path: root/src/mongo/db/repl/master_slave.cpp

/**
*    Copyright (C) 2008 10gen Inc.
*
*    This program is free software: you can redistribute it and/or  modify
*    it under the terms of the GNU Affero General Public License, version 3,
*    as published by the Free Software Foundation.
*
*    This program is distributed in the hope that it will be useful,
*    but WITHOUT ANY WARRANTY; without even the implied warranty of
*    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
*    GNU Affero General Public License for more details.
*
*    You should have received a copy of the GNU Affero General Public License
*    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*
*    As a special exception, the copyright holders give permission to link the
*    code of portions of this program with the OpenSSL library under certain
*    conditions as described in each individual source file and distribute
*    linked combinations including the program with the OpenSSL library. You
*    must comply with the GNU Affero General Public License in all respects for
*    all of the code used other than as permitted herein. If you modify file(s)
*    with this exception, you may extend this exception to your version of the
*    file(s), but you are not obligated to do so. If you do not wish to do so,
*    delete this exception statement from your version. If you delete this
*    exception statement from all source files in the program, then also delete
*    it in the license file.
*/

/* Collections we use:

   local.sources         - indicates what sources we pull from as a "slave", and the last update of each
   local.oplog.$main     - our op log as "master"
   local.dbinfo.<dbname> - no longer used???
   local.pair.startup    - [deprecated] can contain a special value indicating for a pair that we have the master copy.
                           used when replacing other half of the pair which has permanently failed.
   local.pair.sync       - [deprecated] { initialsynccomplete: 1 }
*/

#include "mongo/db/repl/master_slave.h"

#include <pcrecpp.h>

#include <boost/thread/thread.hpp>

#include "mongo/db/auth/authorization_manager.h"
#include "mongo/db/cloner.h"
#include "mongo/db/commands.h"
#include "mongo/db/instance.h"
#include "mongo/db/ops/update.h"
#include "mongo/db/query/internal_plans.h"
#include "mongo/db/repl/oplog.h"
#include "mongo/db/repl/replication_server_status.h"  // replSettings
#include "mongo/db/repl/rs.h" // replLocalAuth()
#include "mongo/db/server_parameters.h"

namespace mongo {

    void pretouchOperation(const BSONObj& op);
    void pretouchN(vector<BSONObj>&, unsigned a, unsigned b);

    /* if 1 sync() is running */
    volatile int syncing = 0;
    static volatile int relinquishSyncingSome = 0;

    /* "dead" means something really bad happened like replication falling completely out of sync.
       when non-null, we are dead and the string is informational
    */
    const char *replAllDead = 0;

    static time_t lastForcedResync = 0;

    /* output by the web console */
    const char *replInfo = "";
    struct ReplInfo {
        ReplInfo(const char *msg) {
            replInfo = msg;
        }
        ~ReplInfo() {
            replInfo = "?";
        }
    };


    /* operator requested resynchronization of replication (on the slave).  { resync : 1 } */
    class CmdResync : public Command {
    public:
        virtual bool slaveOk() const {
            return true;
        }
        virtual bool adminOnly() const {
            return true;
        }
        virtual bool logTheOp() { return false; }
        virtual bool lockGlobally() const { return true; }
        virtual LockType locktype() const { return WRITE; }
        virtual void addRequiredPrivileges(const std::string& dbname,
                                           const BSONObj& cmdObj,
                                           std::vector<Privilege>* out) {
            ActionSet actions;
            actions.addAction(ActionType::resync);
            out->push_back(Privilege(AuthorizationManager::SERVER_RESOURCE_NAME, actions));
        }
        void help(stringstream&h) const { h << "resync (from scratch) an out of date replica slave.\nhttp://dochub.mongodb.org/core/masterslave"; }
        CmdResync() : Command("resync") { }
        virtual bool run(const string& , BSONObj& cmdObj, int, string& errmsg, BSONObjBuilder& result, bool fromRepl) {
            if( cmdLine.usingReplSets() ) {
                errmsg = "resync command not currently supported with replica sets.  See RS102 info in the mongodb documentations";
                result.append("info", "http://dochub.mongodb.org/core/resyncingaverystalereplicasetmember");
                return false;
            }

            if ( cmdObj.getBoolField( "force" ) ) {
                if ( !waitForSyncToFinish( errmsg ) )
                    return false;
                replAllDead = "resync forced";
            }
            if ( !replAllDead ) {
                errmsg = "not dead, no need to resync";
                return false;
            }
            if ( !waitForSyncToFinish( errmsg ) )
                return false;

            ReplSource::forceResyncDead( "client" );
            result.append( "info", "triggered resync for all sources" );
            return true;
        }
        bool waitForSyncToFinish( string &errmsg ) const {
            // Wait for slave thread to finish syncing, so sources will be be
            // reloaded with new saved state on next pass.
            Timer t;
            while ( 1 ) {
                if ( syncing == 0 || t.millis() > 30000 )
                    break;
                {
                    Lock::TempRelease t;
                    relinquishSyncingSome = 1;
                    sleepmillis(1);
                }
            }
            if ( syncing ) {
                errmsg = "timeout waiting for sync() to finish";
                return false;
            }
            return true;
        }
    } cmdResync;


    ReplSource::ReplSource() {
        nClonedThisPass = 0;
        ensureMe();
    }

    ReplSource::ReplSource(BSONObj o) : nClonedThisPass(0) {
        only = o.getStringField("only");
        hostName = o.getStringField("host");
        _sourceName = o.getStringField("source");
        uassert( 10118 ,  "'host' field not set in sources collection object", !hostName.empty() );
        uassert( 10119 ,  "only source='main' allowed for now with replication", sourceName() == "main" );
        BSONElement e = o.getField("syncedTo");
        if ( !e.eoo() ) {
            uassert( 10120 ,  "bad sources 'syncedTo' field value", e.type() == Date || e.type() == Timestamp );
            OpTime tmp( e.date() );
            syncedTo = tmp;
        }

        BSONObj dbsObj = o.getObjectField("dbsNextPass");
        if ( !dbsObj.isEmpty() ) {
            BSONObjIterator i(dbsObj);
            while ( 1 ) {
                BSONElement e = i.next();
                if ( e.eoo() )
                    break;
                addDbNextPass.insert( e.fieldName() );
            }
        }

        dbsObj = o.getObjectField("incompleteCloneDbs");
        if ( !dbsObj.isEmpty() ) {
            BSONObjIterator i(dbsObj);
            while ( 1 ) {
                BSONElement e = i.next();
                if ( e.eoo() )
                    break;
                incompleteCloneDbs.insert( e.fieldName() );
            }
        }
        ensureMe();
    }

    /* Turn our C++ Source object into a BSONObj */
    BSONObj ReplSource::jsobj() {
        BSONObjBuilder b;
        b.append("host", hostName);
        b.append("source", sourceName());
        if ( !only.empty() )
            b.append("only", only);
        if ( !syncedTo.isNull() )
            b.appendTimestamp("syncedTo", syncedTo.asDate());

        BSONObjBuilder dbsNextPassBuilder;
        int n = 0;
        for ( set<string>::iterator i = addDbNextPass.begin(); i != addDbNextPass.end(); i++ ) {
            n++;
            dbsNextPassBuilder.appendBool(*i, 1);
        }
        if ( n )
            b.append("dbsNextPass", dbsNextPassBuilder.done());

        BSONObjBuilder incompleteCloneDbsBuilder;
        n = 0;
        for ( set<string>::iterator i = incompleteCloneDbs.begin(); i != incompleteCloneDbs.end(); i++ ) {
            n++;
            incompleteCloneDbsBuilder.appendBool(*i, 1);
        }
        if ( n )
            b.append("incompleteCloneDbs", incompleteCloneDbsBuilder.done());

        return b.obj();
    }

    void ReplSource::ensureMe() {
        string myname = getHostName();
        {
            Client::WriteContext ctx("local");
            // local.me is an identifier for a server for getLastError w:2+
            if (!Helpers::getSingleton("local.me", _me) ||
                !_me.hasField("host") ||
                _me["host"].String() != myname) {

                // clean out local.me
                Helpers::emptyCollection("local.me");

                // repopulate
                BSONObjBuilder b;
                b.appendOID("_id", 0, true);
                b.append("host", myname);
                _me = b.obj();
                Helpers::putSingleton("local.me", _me);
            }
        }
    }

    void ReplSource::save() {
        BSONObjBuilder b;
        verify( !hostName.empty() );
        b.append("host", hostName);
        // todo: finish allowing multiple source configs.
        // this line doesn't work right when source is null, if that is allowed as it is now:
        //b.append("source", _sourceName);
        BSONObj pattern = b.done();

        BSONObj o = jsobj();
        LOG( 1 ) << "Saving repl source: " << o << endl;

        {
            OpDebug debug;
            Client::Context ctx("local.sources");

            const NamespaceString requestNs("local.sources");
            UpdateRequest request(requestNs);

            request.setQuery(pattern);
            request.setUpdates(o);
            request.setUpsert();

            UpdateResult res = update(request, &debug);

            verify( ! res.modifiers );
            verify( res.numMatched == 1 );
        }
    }

    static void addSourceToList(ReplSource::SourceVector &v, ReplSource& s, ReplSource::SourceVector &old) {
        if ( !s.syncedTo.isNull() ) { // Don't reuse old ReplSource if there was a forced resync.
            for ( ReplSource::SourceVector::iterator i = old.begin(); i != old.end();  ) {
                if ( s == **i ) {
                    v.push_back(*i);
                    old.erase(i);
                    return;
                }
                i++;
            }
        }

        v.push_back( shared_ptr< ReplSource >( new ReplSource( s ) ) );
    }

    /* we reuse our existing objects so that we can keep our existing connection
       and cursor in effect.
    */
    void ReplSource::loadAll(SourceVector &v) {
        Client::Context ctx("local.sources");
        SourceVector old = v;
        v.clear();

        if ( !cmdLine.source.empty() ) {
            // --source <host> specified.
            // check that no items are in sources other than that
            // add if missing
            int n = 0;
            auto_ptr<Runner> runner(InternalPlanner::collectionScan("local.sources"));
            BSONObj obj;
            Runner::RunnerState state;
            while (Runner::RUNNER_ADVANCED == (state = runner->getNext(&obj, NULL))) {
                n++;
                ReplSource tmp(obj);
                if ( tmp.hostName != cmdLine.source ) {
                    log() << "repl: --source " << cmdLine.source << " != " << tmp.hostName << " from local.sources collection" << endl;
                    log() << "repl: for instructions on changing this slave's source, see:" << endl;
                    log() << "http://dochub.mongodb.org/core/masterslave" << endl;
                    log() << "repl: terminating mongod after 30 seconds" << endl;
                    sleepsecs(30);
                    dbexit( EXIT_REPLICATION_ERROR );
                }
                if ( tmp.only != cmdLine.only ) {
                    log() << "--only " << cmdLine.only << " != " << tmp.only << " from local.sources collection" << endl;
                    log() << "terminating after 30 seconds" << endl;
                    sleepsecs(30);
                    dbexit( EXIT_REPLICATION_ERROR );
                }
            }
            uassert(17065, "Internal error reading from local.sources", Runner::RUNNER_EOF == state);
            uassert( 10002 ,  "local.sources collection corrupt?", n<2 );
            if ( n == 0 ) {
                // source missing.  add.
                ReplSource s;
                s.hostName = cmdLine.source;
                s.only = cmdLine.only;
                s.save();
            }
        }
        else {
            try {
                massert( 10384 , "--only requires use of --source", cmdLine.only.empty());
            }
            catch ( ... ) {
                dbexit( EXIT_BADOPTIONS );
            }
        }

        auto_ptr<Runner> runner(InternalPlanner::collectionScan("local.sources"));
        BSONObj obj;
        Runner::RunnerState state;
        while (Runner::RUNNER_ADVANCED == (state = runner->getNext(&obj, NULL))) {
            ReplSource tmp(obj);
            if ( tmp.syncedTo.isNull() ) {
                DBDirectClient c;
                if ( c.exists( "local.oplog.$main" ) ) {
                    BSONObj op = c.findOne( "local.oplog.$main", QUERY( "op" << NE << "n" ).sort( BSON( "$natural" << -1 ) ) );
                    if ( !op.isEmpty() ) {
                        tmp.syncedTo = op[ "ts" ].date();
                    }
                }
            }
            addSourceToList(v, tmp, old);
        }
        uassert(17066, "Internal error reading from local.sources", Runner::RUNNER_EOF == state);
    }

    bool ReplSource::throttledForceResyncDead( const char *requester ) {
        if ( time( 0 ) - lastForcedResync > 600 ) {
            forceResyncDead( requester );
            lastForcedResync = time( 0 );
            return true;
        }
        return false;
    }

    void ReplSource::forceResyncDead( const char *requester ) {
        if ( !replAllDead )
            return;
        SourceVector sources;
        ReplSource::loadAll(sources);
        for( SourceVector::iterator i = sources.begin(); i != sources.end(); ++i ) {
            log() << requester << " forcing resync from "  << (*i)->hostName << endl;
            (*i)->forceResync( requester );
        }
        replAllDead = 0;
    }

    void ReplSource::forceResync( const char *requester ) {
        BSONObj info;
        {
            dbtemprelease t;
            if (!oplogReader.connect(hostName, _me)) {
                msgassertedNoTrace( 14051 , "unable to connect to resync");
            }
            /* todo use getDatabaseNames() method here */
            bool ok = oplogReader.conn()->runCommand( "admin", BSON( "listDatabases" << 1 ), info );
            massert( 10385 ,  "Unable to get database list", ok );
        }
        BSONObjIterator i( info.getField( "databases" ).embeddedObject() );
        while( i.moreWithEOO() ) {
            BSONElement e = i.next();
            if ( e.eoo() )
                break;
            string name = e.embeddedObject().getField( "name" ).valuestr();
            if ( !e.embeddedObject().getBoolField( "empty" ) ) {
                if ( name != "local" ) {
                    if ( only.empty() || only == name ) {
                        resyncDrop( name.c_str(), requester );
                    }
                }
            }
        }
        syncedTo = OpTime();
        addDbNextPass.clear();
        save();
    }

    string ReplSource::resyncDrop( const char *db, const char *requester ) {
        log() << "resync: dropping database " << db << endl;
        Client::Context ctx(db);
        dropDatabase(db);
        return db;
    }

    /* grab initial copy of a database from the master */
    void ReplSource::resync(const std::string& dbName) {
        const std::string db(dbName);   // need local copy of the name, we're dropping the original
        string dummyNs = resyncDrop( db.c_str(), "internal" );
        Client::Context ctx( dummyNs );
        {
            log() << "resync: cloning database " << db << " to get an initial copy" << endl;
            ReplInfo r("resync: cloning a database");
            string errmsg;
            int errCode = 0;
            bool ok = Cloner::cloneFrom(hostName.c_str(), errmsg, cc().database()->name(), false,
                                        /*slaveOk*/ true, /*replauth*/ true, /*snapshot*/false,
                                        /*mayYield*/true, /*mayBeInterrupted*/false, &errCode);
            if ( !ok ) {
                if ( errCode == DatabaseDifferCaseCode ) {
                    resyncDrop( db.c_str(), "internal" );
                    log() << "resync: database " << db << " not valid on the master due to a name conflict, dropping." << endl;
                    return;
                }
                else {
                    problem() << "resync of " << db << " from " << hostName << " failed " << errmsg << endl;
                    throw SyncException();
                }
            }
        }

        log() << "resync: done with initial clone for db: " << db << endl;

        return;
    }
    
    static DatabaseIgnorer ___databaseIgnorer;
    
    void DatabaseIgnorer::doIgnoreUntilAfter( const string &db, const OpTime &futureOplogTime ) {
        if ( futureOplogTime > _ignores[ db ] ) {
            _ignores[ db ] = futureOplogTime;   
        }
    }

    bool DatabaseIgnorer::ignoreAt( const string &db, const OpTime &currentOplogTime ) {
        if ( _ignores[ db ].isNull() ) {
            return false;
        }
        if ( _ignores[ db ] >= currentOplogTime ) {
            return true;
        } else {
            // The ignore state has expired, so clear it.
            _ignores.erase( db );
            return false;
        }
    }

    bool ReplSource::handleDuplicateDbName( const BSONObj &op, const char *ns, const char *db ) {
        if ( dbHolder()._isLoaded( ns, dbpath ) ) {
            // Database is already present.
            return true;   
        }
        BSONElement ts = op.getField( "ts" );
        if ( ( ts.type() == Date || ts.type() == Timestamp ) && ___databaseIgnorer.ignoreAt( db, ts.date() ) ) {
            // Database is ignored due to a previous indication that it is
            // missing from master after optime "ts".
            return false;   
        }
        if ( Database::duplicateUncasedName( false, db, dbpath ).empty() ) {
            // No duplicate database names are present.
            return true;
        }
        
        OpTime lastTime;
        bool dbOk = false;
        {
            dbtemprelease release;
        
            // We always log an operation after executing it (never before), so
            // a database list will always be valid as of an oplog entry generated
            // before it was retrieved.
            
            BSONObj last = oplogReader.findOne( this->ns().c_str(), Query().sort( BSON( "$natural" << -1 ) ) );
            if ( !last.isEmpty() ) {
	            BSONElement ts = last.getField( "ts" );
	            massert( 14032, "Invalid 'ts' in remote log", ts.type() == Date || ts.type() == Timestamp );
	            lastTime = OpTime( ts.date() );
            }

            BSONObj info;
            bool ok = oplogReader.conn()->runCommand( "admin", BSON( "listDatabases" << 1 ), info );
            massert( 14033, "Unable to get database list", ok );
            BSONObjIterator i( info.getField( "databases" ).embeddedObject() );
            while( i.more() ) {
                BSONElement e = i.next();
            
                const char * name = e.embeddedObject().getField( "name" ).valuestr();
                if ( strcasecmp( name, db ) != 0 )
                    continue;
                
                if ( strcmp( name, db ) == 0 ) {
                    // The db exists on master, still need to check that no conflicts exist there.
                    dbOk = true;
                    continue;
                }
                
                // The master has a db name that conflicts with the requested name.
                dbOk = false;
                break;
            }
        }
        
        if ( !dbOk ) {
            ___databaseIgnorer.doIgnoreUntilAfter( db, lastTime );
            incompleteCloneDbs.erase(db);
            addDbNextPass.erase(db);
            return false;   
        }
        
        // Check for duplicates again, since we released the lock above.
        set< string > duplicates;
        Database::duplicateUncasedName( false, db, dbpath, &duplicates );
        
        // The database is present on the master and no conflicting databases
        // are present on the master.  Drop any local conflicts.
        for( set< string >::const_iterator i = duplicates.begin(); i != duplicates.end(); ++i ) {
            ___databaseIgnorer.doIgnoreUntilAfter( *i, lastTime );
            incompleteCloneDbs.erase(*i);
            addDbNextPass.erase(*i);
            Client::Context ctx(*i);
            dropDatabase(*i);
        }
        
        massert( 14034, "Duplicate database names present after attempting to delete duplicates",
                Database::duplicateUncasedName( false, db, dbpath ).empty() );
        return true;
    }

    void ReplSource::applyOperation(const BSONObj& op) {
        try {
            bool failedUpdate = applyOperation_inlock( op );
            if (failedUpdate) {
                Sync sync(hostName);
                if (sync.shouldRetry(op)) {
                    uassert(15914, "Failure retrying initial sync update", !applyOperation_inlock(op));
                }
            }
        }
        catch ( UserException& e ) {
            log() << "sync: caught user assertion " << e << " while applying op: " << op << endl;;
        }
        catch ( DBException& e ) {
            log() << "sync: caught db exception " << e << " while applying op: " << op << endl;;
        }

    }

    /* local.$oplog.main is of the form:
         { ts: ..., op: <optype>, ns: ..., o: <obj> , o2: <extraobj>, b: <boolflag> }
         ...
       see logOp() comments.

       @param alreadyLocked caller already put us in write lock if true
    */
    void ReplSource::sync_pullOpLog_applyOperation(BSONObj& op, bool alreadyLocked) {
        LOG(6) << "processing op: " << op << endl;

        if( op.getStringField("op")[0] == 'n' )
            return;

        char clientName[MaxDatabaseNameLen];
        const char *ns = op.getStringField("ns");
        nsToDatabase(ns, clientName);

        if ( *ns == '.' ) {
            problem() << "skipping bad op in oplog: " << op.toString() << endl;
            return;
        }
        else if ( *ns == 0 ) {
            /*if( op.getStringField("op")[0] != 'n' )*/ {
                problem() << "halting replication, bad op in oplog:\n  " << op.toString() << endl;
                replAllDead = "bad object in oplog";
                throw SyncException();
            }
            //ns = "local.system.x";
            //nsToDatabase(ns, clientName);
        }

        if ( !only.empty() && only != clientName )
            return;

        if( cmdLine.pretouch && !alreadyLocked/*doesn't make sense if in write lock already*/ ) {
            if( cmdLine.pretouch > 1 ) {
                /* note: this is bad - should be put in ReplSource.  but this is first test... */
                static int countdown;
                verify( countdown >= 0 );
                if( countdown > 0 ) {
                    countdown--; // was pretouched on a prev pass
                }
                else {
                    const int m = 4;
                    if( tp.get() == 0 ) {
                        int nthr = min(8, cmdLine.pretouch);
                        nthr = max(nthr, 1);
                        tp.reset( new ThreadPool(nthr) );
                    }
                    vector<BSONObj> v;
                    oplogReader.peek(v, cmdLine.pretouch);
                    unsigned a = 0;
                    while( 1 ) {
                        if( a >= v.size() ) break;
                        unsigned b = a + m - 1; // v[a..b]
                        if( b >= v.size() ) b = v.size() - 1;
                        tp->schedule(pretouchN, v, a, b);
                        DEV cout << "pretouch task: " << a << ".." << b << endl;
                        a += m;
                    }
                    // we do one too...
                    pretouchOperation(op);
                    tp->join();
                    countdown = v.size();
                }
            }
            else {
                pretouchOperation(op);
            }
        }

        scoped_ptr<Lock::GlobalWrite> lk( alreadyLocked ? 0 : new Lock::GlobalWrite() );

        if ( replAllDead ) {
            // hmmm why is this check here and not at top of this function? does it get set between top and here?
            log() << "replAllDead, throwing SyncException: " << replAllDead << endl;
            throw SyncException();
        }

        if ( !handleDuplicateDbName( op, ns, clientName ) ) {
            return;   
        }
                
        Client::Context ctx( ns );
        ctx.getClient()->curop()->reset();

        bool empty = ctx.db()->isEmpty();
        bool incompleteClone = incompleteCloneDbs.count( clientName ) != 0;

        LOG(6) << "ns: " << ns << ", justCreated: " << ctx.justCreated() << ", empty: " << empty << ", incompleteClone: " << incompleteClone << endl;

        // always apply admin command command
        // this is a bit hacky -- the semantics of replication/commands aren't well specified
        if ( strcmp( clientName, "admin" ) == 0 && *op.getStringField( "op" ) == 'c' ) {
            applyOperation( op );
            return;
        }

        if ( ctx.justCreated() || empty || incompleteClone ) {
            // we must add to incomplete list now that setClient has been called
            incompleteCloneDbs.insert( clientName );
            if ( nClonedThisPass ) {
                /* we only clone one database per pass, even if a lot need done.  This helps us
                 avoid overflowing the master's transaction log by doing too much work before going
                 back to read more transactions. (Imagine a scenario of slave startup where we try to
                 clone 100 databases in one pass.)
                 */
                addDbNextPass.insert( clientName );
            }
            else {
                if ( incompleteClone ) {
                    log() << "An earlier initial clone of '" << clientName << "' did not complete, now resyncing." << endl;
                }
                save();
                Client::Context ctx(ns);
                nClonedThisPass++;
                resync(ctx.db()->name());
                addDbNextPass.erase(clientName);
                incompleteCloneDbs.erase( clientName );
            }
            save();
        }
        else {
            applyOperation( op );
            addDbNextPass.erase( clientName );
        }
    }

    void ReplSource::syncToTailOfRemoteLog() {
        string _ns = ns();
        BSONObjBuilder b;
        if ( !only.empty() ) {
            b.appendRegex("ns", string("^") + pcrecpp::RE::QuoteMeta( only ));
        }
        BSONObj last = oplogReader.findOne( _ns.c_str(), Query( b.done() ).sort( BSON( "$natural" << -1 ) ) );
        if ( !last.isEmpty() ) {
            BSONElement ts = last.getField( "ts" );
            massert( 10386 ,  "non Date ts found: " + last.toString(), ts.type() == Date || ts.type() == Timestamp );
            syncedTo = OpTime( ts.date() );
        }
    }

    class ReplApplyBatchSize : public ServerParameter {
    public:
        ReplApplyBatchSize()
            : ServerParameter( ServerParameterSet::getGlobal(), "replApplyBatchSize" ),
              _value( 1 ) {
        }

        int get() const { return _value; }

        virtual void append( BSONObjBuilder& b, const string& name ) {
            b.append( name, _value );
        }

        virtual Status set( const BSONElement& newValuElement ) {
            return set( newValuElement.numberInt() );
        }

        virtual Status set( int b ) {
            if( b < 1 || b > 1024 ) {
                return Status( ErrorCodes::BadValue,
                               "replApplyBatchSize has to be >= 1 and < 1024" );
            }

            if ( replSettings.slavedelay != 0 && b > 1 ) {
                return Status( ErrorCodes::BadValue,
                               "can't use a batch size > 1 with slavedelay" );
            }
            if ( ! replSettings.slave ) {
                return Status( ErrorCodes::BadValue,
                               "can't set replApplyBatchSize on a non-slave machine" );
            }

            _value = b;
            return Status::OK();
        }

        virtual Status setFromString( const string& str ) {
            return set( atoi( str.c_str() ) );
        }

        int _value;

    } replApplyBatchSize;

    /* slave: pull some data from the master's oplog
       note: not yet in db mutex at this point.
       @return -1 error
               0 ok, don't sleep
               1 ok, sleep
    */
    int ReplSource::sync_pullOpLog(int& nApplied) {
        int okResultCode = 1;
        string ns = string("local.oplog.$") + sourceName();
        LOG(2) << "repl: sync_pullOpLog " << ns << " syncedTo:" << syncedTo.toStringLong() << '\n';

        bool tailing = true;
        oplogReader.tailCheck();

        bool initial = syncedTo.isNull();

        if ( !oplogReader.haveCursor() || initial ) {
            if ( initial ) {
                // Important to grab last oplog timestamp before listing databases.
                syncToTailOfRemoteLog();
                BSONObj info;
                bool ok = oplogReader.conn()->runCommand( "admin", BSON( "listDatabases" << 1 ), info );
                massert( 10389 ,  "Unable to get database list", ok );
                BSONObjIterator i( info.getField( "databases" ).embeddedObject() );
                while( i.moreWithEOO() ) {
                    BSONElement e = i.next();
                    if ( e.eoo() )
                        break;
                    string name = e.embeddedObject().getField( "name" ).valuestr();
                    if ( !e.embeddedObject().getBoolField( "empty" ) ) {
                        if ( name != "local" ) {
                            if ( only.empty() || only == name ) {
                                LOG( 2 ) << "adding to 'addDbNextPass': " << name << endl;
                                addDbNextPass.insert( name );
                            }
                        }
                    }
                }
                // obviously global isn't ideal, but non-repl set is old so 
                // keeping it simple
                Lock::GlobalWrite lk;
                save();
            }

            BSONObjBuilder gte;
            gte.appendTimestamp("$gte", syncedTo.asDate());
            BSONObjBuilder query;
            query.append("ts", gte.done());
            if ( !only.empty() ) {
                // note we may here skip a LOT of data table scanning, a lot of work for the master.
                // maybe append "\\." here?
                query.appendRegex("ns", string("^") + pcrecpp::RE::QuoteMeta( only ));
            }
            BSONObj queryObj = query.done();
            // e.g. queryObj = { ts: { $gte: syncedTo } }

            oplogReader.tailingQuery(ns.c_str(), queryObj);
            tailing = false;
        }
        else {
            LOG(2) << "repl: tailing=true\n";
        }

        if( !oplogReader.haveCursor() ) {
            problem() << "repl: dbclient::query returns null (conn closed?)" << endl;
            oplogReader.resetConnection();
            return -1;
        }

        // show any deferred database creates from a previous pass
        {
            set<string>::iterator i = addDbNextPass.begin();
            if ( i != addDbNextPass.end() ) {
                BSONObjBuilder b;
                b.append("ns", *i + '.');
                b.append("op", "db");
                BSONObj op = b.done();
                sync_pullOpLog_applyOperation(op, false);
            }
        }

        if ( !oplogReader.more() ) {
            if ( tailing ) {
                LOG(2) << "repl: tailing & no new activity\n";
                if( oplogReader.awaitCapable() )
                    okResultCode = 0; // don't sleep

            }
            else {
                log() << "repl:   " << ns << " oplog is empty" << endl;
            }
            {
                Lock::GlobalWrite lk;
                save();
            }
            return okResultCode;
        }

        OpTime nextOpTime;
        {
            BSONObj op = oplogReader.next();
            BSONElement ts = op.getField("ts");
            if ( ts.type() != Date && ts.type() != Timestamp ) {
                string err = op.getStringField("$err");
                if ( !err.empty() ) {
                    // 13051 is "tailable cursor requested on non capped collection"
                    if (op.getIntField("code") == 13051) {
                        problem() << "trying to slave off of a non-master" << '\n';
                        massert( 13344 ,  "trying to slave off of a non-master", false );
                    }
                    else {
                        problem() << "repl: $err reading remote oplog: " + err << '\n';
                        massert( 10390 ,  "got $err reading remote oplog", false );
                    }
                }
                else {
                    problem() << "repl: bad object read from remote oplog: " << op.toString() << '\n';
                    massert( 10391 , "repl: bad object read from remote oplog", false);
                }
            }

            nextOpTime = OpTime( ts.date() );
            LOG(2) << "repl: first op time received: " << nextOpTime.toString() << '\n';
            if ( initial ) {
                LOG(1) << "repl:   initial run\n";
            }
            if( tailing ) {
                if( !( syncedTo < nextOpTime ) ) {
                    log() << "repl ASSERTION failed : syncedTo < nextOpTime" << endl;
                    log() << "repl syncTo:     " << syncedTo.toStringLong() << endl;
                    log() << "repl nextOpTime: " << nextOpTime.toStringLong() << endl;
                    verify(false);
                }
                oplogReader.putBack( op ); // op will be processed in the loop below
                nextOpTime = OpTime(); // will reread the op below
            }
            else if ( nextOpTime != syncedTo ) { // didn't get what we queried for - error
                log()
                    << "repl:   nextOpTime " << nextOpTime.toStringLong() << ' '
                    << ((nextOpTime < syncedTo) ? "<??" : ">")
                    << " syncedTo " << syncedTo.toStringLong() << '\n'
                    << "repl:   time diff: " << (nextOpTime.getSecs() - syncedTo.getSecs())
                    << "sec\n"
                    << "repl:   tailing: " << tailing << '\n'
                    << "repl:   data too stale, halting replication" << endl;
                replInfo = replAllDead = "data too stale halted replication";
                verify( syncedTo < nextOpTime );
                throw SyncException();
            }
            else {
                /* t == syncedTo, so the first op was applied previously or it is the first op of initial query and need not be applied. */
            }
        }

        // apply operations
        {
            int n = 0;
            time_t saveLast = time(0);
            while ( 1 ) {

                bool moreInitialSyncsPending = !addDbNextPass.empty() && n; // we need "&& n" to assure we actually process at least one op to get a sync point recorded in the first place.

                if ( moreInitialSyncsPending || !oplogReader.more() ) {
                    Lock::GlobalWrite lk;

                    // NOTE aaron 2011-03-29 This block may be unnecessary, but I'm leaving it in place to avoid changing timing behavior.
                    {
                        dbtemprelease t;
                        if ( !moreInitialSyncsPending && oplogReader.more() ) {
                            continue;
                        }
                        // otherwise, break out of loop so we can set to completed or clone more dbs
                    }
                    
                    if( oplogReader.awaitCapable() && tailing )
                        okResultCode = 0; // don't sleep
                    syncedTo = nextOpTime;
                    save(); // note how far we are synced up to now
                    log() << "repl:   applied " << n << " operations" << endl;
                    nApplied = n;
                    log() << "repl:  end sync_pullOpLog syncedTo: " << syncedTo.toStringLong() << endl;
                    break;
                }
                else {
                }

                OCCASIONALLY if( n > 0 && ( n > 100000 || time(0) - saveLast > 60 ) ) {
                    // periodically note our progress, in case we are doing a lot of work and crash
                    Lock::GlobalWrite lk;
                    syncedTo = nextOpTime;
                    // can't update local log ts since there are pending operations from our peer
                    save();
                    log() << "repl:   checkpoint applied " << n << " operations" << endl;
                    log() << "repl:   syncedTo: " << syncedTo.toStringLong() << endl;
                    saveLast = time(0);
                    n = 0;
                }

                BSONObj op = oplogReader.next();

                int b = replApplyBatchSize.get();
                bool justOne = b == 1;
                scoped_ptr<Lock::GlobalWrite> lk( justOne ? 0 : new Lock::GlobalWrite() );
                while( 1 ) {

                    BSONElement ts = op.getField("ts");
                    if( !( ts.type() == Date || ts.type() == Timestamp ) ) {
                        log() << "sync error: problem querying remote oplog record" << endl;
                        log() << "op: " << op.toString() << endl;
                        log() << "halting replication" << endl;
                        replInfo = replAllDead = "sync error: no ts found querying remote oplog record";
                        throw SyncException();
                    }
                    OpTime last = nextOpTime;
                    nextOpTime = OpTime( ts.date() );
                    if ( !( last < nextOpTime ) ) {
                        log() << "sync error: last applied optime at slave >= nextOpTime from master" << endl;
                        log() << " last:       " << last.toStringLong() << endl;
                        log() << " nextOpTime: " << nextOpTime.toStringLong() << endl;
                        log() << " halting replication" << endl;
                        replInfo = replAllDead = "sync error last >= nextOpTime";
                        uassert( 10123 , "replication error last applied optime at slave >= nextOpTime from master", false);
                    }
                    if ( replSettings.slavedelay && ( unsigned( time( 0 ) ) < nextOpTime.getSecs() + replSettings.slavedelay ) ) {
                        verify( justOne );
                        oplogReader.putBack( op );
                        _sleepAdviceTime = nextOpTime.getSecs() + replSettings.slavedelay + 1;
                        Lock::GlobalWrite lk;
                        if ( n > 0 ) {
                            syncedTo = last;
                            save();
                        }
                        log() << "repl:   applied " << n << " operations" << endl;
                        log() << "repl:   syncedTo: " << syncedTo.toStringLong() << endl;
                        log() << "waiting until: " << _sleepAdviceTime << " to continue" << endl;
                        return okResultCode;
                    }

                    sync_pullOpLog_applyOperation(op, !justOne);
                    n++;

                    if( --b == 0 )
                        break;
                    // if to here, we are doing mulpile applications in a singel write lock acquisition
                    if( !oplogReader.moreInCurrentBatch() ) {
                        // break if no more in batch so we release lock while reading from the master
                        break;
                    }
                    op = oplogReader.next();

                    getDur().commitIfNeeded();
                }
            }
        }

        return okResultCode;
    }


    /* note: not yet in mutex at this point.
       returns >= 0 if ok.  return -1 if you want to reconnect.
       return value of zero indicates no sleep necessary before next call
    */
    int ReplSource::sync(int& nApplied) {
        _sleepAdviceTime = 0;
        ReplInfo r("sync");
        if ( !cmdLine.quiet ) {
            LogstreamBuilder l = log();
            l << "repl: syncing from ";
            if( sourceName() != "main" ) {
                l << "source:" << sourceName() << ' ';
            }
            l << "host:" << hostName << endl;
        }
        nClonedThisPass = 0;

        // FIXME Handle cases where this db isn't on default port, or default port is spec'd in hostName.
        if ( (string("localhost") == hostName || string("127.0.0.1") == hostName) && cmdLine.port == CmdLine::DefaultDBPort ) {
            log() << "repl:   can't sync from self (localhost). sources configuration may be wrong." << endl;
            sleepsecs(5);
            return -1;
        }

        if ( !oplogReader.connect(hostName, _me) ) {
            LOG(4) << "repl:  can't connect to sync source" << endl;
            return -1;
        }

        return sync_pullOpLog(nApplied);
    }

    /* --------------------------------------------------------------*/

    /*
    TODO:
    _ source has autoptr to the cursor
    _ reuse that cursor when we can
    */

    /* returns: # of seconds to sleep before next pass
                0 = no sleep recommended
                1 = special sentinel indicating adaptive sleep recommended
    */
    int _replMain(ReplSource::SourceVector& sources, int& nApplied) {
        {
            ReplInfo r("replMain load sources");
            Lock::GlobalWrite lk;
            ReplSource::loadAll(sources);
            replSettings.fastsync = false; // only need this param for initial reset
        }

        if ( sources.empty() ) {
            /* replication is not configured yet (for --slave) in local.sources.  Poll for config it
            every 20 seconds.
            */
            log() << "no source given, add a master to local.sources to start replication" << endl;
            return 20;
        }

        int sleepAdvice = 1;
        for ( ReplSource::SourceVector::iterator i = sources.begin(); i != sources.end(); i++ ) {
            ReplSource *s = i->get();
            int res = -1;
            try {
                res = s->sync(nApplied);
                bool moreToSync = s->haveMoreDbsToSync();
                if( res < 0 ) {
                    sleepAdvice = 3;
                }
                else if( moreToSync ) {
                    sleepAdvice = 0;
                }
                else if ( s->sleepAdvice() ) {
                    sleepAdvice = s->sleepAdvice();
                }
                else
                    sleepAdvice = res;
            }
            catch ( const SyncException& ) {
                log() << "caught SyncException" << endl;
                return 10;
            }
            catch ( AssertionException& e ) {
                if ( e.severe() ) {
                    log() << "replMain AssertionException " << e.what() << endl;
                    return 60;
                }
                else {
                    log() << "repl: AssertionException " << e.what() << endl;
                }
                replInfo = "replMain caught AssertionException";
            }
            catch ( const DBException& e ) {
                log() << "repl: DBException " << e.what() << endl;
                replInfo = "replMain caught DBException";
            }
            catch ( const std::exception &e ) {
                log() << "repl: std::exception " << e.what() << endl;
                replInfo = "replMain caught std::exception";
            }
            catch ( ... ) {
                log() << "unexpected exception during replication.  replication will halt" << endl;
                replAllDead = "caught unexpected exception during replication";
            }
            if ( res < 0 )
                s->oplogReader.resetConnection();
        }
        return sleepAdvice;
    }

    void replMain() {
        ReplSource::SourceVector sources;
        while ( 1 ) {
            int s = 0;
            {
                Lock::GlobalWrite lk;
                if ( replAllDead ) {
                    // throttledForceResyncDead can throw
                    if ( !replSettings.autoresync || !ReplSource::throttledForceResyncDead( "auto" ) ) {
                        log() << "all sources dead: " << replAllDead << ", sleeping for 5 seconds" << endl;
                        break;
                    }
                }
                verify( syncing == 0 ); // i.e., there is only one sync thread running. we will want to change/fix this.
                syncing++;
            }
            try {
                int nApplied = 0;
                s = _replMain(sources, nApplied);
                if( s == 1 ) {
                    if( nApplied == 0 ) s = 2;
                    else if( nApplied > 100 ) {
                        // sleep very little - just enough that we aren't truly hammering master
                        sleepmillis(75);
                        s = 0;
                    }
                }
            }
            catch (...) {
                out() << "caught exception in _replMain" << endl;
                s = 4;
            }
            {
                Lock::GlobalWrite lk;
                verify( syncing == 1 );
                syncing--;
            }

            if( relinquishSyncingSome )  {
                relinquishSyncingSome = 0;
                s = 1; // sleep before going back in to syncing=1
            }

            if ( s ) {
                stringstream ss;
                ss << "repl: sleep " << s << " sec before next pass";
                string msg = ss.str();
                if ( ! cmdLine.quiet )
                    log() << msg << endl;
                ReplInfo r(msg.c_str());
                sleepsecs(s);
            }
        }
    }

    static void replMasterThread() {
        sleepsecs(4);
        Client::initThread("replmaster");
        int toSleep = 10;
        while( 1 ) {

            sleepsecs( toSleep );
            /* write a keep-alive like entry to the log.  this will make things like
               printReplicationStatus() and printSlaveReplicationStatus() stay up-to-date
               even when things are idle.
            */
            {
                writelocktry lk(1);
                if ( lk.got() ) {
                    toSleep = 10;

                    replLocalAuth();

                    try {
                        logKeepalive();
                    }
                    catch(...) {
                        log() << "caught exception in replMasterThread()" << endl;
                    }
                }
                else {
                    LOG(5) << "couldn't logKeepalive" << endl;
                    toSleep = 1;
                }
            }
        }
    }

    void replSlaveThread() {
        sleepsecs(1);
        Client::initThread("replslave");

        {
            Lock::GlobalWrite lk;
            replLocalAuth();
        }

        while ( 1 ) {
            try {
                replMain();
                sleepsecs(5);
            }
            catch ( AssertionException& ) {
                ReplInfo r("Assertion in replSlaveThread(): sleeping 5 minutes before retry");
                problem() << "Assertion in replSlaveThread(): sleeping 5 minutes before retry" << endl;
                sleepsecs(300);
            }
            catch ( DBException& e ) {
                problem() << "exception in replSlaveThread(): " << e.what()
                          << ", sleeping 5 minutes before retry" << endl;
                sleepsecs(300);
            }
            catch ( ... ) {
                problem() << "error in replSlaveThread(): sleeping 5 minutes before retry" << endl;
                sleepsecs(300);
            }
        }
    }

    void startMasterSlave() {

        oldRepl();

        if( !replSettings.slave && !replSettings.master )
            return;

        {
            Lock::GlobalWrite lk;
            replLocalAuth();
        }

        if ( replSettings.slave ) {
            verify( replSettings.slave == SimpleSlave );
            LOG(1) << "slave=true" << endl;
            boost::thread repl_thread(replSlaveThread);
        }

        if ( replSettings.master ) {
            LOG(1) << "master=true" << endl;
            replSettings.master = true;
            createOplog();
            boost::thread t(replMasterThread);
        }

        while( replSettings.fastsync ) // don't allow writes until we've set up from log
            sleepmillis( 50 );
    }
    int _dummy_z;

    void pretouchN(vector<BSONObj>& v, unsigned a, unsigned b) {
        DEV verify( ! Lock::isW() );

        Client *c = currentClient.get();
        if( c == 0 ) {
            Client::initThread("pretouchN");
            c = &cc();
        }

        Lock::GlobalRead lk;
        for( unsigned i = a; i <= b; i++ ) {
            const BSONObj& op = v[i];
            const char *which = "o";
            const char *opType = op.getStringField("op");
            if ( *opType == 'i' )
                ;
            else if( *opType == 'u' )
                which = "o2";
            else
                continue;
            /* todo : other operations */

            try {
                BSONObj o = op.getObjectField(which);
                BSONElement _id;
                if( o.getObjectID(_id) ) {
                    const char *ns = op.getStringField("ns");
                    BSONObjBuilder b;
                    b.append(_id);
                    BSONObj result;
                    Client::Context ctx( ns );
                    if( Helpers::findById(cc(), ns, b.done(), result) )
                        _dummy_z += result.objsize(); // touch
                }
            }
            catch( DBException& e ) {
                log() << "ignoring assertion in pretouchN() " << a << ' ' << b << ' ' << i << ' ' << e.toString() << endl;
            }
        }
    }

    void pretouchOperation(const BSONObj& op) {

        if( Lock::somethingWriteLocked() )
            return; // no point pretouching if write locked. not sure if this will ever fire, but just in case.

        const char *which = "o";
        const char *opType = op.getStringField("op");
        if ( *opType == 'i' )
            ;
        else if( *opType == 'u' )
            which = "o2";
        else
            return;
        /* todo : other operations */

        try {
            BSONObj o = op.getObjectField(which);
            BSONElement _id;
            if( o.getObjectID(_id) ) {
                const char *ns = op.getStringField("ns");
                BSONObjBuilder b;
                b.append(_id);
                BSONObj result;
                Client::ReadContext ctx( ns );
                if( Helpers::findById(cc(), ns, b.done(), result) )
                    _dummy_z += result.objsize(); // touch
            }
        }
        catch( DBException& ) {
            log() << "ignoring assertion in pretouchOperation()" << endl;
        }
    }

} // namespace mongo