1<?xml version="1.0" encoding="UTF-8" standalone="no"?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml"> 4 <head> 5 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> 6 <title>Chapter��3.��The DB Replication Framework</title> 7 <link rel="stylesheet" href="gettingStarted.css" type="text/css" /> 8 <meta name="generator" content="DocBook XSL Stylesheets V1.62.4" /> 9 <link rel="home" href="index.html" title="Getting Started with Replicated Berkeley DB Applications" /> 10 <link rel="up" href="index.html" title="Getting Started with Replicated Berkeley DB Applications" /> 11 <link rel="previous" href="simpleprogramlisting.html" title="Program Listing" /> 12 <link rel="next" href="repmgr_init_example_c.html" title="Adding the Replication Framework to SimpleTxn " /> 13 </head> 14 <body> 15 <div class="navheader"> 16 <table width="100%" summary="Navigation header"> 17 <tr> 18 <th colspan="3" align="center">Chapter��3.��The DB Replication Framework</th> 19 </tr> 20 <tr> 21 <td width="20%" align="left"><a accesskey="p" href="simpleprogramlisting.html">Prev</a>��</td> 22 <th width="60%" align="center">��</th> 23 <td width="20%" align="right">��<a accesskey="n" href="repmgr_init_example_c.html">Next</a></td> 24 </tr> 25 </table> 26 <hr /> 27 </div> 28 <div class="chapter" lang="en" xml:lang="en"> 29 <div class="titlepage"> 30 <div> 31 <div> 32 <h2 class="title"><a id="repapp"></a>Chapter��3.��The DB Replication Framework</h2> 33 </div> 34 </div> 35 <div></div> 36 </div> 37 <div class="toc"> 38 <p> 39 <b>Table of Contents</b> 40 </p> 41 <dl> 42 <dt> 43 <span class="sect1"> 44 <a href="repapp.html#rep_init_code"> 45 Starting and Stopping Replication 46 </a> 47 </span> 48 </dt> 49 <dd> 50 <dl> 51 <dt> 52 <span class="sect2"> 53 <a href="repapp.html#election_flags">Managing Election Policies</a> 54 </span> 55 </dt> 56 <dt> 57 <span class="sect2"> 58 <a href="repapp.html#thread_count">Selecting the Number of Threads</a> 59 </span> 60 </dt> 61 </dl> 62 </dd> 63 <dt> 64 <span class="sect1"> 65 <a href="repmgr_init_example_c.html">Adding the Replication Framework to 66 67 SimpleTxn 68 </a> 69 </span> 70 </dt> 71 <dt> 72 <span class="sect1"> 73 <a href="fwrkpermmessage.html">Permanent Message Handling</a> 74 </span> 75 </dt> 76 <dd> 77 <dl> 78 <dt> 79 <span class="sect2"> 80 <a href="fwrkpermmessage.html#fmwrkpermpolicy">Identifying Permanent Message Policies</a> 81 </span> 82 </dt> 83 <dt> 84 <span class="sect2"> 85 <a href="fwrkpermmessage.html#fmwrkpermtimeout">Setting the Permanent Message Timeout</a> 86 </span> 87 </dt> 88 <dt> 89 <span class="sect2"> 90 <a href="fwrkpermmessage.html#perm2fmwrkexample">Adding a Permanent Message Policy to 91 92 93 RepQuoteExample 94 </a> 95 </span> 96 </dt> 97 </dl> 98 </dd> 99 <dt> 100 <span class="sect1"> 101 <a href="electiontimes.html">Managing Election Times</a> 102 </span> 103 </dt> 104 <dd> 105 <dl> 106 <dt> 107 <span class="sect2"> 108 <a href="electiontimes.html#electiontimeout">Managing Election Timeouts</a> 109 </span> 110 </dt> 111 <dt> 112 <span class="sect2"> 113 <a href="electiontimes.html#electretrytime">Managing Election Retry Times</a> 114 </span> 115 </dt> 116 </dl> 117 </dd> 118 <dt> 119 <span class="sect1"> 120 <a href="fmwrkconnectretry.html">Managing Connection Retries</a> 121 </span> 122 </dt> 123 <dt> 124 <span class="sect1"> 125 <a href="heartbeats.html">Managing Heartbeats</a> 126 </span> 127 </dt> 128 </dl> 129 </div> 130 <p> 131 The easiest way to add replication to your transactional 132 application is to use the replication framework. The replication framework provides a comprehensive 133 communications layer that enables replication. For a brief listing 134 of the replication framework's feature set, see 135 <a href="apioverview.html#repframeworkoverview">Replication Framework Overview</a>. 136 </p> 137 <p> 138 To use the replication framework, you make use of special methods off the 139 <tt class="classname">Environment</tt> and 140 <tt class="methodname">EnvironmentConfig</tt> 141 classes. You also use a series of related classes to perform 142 your implementation. For example, in order to detect whether 143 your code is running as a master or a replica, you must 144 implement <tt class="classname">com.sleepycat.db.EventHandler</tt>. 145 (see <a href="fwrkmasterreplica.html#determinestate">Determining State</a>). 146 That is: 147 </p> 148 <div class="orderedlist"> 149 <ol type="1"> 150 <li> 151 <p> 152 Create an environment handle as normal. 153 </p> 154 </li> 155 <li> 156 <p> 157 Configure your environment handle as 158 needed (e.g. set the error file and 159 error prefix values, if desired). 160 </p> 161 </li> 162 <li> 163 <p> 164 Use the replication framework replication methods to 165 configure the replication framework. Using these 166 methods causes DB to know that you 167 are using the replication framework. 168 </p> 169 <p> 170 Configuring the replication framework 171 entails setting its replication 172 priority, setting the TCP/IP address 173 that this replication environment will use for 174 incoming replication messages, identify 175 TCP/IP addresses of other replication 176 environments, setting the number of 177 replication environments in the 178 replication group, and so forth. These actions are 179 discussed throughout the remainder of 180 this chapter. 181 </p> 182 </li> 183 <li> 184 <p> 185 Open your environment handle. When you 186 do this, be sure to specify 187 188 189 190 <span> 191 <tt class="methodname">EnvironmentConfig.setInitializeReplication()</tt> 192 when you configure your environment 193 handle. This is in addition to the 194 configuration that you would normally 195 use for a transactional application. 196 This causes replication to be 197 initialized for the application. 198 </span> 199 </p> 200 </li> 201 <li> 202 <p> 203 Start replication by calling 204 205 206 <span><tt class="methodname">Environment.replicationManagerStart()</tt>.</span> 207 </p> 208 </li> 209 <li> 210 <p> 211 Open your databases as needed. Masters 212 must open their databases for read 213 and write activity. Replicas can open 214 their databases for read-only activity, but 215 doing so means they must re-open the 216 databases if the replica ever becomes a 217 master. Either way, replicas should never attempt to 218 write to the database(s) directly. 219 </p> 220 </li> 221 </ol> 222 </div> 223 <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"> 224 <h3 class="title">Note</h3> 225 <p> 226 The replication framework allows you to only use one 227 environment handle per process. 228 </p> 229 </div> 230 <p> 231 When you are ready to shut down your application: 232 </p> 233 <div class="orderedlist"> 234 <ol type="1"> 235 <li> 236 <p> 237 Close your databases 238 </p> 239 </li> 240 <li> 241 <p> 242 Close your environment. This causes 243 replication to stop as well. 244 </p> 245 </li> 246 </ol> 247 </div> 248 <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"> 249 <h3 class="title">Note</h3> 250 <p> 251 Before you can use the replication framework, you may have to 252 enable it in your DB library. This is 253 <span class="emphasis"><em>not</em></span> a requirement for 254 Microsoft Windows systems, or Unix systems that 255 use pthread mutexes by default. Other systems, 256 notably BSD and BSD-derived systems (such as 257 Mac OS X), must enable the replication framework when you 258 configure the DB build. 259 </p> 260 <p> 261 You do this by <span class="emphasis"><em>not</em></span> 262 disabling replication and by configuring the 263 library with POSIX threads support. In other 264 words, replication must be turned on in the 265 build (it is by default), and POSIX thread 266 support must be enabled if it is not already by 267 default. To do this, use the 268 <tt class="literal">--enable-pthread_api</tt> switch 269 on the configure script. 270 </p> 271 <p> 272 For example: 273 </p> 274 <pre class="programlisting">../dist/configure --enable-pthread-api</pre> 275 </div> 276 <div class="sect1" lang="en" xml:lang="en"> 277 <div class="titlepage"> 278 <div> 279 <div> 280 <h2 class="title" style="clear: both"><a id="rep_init_code"></a> 281 Starting and Stopping Replication 282 </h2> 283 </div> 284 </div> 285 <div></div> 286 </div> 287 <p> 288 As described above, you introduce replication to an 289 application by starting with a transactional 290 application, performing some basic replication 291 configuration, and then starting replication using 292 293 294 <span><tt class="methodname">Environment.replicationManagerStart()</tt>.</span> 295 </p> 296 <p> 297 You stop replication by closing your environment 298 cleanly, as is normal for an DB application. 299 </p> 300 <p> 301 For example, the following code fragment initializes, then 302 stops and starts replication. Note that other replication 303 activities are omitted for brevity. 304 </p> 305 <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"> 306 <h3 class="title">Note</h3> 307 <p> 308 Note that the following code fragment would be part of a larger 309 class that must implement 310 <tt class="classname">com.sleepycat.db.EventHandler</tt>. This 311 class is used to track state changes between master and 312 replica. We put off that implementation for the moment, but the 313 point remains that the following code fragment would be 314 contained in a method or two that you would include in your 315 <tt class="classname">com.sleepycat.db.EventHandler</tt> 316 implementation. 317 </p> 318 </div> 319 <pre class="programlisting"> 320import com.sleepycat.db.Environment; 321import com.sleepycat.db.EnvironmentConfig; 322import com.sleepycat.db.ReplicationHostAddress; 323import com.sleepycat.db.ReplicationManagerStartPolicy; 324 325... 326 String progname = "example_replication"; 327 String envHome = "TESTDIR"; 328 int cachesize = 10 * 1024 * 1024; 329 330 Environment dbenv; 331 ReplicationHostAddress thisHost; 332 String listenHost = "mymachine.sleepycat.com"; 333 int listenPort = 8080; 334 335 ReplicationHostAddress otherReplica; 336 String otherHost = "anothermachine.sleepycat.com"; 337 int otherPort = 8081; 338 339 340try { 341 // Configure the environment handle 342 EnvironmentConfig envConfig = new EnvironmentConfig(); 343 envConfig.setErrorStream(System.err); 344 envConfig.setErrorPrefix(progname); 345 envConfig.setCacheSize(cachesize); 346 envConfig.setTxnNoSync(true); 347 348 // Identify the local replication site. This is the local hostname and 349 // port that this replication participant will use to receive 350 // incoming replication messages. Note that this can be 351 // performed only once for the application. It is required. 352 thisHost = new ReplicationHostAddress(listenHost, listenPort); 353 envConfig.setReplicationManagerLocalSite(thisHost); 354 355 // Set this application's priority. This is used for elections. 356 // 357 // Set this number to a positive integer, or 0 if you do not want 358 // this site to be able to become a master. 359 envConfig.setReplicationPriority(100); 360 // Add a site to the list of replication environments known to 361 // this application. 362 otherReplica = new ReplicationHostAddress(otherHost, otherPort); 363 envConfig.replicationManagerAddRemoteSite(otherReplica); 364 365 // Identify the number of sites in the replication group. This is 366 // necessary so that elections and permanent message 367 // handling can be performed correctly. 368 envConfig.setReplicationNumSites(2); 369 370 // Configure the environment's subsystems. Note that we initialize 371 // replication. This is required. 372 envConfig.setAllowCreate(true); 373 envConfig.setRunRecovery(true); 374 envConfig.setThreaded(true); 375 envConfig.setInitializeReplication(true); 376 envConfig.setInitializeLocking(true); 377 envConfig.setInitializeLogging(true); 378 envConfig.setInitializeCache(true); 379 envConfig.setTransactional(true); 380 381 // Missing from this is where we set the event handle and the 382 // acknowledgement policy. We discuss these things later in this 383 // book. 384 385 // Open our environment handle. 386 try { 387 dbenv = new Environment(envHome, envConfig); 388 } catch(FileNotFoundException e) { 389 System.err.println("FileNotFound exception: " + e.toString()); 390 System.err.println( 391 "Ensure that the environment directory is pre-created."); 392 } 393 394 // Start the replication framework such that it has three threads. 395 dbenv.replicationManagerStart(3, 396 ReplicationManagerStartPolicy.REP_ELECTION); 397 398 ////////////////////////////////////////////////// 399 // All other application code goes here, including 400 // database opens. 401 ////////////////////////////////////////////////// 402 403} catch (DatabaseException dbe) { 404 // Error handling goes here 405} 406 407// Close out your application here. 408try { 409 // Make sure all your databases are closed. 410 411 // Closing your environment stops replication. 412 dbenv.close(); 413} catch (DatabaseException dbe) { 414 // Error handling here. 415} 416 417// All done. </pre> 418 <div class="sect2" lang="en" xml:lang="en"> 419 <div class="titlepage"> 420 <div> 421 <div> 422 <h3 class="title"><a id="election_flags"></a>Managing Election Policies</h3> 423 </div> 424 </div> 425 <div></div> 426 </div> 427 <p> 428 Before continuing, it is worth taking a look at the 429 430 431 432 <span> 433 startup election options that you can set for 434 replication. You set these using the 435 <tt class="classname">ReplicationManagerStartPolicy</tt> 436 class that you pass to the 437 <tt class="methodname">Environment.replicationManagerStart()</tt> 438 method. 439 440 </span> 441 </p> 442 <p> 443 In the previous example, we specified 444 445 <tt class="literal">ReplicationManagerStartPolicy.REP_ELECTION</tt> 446 when we started replication. This causes the 447 application to try to find a master upon startup. If it 448 cannot, it calls for an election. In the event an 449 election is held, the environment receiving the most number of 450 votes will become the master. 451 </p> 452 <p> 453 There's some important points to make here: 454 </p> 455 <div class="itemizedlist"> 456 <ul type="disc"> 457 <li> 458 <p> 459 This 460 461 <span>option</span> 462 only requires that other 463 environments in the replication group 464 participate in the vote. There is no 465 requirement that 466 <span class="emphasis"><em>all</em></span> such 467 environments participate. In other 468 words, if an environment 469 starts up, it can call for an 470 election, and select a master, even 471 if all other environment have not yet 472 joined the replication group. 473 </p> 474 </li> 475 <li> 476 <p> 477 It only requires a simple majority of 478 participating environments to elect a master. The number of 479 environments used to calculate the simple 480 majority is based on the value set for 481 482 483 484 <span><tt class="methodname">EnvironmentConfig.setReplicationNumSites()</tt>.</span> 485 486 487 This is always true of elections held using the replication framework. 488 </p> 489 </li> 490 <li> 491 <p> 492 As always, the environment participating in the election with the most 493 up-to-date log files is selected as 494 master. If an environment with better log files 495 has not yet joined the replication 496 group, it may not become the master. 497 </p> 498 </li> 499 </ul> 500 </div> 501 <p> 502 Any one of these points may be enough to cause a 503 less-than-optimum environment to be selected as master. 504 Therefore, to give you a better degree of control over 505 which environment becomes a master at application startup, 506 the replication framework offers the following start-up 507 508 <span>options:</span> 509 </p> 510 <div class="informaltable"> 511 <table border="1" width="80%"> 512 <colgroup> 513 <col /> 514 <col /> 515 </colgroup> 516 <thead> 517 <tr> 518 <th>Option</th> 519 <th>Description</th> 520 </tr> 521 </thead> 522 <tbody> 523 <tr> 524 <td> 525 <tt class="literal">ReplicationManagerStartPolicy. REP_MASTER</tt> 526 </td> 527 <td> 528 <p> 529 The application starts up and declares itself to be a master 530 without calling for an election. It is an error for more 531 than one environment to start up using this flag, or for 532 an environment 533 to use this flag when a master already exists. 534 </p> 535 <p> 536 Note that no replication group should 537 <span class="emphasis"><em>ever</em></span> operate with more than 538 one master. 539 </p> 540 <p> 541 In the event that a environment attempts to become a 542 master when a master already exists, the 543 replication code will resolve the problem by 544 holding an election. Note, however, that there 545 is always a possibility of data loss in the face 546 of duplicate masters, because once a master is 547 selected, the environment that loses the election will 548 have to roll back any transactions committed 549 until it is in sync with the "real" master. 550 </p> 551 </td> 552 </tr> 553 <tr> 554 <td> 555 <tt class="literal">ReplicationManagerStartPolicy. REP_CLIENT</tt> 556 </td> 557 <td> 558 <p> 559 The application starts up and declares 560 itself to be a replica without calling for 561 an election. Note that the application 562 can still become a master if a subsequent 563 application starts up, calls for an 564 election, and this application is elected 565 master. 566 </p> 567 </td> 568 </tr> 569 <tr> 570 <td> 571 <tt class="literal">ReplicationManagerStartPolicy. REP_ELECTION</tt> 572 </td> 573 <td> 574 <p> 575 As described above, the application starts up, 576 looks for a master, and if one is not found calls 577 for an election. 578 </p> 579 </td> 580 </tr> 581 <tr> 582 <td> 583 <tt class="literal">ReplicationManagerStartPolicy. REP_FULL_ELECTION</tt> 584 </td> 585 <td> 586 <p> 587 Identical to 588 589 <tt class="literal">ReplicationManagerStartPolicy. REP_ELECTION</tt> 590 except that the election requires all 591 known members of the replication group to 592 participate. If a given environment has not yet 593 started but it is included in the 594 replication group count (using 595 596 597 <span><tt class="methodname">EnvironmentConfig.setReplicationNumSites()</tt>)</span> 598 then a master can not be elected. 599 </p> 600 </td> 601 </tr> 602 </tbody> 603 </table> 604 </div> 605 </div> 606 <div class="sect2" lang="en" xml:lang="en"> 607 <div class="titlepage"> 608 <div> 609 <div> 610 <h3 class="title"><a id="thread_count"></a>Selecting the Number of Threads</h3> 611 </div> 612 </div> 613 <div></div> 614 </div> 615 <p> 616 Under the hood, the replication framework is threaded and you can 617 control the number of threads used to process messages received from 618 other replicas. The threads that the replication framework uses are: 619 </p> 620 <div class="itemizedlist"> 621 <ul type="disc"> 622 <li> 623 <p> 624 Incoming message thread. This thread 625 receives messages from the site's 626 socket and passes those messages to 627 message processing threads (see below) 628 for handling. 629 </p> 630 </li> 631 <li> 632 <p> 633 Outgoing message thread. Outgoing 634 are performed in whatever thread 635 performed a write to the database(s). 636 That is, the thread that called, for 637 example, 638 639 640 <tt class="methodname">Database.put()</tt> 641 is the thread that writes replication messages 642 about that fact to the socket. 643 </p> 644 <p> 645 Note that if this write activity would 646 cause the thread to be blocked due to 647 some condition on the socket, the replication framework 648 will hand the outgoing message to the 649 incoming message thread, and it will 650 then write the message to the socket. 651 This prevents your database write 652 threads from blocking due to abnormal 653 network I/O conditions. 654 </p> 655 </li> 656 <li> 657 <p> 658 Message processing threads are 659 responsible for parsing and then 660 responding to incoming replication 661 messages. Typically, a response will 662 include write activity to your 663 database(s), so these threads can be 664 busy performing disk I/O. 665 </p> 666 </li> 667 </ul> 668 </div> 669 <p> 670 Of these threads, the only ones that you have any 671 configuration control over are the message processing 672 threads. In this case, you can determine how many 673 of these threads you want to run. 674 </p> 675 <p> 676 It is always a bit of an art to decide on a thread count, 677 but the short answer is you probably do not need more 678 than three threads here, and it is likely that one will 679 suffice. That said, the best thing to do is set your 680 thread count to a fairly low number and then increase 681 it if it appears that your application will benefit 682 from the additional threads. 683 </p> 684 </div> 685 </div> 686 </div> 687 <div class="navfooter"> 688 <hr /> 689 <table width="100%" summary="Navigation footer"> 690 <tr> 691 <td width="40%" align="left"><a accesskey="p" href="simpleprogramlisting.html">Prev</a>��</td> 692 <td width="20%" align="center"> 693 <a accesskey="u" href="index.html">Up</a> 694 </td> 695 <td width="40%" align="right">��<a accesskey="n" href="repmgr_init_example_c.html">Next</a></td> 696 </tr> 697 <tr> 698 <td width="40%" align="left" valign="top">Program Listing��</td> 699 <td width="20%" align="center"> 700 <a accesskey="h" href="index.html">Home</a> 701 </td> 702 <td width="40%" align="right" valign="top">��Adding the Replication Framework to 703 704 SimpleTxn 705 </td> 706 </tr> 707 </table> 708 </div> 709 </body> 710</html> 711