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EOSIO appears to make use of the Chainbase database for various functions in the codebase.

What is this used for and how does it relate to the RAM concept in EOSIO? Is it simply the database used to manage RAM data, or does it perform additional functions related to the current or past state of EOSIO? Can it be accessed from smart contracts via the C++ API?

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1.What is chainbase used for? we can find the answer in : about chainbase

2.when we add a new object (i.e., row) to the table by Multi-Index such as

 class addressbook: contract {
       struct address {
          uint64_t account_name;
          string first_name;
          string last_name;
          string street;
          string city;
          string state;
          uint64_t primary_key() const { return account_name; }
          EOSLIB_SERIALIZE( address, (account_name)(first_name)(last_name)(street)(city)(state) )
       };
       public:
         addressbook(account_name self):contract(self) {}
         typedef eosio::multi_index< N(address), address > address_index;
         void myaction() {
           address_index addresses(_self, _self); // code, scope
           // add to table, first argument is account to bill for storage
           addresses.emplace(_self, [&](auto& address) {
             address.account_name = N(dan);
             address.first_name = "Daniel";
             address.last_name = "Larimer";
             address.street = "1 EOS Way";
             address.city = "Blacksburg";
             address.state = "VA";
           });
         }
     }

when we use emplace call the funtion db_store_i64 ,then we can find db_store_i64 in apply_context.cpp

int apply_context::db_store_i64( uint64_t code, uint64_t scope, uint64_t table, const account_name& payer, uint64_t id, const char* buffer, size_t buffer_size ) {
//   require_write_lock( scope );
   const auto& tab = find_or_create_table( code, scope, table, payer );
   auto tableid = tab.id;

   EOS_ASSERT( payer != account_name(), invalid_table_payer, "must specify a valid account to pay for new record" );

   const auto& obj = db.create<key_value_object>( [&]( auto& o ) {
      o.t_id        = tableid;
      o.primary_key = id;
      o.value.resize( buffer_size );
      o.payer       = payer;
      memcpy( o.value.data(), buffer, buffer_size );
   });

   db.modify( tab, [&]( auto& t ) {
     ++t.count;
   });

db is a object of chianbase ,after a series operate the data has inserted to the db,then we can get the info how we use or update our RAM used by this function update_db_usage

void apply_context::update_db_usage( const account_name& payer, int64_t delta ) {
   if( delta > 0 ) {
      if( !(privileged || payer == account_name(receiver)) ) {
         require_authorization( payer );
      }
   }
   trx_context.add_ram_usage(payer, delta);
}

above all is all i know about chainbase.

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