本文接下来先分析CandidatesProcessor处理器，我们称之为候选处理器，该处理器的功能是对网页抽取的外链进行过滤，通过过滤的链接则添加到Frontier边界部件的BdbWorkQueue工作队列；CandidatesProcessor处理器对CrawlURI candidate对象的过滤功能是通过CandidateChain处理器链处理的，该处理器链包括两大处理器，分别为org.archive.crawler.prefetch.CandidateScoper处理器与org.archive.crawler.prefetch.FrontierPreparer处理器
CandidatesProcessor处理器的处理方法如下：
/* (non-Javadoc) * @see org.archive.modules.Processor#innerProcess(org.archive.modules.CrawlURI) */ @Override protected void innerProcess(final CrawlURI curi) throws InterruptedException { // Handle any prerequisites when S_DEFERRED for prereqs //处理先决条件 if (curi.hasPrerequisiteUri() && curi.getFetchStatus() == S_DEFERRED) { CrawlURI prereq = curi.getPrerequisiteUri(); prereq.setFullVia(curi); sheetOverlaysManager.applyOverlaysTo(prereq); try { KeyedProperties.clearOverridesFrom(curi); KeyedProperties.loadOverridesFrom(prereq); //Candidate处理器链 getCandidateChain().process(prereq, null); if(prereq.getFetchStatus()>=0) { //System.out.println("prereq:"+prereq.toString()); frontier.schedule(prereq); } else { curi.setFetchStatus(S_PREREQUISITE_UNSCHEDULABLE_FAILURE); } } finally { KeyedProperties.clearOverridesFrom(prereq); KeyedProperties.loadOverridesFrom(curi); } return; } // Don't consider candidate links of error pages //通常为先决条件 if (curi.getFetchStatus() < 200 || curi.getFetchStatus() >= 400) { curi.getOutLinks().clear(); return; } //遍历外链 for (Link wref: curi.getOutLinks()) { CrawlURI candidate; try { //根据外链构造CrawlURI对象 candidate = curi.createCrawlURI(curi.getBaseURI(),wref); // at least for duration of candidatechain, offer // access to full CrawlURI of via candidate.setFullVia(curi); } catch (URIException e) { loggerModule.logUriError(e, curi.getUURI(), wref.getDestination().toString()); continue; } sheetOverlaysManager.applyOverlaysTo(candidate); try { KeyedProperties.clearOverridesFrom(curi); KeyedProperties.loadOverridesFrom(candidate); //从种子CrawlURI curi跳转过来的CrawlURI candidate设置为种子 if(getSeedsRedirectNewSeeds() && curi.isSeed() && wref.getHopType() == Hop.REFER && candidate.getHopCount() < SEEDS_REDIRECT_NEW_SEEDS_MAX_HOPS) { candidate.setSeed(true); } getCandidateChain().process(candidate, null); if(candidate.getFetchStatus()>=0) { //seed if(checkForSeedPromotion(candidate)) { /* * We want to guarantee crawling of seed version of * CrawlURI even if same url has already been enqueued, * see https://webarchive.jira.com/browse/HER-1891 */ candidate.setForceFetch(true); //System.out.println("candidate addSeed:"+candidate.toString()); getSeeds().addSeed(candidate); } else { //System.out.println("candidate:"+candidate.toString()); frontier.schedule(candidate); } //候选链接记录 curi.getOutCandidates().add(candidate); } } finally { KeyedProperties.clearOverridesFrom(candidate); KeyedProperties.loadOverridesFrom(curi); } } curi.getOutLinks().clear(); }
我在代码里面已经加了注释，该方法首先是判断当前CrawlURI curi对象是否存在先决条件，如果存在，则将先决条件进入CandidateChain处理器链，如果符合条件（prereq.getFetchStatus()>=0），则将该先决条件添加到Frontier边界部件的BdbWorkQueue工作队列
后面部分是遍历CrawlURI curi对象的外链，根据当前CrawlURI curi对象和外链链接构建CrawlURI candidate对象，然后同样进入CandidateChain处理器链，通过过滤的CrawlURI candidate对象同样添加到Frontier边界部件的BdbWorkQueue工作队列，最后清空外链
CrawlURI candidate对象是怎么创建的，调用当前CrawlURI curi对象的CrawlURI createCrawlURI(UURI baseUURI, Link link)方法
/** * Utility method for creation of CandidateURIs found extracting * links from this CrawlURI. * @param baseUURI BaseUURI for <code>link</code>. * @param link Link to wrap CandidateURI in. * @return New candidateURI wrapper around <code>link</code>. * @throws URIException */ public CrawlURI createCrawlURI(UURI baseUURI, Link link) throws URIException { UURI u = (link.getDestination() instanceof UURI)? (UURI)link.getDestination(): UURIFactory.getInstance(baseUURI, link.getDestination().toString()); CrawlURI newCaURI = new CrawlURI(u, extendHopsPath(getPathFromSeed(),link.getHopType().getHopChar()), getUURI(), link.getContext()); newCaURI.inheritFrom(this); return newCaURI; }
新的CrawlURI candidate对象的String pathFromSeed属性是由其父级CrawlURI curi对象的String pathFromSeed属性和当前链接的Hop hop属性构建出来的
/** * Extend a 'hopsPath' (pathFromSeed string of single-character hop-type symbols), * keeping the number of displayed hop-types under MAX_HOPS_DISPLAYED. For longer * hops paths, precede the string with a integer and '+', then the displayed * hops. * * @param pathFromSeed * @param hopChar * @return */ public static String extendHopsPath(String pathFromSeed, char hopChar) { if(pathFromSeed.length()<MAX_HOPS_DISPLAYED) { return pathFromSeed + hopChar; } int plusIndex = pathFromSeed.indexOf('+'); int prevOverflow = (plusIndex<0) ? 0 : Integer.parseInt(pathFromSeed.substring(0,plusIndex)); return (prevOverflow+1)+"+"+pathFromSeed.substring(plusIndex+2)+hopChar; }
里面的boolean checkForSeedPromotion(CrawlURI curi)方法检查CrawlURI curi对象是否seed种子（从种子URL跳转的）
/** * Check if the URI needs special 'discovered seed' treatment. * * @param curi */ protected boolean checkForSeedPromotion(CrawlURI curi) { if (curi.isSeed() && curi.getVia() != null && curi.flattenVia().length() > 0) { // The only way a seed can have a non-empty via is if it is the // result of a seed redirect. Returning true here schedules it // via the seeds module, so it may affect scope and be logged // as 'discovered' seed. // // This is a feature. This is handling for case where a seed // gets immediately redirected to another page. What we're doing is // treating the immediate redirect target as a seed. // And it needs rapid scheduling. //设置调度等级 if (curi.getSchedulingDirective() == SchedulingConstants.NORMAL) { curi.setSchedulingDirective(SchedulingConstants.MEDIUM); } return true; } return false; }
CandidateChain处理器链的第一个处理器为CandidateScoper，该处理器继承自Scoper类，对当前CrawlURI caUri对象的范围判断是通过调用DecideRule scope成员的DecideResult decisionFor(CrawlURI uri)方法返回的结果进项判断的，代码比较简单（关于DecideRule类，我在前面的文章已经有过分析，里面是通过迭代调用DecideRule类型集合的成员的方法，这里不再重复）
/** * Schedule the given {@link CrawlURI CrawlURI} with the Frontier. * @param caUri The CrawlURI to be scheduled. * @return true if CrawlURI was accepted by crawl scope, false * otherwise. */ protected boolean isInScope(CrawlURI caUri) { boolean result = false; //System.out.println(this.getClass().getName()+":"+"scope name:"+scope.getClass().getName()); DecideResult dr = scope.decisionFor(caUri); if (dr == DecideResult.ACCEPT) { result = true; if (fileLogger != null) { fileLogger.info("ACCEPT " + caUri); } } else { outOfScope(caUri); } return result; }
CandidateChain处理器链的第二个处理器为FrontierPreparer，该处理器的功能为当前CrawlURI uri对象在进入边界部件Frontier之前设置相关策略（该处理器前面文章已解析，这里不再重复）
/** * Apply all configured policies to CrawlURI * * @param curi CrawlURI */ public void prepare(CrawlURI curi) { // set schedulingDirective curi.setSchedulingDirective(getSchedulingDirective(curi)); // set canonicalized version curi.setCanonicalString(canonicalize(curi)); // set queue key curi.setClassKey(getClassKey(curi)); // set cost curi.setHolderCost(getCost(curi)); // set URI precedence getUriPrecedencePolicy().uriScheduled(curi); }
接下来要分析的处理器为DispositionProcessor，我们可以称之为后置处理器，其主要功能为更新服务器信息和设置队列延迟时间
@Override protected void innerProcess(CrawlURI puri) { CrawlURI curi = (CrawlURI)puri; // Tally per-server, per-host, per-frontier-class running totals CrawlServer server = serverCache.getServerFor(curi.getUURI()); String scheme = curi.getUURI().getScheme().toLowerCase(); if (scheme.equals("http") || scheme.equals("https") && server != null) { // Update connection problems counter if(curi.getFetchStatus() == S_CONNECT_FAILED || curi.getFetchStatus() == S_CONNECT_LOST ) { server.incrementConsecutiveConnectionErrors(); } else if (curi.getFetchStatus() > 0){ server.resetConsecutiveConnectionErrors(); } // Update robots info try { if ("/robots.txt".equals(curi.getUURI().getPath()) && curi.getFetchStatus() != S_DEFERRED) { // shortcut retries w/ DEEMED when ignore-all if (metadata.getRobotsPolicy() instanceof IgnoreRobotsPolicy) { if(curi.getFetchStatus() < 0 && curi.getFetchStatus()!=S_DEFERRED) { // prevent the rest of the usual retries curi.setFetchStatus(S_DEEMED_NOT_FOUND); } } // Update server with robots info // NOTE: in some cases the curi's status can be changed here server.updateRobots(curi); } } catch (URIException e) { logger.severe("Failed get path on " + curi.getUURI()); } } // set politeness delay curi.setPolitenessDelay(politenessDelayFor(curi)); // consider operator-set force-retire if (getForceRetire()) { curi.setForceRetire(true); } // TODO: set other disposition decisions // success, failure, retry(retry-delay) }
计算队列延迟时间的方法如下
/** * Update any scheduling structures with the new information in this * CrawlURI. Chiefly means make necessary arrangements for no other URIs at * the same host to be visited within the appropriate politeness window. * * @param curi * The CrawlURI * @return millisecond politeness delay */ protected long politenessDelayFor(CrawlURI curi) { long durationToWait = 0; Map<String,Object> cdata = curi.getData(); if (cdata.containsKey(A_FETCH_BEGAN_TIME) && cdata.containsKey(A_FETCH_COMPLETED_TIME)) { long completeTime = curi.getFetchCompletedTime(); long durationTaken = (completeTime - curi.getFetchBeginTime()); durationToWait = (long)(getDelayFactor() * durationTaken); long minDelay = getMinDelayMs(); if (minDelay > durationToWait) { // wait at least the minimum durationToWait = minDelay; } long maxDelay = getMaxDelayMs(); if (durationToWait > maxDelay) { // wait no more than the maximum durationToWait = maxDelay; } long respectThreshold = getRespectCrawlDelayUpToSeconds() * 1000; if (durationToWait<respectThreshold) { // may need to extend wait CrawlServer s = getServerCache().getServerFor(curi.getUURI()); String ua = curi.getUserAgent(); if (ua == null) { ua = metadata.getUserAgent(); } Robotstxt rep = s.getRobotstxt(); if (rep != null) { long crawlDelay = (long)(1000 * rep.getDirectivesFor(ua).getCrawlDelay()); crawlDelay = (crawlDelay > respectThreshold) ? respectThreshold : crawlDelay; if (crawlDelay > durationToWait) { // wait at least the directive crawl-delay durationToWait = crawlDelay; } } } long now = System.currentTimeMillis(); int maxBandwidthKB = getMaxPerHostBandwidthUsageKbSec(); if (maxBandwidthKB > 0) { // Enforce bandwidth limit ServerCache cache = this.getServerCache(); CrawlHost host = cache.getHostFor(curi.getUURI()); long minDurationToWait = host.getEarliestNextURIEmitTime() - now; float maxBandwidth = maxBandwidthKB * 1.024F; // kilo factor long processedBytes = curi.getContentSize(); host .setEarliestNextURIEmitTime((long)(processedBytes / maxBandwidth) + now); if (minDurationToWait > durationToWait) { durationToWait = minDurationToWait; } } } return durationToWait; }
如果我们需要更改队列延迟时间，可以在配置文件crawler-beans.cxml里面设置相关参数
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本系列Heritrix 3.1.0 源码解析系本人原创
转载请注明出处 博客园 刺猬的温驯
本文链接 http://www.cnblogs.com/chenying99/archive/2013/05/07/3065205.html
