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1 | NAME |
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2 | BDB - Asynchronous Berkeley DB access |
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3 | |
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4 | SYNOPSIS |
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5 | use BDB; |
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6 | |
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7 | DESCRIPTION |
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8 | See the BerkeleyDB documentation |
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9 | (<http://www.oracle.com/technology/documentation/berkeley-db/db/index.ht |
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10 | ml>). The BDB API is very similar to the C API (the translation has been |
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11 | very faithful). |
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12 | |
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13 | See also the example sections in the document below and possibly the eg/ |
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14 | subdirectory of the BDB distribution. Last not least see the IO::AIO |
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15 | documentation, as that module uses almost the same asynchronous request |
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16 | model as this module. |
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17 | |
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18 | I know this is woefully inadequate documentation. Send a patch! |
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19 | |
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20 | REQUEST ANATOMY AND LIFETIME |
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21 | Every request method creates a request. which is a C data structure not |
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22 | directly visible to Perl. |
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23 | |
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24 | During their existance, bdb requests travel through the following |
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25 | states, in order: |
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26 | |
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27 | ready |
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28 | Immediately after a request is created it is put into the ready |
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29 | state, waiting for a thread to execute it. |
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30 | |
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31 | execute |
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32 | A thread has accepted the request for processing and is currently |
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33 | executing it (e.g. blocking in read). |
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34 | |
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35 | pending |
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36 | The request has been executed and is waiting for result processing. |
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37 | |
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38 | While request submission and execution is fully asynchronous, result |
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39 | processing is not and relies on the perl interpreter calling |
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40 | "poll_cb" (or another function with the same effect). |
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41 | |
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42 | result |
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43 | The request results are processed synchronously by "poll_cb". |
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44 | |
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45 | The "poll_cb" function will process all outstanding aio requests by |
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46 | calling their callbacks, freeing memory associated with them and |
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47 | managing any groups they are contained in. |
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48 | |
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49 | done |
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50 | Request has reached the end of its lifetime and holds no resources |
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51 | anymore (except possibly for the Perl object, but its connection to |
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52 | the actual aio request is severed and calling its methods will |
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53 | either do nothing or result in a runtime error). |
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54 | |
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55 | BERKELEYDB FUNCTIONS |
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56 | All of these are functions. The create functions simply return a new |
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57 | object and never block. All the remaining functions all take an optional |
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58 | callback as last argument. If it is missing, then the fucntion will be |
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59 | executed synchronously. |
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60 | |
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61 | BDB functions that cannot block (mostly functions that manipulate |
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62 | settings) are method calls on the relevant objects, so the rule of thumb |
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63 | is: if its a method, its not blocking, if its a function, it takes a |
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64 | callback as last argument. |
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65 | |
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66 | In the following, $int signifies an integer return value, "octetstring" |
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67 | is a "binary string" (i.e. a perl string with no character indices |
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68 | >255), "U32" is an unsigned 32 bit integer, "int" is some integer, "NV" |
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69 | is a floating point value. |
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70 | |
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71 | The "SV *" types are generic perl scalars (for input and output of data |
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72 | values), and the "SV *callback" is the optional callback function to |
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73 | call when the request is completed. |
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74 | |
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75 | The various "DB_ENV" etc. arguments are handles return by |
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76 | "db_env_create", "db_create", "txn_begin" and so on. If they have an |
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77 | appended "_ornull" this means they are optional and you can pass "undef" |
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78 | for them, resulting a NULL pointer on the C level. |
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79 | |
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80 | BDB functions |
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81 | Functions in the BDB namespace, exported by default: |
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82 | |
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83 | $env = db_env_create (U32 env_flags = 0) |
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84 | flags: RPCCLIENT |
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85 | |
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86 | db_env_open (DB_ENV *env, octetstring db_home, U32 open_flags, int mode, SV *callback = &PL_sv_undef) |
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87 | open_flags: INIT_CDB INIT_LOCK INIT_LOG INIT_MPOOL INIT_REP INIT_TXN RECOVER RECOVER_FATAL USE_ENVIRON USE_ENVIRON_ROOT CREATE LOCKDOWN PRIVATE REGISTER SYSTEM_MEM |
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88 | db_env_close (DB_ENV *env, U32 flags = 0, SV *callback = &PL_sv_undef) |
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89 | db_env_txn_checkpoint (DB_ENV *env, U32 kbyte = 0, U32 min = 0, U32 flags = 0, SV *callback = &PL_sv_undef) |
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90 | flags: FORCE |
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91 | db_env_lock_detect (DB_ENV *env, U32 flags = 0, U32 atype = DB_LOCK_DEFAULT, SV *dummy = 0, SV *callback = &PL_sv_undef) |
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92 | atype: LOCK_DEFAULT LOCK_EXPIRE LOCK_MAXLOCKS LOCK_MAXWRITE LOCK_MINLOCKS LOCK_MINWRITE LOCK_OLDEST LOCK_RANDOM LOCK_YOUNGEST |
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93 | db_env_memp_sync (DB_ENV *env, SV *dummy = 0, SV *callback = &PL_sv_undef) |
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94 | db_env_memp_trickle (DB_ENV *env, int percent, SV *dummy = 0, SV *callback = &PL_sv_undef) |
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95 | |
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96 | $db = db_create (DB_ENV *env = 0, U32 flags = 0) |
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97 | flags: XA_CREATE |
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98 | |
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99 | db_open (DB *db, DB_TXN_ornull *txnid, octetstring file, octetstring database, int type, U32 flags, int mode, SV *callback = &PL_sv_undef) |
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100 | flags: AUTO_COMMIT CREATE EXCL MULTIVERSION NOMMAP RDONLY READ_UNCOMMITTED THREAD TRUNCATE |
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101 | db_close (DB *db, U32 flags = 0, SV *callback = &PL_sv_undef) |
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102 | flags: DB_NOSYNC |
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103 | db_compact (DB *db, DB_TXN_ornull *txn = 0, SV *start = 0, SV *stop = 0, SV *unused1 = 0, U32 flags = DB_FREE_SPACE, SV *unused2 = 0, SV *callback = &PL_sv_undef) |
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104 | flags: FREELIST_ONLY FREE_SPACE |
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105 | db_sync (DB *db, U32 flags = 0, SV *callback = &PL_sv_undef) |
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106 | db_key_range (DB *db, DB_TXN_ornull *txn, SV *key, SV *key_range, U32 flags = 0, SV *callback = &PL_sv_undef) |
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107 | db_put (DB *db, DB_TXN_ornull *txn, SV *key, SV *data, U32 flags = 0, SV *callback = &PL_sv_undef) |
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108 | flags: APPEND NODUPDATA NOOVERWRITE |
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109 | db_get (DB *db, DB_TXN_ornull *txn, SV *key, SV *data, U32 flags = 0, SV *callback = &PL_sv_undef) |
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110 | flags: CONSUME CONSUME_WAIT GET_BOTH SET_RECNO MULTIPLE READ_COMMITTED READ_UNCOMMITTED RMW |
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111 | db_pget (DB *db, DB_TXN_ornull *txn, SV *key, SV *pkey, SV *data, U32 flags = 0, SV *callback = &PL_sv_undef) |
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112 | flags: CONSUME CONSUME_WAIT GET_BOTH SET_RECNO MULTIPLE READ_COMMITTED READ_UNCOMMITTED RMW |
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113 | db_del (DB *db, DB_TXN_ornull *txn, SV *key, U32 flags = 0, SV *callback = &PL_sv_undef) |
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114 | db_txn_commit (DB_TXN *txn, U32 flags = 0, SV *callback = &PL_sv_undef) |
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115 | flags: TXN_NOSYNC TXN_SYNC |
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116 | db_txn_abort (DB_TXN *txn, SV *callback = &PL_sv_undef) |
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117 | |
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118 | db_c_close (DBC *dbc, SV *callback = &PL_sv_undef) |
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119 | db_c_count (DBC *dbc, SV *count, U32 flags = 0, SV *callback = &PL_sv_undef) |
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120 | db_c_put (DBC *dbc, SV *key, SV *data, U32 flags = 0, SV *callback = &PL_sv_undef) |
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121 | flags: AFTER BEFORE CURRENT KEYFIRST KEYLAST NODUPDATA |
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122 | db_c_get (DBC *dbc, SV *key, SV *data, U32 flags = 0, SV *callback = &PL_sv_undef) |
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123 | flags: CURRENT FIRST GET_BOTH GET_BOTH_RANGE GET_RECNO JOIN_ITEM LAST NEXT NEXT_DUP NEXT_NODUP PREV PREV_DUP PREV_NODUP SET SET_RANGE SET_RECNO READ_UNCOMMITTED MULTIPLE MULTIPLE_KEY RMW |
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124 | db_c_pget (DBC *dbc, SV *key, SV *pkey, SV *data, U32 flags = 0, SV *callback = &PL_sv_undef) |
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125 | db_c_del (DBC *dbc, U32 flags = 0, SV *callback = &PL_sv_undef) |
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126 | |
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127 | db_sequence_open (DB_SEQUENCE *seq, DB_TXN_ornull *txnid, SV *key, U32 flags = 0, SV *callback = &PL_sv_undef) |
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128 | flags: CREATE EXCL |
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129 | db_sequence_close (DB_SEQUENCE *seq, U32 flags = 0, SV *callback = &PL_sv_undef) |
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130 | db_sequence_get (DB_SEQUENCE *seq, DB_TXN_ornull *txnid, int delta, SV *seq_value, U32 flags = DB_TXN_NOSYNC, SV *callback = &PL_sv_undef) |
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131 | flags: TXN_NOSYNC |
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132 | db_sequence_remove (DB_SEQUENCE *seq, DB_TXN_ornull *txnid = 0, U32 flags = 0, SV *callback = &PL_sv_undef) |
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133 | flags: TXN_NOSYNC |
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134 | |
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135 | db_txn_finish (DB_TXN *txn, U32 flags = 0, SV *callback = &PL_sv_undef) |
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136 | This is not actually a Berkeley DB function but a BDB module extension. |
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137 | The background for this exytension is: It is very annoying to have to |
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138 | check every single BDB function for error returns and provide a codepath |
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139 | out of your transaction. While the BDB module still makes this possible, |
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140 | it contains the following extensions: |
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141 | |
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142 | When a transaction-protected function returns any operating system error |
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143 | (errno > 0), BDB will set the "TXN_DEADLOCK" flag on the transaction. |
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144 | This flag is also set by Berkeley DB functions themselves when an |
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145 | operation fails with LOCK_DEADLOCK, and it causes all further operations |
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146 | on that transaction (including "db_txn_commit") to fail. |
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147 | |
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148 | The "db_txn_finish" request will look at this flag, and, if it is set, |
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149 | will automatically call "db_txn_abort" (setting errno to "LOCK_DEADLOCK" |
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150 | if it isn't set to something else yet). If it isn't set, it will call |
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151 | "db_txn_commit" and return the error normally. |
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152 | |
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153 | How to use this? Easy: just write your transaction normally: |
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154 | |
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155 | my $txn = $db_env->txn_begin; |
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156 | db_get $db, $txn, "key", my $data; |
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157 | db_put $db, $txn, "key", $data + 1 unless $! == BDB::NOTFOUND; |
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158 | db_txn_finish $txn; |
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159 | die "transaction failed" if $!; |
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160 | |
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161 | That is, handle only the expected errors. If something unexpected |
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162 | happens (EIO, LOCK_NOTGRANTED or a deadlock in either db_get or db_put), |
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163 | then the remaining requests (db_put in this case) will simply be skipped |
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164 | (they will fail with LOCK_DEADLOCK) and the transaction will be aborted. |
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165 | |
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166 | You can use the "$txn->failed" method to check wether a transaction has |
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167 | failed in this way and abort further processing (excluding |
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168 | "db_txn_finish"). |
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169 | |
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170 | DB_ENV/database environment methods |
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171 | Methods available on DB_ENV/$env handles: |
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172 | |
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173 | DESTROY (DB_ENV_ornull *env) |
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174 | CODE: |
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175 | if (env) |
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176 | env->close (env, 0); |
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177 | |
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178 | $int = $env->set_data_dir (const char *dir) |
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179 | $int = $env->set_tmp_dir (const char *dir) |
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180 | $int = $env->set_lg_dir (const char *dir) |
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181 | $int = $env->set_shm_key (long shm_key) |
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182 | $int = $env->set_cachesize (U32 gbytes, U32 bytes, int ncache = 0) |
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183 | $int = $env->set_flags (U32 flags, int onoff) |
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184 | $env->set_errfile (FILE *errfile = 0) |
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185 | $env->set_msgfile (FILE *msgfile = 0) |
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186 | $int = $env->set_verbose (U32 which, int onoff = 1) |
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187 | $int = $env->set_encrypt (const char *password, U32 flags = 0) |
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188 | $int = $env->set_timeout (NV timeout_seconds, U32 flags = SET_TXN_TIMEOUT) |
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189 | $int = $env->set_mp_max_openfd (int maxopenfd); |
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190 | $int = $env->set_mp_max_write (int maxwrite, int maxwrite_sleep); |
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191 | $int = $env->set_mp_mmapsize (int mmapsize_mb) |
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192 | $int = $env->set_lk_detect (U32 detect = DB_LOCK_DEFAULT) |
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193 | $int = $env->set_lk_max_lockers (U32 max) |
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194 | $int = $env->set_lk_max_locks (U32 max) |
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195 | $int = $env->set_lk_max_objects (U32 max) |
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196 | $int = $env->set_lg_bsize (U32 max) |
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197 | $int = $env->set_lg_max (U32 max) |
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198 | |
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199 | $txn = $env->txn_begin (DB_TXN_ornull *parent = 0, U32 flags = 0) |
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200 | flags: READ_COMMITTED READ_UNCOMMITTED TXN_NOSYNC TXN_NOWAIT TXN_SNAPSHOT TXN_SYNC TXN_WAIT TXN_WRITE_NOSYNC |
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201 | |
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202 | Example: |
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203 | use AnyEvent; |
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204 | use BDB; |
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205 | |
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206 | our $FH; open $FH, "<&=" . BDB::poll_fileno; |
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207 | our $WATCHER = AnyEvent->io (fh => $FH, poll => 'r', cb => \&BDB::poll_cb); |
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208 | |
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209 | BDB::min_parallel 8; |
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210 | |
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211 | my $env = db_env_create; |
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212 | |
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213 | mkdir "bdtest", 0700; |
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214 | db_env_open |
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215 | $env, |
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216 | "bdtest", |
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217 | BDB::INIT_LOCK | BDB::INIT_LOG | BDB::INIT_MPOOL | BDB::INIT_TXN | BDB::RECOVER | BDB::USE_ENVIRON | BDB::CREATE, |
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218 | 0600; |
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219 | |
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220 | $env->set_flags (BDB::AUTO_COMMIT | BDB::TXN_NOSYNC, 1); |
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221 | |
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222 | DB/database methods |
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223 | Methods available on DB/$db handles: |
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224 | |
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225 | DESTROY (DB_ornull *db) |
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226 | CODE: |
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227 | if (db) |
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228 | { |
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229 | SV *env = (SV *)db->app_private; |
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230 | db->close (db, 0); |
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231 | SvREFCNT_dec (env); |
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232 | } |
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233 | |
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234 | $int = $db->set_cachesize (U32 gbytes, U32 bytes, int ncache = 0) |
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235 | $int = $db->set_flags (U32 flags) |
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236 | flags: CHKSUM ENCRYPT TXN_NOT_DURABLE |
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237 | Btree: DUP DUPSORT RECNUM REVSPLITOFF |
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238 | Hash: DUP DUPSORT |
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239 | Queue: INORDER |
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240 | Recno: RENUMBER SNAPSHOT |
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241 | |
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242 | $int = $db->set_encrypt (const char *password, U32 flags) |
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243 | $int = $db->set_lorder (int lorder) |
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244 | $int = $db->set_bt_minkey (U32 minkey) |
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245 | $int = $db->set_re_delim (int delim) |
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246 | $int = $db->set_re_pad (int re_pad) |
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247 | $int = $db->set_re_source (char *source) |
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248 | $int = $db->set_re_len (U32 re_len) |
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249 | $int = $db->set_h_ffactor (U32 h_ffactor) |
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250 | $int = $db->set_h_nelem (U32 h_nelem) |
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251 | $int = $db->set_q_extentsize (U32 extentsize) |
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252 | |
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253 | $dbc = $db->cursor (DB_TXN_ornull *txn = 0, U32 flags = 0) |
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254 | flags: READ_COMMITTED READ_UNCOMMITTED WRITECURSOR TXN_SNAPSHOT |
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255 | $seq = $db->sequence (U32 flags = 0) |
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256 | |
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257 | Example: |
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258 | my $db = db_create $env; |
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259 | db_open $db, undef, "table", undef, BDB::BTREE, BDB::AUTO_COMMIT | BDB::CREATE | BDB::READ_UNCOMMITTED, 0600; |
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260 | |
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261 | for (1..1000) { |
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262 | db_put $db, undef, "key $_", "data $_"; |
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263 | |
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264 | db_key_range $db, undef, "key $_", my $keyrange; |
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265 | my ($lt, $eq, $gt) = @$keyrange; |
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266 | } |
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267 | |
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268 | db_del $db, undef, "key $_" for 1..1000; |
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269 | |
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270 | db_sync $db; |
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271 | |
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272 | DB_TXN/transaction methods |
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273 | Methods available on DB_TXN/$txn handles: |
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274 | |
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275 | DESTROY (DB_TXN_ornull *txn) |
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276 | CODE: |
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277 | if (txn) |
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278 | txn->abort (txn); |
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279 | |
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280 | $int = $txn->set_timeout (NV timeout_seconds, U32 flags = SET_TXN_TIMEOUT) |
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281 | flags: SET_LOCK_TIMEOUT SET_TXN_TIMEOUT |
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282 | |
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283 | $bool = $txn->failed |
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284 | # see db_txn_finish documentation, above |
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285 | |
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286 | DBC/cursor methods |
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287 | Methods available on DBC/$dbc handles: |
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288 | |
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289 | DESTROY (DBC_ornull *dbc) |
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290 | CODE: |
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291 | if (dbc) |
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292 | dbc->c_close (dbc); |
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293 | |
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294 | Example: |
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295 | my $c = $db->cursor; |
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296 | |
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297 | for (;;) { |
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298 | db_c_get $c, my $key, my $data, BDB::NEXT; |
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299 | warn "<$!,$key,$data>"; |
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300 | last if $!; |
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301 | } |
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302 | |
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303 | db_c_close $c; |
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304 | |
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305 | DB_SEQUENCE/sequence methods |
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306 | Methods available on DB_SEQUENCE/$seq handles: |
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307 | |
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308 | DESTROY (DB_SEQUENCE_ornull *seq) |
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309 | CODE: |
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310 | if (seq) |
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311 | seq->close (seq, 0); |
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312 | |
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313 | $int = $seq->initial_value (db_seq_t value) |
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314 | $int = $seq->set_cachesize (U32 size) |
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315 | $int = $seq->set_flags (U32 flags) |
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316 | flags: SEQ_DEC SEQ_INC SEQ_WRAP |
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317 | $int = $seq->set_range (db_seq_t min, db_seq_t max) |
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318 | |
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319 | Example: |
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320 | my $seq = $db->sequence; |
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321 | |
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322 | db_sequence_open $seq, undef, "seq", BDB::CREATE; |
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323 | db_sequence_get $seq, undef, 1, my $value; |
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324 | |
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325 | SUPPORT FUNCTIONS |
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326 | EVENT PROCESSING AND EVENT LOOP INTEGRATION |
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327 | $fileno = BDB::poll_fileno |
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328 | Return the *request result pipe file descriptor*. This filehandle |
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329 | must be polled for reading by some mechanism outside this module |
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330 | (e.g. Event or select, see below or the SYNOPSIS). If the pipe |
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331 | becomes readable you have to call "poll_cb" to check the results. |
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332 | |
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333 | See "poll_cb" for an example. |
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334 | |
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335 | BDB::poll_cb |
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336 | Process some outstanding events on the result pipe. You have to call |
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337 | this regularly. Returns the number of events processed. Returns |
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338 | immediately when no events are outstanding. The amount of events |
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339 | processed depends on the settings of "BDB::max_poll_req" and |
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340 | "BDB::max_poll_time". |
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341 | |
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342 | If not all requests were processed for whatever reason, the |
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343 | filehandle will still be ready when "poll_cb" returns. |
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344 | |
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345 | Example: Install an Event watcher that automatically calls |
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346 | BDB::poll_cb with high priority: |
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347 | |
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348 | Event->io (fd => BDB::poll_fileno, |
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349 | poll => 'r', async => 1, |
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350 | cb => \&BDB::poll_cb); |
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351 | |
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352 | BDB::max_poll_reqs $nreqs |
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353 | BDB::max_poll_time $seconds |
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354 | These set the maximum number of requests (default 0, meaning |
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355 | infinity) that are being processed by "BDB::poll_cb" in one call, |
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356 | respectively the maximum amount of time (default 0, meaning |
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357 | infinity) spent in "BDB::poll_cb" to process requests (more |
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358 | correctly the mininum amount of time "poll_cb" is allowed to use). |
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359 | |
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360 | Setting "max_poll_time" to a non-zero value creates an overhead of |
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361 | one syscall per request processed, which is not normally a problem |
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362 | unless your callbacks are really really fast or your OS is really |
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363 | really slow (I am not mentioning Solaris here). Using |
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364 | "max_poll_reqs" incurs no overhead. |
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365 | |
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366 | Setting these is useful if you want to ensure some level of |
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367 | interactiveness when perl is not fast enough to process all requests |
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368 | in time. |
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369 | |
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370 | For interactive programs, values such as 0.01 to 0.1 should be fine. |
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371 | |
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372 | Example: Install an Event watcher that automatically calls |
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373 | BDB::poll_cb with low priority, to ensure that other parts of the |
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374 | program get the CPU sometimes even under high AIO load. |
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375 | |
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376 | # try not to spend much more than 0.1s in poll_cb |
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377 | BDB::max_poll_time 0.1; |
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378 | |
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379 | # use a low priority so other tasks have priority |
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380 | Event->io (fd => BDB::poll_fileno, |
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381 | poll => 'r', nice => 1, |
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382 | cb => &BDB::poll_cb); |
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383 | |
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384 | BDB::poll_wait |
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385 | If there are any outstanding requests and none of them in the result |
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386 | phase, wait till the result filehandle becomes ready for reading |
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387 | (simply does a "select" on the filehandle. This is useful if you |
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388 | want to synchronously wait for some requests to finish). |
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389 | |
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390 | See "nreqs" for an example. |
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391 | |
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392 | BDB::poll |
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393 | Waits until some requests have been handled. |
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394 | |
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395 | Returns the number of requests processed, but is otherwise strictly |
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396 | equivalent to: |
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397 | |
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398 | BDB::poll_wait, BDB::poll_cb |
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399 | |
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400 | BDB::flush |
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401 | Wait till all outstanding AIO requests have been handled. |
|
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402 | |
|
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403 | Strictly equivalent to: |
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404 | |
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405 | BDB::poll_wait, BDB::poll_cb |
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406 | while BDB::nreqs; |
|
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407 | |
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408 | CONTROLLING THE NUMBER OF THREADS |
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409 | BDB::min_parallel $nthreads |
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410 | Set the minimum number of AIO threads to $nthreads. The current |
|
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411 | default is 8, which means eight asynchronous operations can execute |
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412 | concurrently at any one time (the number of outstanding requests, |
|
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413 | however, is unlimited). |
|
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414 | |
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415 | BDB starts threads only on demand, when an AIO request is queued and |
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416 | no free thread exists. Please note that queueing up a hundred |
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417 | requests can create demand for a hundred threads, even if it turns |
|
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418 | out that everything is in the cache and could have been processed |
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419 | faster by a single thread. |
|
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420 | |
|
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421 | It is recommended to keep the number of threads relatively low, as |
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422 | some Linux kernel versions will scale negatively with the number of |
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423 | threads (higher parallelity => MUCH higher latency). With current |
|
|
424 | Linux 2.6 versions, 4-32 threads should be fine. |
|
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425 | |
|
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426 | Under most circumstances you don't need to call this function, as |
|
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427 | the module selects a default that is suitable for low to moderate |
|
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428 | load. |
|
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429 | |
|
|
430 | BDB::max_parallel $nthreads |
|
|
431 | Sets the maximum number of AIO threads to $nthreads. If more than |
|
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432 | the specified number of threads are currently running, this function |
|
|
433 | kills them. This function blocks until the limit is reached. |
|
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434 | |
|
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435 | While $nthreads are zero, aio requests get queued but not executed |
|
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436 | until the number of threads has been increased again. |
|
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437 | |
|
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438 | This module automatically runs "max_parallel 0" at program end, to |
|
|
439 | ensure that all threads are killed and that there are no outstanding |
|
|
440 | requests. |
|
|
441 | |
|
|
442 | Under normal circumstances you don't need to call this function. |
|
|
443 | |
|
|
444 | BDB::max_idle $nthreads |
|
|
445 | Limit the number of threads (default: 4) that are allowed to idle |
|
|
446 | (i.e., threads that did not get a request to process within 10 |
|
|
447 | seconds). That means if a thread becomes idle while $nthreads other |
|
|
448 | threads are also idle, it will free its resources and exit. |
|
|
449 | |
|
|
450 | This is useful when you allow a large number of threads (e.g. 100 or |
|
|
451 | 1000) to allow for extremely high load situations, but want to free |
|
|
452 | resources under normal circumstances (1000 threads can easily |
|
|
453 | consume 30MB of RAM). |
|
|
454 | |
|
|
455 | The default is probably ok in most situations, especially if thread |
|
|
456 | creation is fast. If thread creation is very slow on your system you |
|
|
457 | might want to use larger values. |
|
|
458 | |
|
|
459 | $oldmaxreqs = BDB::max_outstanding $maxreqs |
|
|
460 | This is a very bad function to use in interactive programs because |
|
|
461 | it blocks, and a bad way to reduce concurrency because it is |
|
|
462 | inexact: Better use an "aio_group" together with a feed callback. |
|
|
463 | |
|
|
464 | Sets the maximum number of outstanding requests to $nreqs. If you to |
|
|
465 | queue up more than this number of requests, the next call to the |
|
|
466 | "poll_cb" (and "poll_some" and other functions calling "poll_cb") |
|
|
467 | function will block until the limit is no longer exceeded. |
|
|
468 | |
|
|
469 | The default value is very large, so there is no practical limit on |
|
|
470 | the number of outstanding requests. |
|
|
471 | |
|
|
472 | You can still queue as many requests as you want. Therefore, |
|
|
473 | "max_oustsanding" is mainly useful in simple scripts (with low |
|
|
474 | values) or as a stop gap to shield against fatal memory overflow |
|
|
475 | (with large values). |
|
|
476 | |
|
|
477 | BDB::set_sync_prepare $cb |
|
|
478 | Sets a callback that is called whenever a request is created without |
|
|
479 | an explicit callback. It has to return two code references. The |
|
|
480 | first is used as the request callback, and the second is called to |
|
|
481 | wait until the first callback has been called. The default |
|
|
482 | implementation works like this: |
|
|
483 | |
|
|
484 | sub { |
|
|
485 | my $status; |
|
|
486 | ( |
|
|
487 | sub { $status = $! }, |
|
|
488 | sub { BDB::poll while !defined $status; $! = $status }, |
|
|
489 | ) |
|
|
490 | } |
|
|
491 | |
|
|
492 | STATISTICAL INFORMATION |
|
|
493 | BDB::nreqs |
|
|
494 | Returns the number of requests currently in the ready, execute or |
|
|
495 | pending states (i.e. for which their callback has not been invoked |
|
|
496 | yet). |
|
|
497 | |
|
|
498 | Example: wait till there are no outstanding requests anymore: |
|
|
499 | |
|
|
500 | BDB::poll_wait, BDB::poll_cb |
|
|
501 | while BDB::nreqs; |
|
|
502 | |
|
|
503 | BDB::nready |
|
|
504 | Returns the number of requests currently in the ready state (not yet |
|
|
505 | executed). |
|
|
506 | |
|
|
507 | BDB::npending |
|
|
508 | Returns the number of requests currently in the pending state |
|
|
509 | (executed, but not yet processed by poll_cb). |
|
|
510 | |
|
|
511 | FORK BEHAVIOUR |
|
|
512 | This module should do "the right thing" when the process using it forks: |
|
|
513 | |
|
|
514 | Before the fork, IO::AIO enters a quiescent state where no requests can |
|
|
515 | be added in other threads and no results will be processed. After the |
|
|
516 | fork the parent simply leaves the quiescent state and continues |
|
|
517 | request/result processing, while the child frees the request/result |
|
|
518 | queue (so that the requests started before the fork will only be handled |
|
|
519 | in the parent). Threads will be started on demand until the limit set in |
|
|
520 | the parent process has been reached again. |
|
|
521 | |
|
|
522 | In short: the parent will, after a short pause, continue as if fork had |
|
|
523 | not been called, while the child will act as if IO::AIO has not been |
|
|
524 | used yet. |
|
|
525 | |
|
|
526 | MEMORY USAGE |
|
|
527 | Per-request usage: |
|
|
528 | |
|
|
529 | Each aio request uses - depending on your architecture - around 100-200 |
|
|
530 | bytes of memory. In addition, stat requests need a stat buffer (possibly |
|
|
531 | a few hundred bytes), readdir requires a result buffer and so on. Perl |
|
|
532 | scalars and other data passed into aio requests will also be locked and |
|
|
533 | will consume memory till the request has entered the done state. |
|
|
534 | |
|
|
535 | This is not awfully much, so queuing lots of requests is not usually a |
|
|
536 | problem. |
|
|
537 | |
|
|
538 | Per-thread usage: |
|
|
539 | |
|
|
540 | In the execution phase, some aio requests require more memory for |
|
|
541 | temporary buffers, and each thread requires a stack and other data |
|
|
542 | structures (usually around 16k-128k, depending on the OS). |
|
|
543 | |
|
|
544 | KNOWN BUGS |
|
|
545 | Known bugs will be fixed in the next release, except: |
|
|
546 | |
|
|
547 | If you use a transaction in any request, and the request returns |
|
|
548 | with an operating system error or DB_LOCK_NOTGRANTED, the internal |
|
|
549 | TXN_DEADLOCK flag will be set on the transaction. See C<db_txn_finish>, |
|
|
550 | above. |
|
|
551 | |
|
|
552 | SEE ALSO |
|
|
553 | Coro::AIO. |
|
|
554 | |
|
|
555 | AUTHOR |
|
|
556 | Marc Lehmann <schmorp@schmorp.de> |
|
|
557 | http://home.schmorp.de/ |
|
|
558 | |