession(),
             "statistics:" + getURI(), "statistics=(fast)", WT_STAT_DSRC_BLOCK_SIZE);
@@ -447,7 +447,9 @@ namespace {
         invariant( realRecoveryUnit );
         WiredTigerSessionCache* sc = realRecoveryUnit->getSessionCache();
         txn->setRecoveryUnit( new WiredTigerRecoveryUnit( sc ) );
-        WT_SESSION* session = WiredTigerRecoveryUnit::get(txn)->getSession()->getSession();
+
+        WiredTigerRecoveryUnit::get(txn)->markNoTicketRequired(); // realRecoveryUnit already has
+        WT_SESSION* session = WiredTigerRecoveryUnit::get(txn)->getSession(txn)->getSession();
 
         int64_t dataSize = _dataSize.load();
         int64_t numRecords = _numRecords.load();
@@ -783,7 +785,7 @@ namespace {
 
         output->appendNumber( "nrecords", nrecords );
 
-        WiredTigerSession* session = WiredTigerRecoveryUnit::get(txn)->getSession();
+        WiredTigerSession* session = WiredTigerRecoveryUnit::get(txn)->getSession(txn);
         WT_SESSION* s = session->getSession();
         BSONObjBuilder bob(output->subobjStart(kWiredTigerEngineName));
         Status status = WiredTigerUtil::exportTableToBSON(s, "statistics:" + getURI(),
@@ -804,7 +806,7 @@ namespace {
             result->appendIntOrLL( "max", _cappedMaxDocs );
             result->appendIntOrLL( "maxSize", static_cast<long long>(_cappedMaxSize / scale) );
         }
-        WiredTigerSession* session = WiredTigerRecoveryUnit::get(txn)->getSession();
+        WiredTigerSession* session = WiredTigerRecoveryUnit::get(txn)->getSession(txn);
         WT_SESSION* s = session->getSession();
         BSONObjBuilder bob(result->subobjStart(kWiredTigerEngineName));
         {
@@ -1198,7 +1200,7 @@ namespace {
         invariant( _savedRecoveryUnit == txn->recoveryUnit() );
         if ( needRestore || !wt_keeptxnopen() ) {
             // This will ensure an active session exists, so any restored cursors will bind to it
-            invariant(WiredTigerRecoveryUnit::get(txn)->getSession() == _cursor->getSession());
+            invariant(WiredTigerRecoveryUnit::get(txn)->getSession(txn) == _cursor->getSession());
 
             RecordId saved = _lastLoc;
             _locate(_lastLoc, false);
diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_record_store_mongod.cpp b/src/mongo/db/storage/wiredtiger/wiredtiger_record_store_mongod.cpp
index ab9099c2ea9..d2c0f30869b 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_record_store_mongod.cpp
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_record_store_mongod.cpp
@@ -33,12 +33,14 @@
 
 #include "mongo/platform/basic.h"
 
+#include "mongo/base/checked_cast.h"
 #include "mongo/db/client.h"
 #include "mongo/db/concurrency/d_concurrency.h"
 #include "mongo/db/global_environment_experiment.h"
 #include "mongo/db/namespace_string.h"
 #include "mongo/db/operation_context_impl.h"
 #include "mongo/db/storage/wiredtiger/wiredtiger_record_store.h"
+#include "mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.h"
 #include "mongo/util/background.h"
 #include "mongo/util/log.h"
 
@@ -67,6 +69,7 @@ namespace mongo {
                 }
 
                 OperationContextImpl txn;
+                checked_cast<WiredTigerRecoveryUnit*>(txn.recoveryUnit())->markNoTicketRequired();
 
                 try {
                     Lock::DBLock dbLock(txn.lockState(), _ns.db(), MODE_IX);
diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_record_store_test.cpp b/src/mongo/db/storage/wiredtiger/wiredtiger_record_store_test.cpp
index 28b74c5908c..9a0f2d5e110 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_record_store_test.cpp
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_record_store_test.cpp
@@ -100,7 +100,7 @@ namespace mongo {
 
             {
                 WriteUnitOfWork uow(&txn);
-                WT_SESSION* s = ru->getSession()->getSession();
+                WT_SESSION* s = ru->getSession(&txn)->getSession();
                 invariantWTOK( s->create( s, uri.c_str(), config.c_str() ) );
                 uow.commit();
             }
@@ -126,7 +126,7 @@ namespace mongo {
 
             {
                 WriteUnitOfWork uow(&txn);
-                WT_SESSION* s = ru->getSession()->getSession();
+                WT_SESSION* s = ru->getSession(&txn)->getSession();
                 invariantWTOK( s->create( s, uri.c_str(), config.c_str() ) );
                 uow.commit();
             }
@@ -326,14 +326,15 @@ namespace mongo {
 
             {
                 WriteUnitOfWork uow( opCtx.get() );
-                WT_SESSION* s = ru->getSession()->getSession();
+                WT_SESSION* s = ru->getSession(opCtx.get())->getSession();
                 invariantWTOK( s->create( s, indexUri.c_str(), "" ) );
                 uow.commit();
             }
 
             {
                 WriteUnitOfWork uow( opCtx.get() );
-                ss.storeInto( WiredTigerRecoveryUnit::get( opCtx.get() )->getSession(), indexUri );
+                ss.storeInto( WiredTigerRecoveryUnit::get( opCtx.get() )->getSession(opCtx.get()),
+                              indexUri );
                 uow.commit();
             }
         }
@@ -341,7 +342,8 @@ namespace mongo {
         {
             scoped_ptr<OperationContext> opCtx( harnessHelper->newOperationContext() );
             WiredTigerSizeStorer ss2;
-            ss2.loadFrom( WiredTigerRecoveryUnit::get( opCtx.get() )->getSession(), indexUri );
+            ss2.loadFrom( WiredTigerRecoveryUnit::get( opCtx.get() )->getSession(opCtx.get()),
+                          indexUri );
             long long numRecords;
             long long dataSize;
             ss2.load( uri, &numRecords, &dataSize );
diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.cpp b/src/mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.cpp
index 93dd31e6528..a248f84368a 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.cpp
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.cpp
@@ -34,11 +34,16 @@
 #include <boost/thread/mutex.hpp>
 
 #include "mongo/base/checked_cast.h"
+#include "mongo/base/init.h"
 #include "mongo/bson/bsonobjbuilder.h"
+#include "mongo/db/commands/server_status_metric.h"
+#include "mongo/db/server_parameters.h"
 #include "mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.h"
 #include "mongo/db/storage/wiredtiger/wiredtiger_session_cache.h"
 #include "mongo/db/storage/wiredtiger/wiredtiger_util.h"
+#include "mongo/util/concurrency/ticketholder.h"
 #include "mongo/util/log.h"
+#include "mongo/util/mongoutils/str.h"
 #include "mongo/util/stacktrace.h"
 
 namespace mongo {
@@ -82,7 +87,8 @@ namespace mongo {
         _myTransactionCount( 0 ),
         _everStartedWrite( false ),
         _currentlySquirreled( false ),
-        _syncing( false ) {
+        _syncing( false ),
+        _noTicketNeeded( false ) {
     }
 
     WiredTigerRecoveryUnit::~WiredTigerRecoveryUnit() {
@@ -95,11 +101,13 @@ namespace mongo {
     }
 
     void WiredTigerRecoveryUnit::reportState( BSONObjBuilder* b ) const {
-        b->append( "wt_depth", _depth );
-        b->append( "wt_active", _active );
-        b->append( "wt_everStartedWrite", _everStartedWrite );
-        if ( _active )
-            b->append( "wt_millisSinceCommit", _timer.millis() );
+        b->append("wt_depth", _depth);
+        b->append("wt_active", _active);
+        b->append("wt_everStartedWrite", _everStartedWrite);
+        b->append("wt_hasTicket", _ticket.hasTicket());
+        b->appendNumber("wt_myTransactionCount", static_cast<long long>(_myTransactionCount));
+        if (_active)
+            b->append("wt_millisSinceCommit", _timer.millis());
     }
 
     void WiredTigerRecoveryUnit::_commit() {
@@ -125,10 +133,11 @@ namespace mongo {
         _changes.clear();
     }
 
-    void WiredTigerRecoveryUnit::beginUnitOfWork() {
+    void WiredTigerRecoveryUnit::beginUnitOfWork(OperationContext* opCtx) {
         invariant( !_currentlySquirreled );
         _depth++;
         _everStartedWrite = true;
+        _getTicket(opCtx);
     }
 
     void WiredTigerRecoveryUnit::commitUnitOfWork() {
@@ -176,13 +185,13 @@ namespace mongo {
         fassert( 28575, _active );
     }
 
-    WiredTigerSession* WiredTigerRecoveryUnit::getSession() {
+    WiredTigerSession* WiredTigerRecoveryUnit::getSession(OperationContext* opCtx) {
         if ( !_session ) {
             _session = _sessionCache->getSession();
         }
 
         if ( !_active ) {
-            _txnOpen();
+            _txnOpen(opCtx);
         }
         return _session;
     }
@@ -198,6 +207,81 @@ namespace mongo {
         _oplogReadTill = loc;
     }
 
+    namespace {
+
+
+        class TicketServerParameter : public ServerParameter {
+            MONGO_DISALLOW_COPYING(TicketServerParameter);
+        public:
+            TicketServerParameter(TicketHolder* holder, const std::string& name)
+                : ServerParameter(ServerParameterSet::getGlobal(),
+                                  name,
+                                  true,
+                                  true),
+                  _holder( holder ) {
+            }
+
+            virtual void append(OperationContext* txn, BSONObjBuilder& b, const std::string& name) {
+                b.append(name, _holder->outof());
+            }
+
+            virtual Status set( const BSONElement& newValueElement ) {
+                if (!newValueElement.isNumber())
+                    return Status(ErrorCodes::BadValue,
+                                  str::stream() << name() << " has to be a number");
+                return _set(newValueElement.numberInt());
+            }
+
+            virtual Status setFromString( const std::string& str ) {
+                int num = 0;
+                Status status = parseNumberFromString(str, &num);
+                if (!status.isOK())
+                    return status;
+                return _set(num);
+            }
+
+            Status _set(int newNum) {
+                if (newNum <= 0) {
+                    return Status(ErrorCodes::BadValue,
+                                  str::stream() << name() << " has to be > 0");
+                }
+
+                return _holder->resize(newNum);
+            }
+
+        private:
+            TicketHolder* _holder;
+        };
+
+        TicketHolder openWriteTransaction(128);
+        TicketServerParameter openWriteTransactionParam(&openWriteTransaction,
+                                                        "wiredTigerConcurrentWriteTransactions");
+
+        TicketHolder openReadTransaction(256);
+        TicketServerParameter openReadTransactionParam(&openReadTransaction,
+                                                       "wiredTigerConcurrentReadTransactions");
+
+    }
+
+    void WiredTigerRecoveryUnit::appendGlobalStats(BSONObjBuilder& b) {
+        BSONObjBuilder bb(b.subobjStart("concurrentTransactions"));
+        {
+            BSONObjBuilder bbb(bb.subobjStart("write"));
+            bbb.append("out", openWriteTransaction.used());
+            bbb.append("available", openWriteTransaction.available());
+            bbb.append("totalTickets", openWriteTransaction.outof());
+            bbb.done();
+        }
+        {
+            BSONObjBuilder bbb(bb.subobjStart("read"));
+            bbb.append("out", openReadTransaction.used());
+            bbb.append("available", openReadTransaction.available());
+            bbb.append("totalTickets", openReadTransaction.outof());
+            bbb.done();
+        }
+        bb.done();
+    }
+
     void WiredTigerRecoveryUnit::_txnClose( bool commit ) {
         invariant( _active );
         WT_SESSION *s = _session->getSession();
@@ -213,14 +297,49 @@ namespace mongo {
         }
         _active = false;
         _myTransactionCount++;
+        _ticket.reset(NULL);
     }
 
     uint64_t WiredTigerRecoveryUnit::getMyTransactionCount() const {
         return _myTransactionCount;
     }
 
-    void WiredTigerRecoveryUnit::_txnOpen() {
+    void WiredTigerRecoveryUnit::markNoTicketRequired() {
+        invariant(!_ticket.hasTicket());
+        _noTicketNeeded = true;
+    }
+
+    void WiredTigerRecoveryUnit::_getTicket(OperationContext* opCtx) {
+        // already have a ticket
+        if (_ticket.hasTicket())
+            return;
+
+        if (_noTicketNeeded)
+            return;
+
+        bool writeLocked;
+
+        // If we have a strong lock, waiting for a ticket can cause a deadlock.
+        if (opCtx != NULL &&
+            opCtx->lockState() != NULL) {
+            if (opCtx->lockState()->hasStrongLocks())
+                return;
+            writeLocked = opCtx->lockState()->isWriteLocked();
+        }
+        else {
+            writeLocked = _everStartedWrite;
+        }
+
+        TicketHolder* holder = writeLocked ? &openWriteTransaction : &openReadTransaction;
+
+        holder->waitForTicket();
+        _ticket.reset(holder);
+    }
+
+    void WiredTigerRecoveryUnit::_txnOpen(OperationContext* opCtx) {
         invariant( !_active );
+        _getTicket(opCtx);
+
         WT_SESSION *s = _session->getSession();
         _syncing = _syncing || awaitCommitData.numWaitingForSync.load() > 0;
         invariantWTOK( s->begin_transaction(s, _syncing ? "sync=true" : NULL) );
@@ -247,24 +366,10 @@ namespace mongo {
     WiredTigerCursor::WiredTigerCursor(const std::string& uri,
                                        uint64_t id,
                                        bool forRecordStore,
-                                       WiredTigerRecoveryUnit* ru) {
-        _init( uri, id, forRecordStore, ru );
-    }
-
-    WiredTigerCursor::WiredTigerCursor(const std::string& uri,
-                                       uint64_t id,
-                                       bool forRecordStore,
                                        OperationContext* txn) {
-        _init( uri, id, forRecordStore, WiredTigerRecoveryUnit::get( txn ) );
-    }
-
-    void WiredTigerCursor::_init( const std::string& uri,
-                                  uint64_t id,
-                                  bool forRecordStore,
-                                  WiredTigerRecoveryUnit* ru ) {
         _uriID = id;
-        _ru = ru;
-        _session = _ru->getSession();
+        _ru = WiredTigerRecoveryUnit::get( txn );
+        _session = _ru->getSession(txn);
         _cursor = _session->getCursor( uri, id, forRecordStore );
         if ( !_cursor ) {
             error() << "no cursor for uri: " << uri;
@@ -272,7 +377,6 @@ namespace mongo {
     }
 
     WiredTigerCursor::~WiredTigerCursor() {
-        invariant( _session == _ru->getSession() );
         _session->releaseCursor( _uriID, _cursor );
         _cursor = NULL;
     }
diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.h b/src/mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.h
index ed2b77bff00..e1247457420 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.h
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.h
@@ -40,6 +40,7 @@
 #include "mongo/db/operation_context.h"
 #include "mongo/db/record_id.h"
 #include "mongo/db/storage/recovery_unit.h"
+#include "mongo/util/concurrency/ticketholder.h"
 #include "mongo/util/timer.h"
 
 namespace mongo {
@@ -56,7 +57,7 @@ namespace mongo {
 
         virtual void reportState( BSONObjBuilder* b ) const;
 
-        virtual void beginUnitOfWork();
+        virtual void beginUnitOfWork(OperationContext* opCtx);
 
         virtual void commitUnitOfWork();
 
@@ -81,7 +82,7 @@ namespace mongo {
 
         // ---- WT STUFF
 
-        WiredTigerSession* getSession();
+        WiredTigerSession* getSession(OperationContext* opCtx);
         WiredTigerSessionCache* getSessionCache() { return _sessionCache; }
         bool inActiveTxn() const { return _active; }
         void assertInActiveTxn() const;
@@ -92,15 +93,18 @@ namespace mongo {
         void setOplogReadTill( const RecordId& loc );
         RecordId getOplogReadTill() const { return _oplogReadTill; }
 
+        void markNoTicketRequired();
+
         static WiredTigerRecoveryUnit* get(OperationContext *txn);
 
+        static void appendGlobalStats(BSONObjBuilder& b);
     private:
 
         void _abort();
         void _commit();
 
         void _txnClose( bool commit );
-        void _txnOpen();
+        void _txnOpen(OperationContext* opCtx);
 
         WiredTigerSessionCache* _sessionCache; // not owned
         WiredTigerSession* _session; // owned, but from pool
@@ -116,6 +120,10 @@ namespace mongo {
 
         typedef OwnedPointerVector<Change> Changes;
         Changes _changes;
+
+        bool _noTicketNeeded;
+        void _getTicket(OperationContext* opCtx);
+        TicketHolderReleaser _ticket;
     };
 
     /**
@@ -127,10 +135,7 @@ namespace mongo {
                          uint64_t uriID,
                          bool forRecordStore,
                          OperationContext* txn);
-        WiredTigerCursor(const std::string& uri,
-                         uint64_t uriID,
-                         bool forRecordStore,
-                         WiredTigerRecoveryUnit* ru);
+
         ~WiredTigerCursor();
 
 
@@ -149,11 +154,6 @@ namespace mongo {
         void assertInActiveTxn() const { _ru->assertInActiveTxn(); }
 
     private:
-        void _init( const std::string& uri,
-                    uint64_t uriID,
-                    bool forRecordStore,
-                    WiredTigerRecoveryUnit* ru );
-
         uint64_t _uriID;
         WiredTigerRecoveryUnit* _ru; // not owned
         WiredTigerSession* _session;
diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_server_status.cpp b/src/mongo/db/storage/wiredtiger/wiredtiger_server_status.cpp
index a640793cc29..25f68dfeb72 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_server_status.cpp
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_server_status.cpp
@@ -62,7 +62,7 @@ namespace mongo {
                 const BSONElement& configElement) const {
 
         WiredTigerSession* session =
-            checked_cast<WiredTigerRecoveryUnit*>(txn->recoveryUnit())->getSession();
+            checked_cast<WiredTigerRecoveryUnit*>(txn->recoveryUnit())->getSession(txn);
         invariant(session);
 
         WT_SESSION* s = session->getSession();
@@ -78,6 +78,8 @@ namespace mongo {
             bob.append("reason", status.reason());
         }
 
+        WiredTigerRecoveryUnit::appendGlobalStats(bob);
+
         return bob.obj();
     }
 
diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_util_test.cpp b/src/mongo/db/storage/wiredtiger/wiredtiger_util_test.cpp
index cffc1a01284..e35ecd35f2f 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_util_test.cpp
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_util_test.cpp
@@ -112,7 +112,7 @@ namespace mongo {
 
         void createSession(const char* config) {
             WT_SESSION* wtSession =
-                WiredTigerRecoveryUnit::get(_opCtx.get())->getSession()->getSession();
+                WiredTigerRecoveryUnit::get(_opCtx.get())->getSession(_opCtx.get())->getSession();
             ASSERT_OK(wtRCToStatus(wtSession->create(wtSession, getURI(), config)));
         }
     private:
@@ -262,7 +262,7 @@ namespace mongo {
     TEST(WiredTigerUtilTest, GetStatisticsValueMissingTable) {
         WiredTigerUtilHarnessHelper harnessHelper("statistics=(all)");
         WiredTigerRecoveryUnit recoveryUnit(harnessHelper.getSessionCache());
-        WiredTigerSession* session = recoveryUnit.getSession();
+        WiredTigerSession* session = recoveryUnit.getSession(NULL);
         StatusWith<uint64_t> result = WiredTigerUtil::getStatisticsValue(session->getSession(),
             "statistics:table:no_such_table", "statistics=(fast)", WT_STAT_DSRC_BLOCK_SIZE);
         ASSERT_NOT_OK(result.getStatus());
@@ -272,7 +272,7 @@ namespace mongo {
     TEST(WiredTigerUtilTest, GetStatisticsValueStatisticsDisabled) {
         WiredTigerUtilHarnessHelper harnessHelper("statistics=(none)");
         WiredTigerRecoveryUnit recoveryUnit(harnessHelper.getSessionCache());
-        WiredTigerSession* session = recoveryUnit.getSession();
+        WiredTigerSession* session = recoveryUnit.getSession(NULL);
         WT_SESSION* wtSession = session->getSession();
         ASSERT_OK(wtRCToStatus(wtSession->create(wtSession, "table:mytable", NULL)));
         StatusWith<uint64_t> result = WiredTigerUtil::getStatisticsValue(session->getSession(),
@@ -284,7 +284,7 @@ namespace mongo {
     TEST(WiredTigerUtilTest, GetStatisticsValueInvalidKey) {
         WiredTigerUtilHarnessHelper harnessHelper("statistics=(all)");
         WiredTigerRecoveryUnit recoveryUnit(harnessHelper.getSessionCache());
-        WiredTigerSession* session = recoveryUnit.getSession();
+        WiredTigerSession* session = recoveryUnit.getSession(NULL);
         WT_SESSION* wtSession = session->getSession();
         ASSERT_OK(wtRCToStatus(wtSession->create(wtSession, "table:mytable", NULL)));
         // Use connection statistics key which does not apply to a table.
@@ -297,7 +297,7 @@ namespace mongo {
     TEST(WiredTigerUtilTest, GetStatisticsValueValidKey) {
         WiredTigerUtilHarnessHelper harnessHelper("statistics=(all)");
         WiredTigerRecoveryUnit recoveryUnit(harnessHelper.getSessionCache());
-        WiredTigerSession* session = recoveryUnit.getSession();
+        WiredTigerSession* session = recoveryUnit.getSession(NULL);
         WT_SESSION* wtSession = session->getSession();
         ASSERT_OK(wtRCToStatus(wtSession->create(wtSession, "table:mytable", NULL)));
         // Use connection statistics key which does not apply to a table.
@@ -311,7 +311,7 @@ namespace mongo {
     TEST(WiredTigerUtilTest, GetStatisticsValueAsUInt8) {
         WiredTigerUtilHarnessHelper harnessHelper("statistics=(all)");
         WiredTigerRecoveryUnit recoveryUnit(harnessHelper.getSessionCache());
-        WiredTigerSession* session = recoveryUnit.getSession();
+        WiredTigerSession* session = recoveryUnit.getSession(NULL);
         WT_SESSION* wtSession = session->getSession();
         ASSERT_OK(wtRCToStatus(wtSession->create(wtSession, "table:mytable", NULL)));