hadoop 2.6.3 CachedDNSToSwitchMapping源代码分析

CachedDNSToSwitchMapping的作用是对已经解析了的ip地址和rack的地址的映射进行缓冲。

/**
* A cached implementation of DNSToSwitchMapping that takes an
* raw DNSToSwitchMapping and stores the resolved network location in
* a cache. The following calls to a resolved network location
* will get its location from the cache.
*
*/

里面有两个变量，cache用来存储ip地址和network location的地址的映射,第二个用来对未缓冲的ip 地址进行解析。

private Map<String, String> cache = new ConcurrentHashMap<String, String>();

/**
* The uncached mapping
*/
protected final DNSToSwitchMapping rawMapping;

在resolve，没有缓冲的ip，调用rawMapping.resolve方法来解析它的networklocation.

@Override
public List<String> resolve(List<String> names) {
// normalize all input names to be in the form of IP addresses
names = NetUtils.normalizeHostNames(names);

List <String> result = new ArrayList<String>(names.size());
if (names.isEmpty()) {
return result;
}

List<String> uncachedHosts = getUncachedHosts(names);

// Resolve the uncached hosts
List<String> resolvedHosts = rawMapping.resolve(uncachedHosts);
//cache them
cacheResolvedHosts(uncachedHosts, resolvedHosts);
//now look up the entire list in the cache
return getCachedHosts(names);

}

getUncachedHosts，判断ip是否在缓冲里。

/**
* @param names a list of hostnames to probe for being cached
* @return the hosts from 'names' that have not been cached previously
*/
private List<String> getUncachedHosts(List<String> names) {
// find out all names without cached resolved location
List<String> unCachedHosts = new ArrayList<String>(names.size());
for (String name : names) {
if (cache.get(name) == null) {
unCachedHosts.add(name);
}
}
return unCachedHosts;
}

cacheResolvedHosts对新解析的ip和networklocation进行缓冲，两个list的大小必须相等。

/**
* Caches the resolved host:rack mappings. The two list
* parameters must be of equal size.
*
* @param uncachedHosts a list of hosts that were uncached
* @param resolvedHosts a list of resolved host entries where the element
* at index(i) is the resolved value for the entry in uncachedHosts[i]
*/
private void cacheResolvedHosts(List<String> uncachedHosts,
List<String> resolvedHosts) {
// Cache the result
if (resolvedHosts != null) {
for (int i=0; i<uncachedHosts.size(); i++) {
cache.put(uncachedHosts.get(i), resolvedHosts.get(i));
}
}
}

其它的方法非常简单，如下：

/**
* Get the (host x switch) map.
* @return a copy of the cached map of hosts to rack
*/
@Override
public Map<String, String> getSwitchMap() {
Map<String, String > switchMap = new HashMap<String, String>(cache);
return switchMap;
}

@Override
public String toString() {
return "cached switch mapping relaying to " + rawMapping;
}

/**
* Delegate the switch topology query to the raw mapping, via
* {@link AbstractDNSToSwitchMapping#isMappingSingleSwitch(DNSToSwitchMapping)}
* @return true iff the raw mapper is considered single-switch.
*/
@Override
public boolean isSingleSwitch() {
return isMappingSingleSwitch(rawMapping);
}

@Override
public void reloadCachedMappings() {
cache.clear();
}

@Override
public void reloadCachedMappings(List<String> names) {
for (String name : names) {
cache.remove(name);
}
}