2.4.3 Handling Errors and Retrying Transactions

This program demonstrates handling errors and retrying failed transactions using the NDB API.

The source code for this example can be found in storage/ndb/ndbapi-examples/ndbapi_retries/ndbapi_retries.cpp in the MySQL Cluster source tree.

There are many ways to program using the NDB API. In this example, we perform two inserts in the same transaction using NdbTransaction::execute(NoCommit).

In NDB API applications, there are two types of failures to be taken into account:

  1. Transaction failures: If nonpermanent, these can be handled by re-executing the transaction.

  2. Application errors: These are indicated by APIERROR; they must be handled by the application programmer.

// 
//  ndbapi_retries.cpp: Error handling and transaction retries
//
//  There are many ways to program using the NDB API.  In this example
//  we execute two inserts in the same transaction using 
//  NdbConnection::execute(NoCommit).
// 
//  Transaction failing is handled by re-executing the transaction
//  in case of non-permanent transaction errors.
//  Application errors (i.e. errors at points marked with APIERROR) 
//  should be handled by the application programmer.

#include <mysql.h>
#include <mysqld_error.h>
#include <NdbApi.hpp>

// Used for cout
#include <iostream>  

// Used for sleep (use your own version of sleep)
#include <unistd.h>
#define TIME_TO_SLEEP_BETWEEN_TRANSACTION_RETRIES 1

#define PRINT_ERROR(code,msg) \
  std::cout << "Error in " << __FILE__ << ", line: " << __LINE__ \
            << ", code: " << code \
            << ", msg: " << msg << "." << std::endl
#define MYSQLERROR(mysql) { \
  PRINT_ERROR(mysql_errno(&mysql),mysql_error(&mysql)); \
  exit(-1); }

//
//  APIERROR prints an NdbError object
//
#define APIERROR(error) \
  { std::cout << "API ERROR: " << error.code << " " << error.message \
              << std::endl \
              << "           " << "Status: " << error.status \
              << ", Classification: " << error.classification << std::endl\
              << "           " << "File: " << __FILE__ \
              << " (Line: " << __LINE__ << ")" << std::endl \
              ; \
  }

//
//  TRANSERROR prints all error info regarding an NdbTransaction
//
#define TRANSERROR(ndbTransaction) \
  { NdbError error = ndbTransaction->getNdbError(); \
    std::cout << "TRANS ERROR: " << error.code << " " << error.message \
              << std::endl \
              << "           " << "Status: " << error.status \
              << ", Classification: " << error.classification << std::endl \
              << "           " << "File: " << __FILE__ \
              << " (Line: " << __LINE__ << ")" << std::endl \
              ; \
    printTransactionError(ndbTransaction); \
  }

void printTransactionError(NdbTransaction *ndbTransaction) {
  const NdbOperation *ndbOp = NULL;
  int i=0;

  /****************************************************************
   * Print NdbError object of every operations in the transaction *
   ****************************************************************/
  while ((ndbOp = ndbTransaction->getNextCompletedOperation(ndbOp)) != NULL) {
    NdbError error = ndbOp->getNdbError();
    std::cout << "           OPERATION " << i+1 << ": " 
	      << error.code << " " << error.message << std::endl
	      << "           Status: " << error.status 
	      << ", Classification: " << error.classification << std::endl;
    i++;
  }
}


//
//  Example insert
//  @param myNdb          Ndb object representing NDB Cluster
//  @param myTransaction  NdbTransaction used for transaction
//  @param myTable        Table to insert into
//  @param error          NdbError object returned in case of errors
//  @return -1 in case of failures, 0 otherwise
//
int insert(int transactionId, NdbTransaction* myTransaction,
	   const NdbDictionary::Table *myTable) {
  NdbOperation	 *myOperation;          // For other operations

  myOperation = myTransaction->getNdbOperation(myTable);
  if (myOperation == NULL) return -1;
  
  if (myOperation->insertTuple() ||  
      myOperation->equal("ATTR1", transactionId) ||
      myOperation->setValue("ATTR2", transactionId)) {
    APIERROR(myOperation->getNdbError());
    exit(-1);
  }

  return myTransaction->execute(NdbTransaction::NoCommit);
}


//
//  Execute function which re-executes (tries 10 times) the transaction 
//  if there are temporary errors (e.g. the NDB Cluster is overloaded).
//  @return -1 failure, 1 success
//
int executeInsertTransaction(int transactionId, Ndb* myNdb,
			     const NdbDictionary::Table *myTable) {
  int result = 0;                       // No result yet
  int noOfRetriesLeft = 10;
  NdbTransaction	 *myTransaction;         // For other transactions
  NdbError ndberror;
  
  while (noOfRetriesLeft > 0 && !result) {
    
    /*********************************
     * Start and execute transaction *
     *********************************/
    myTransaction = myNdb->startTransaction();
    if (myTransaction == NULL) {
      APIERROR(myNdb->getNdbError());
      ndberror = myNdb->getNdbError();
      result = -1;  // Failure
    } else if (insert(transactionId, myTransaction, myTable) || 
	       insert(10000+transactionId, myTransaction, myTable) ||
	       myTransaction->execute(NdbTransaction::Commit)) {
      TRANSERROR(myTransaction);
      ndberror = myTransaction->getNdbError();
      result = -1;  // Failure
    } else {
      result = 1;   // Success
    }

    /**********************************
     * If failure, then analyze error *
     **********************************/
    if (result == -1) {                 
      switch (ndberror.status) {
      case NdbError::Success:
	break;
      case NdbError::TemporaryError:
	std::cout << "Retrying transaction..." << std::endl;
	sleep(TIME_TO_SLEEP_BETWEEN_TRANSACTION_RETRIES);
	--noOfRetriesLeft;
	result = 0;   // No completed transaction yet
	break;
	
      case NdbError::UnknownResult:
      case NdbError::PermanentError:
	std::cout << "No retry of transaction..." << std::endl;
	result = -1;  // Permanent failure
	break;
      }
    }

    /*********************
     * Close transaction *
     *********************/
    if (myTransaction != NULL) {
      myNdb->closeTransaction(myTransaction);
    }
  }

  if (result != 1) exit(-1);
  return result;
}

/*********************************************************
 * Create a table named api_retries if it does not exist *
 *********************************************************/
static void create_table(MYSQL &mysql)
{
  while(mysql_query(&mysql, 
		  "CREATE TABLE "
		  "  api_retries"
		  "    (ATTR1 INT UNSIGNED NOT NULL PRIMARY KEY,"
		  "     ATTR2 INT UNSIGNED NOT NULL)"
		  "  ENGINE=NDB"))
  {
    if (mysql_errno(&mysql) == ER_TABLE_EXISTS_ERROR)
    {
      std::cout << "MySQL Cluster already has example table: api_scan. "
	     << "Dropping it..." << std::endl; 
        mysql_query(&mysql, "DROP TABLE api_retries");
    }
    else MYSQLERROR(mysql);
  }
}


int main(int argc, char** argv)
{
  if (argc != 3)
  {
    std::cout << "Arguments are <socket mysqld> <connect_string cluster>.\n";
    exit(-1);
  }
  char * mysqld_sock  = argv[1];
  const char *connection_string = argv[2];
  ndb_init();

  Ndb_cluster_connection *cluster_connection=
    new Ndb_cluster_connection(connection_string); // Object representing the cluster

  int r= cluster_connection->connect(5 /* retries               */,
				     3 /* delay between retries */,
				     1 /* verbose               */);
  if (r > 0)
  {
    std::cout
      << "Cluster connect failed, possibly resolved with more retries.\n";
    exit(-1);
  }
  else if (r < 0)
  {
    std::cout
      << "Cluster connect failed.\n";
    exit(-1);
  }
					   
  if (cluster_connection->wait_until_ready(30,30))
  {
    std::cout << "Cluster was not ready within 30 secs." << std::endl;
    exit(-1);
  }
  // connect to mysql server
  MYSQL mysql;
  if ( !mysql_init(&mysql) ) {
    std::cout << "mysql_init failed\n";
    exit(-1);
  }
  if ( !mysql_real_connect(&mysql, "localhost", "root", "", "",
			   0, mysqld_sock, 0) )
    MYSQLERROR(mysql);
  
  /********************************************
   * Connect to database via mysql-c          *
   ********************************************/
  mysql_query(&mysql, "CREATE DATABASE ndb_examples");
  if (mysql_query(&mysql, "USE ndb_examples") != 0) MYSQLERROR(mysql);
  create_table(mysql);

  Ndb* myNdb= new Ndb( cluster_connection,
		       "ndb_examples" );  // Object representing the database
  
  if (myNdb->init() == -1) {
    APIERROR(myNdb->getNdbError());
    exit(-1);
  }

  const NdbDictionary::Dictionary* myDict= myNdb->getDictionary();
  const NdbDictionary::Table *myTable= myDict->getTable("api_retries");
  if (myTable == NULL)
  {
    APIERROR(myDict->getNdbError());
    return -1;
  }
  /************************************
   * Execute some insert transactions *
   ************************************/
   
  std::cout << "Ready to insert rows.  You will see notices for temporary "
    "errors, permenant errors, and retries. \n";
  for (int i = 10000; i < 20000; i++) {
    executeInsertTransaction(i, myNdb, myTable);
  }
  std::cout << "Done.\n";
  
  delete myNdb;
  delete cluster_connection;
    
  ndb_end(0);
  return 0;
}