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 Manager</title> 7 <link rel="stylesheet" href="gettingStarted.css" type="text/css" /> 8 <meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /> 9 <link rel="start" 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="prev" href="simpleprogramlisting.html" title="Program Listing" /> 12 <link rel="next" href="repmgr_init_example_c.html" title="Adding the Replication Manager 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 Manager</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 Manager</h2> 33 </div> 34 </div> 35 </div> 36 <div class="toc"> 37 <p> 38 <b>Table of Contents</b> 39 </p> 40 <dl> 41 <dt> 42 <span class="sect1"> 43 <a href="repapp.html#rep_init_code"> 44 Starting and Stopping Replication 45 </a> 46 </span> 47 </dt> 48 <dd> 49 <dl> 50 <dt> 51 <span class="sect2"> 52 <a href="repapp.html#election_flags">Managing Election Policies</a> 53 </span> 54 </dt> 55 <dt> 56 <span class="sect2"> 57 <a href="repapp.html#thread_count">Selecting the Number of Threads</a> 58 </span> 59 </dt> 60 </dl> 61 </dd> 62 <dt> 63 <span class="sect1"> 64 <a href="repmgr_init_example_c.html">Adding the Replication Manager to 65 66 67 <span>SimpleTxn</span> 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 <span>RepQuoteExampleGSG</span> 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 Manager. The Replication Manager provides a comprehensive 133 communications layer that enables replication. For a brief listing 134 of the Replication Manager's feature set, see 135 <a class="xref" href="apioverview.html#repframeworkoverview" title="Replication Manager Overview">Replication Manager Overview</a>. 136 </p> 137 <p> 138 To use the Replication Manager, you make use of special methods off the 139 <code class="classname">Environment</code> and 140 <code class="methodname">EnvironmentConfig</code> 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 <code class="classname">com.sleepycat.db.EventHandler</code>. 145 (see <a class="xref" href="fwrkmasterreplica.html#determinestate" title="Determining State">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 Manager replication methods to 165 configure the Replication Manager. Using these 166 methods causes DB to know that you 167 are using the Replication Manager. 168 </p> 169 <p> 170 Configuring the Replication Manager entails setting the replication 171 environment's priority, setting the TCP/IP address 172 that this replication environment will use for 173 incoming replication messages, identifying 174 TCP/IP addresses of other replication 175 environments, setting the number of 176 replication environments in the 177 replication group, and so forth. These actions are 178 discussed throughout the remainder of 179 this chapter. 180 </p> 181 </li> 182 <li> 183 <p> 184 Open your environment handle. When you 185 do this, be sure to specify 186 187 188 189 <span> 190 <code class="methodname">EnvironmentConfig.setInitializeReplication()</code> 191 when you configure your environment 192 handle. This is in addition to the 193 configuration that you would normally 194 use for a transactional application. 195 This causes replication to be 196 initialized for the application. 197 </span> 198 </p> 199 </li> 200 <li> 201 <p> 202 Start replication by calling 203 204 205 <span><code class="methodname">Environment.replicationManagerStart()</code>.</span> 206 </p> 207 </li> 208 <li> 209 <p> 210 Open your databases as needed. Masters 211 must open their databases for read 212 and write activity. Replicas can open 213 their databases for read-only activity, but 214 doing so means they must re-open the 215 databases if the replica ever becomes a 216 master. Either way, replicas should never attempt to 217 write to the database(s) directly. 218 </p> 219 </li> 220 </ol> 221 </div> 222 <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"> 223 <h3 class="title">Note</h3> 224 <p> 225 The Replication Manager allows you to only use one 226 environment handle per process. 227 </p> 228 </div> 229 <p> 230 When you are ready to shut down your application: 231 </p> 232 <div class="orderedlist"> 233 <ol type="1"> 234 <li> 235 <p> 236 Close your databases 237 </p> 238 </li> 239 <li> 240 <p> 241 Close your environment. This causes 242 replication to stop as well. 243 </p> 244 </li> 245 </ol> 246 </div> 247 <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"> 248 <h3 class="title">Note</h3> 249 <p> 250 Before you can use the Replication Manager, you may have to 251 enable it in your DB library. This is 252 <span class="emphasis"><em>not</em></span> a requirement for 253 Microsoft Windows systems, or Unix systems that 254 use pthread mutexes by default. Other systems, 255 notably BSD and BSD-derived systems (such as 256 Mac OS X), must enable the Replication Manager when you 257 configure the DB build. 258 </p> 259 <p> 260 You do this by <span class="emphasis"><em>not</em></span> 261 disabling replication and by configuring the 262 library with POSIX threads support. In other 263 words, replication must be turned on in the 264 build (it is by default), and POSIX thread 265 support must be enabled if it is not already by 266 default. To do this, use the 267 <code class="literal">--enable-pthread_api</code> switch 268 on the configure script. 269 </p> 270 <p> 271 For example: 272 </p> 273 <pre class="programlisting">../dist/configure --enable-pthread-api</pre> 274 </div> 275 <div class="sect1" lang="en" xml:lang="en"> 276 <div class="titlepage"> 277 <div> 278 <div> 279 <h2 class="title" style="clear: both"><a id="rep_init_code"></a> 280 Starting and Stopping Replication 281 </h2> 282 </div> 283 </div> 284 </div> 285 <div class="toc"> 286 <dl> 287 <dt> 288 <span class="sect2"> 289 <a href="repapp.html#election_flags">Managing Election Policies</a> 290 </span> 291 </dt> 292 <dt> 293 <span class="sect2"> 294 <a href="repapp.html#thread_count">Selecting the Number of Threads</a> 295 </span> 296 </dt> 297 </dl> 298 </div> 299 <p> 300 As described above, you introduce replication to an 301 application by starting with a transactional 302 application, performing some basic replication 303 configuration, and then starting replication using 304 305 306 <span><code class="methodname">Environment.replicationManagerStart()</code>.</span> 307 </p> 308 <p> 309 You stop replication by closing your environment 310 cleanly in the same way you would for any DB application. 311 </p> 312 <p> 313 For example, the following code fragment initializes, then 314 stops and starts replication. Note that other replication 315 activities are omitted for brevity. 316 </p> 317 <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"> 318 <h3 class="title">Note</h3> 319 <p> 320 Note that the following code fragment would be part of a larger 321 class that must implement 322 <code class="classname">com.sleepycat.db.EventHandler</code>. This 323 class is used to track state changes between master and 324 replica. We put off that implementation for the moment, but the 325 point remains that the following code fragment would be 326 contained in a method or two that you would include in your 327 <code class="classname">com.sleepycat.db.EventHandler</code> 328 implementation. 329 </p> 330 </div> 331 <pre class="programlisting"> 332import com.sleepycat.db.Environment; 333import com.sleepycat.db.EnvironmentConfig; 334import com.sleepycat.db.ReplicationHostAddress; 335import com.sleepycat.db.ReplicationManagerStartPolicy; 336 337... 338 String progname = "example_replication"; 339 String envHome = "TESTDIR"; 340 int cachesize = 10 * 1024 * 1024; 341 342 Environment dbenv; 343 ReplicationHostAddress thisHost; 344 String listenHost = "mymachine.sleepycat.com"; 345 int listenPort = 8080; 346 347 ReplicationHostAddress otherReplica; 348 String otherHost = "anothermachine.sleepycat.com"; 349 int otherPort = 8081; 350 351 352try { 353 // Configure the environment handle 354 EnvironmentConfig envConfig = new EnvironmentConfig(); 355 envConfig.setErrorStream(System.err); 356 envConfig.setErrorPrefix(progname); 357 envConfig.setCacheSize(cachesize); 358 envConfig.setTxnNoSync(true); 359 360 // Identify the local replication site. This is the local hostname 361 // and port that this replication participant will use to receive 362 // incoming replication messages. Note that this can be 363 // performed only once for the application. It is required. 364 thisHost = new ReplicationHostAddress(listenHost, listenPort); 365 envConfig.setReplicationManagerLocalSite(thisHost); 366 367 // Set this application's priority. This is used for elections. 368 // 369 // Set this number to a positive integer, or 0 if you do not want 370 // this site to be able to become a master. 371 envConfig.setReplicationPriority(100); 372 // Add a site to the list of replication environments known to 373 // this application. 374 otherReplica = new ReplicationHostAddress(otherHost, otherPort); 375 envConfig.replicationManagerAddRemoteSite(otherReplica); 376 377 // Identify the number of sites in the replication group. This is 378 // necessary so that elections and permanent message 379 // handling can be performed correctly. 380 envConfig.setReplicationNumSites(2); 381 382 // Configure the environment's subsystems. Note that we initialize 383 // replication. This is required. 384 envConfig.setAllowCreate(true); 385 envConfig.setRunRecovery(true); 386 envConfig.setThreaded(true); 387 envConfig.setInitializeReplication(true); 388 envConfig.setInitializeLocking(true); 389 envConfig.setInitializeLogging(true); 390 envConfig.setInitializeCache(true); 391 envConfig.setTransactional(true); 392 393 // Missing from this is where we set the event handle and the 394 // acknowledgement policy. We discuss these things later in this 395 // book. 396 397 // Open our environment handle. 398 try { 399 dbenv = new Environment(envHome, envConfig); 400 } catch(FileNotFoundException e) { 401 System.err.println("FileNotFound exception: " + e.toString()); 402 System.err.println( 403 "Ensure that the environment directory is pre-created."); 404 } 405 406 // Start the replication manager such that it has three threads. 407 dbenv.replicationManagerStart(3, 408 ReplicationManagerStartPolicy.REP_ELECTION); 409 410 ////////////////////////////////////////////////// 411 // All other application code goes here, including 412 // database opens. 413 ////////////////////////////////////////////////// 414 415} catch (DatabaseException dbe) { 416 // Error handling goes here 417} 418 419// Close out your application here. 420try { 421 // Make sure all your databases are closed. 422 423 // Closing your environment stops replication. 424 dbenv.close(); 425} catch (DatabaseException dbe) { 426 // Error handling here. 427} 428 429// All done. </pre> 430 <div class="sect2" lang="en" xml:lang="en"> 431 <div class="titlepage"> 432 <div> 433 <div> 434 <h3 class="title"><a id="election_flags"></a>Managing Election Policies</h3> 435 </div> 436 </div> 437 </div> 438 <p> 439 Before continuing, it is worth taking a look at the 440 441 442 443 <span> 444 startup election options that you can set for 445 replication. You set these using the 446 <code class="classname">ReplicationManagerStartPolicy</code> 447 class that you pass to the 448 <code class="methodname">Environment.replicationManagerStart()</code> 449 method. 450 451 </span> 452 </p> 453 <p> 454 In the previous example, we specified 455 456 <code class="literal">ReplicationManagerStartPolicy.REP_ELECTION</code> 457 when we started replication. This causes the 458 application to try to find a master upon startup. If it 459 cannot, it calls for an election. In the event an 460 election is held, the environment receiving the most number of 461 votes will become the master. 462 </p> 463 <p> 464 There's some important points to make here: 465 </p> 466 <div class="itemizedlist"> 467 <ul type="disc"> 468 <li> 469 <p> 470 This 471 472 <span>option</span> 473 only requires that other 474 environments in the replication group 475 participate in the vote. There is no 476 requirement that 477 <span class="emphasis"><em>all</em></span> such 478 environments participate. In other 479 words, if an environment 480 starts up, it can call for an 481 election, and select a master, even 482 if all other environment have not yet 483 joined the replication group. 484 </p> 485 </li> 486 <li> 487 <p> 488 It only requires a simple majority of 489 participating environments to elect a master. The number of 490 environments used to calculate the simple 491 majority is based on the value set for 492 493 494 495 <span><code class="methodname">EnvironmentConfig.setReplicationNumSites()</code>.</span> 496 497 498 This is always true of elections held using the Replication Manager. 499 </p> 500 </li> 501 <li> 502 <p> 503 As always, the environment participating in the election with the most 504 up-to-date log files is selected as 505 master. If an environment with more recent log files 506 has not yet joined the replication 507 group, it may not become the master. 508 </p> 509 </li> 510 </ul> 511 </div> 512 <p> 513 Any one of these points may be enough to cause a 514 less-than-optimum environment to be selected as master. 515 Therefore, to give you a better degree of control over 516 which environment becomes a master at application startup, 517 the Replication Manager offers the following start-up 518 519 <span>options:</span> 520 </p> 521 <div class="informaltable"> 522 <table border="1" width="80%"> 523 <colgroup> 524 <col /> 525 <col /> 526 </colgroup> 527 <thead> 528 <tr> 529 <th>Option</th> 530 <th>Description</th> 531 </tr> 532 </thead> 533 <tbody> 534 <tr> 535 <td> 536 <code class="literal">ReplicationManagerStartPolicy.REP_MASTER</code> 537 </td> 538 <td> 539 <p> 540 The application starts up and declares the environment to be a master 541 without calling for an election. It is an error for more 542 than one environment to start up using this flag, or for 543 an environment 544 to use this flag when a master already exists. 545 </p> 546 <p> 547 Note that no replication group should 548 <span class="emphasis"><em>ever</em></span> operate with more than 549 one master. 550 </p> 551 <p> 552 In the event that a environment attempts to become a 553 master when a master already exists, the 554 replication code will resolve the problem by 555 holding an election. Note, however, that there 556 is always a possibility of data loss in the face 557 of duplicate masters, because once a master is 558 selected, the environment that loses the election will 559 have to roll back any transactions committed 560 until it is in sync with the "real" master. 561 </p> 562 563 </td> 564 </tr> 565 <tr> 566 <td> 567 <code class="literal">ReplicationManagerStartPolicy.REP_CLIENT</code> 568 </td> 569 <td> 570 <p> 571 The application starts up and declares 572 the environment to be a replica without calling for 573 an election. Note that the environment 574 can still become a master if a subsequent 575 application starts up, calls for an 576 election, and this environment is elected 577 master. 578 </p> 579 </td> 580 </tr> 581 <tr> 582 <td> 583 <code class="literal">ReplicationManagerStartPolicy.REP_ELECTION</code> 584 </td> 585 <td> 586 <p> 587 As described above, the application starts up, 588 looks for a master, and if one is not found calls 589 for an election. 590 </p> 591 </td> 592 </tr> 593 </tbody> 594 </table> 595 </div> 596 </div> 597 <div class="sect2" lang="en" xml:lang="en"> 598 <div class="titlepage"> 599 <div> 600 <div> 601 <h3 class="title"><a id="thread_count"></a>Selecting the Number of Threads</h3> 602 </div> 603 </div> 604 </div> 605 <p> 606 Under the hood, the Replication Manager is threaded and you can 607 control the number of threads used to process messages received from 608 other replicas. The threads that the Replication Manager uses are: 609 </p> 610 <div class="itemizedlist"> 611 <ul type="disc"> 612 <li> 613 <p> 614 Incoming message thread. This thread 615 receives messages from the site's 616 socket and passes those messages to 617 message processing threads (see below) 618 for handling. 619 </p> 620 </li> 621 <li> 622 <p> 623 Outgoing message thread. Outgoing 624 messages are sent from whatever thread 625 performed a write to the database(s). 626 That is, the thread that called, for 627 example, 628 629 630 <code class="methodname">Database.put()</code> 631 is the thread that writes replication messages 632 about that fact to the socket. 633 </p> 634 <p> 635 Note that if this write activity would 636 cause the thread to be blocked due to 637 some condition on the socket, the Replication Manager 638 will hand the outgoing message to the 639 incoming message thread, and it will 640 then write the message to the socket. 641 This prevents your database write 642 threads from blocking due to abnormal 643 network I/O conditions. 644 </p> 645 </li> 646 <li> 647 <p> 648 Message processing threads are 649 responsible for parsing and then 650 responding to incoming replication 651 messages. Typically, a response will 652 include write activity to your 653 database(s), so these threads can be 654 busy performing disk I/O. 655 </p> 656 </li> 657 </ul> 658 </div> 659 <p> 660 Of these threads, the only ones that you have any 661 configuration control over are the message processing 662 threads. In this case, you can determine how many 663 of these threads you want to run. 664 </p> 665 <p> 666 It is always a bit of an art to decide on a thread count, 667 but the short answer is you probably do not need more 668 than three threads here, and it is likely that one will 669 suffice. That said, the best thing to do is set your 670 thread count to a fairly low number and then increase 671 it if it appears that your application will benefit 672 from the additional threads. 673 </p> 674 </div> 675 </div> 676 </div> 677 <div class="navfooter"> 678 <hr /> 679 <table width="100%" summary="Navigation footer"> 680 <tr> 681 <td width="40%" align="left"><a accesskey="p" href="simpleprogramlisting.html">Prev</a>��</td> 682 <td width="20%" align="center">��</td> 683 <td width="40%" align="right">��<a accesskey="n" href="repmgr_init_example_c.html">Next</a></td> 684 </tr> 685 <tr> 686 <td width="40%" align="left" valign="top">Program Listing��</td> 687 <td width="20%" align="center"> 688 <a accesskey="h" href="index.html">Home</a> 689 </td> 690 <td width="40%" align="right" valign="top">��Adding the Replication Manager to 691 692 693 <span>SimpleTxn</span> 694 </td> 695 </tr> 696 </table> 697 </div> 698 </body> 699</html> 700