EOS合約開發第十七章-合約通訊程式設計
合約通訊程式設計
一、EOS通知概述
我們首先看一看eosio.token合約中issue的通知。跳過基本的合約和賬戶配置，我們直接進入eosio.token合約，首先建立一個token：
[[email protected] nodeos1]$ cleos --wallet-url http://localhost:9800 --url http://localhost:9800 push action eosio.token create '{"issuer":"eosio","maximum_supply":"100000000000.0000 EOS","can_freeze":0,"can_recall":0,"can_whitelist":0}' -p eosio.token executed transaction: 756ecf4050184dd06c7f27d77cca3864e41fa3c319b23c326180d7cc386b62d9 120 bytes 6597 us # eosio.token <= eosio.token::create {"issuer":"eosio","maximum_supply":"100000000000.0000 EOS","can_freeze":0,"can_recall":0,"can_whitel... warning: transaction executed locally, but may not be confirmed by the network yet
然後eosio向配置好的賬戶helloworld發行token：
[[email protected] dice]$ cleos --wallet-url http://localhost:9800 --url http://localhost:9800 push action eosio.token issue '{"to":"helloworld", "quantity":"100.0000 EOS", "memo":"m"}' -p eosio executed transaction: 732e883ae259644de6c4442a0b4007fdbc0d1b95157c5b000a3f7c9cbb1eecf7 120 bytes 1871 us # eosio.token <= eosio.token::issue {"to":"helloworld","quantity":"100.0000 EOS","memo":"m"} >> issueeosio balance: 100.0000 EOS # eosio.token <= eosio.token::transfer {"from":"eosio","to":"helloworld","quantity":"100.0000 EOS","memo":"m"} >> transfer from eosio to helloworld 100.0000 EOS # eosio <= eosio.token::transfer {"from":"eosio","to":"helloworld","quantity":"100.0000 EOS","memo":"m"} # helloworld <= eosio.token::transfer {"from":"eosio","to":"helloworld","quantity":"100.0000 EOS","memo":"m"} warning: transaction executed locally, but may not be confirmed by the network yet
從以上issue合約執行的日誌中可以看出賬戶之間存在多次的通知，可以用以下圖來描述整個訊息流：
eosio.token合約首先收到issue action的訊息，在eosio.token合約執行時發起了一個新的transfer action訊息給eosio.token合約，即發給自己，如下是issue action程式碼片段：
void token::issue( account_name to, asset quantity, string memo ) { print( "issue" ); auto sym = quantity.symbol; eosio_assert( sym.is_valid(), "invalid symbol name" ); auto sym_name = sym.name(); stats statstable( _self, sym_name ); auto existing = statstable.find( sym_name ); eosio_assert( existing != statstable.end(), "token with symbol does not exist, create token before issue" ); const auto& st = *existing; require_auth( st.issuer ); eosio_assert( quantity.is_valid(), "invalid quantity" ); eosio_assert( quantity.amount > 0, "must issue positive quantity" ); eosio_assert( quantity.symbol == st.supply.symbol, "symbol precision mismatch" ); eosio_assert( quantity.amount <= st.max_supply.amount - st.supply.amount, "quantity exceeds available supply"); statstable.modify( st, 0, [&]( auto& s ) { s.supply += quantity; }); add_balance( st.issuer, quantity, st, st.issuer ); if( to != st.issuer ) { SEND_INLINE_ACTION( *this, transfer, {st.issuer,N(active)}, {st.issuer, to, quantity, memo} ); } }
我可以看到在第27行，傳送了一個action，採用的是inline方式（這是eos兩種通訊方式的一種，還有一種是Deferred ），傳送到自己即eosio.token的transfer action上。
以下是eosio.token的transfer action程式碼片段：
void token::transfer( account_name from,
account_name to,
asset quantity,
string /*memo*/ )
{
print( "transfer from ", eosio::name{from}, " to ", eosio::name{to}, " ", quantity, "\n" );
eosio_assert( from != to, "cannot transfer to self" );
require_auth( from );
eosio_assert( is_account( to ), "to account does not exist");
auto sym = quantity.symbol.name();
stats statstable( _self, sym );
const auto& st = statstable.get( sym );
require_recipient( from );
require_recipient( to );
eosio_assert( quantity.is_valid(), "invalid quantity" );
eosio_assert( quantity.amount > 0, "must transfer positive quantity" );
eosio_assert( quantity.symbol == st.supply.symbol, "symbol precision mismatch" );
sub_balance( from, quantity, st );
add_balance( to, quantity, st, from );
}從第14、15行可以看到其給from和to（即eosio和helloworld）各發送一個通知訊息。
那麼該通知資訊會被eosio和helloworld兩個賬戶收到嗎？如果賬戶能收到通知訊息，通知訊息傳送給賬戶時的引數又是什麼呢？那麼賬戶怎麼處理通知訊息呢？通過以下程式設計來深入瞭解EOS的通知訊息。
二、EOS通知程式設計
我們建立三個合約，模擬兩個遊戲玩家在對戰平臺玩遊戲的場景，遊戲玩家1執行合約action後，遊戲平臺將收到玩家1請求，平臺合約將執行玩家1的請求，同時將結果分別通知給遊戲玩家1和遊戲玩家2以通知其更新資料，遊戲玩家完成更新後再通知平臺合約已完成更新。在每個合約入口我們都首先將合約引數寫入資料庫中以跟蹤整個通知流程。
遊戲平臺合約：
/**
* * The apply() methods must have C calling convention so that the blockchain can lookup and
* * call these methods.
* */
#include <eosiolib/eosio.hpp>
extern "C" {
struct gameinfo {
uint64_t id;
account_name player1;
uint64_t x1;
uint64_t y1;
account_name player2;
uint64_t x2;
uint64_t y2;
uint64_t info;
uint64_t primary_key()const { return id; }
//EOSLIB_SERIALIZE( gameinfo, (id)(player)(x)(y) )
};
typedef eosio::multi_index< N(gameinfo), gameinfo> gameinfo_index;
struct notifyinfo {
uint64_t id;
account_name receiver;
account_name code;
account_name action;
uint64_t primary_key()const { return id; }
//EOSLIB_SERIALIZE( notifyinfo, (id)(receiver)(code)(action) )
};
typedef eosio::multi_index< N(notifyinfo), notifyinfo> notifyinfo_index;
struct dummy_action {
account_name player1;
account_name player2;
uint64_t info;
};
/// The apply method implements the dispatch of events to this contract
void apply( uint64_t receiver, uint64_t code, uint64_t action ) {
//
auto _self = receiver;
{
notifyinfo_index notifyinfos(_self, _self);
auto new_notifyinfo_itr = notifyinfos.emplace(_self, [&](auto& info){
info.id = notifyinfos.available_primary_key();
info.receiver = receiver;
info.code = code;
info.action = action;
});
}
//
if(receiver == code && action == N(creategame)) {
dummy_action dummy = eosio::unpack_action_data<dummy_action>();
gameinfo_index gameinfos(_self, _self);
auto new_gameinfos_itr = gameinfos.emplace(_self, [&](auto& info){
info.id = gameinfos.available_primary_key();
info.player1 = dummy.player1;
info.x1 = 0;
info.y1 = 0;
info.player2 = dummy.player2;
info.x2 = 0;
info.y2 = 0;
info.info = dummy.info;
});
//
require_recipient(dummy.player1);
require_recipient(dummy.player2);
} else if(action == N(creategame)) {
// do something....
//
} else if(receiver == code && action == N(reset)) {
gameinfo_index gameinfos(_self, _self);
auto cur_gameinfos_itr = gameinfos.begin();
while(cur_gameinfos_itr != gameinfos.end()) {
cur_gameinfos_itr = gameinfos.erase(cur_gameinfos_itr);
}
notifyinfo_index notifyinfos(_self, _self);
auto cur_notifyinfos_itr = notifyinfos.begin();
while(cur_notifyinfos_itr != notifyinfos.end()) {
cur_notifyinfos_itr = notifyinfos.erase(cur_notifyinfos_itr);
}
}
}
} // extern "C"使用者合約程式碼：
/**
* * The apply() methods must have C calling convention so that the blockchain can lookup and
* * call these methods.
* */
#include <eosiolib/eosio.hpp>
extern "C" {
struct gameinfo {
uint64_t id;
account_name player1;
uint64_t x1;
uint64_t y1;
account_name player2;
uint64_t x2;
uint64_t y2;
uint64_t info;
uint64_t primary_key()const { return id; }
//EOSLIB_SERIALIZE( gameinfo, (id)(player)(x)(y) )
};
typedef eosio::multi_index< N(gameinfo), gameinfo> gameinfo_index;
struct notifyinfo {
uint64_t id;
account_name receiver;
account_name code;
account_name action;
uint64_t primary_key()const { return id; }
//EOSLIB_SERIALIZE( notifyinfo, (id)(receiver)(code)(action) )
};
typedef eosio::multi_index< N(notifyinfo), notifyinfo> notifyinfo_index;
struct dummy_action {
account_name player1;
account_name player2;
uint64_t info;
};
/// The apply method implements the dispatch of events to this contract
void apply( uint64_t receiver, uint64_t code, uint64_t action ) {
auto _self = receiver;
{
notifyinfo_index notifyinfos(_self, _self);
auto new_notifyinfo_itr = notifyinfos.emplace(_self, [&](auto& info){
info.id = notifyinfos.available_primary_key();
info.receiver = receiver;
info.code = code;
info.action = action;
});
}
//
if(action == N(creategame)) {
dummy_action dummy = eosio::unpack_action_data<dummy_action>();
gameinfo_index gameinfos(_self, _self);
auto new_gameinfos_itr = gameinfos.emplace(_self, [&](auto& info){
info.id = gameinfos.available_primary_key();
info.player1 = dummy.player1;
info.x1 = 0;
info.y1 = 0;
info.player2 = dummy.player2;
info.x2 = 0;
info.y2 = 0;
info.info = dummy.info;
});
//
require_recipient(code);
}
}
} // extern "C"我們將建立三個賬戶，分別運行遊戲平臺合約和兩個遊戲玩家合約：
[[email protected] test_notify_s]$ cleos --wallet-url http://localhost:9800 --url http://localhost:9800 create account eosio game.s EOS6MRyAjQq8ud7hVNYcfnVPJqcVpscN5So8BhtHuGYqET5GDW5CV EOS6MRyAjQq8ud7hVNYcfnVPJqcVpscN5So8BhtHuGYqET5GDW5CV
executed transaction: c7dc538ecf56ba612eae50eb266848e1ae6a9907cbbab62ae8ade48018c38644 200 bytes 451 us
# eosio <= eosio::newaccount {"creator":"eosio","name":"game.s","owner":{"threshold":1,"keys":[{"key":"EOS6MRyAjQq8ud7hVNYcfnVPJq...
warning: transaction executed locally, but may not be confirmed by the network yet
[[email protected] test_notify_s]$ cleos --wallet-url http://localhost:9800 --url http://localhost:9800 create account eosio game.c.1 EOS6MRyAjQq8ud7hVNYcfnVPJqcVpscN5So8BhtHuGYqET5GDW5CV EOS6MRyAjQq8ud7hVNYcfnVPJqcVpscN5So8BhtHuGYqET5GDW5CV
executed transaction: f44bdfa622c06fb170e5b564bca0fd52f69d4cfc068bad9546a820ad822d6ece 200 bytes 403 us
# eosio <= eosio::newaccount {"creator":"eosio","name":"game.c.1","owner":{"threshold":1,"keys":[{"key":"EOS6MRyAjQq8ud7hVNYcfnVP...
warning: transaction executed locally, but may not be confirmed by the network yet
[[email protected] test_notify_s]$ cleos --wallet-url http://localhost:9800 --url http://localhost:9800 create account eosio game.c.2 EOS6MRyAjQq8ud7hVNYcfnVPJqcVpscN5So8BhtHuGYqET5GDW5CV EOS6MRyAjQq8ud7hVNYcfnVPJqcVpscN5So8BhtHuGYqET5GDW5CV
executed transaction: c52b61f22f6df8b46ac7a8623e3114404cfc2dceec81363abfee094f7e450c28 200 bytes 476 us
# eosio <= eosio::newaccount {"creator":"eosio","name":"game.c.2","owner":{"threshold":1,"keys":[{"key":"EOS6MRyAjQq8ud7hVNYcfnVP...
warning: transaction executed locally, but may not be confirmed by the network yet在遊戲平臺賬戶上部署遊戲平臺合約：
[[email protected] test_notify_s]$ cleos --wallet-url http://localhost:9800 --url http://localhost:9800 set contract game.s /home/kingnet/tangy/eos/mycontracts/test_notify_s/
Reading WAST/WASM from /home/kingnet/tangy/eos/mycontracts/test_notify_s/test_notify_s.wasm...
Using already assembled WASM...
Publishing contract...
executed transaction: 240396da31c7d4d193befc07bfe4db4956a250e0760a0156adeae8d3979a7017 3848 bytes 1273 us
# eosio <= eosio::setcode {"account":"game.s","vmtype":0,"vmversion":0,"code":"0061736d0100000001560f6000006000017e60027e7e006...
# eosio <= eosio::setabi {"account":"game.s","abi":{"types":[],"structs":[{"name":"creategame","base":"","fields":[{"name":"p...
warning: transaction executed locally, but may not be confirmed by the network yet分別在兩個遊戲玩家賬戶張部署遊戲客戶端合約：
[[email protected] test_notify_g1]$ cleos --wallet-url http://localhost:9800 --url http://localhost:9800 set contract game.c.1 /home/kingnet/tangy/eos/mycontracts/test_notify_g1
Reading WAST/WASM from /home/kingnet/tangy/eos/mycontracts/test_notify_g1/test_notify_g1.wasm...
Using already assembled WASM...
Publishing contract...
executed transaction: 1c1b448f52cd4bca5c251cacf56c93ffaa49abfb2a03d7e065b18068f522cb9b 3368 bytes 1201 us
# eosio <= eosio::setcode {"account":"game.c.1","vmtype":0,"vmversion":0,"code":"0061736d0100000001560f6000006000017e60027e7e0...
# eosio <= eosio::setabi {"account":"game.c.1","abi":{"types":[],"structs":[{"name":"gameinfo","base":"","fields":[{"name":"i...
warning: transaction executed locally, but may not be confirmed by the network yet
[[email protected] test_notify_g1]$ cleos --wallet-url http://localhost:9800 --url http://localhost:9800 set contract game.c.2 /home/kingnet/tangy/eos/mycontracts/test_notify_g1
Reading WAST/WASM from /home/kingnet/tangy/eos/mycontracts/test_notify_g1/test_notify_g1.wasm...
Using already assembled WASM...
Publishing contract...
executed transaction: d3456d22a8ea450edbf1ee014833dbd00aecc6fd22883fbe500460681bae653b 3368 bytes 1000 us
# eosio <= eosio::setcode {"account":"game.c.2","vmtype":0,"vmversion":0,"code":"0061736d0100000001560f6000006000017e60027e7e0...
# eosio <= eosio::setabi {"account":"game.c.2","abi":{"types":[],"structs":[{"name":"gameinfo","base":"","fields":[{"name":"i...
warning: transaction executed locally, but may not be confirmed by the network yet我們接下來執行一次遊戲建立操作：
[[email protected] test_notify_g1]$ cleos --wallet-url http://localhost:9800 --url http://localhost:9800 push action game.s creategame '{"player1":"game.c.1","player2":"game.c.2","info":1}' -p game.s
executed transaction: 5048ffd4082325ca9f72ad9ac2c6fd7f3a6befbf87766343b064b57b6fac2fd5 120 bytes 4871 us
# game.s <= game.s::creategame {"player1":"game.c.1","player2":"game.c.2","info":1}
# game.c.1 <= game.s::creategame {"player1":"game.c.1","player2":"game.c.2","info":1}
# game.c.2 <= game.s::creategame {"player1":"game.c.1","player2":"game.c.2","info":1}
warning: transaction executed locally, but may not be confirmed by the network yet從以上執行的結果可以看到，creategame這個action首先發給了game.s，這是我們發起的，這個action訊息又轉發給了game.c.1和game.c.2，這是我們在creategame這個action中傳送的通知訊息。
我們可以繼續看看每個合約賬戶上收到的action通知訊息：
[[email protected] test_notify_g1]$ cleos --wallet-url http://localhost:9800 --url http://localhost:9800 get actions game.s
# seq when contract::action => receiver trx id... args
================================================================================================================
# 0 2018-05-17T09:56:55.500 eosio::setcode => eosio 240396da... {"account":"game.s","vmtype":0,"vmversion":0,"code":"0061736...
# 1 2018-05-17T09:56:55.500 eosio::setabi => eosio 240396da... {"account":"game.s","abi":{"types":[],"structs":[{"name":"cr...
# 2 2018-05-17T10:03:41.000 game.s::creategame => game.s 5048ffd4... {"player1":"game.c.1","player2":"game.c.2","info":1}...
# 3 2018-05-17T10:03:41.000 game.s::creategame => game.c.1 5048ffd4... {"player1":"game.c.1","player2":"game.c.2","info":1}...
# 4 2018-05-17T10:03:41.000 game.s::creategame => game.c.2 5048ffd4... {"player1":"game.c.1","player2":"game.c.2","info":1}...
[[email protected] test_notify_g1]$ cleos --wallet-url http://localhost:9800 --url http://localhost:9800 get actions game.c.1
# seq when contract::action => receiver trx id... args
================================================================================================================
# 0 2018-05-17T09:59:50.500 eosio::setcode => eosio 1c1b448f... {"account":"game.c.1","vmtype":0,"vmversion":0,"code":"00617...
# 1 2018-05-17T09:59:50.500 eosio::setabi => eosio 1c1b448f... {"account":"game.c.1","abi":{"types":[],"structs":[{"name":"...
# 2 2018-05-17T10:03:41.000 game.s::creategame => game.c.1 5048ffd4... {"player1":"game.c.1","player2":"game.c.2","info":1}...
[[email protected] test_notify_g1]$ cleos --wallet-url http://localhost:9800 --url http://localhost:9800 get actions game.c.2
# seq when contract::action => receiver trx id... args
================================================================================================================
# 0 2018-05-17T10:00:05.000 eosio::setcode => eosio d3456d22... {"account":"game.c.2","vmtype":0,"vmversion":0,"code":"00617...
# 1 2018-05-17T10:00:05.000 eosio::setabi => eosio d3456d22... {"account":"game.c.2","abi":{"types":[],"structs":[{"name":"...
# 2 2018-05-17T10:03:41.000 game.s::creategame => game.c.2 5048ffd4... {"player1":"game.c.1","player2":"game.c.2","info":1}...我們在入口跟蹤了通知訊息並寫入了資料庫，如果檢視資料庫可以得出一致的訊息，同時我們在收到通知後也正確解析了請求的引數，檢視資料庫可以得到資料庫中三個表的內容完全一致。
我們現在可以回答上面關於eos通知訊息的問題了：
1. 向一個合約傳送的通知會被這個合約接收並在應用層處理嗎？
一個合約向另一個合約發出的通知訊息會被另一個合約收到，不管有沒有ABI定義介面，都會在應用層收到。
2. 引數是什麼呢？
一個合約向另一個合約傳送的通知訊息就是本合約收到的訊息，包括code、action和引數
3. 賬戶如何處理該通知訊息呢？
和處理正常的訊息一樣，根據code、action和引數進行邏輯處理
三、拒絕接受轉賬交易及DoS
我們目前已經清楚了eos合約通知機制，同時也知道了給一個賬戶轉賬交易會給該賬戶訊息通知，其實我們還有一個疑問，如果賬戶合約處理通知訊息失敗了，那麼這個交易狀態會回滾麼？
我們現在開發一個合約，合約處理交易的transfer事件通知，在通知處理程式中，我們丟擲一個異常，使得該通知處理失敗。同時我們照樣在入口記錄所有收到的action。
我們會將該合約部署在賬戶test.notify上。我們轉賬給test.notify，那麼eosio.token合約的transfer action會被執行，在eosio.token合約執行轉賬時，給向目標賬戶test.notify傳送事件通知，但事件通知處理失敗了，到底這個轉賬交易會回滾麼？
拒絕接收轉賬合約：
/**
* * The apply() methods must have C calling convention so that the blockchain can lookup and
* * call these methods.
* */
#include <eosiolib/eosio.hpp>
extern "C" {
struct notifyinfo {
uint64_t id;
account_name receiver;
account_name code;
account_name action;
uint64_t primary_key()const { return id; }
//EOSLIB_SERIALIZE( notifyinfo, (id)(receiver)(code)(action) )
};
typedef eosio::multi_index< N(notifyinfo), notifyinfo> notifyinfo_index;
/// The apply method implements the dispatch of events to this contract
void apply( uint64_t receiver, uint64_t code, uint64_t action ) {
auto _self = receiver;
notifyinfo_index notifyinfos(_self, _self);
auto new_notifyinfo_itr = notifyinfos.emplace(_self, [&](auto& info){
info.id = notifyinfos.available_primary_key();
info.receiver = receiver;
info.code = code;
info.action = action;
});
if(action == N(transfer)) {
eosio_assert("reject transfer!");
}
if(receiver == code && action == N(reset)) {
auto cur_notify_itr = notifyinfos.begin();
while(cur_notify_itr != notifyinfos.end()) {
cur_notify_itr = notifyinfos.erase(cur_notify_itr);
}
}
}
} // extern "C"我們執行轉賬操作並檢查結果：
[[email protected] test_notify]$ cleos --wallet-url http://localhost:9800 --url http://localhost:9800 create account eosio test.notify EOS6MRyAjQq8ud7hVNYcfnVPJqcVpscN5So8BhtHuGYqET5GDW5CV EOS6MRyAjQq8ud7hVNYcfnVPJqcVpscN5So8BhtHuGYqET5GDW5CV
executed transaction: 4576dec103dab2abfbfff775f8208c8fff46996cc9d3717d576574544f381d72 200 bytes 476 us
# eosio <= eosio::newaccount {"creator":"eosio","name":"test.notify","owner":{"threshold":1,"keys":[{"key":"EOS6MRyAjQq8ud7hVNYcf...
warning: transaction executed locally, but may not be confirmed by the network yet
[[email protected] test_notify]$ cleos --wallet-url http://localhost:9800 --url http://localhost:9800 set contract test.notify /home/kingnet/tangy/eos/mycontracts/test_notify
Reading WAST/WASM from /home/kingnet/tangy/eos/mycontracts/test_notify/test_notify.wasm...
Using already assembled WASM...
Publishing contract...
executed transaction: 7bd410a214ed2700eea182bebcdf94304ee49911e8f0c53dcfa58068150067d4 5048 bytes 1200 us
# eosio <= eosio::setcode {"account":"test.notify","vmtype":0,"vmversion":0,"code":"0061736d0100000001520e6000006000017e60027e...
# eosio <= eosio::setabi {"account":"test.notify","abi":{"types":[],"structs":[{"name":"hi","base":"","fields":[{"name":"user...
warning: transaction executed locally, but may not be confirmed by the network yet
[[email protected] test_notify]$ cleos --wallet-url http://localhost:9800 --url http://localhost:9800 push action eosio.token issue '{"to":"test.notify","quantity":"1001.0000 EOS", "memo":"m"}' -p eosio
executed transaction: 995b01cd4206ee19894400f09d2d0bbbba1b5d9e74010438447387e4be1f226e 120 bytes 10667 us
# eosio.token <= eosio.token::issue {"to":"test.notify","quantity":"1001.0000 EOS","memo":"m"}
>> issueeosio balance: 1001.0000 EOS
# eosio.token <= eosio.token::transfer {"from":"eosio","to":"test.notify","quantity":"1001.0000 EOS","memo":"m"}
>> transfer from eosio to test.notify 1.0000 EOS
# eosio <= eosio.token::transfer {"from":"eosio","to":"test.notify","quantity":"1001.0000 EOS","memo":"m"}
# test.notify <= eosio.token::transfer {"from":"eosio","to":"test.notify","quantity":"1001.0000 EOS","memo":"m"}
warning: transaction executed locally, but may not be confirmed by the network yet
[[email protected] test_notify]$ cleos --wallet-url http://localhost:9800 --url http://localhost:9800 get currency balance eosio.token test.notify
1001.0000 EOS
[[email protected] test_notify]$ cleos --wallet-url http://localhost:9800 --url http://localhost:9800 push action eosio.token transfer '{"from":"alice","to":"test.notify","quantity":"1.0000 EOS","memo":"m"}' -p alice
1566956ms thread-0 main.cpp:2316 main ] Failed with error: Assert Exception (10)
condition: assertion failed: reject transfer!
[[email protected] test_notify]$ cleos --wallet-url http://localhost:9800 --url http://localhost:9800 get currency balance eosio.token test.notify
1001.0000 EOS我們可以看到轉賬失敗了，這個失敗是由轉賬的接收方處理轉賬通知事件時給出錯誤應答引起的，這樣就可以讓一個賬戶拒絕接收轉賬。
這個例子演示了時事件通知處理失敗會導致整個交易回滾，如果一個transaction中包含了許多的action，但只要某一個賬戶處理action的通知事件失敗將引起整個交易回滾，這樣某一個賬戶可以進行DoS（拒絕服務攻擊）。
下一章介紹EOS合約中通訊程式設計。
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