springboot之HTTP鏈接池

服務間基於HTTP通訊相對於grpc、dubbo之類的通訊效率要低得多，一方面是後者的傳輸數據結構緊湊，使用了序列化和壓縮；另外一方面，後者使用了TCP鏈接池，而前者默認狀況下每一次服務間的通訊會建立一個新的HTTP請求，會產生不小的性能消耗，對於須要額外非對稱加密的HTTPS請求，性能消耗更加嚴重。

非鏈接池

默認狀況下springboot的RestTemplate使用的org.springframework.http.client.SimpleClientHttpRequestFactory，即對每一次HTTP請求均新建一個新的TCP鏈接，請求結束後則關閉該鏈接。

RestTemplate restTemplate = new RestTemplate();
for(int i=0;i<100;i++) {
	ResponseEntity<String> responseEntity = restTemplate.getForEntity("https://wakzz.cn", String.class);
	System.out.println(responseEntity.getBody());
}
複製代碼

經過抓包以下圖，每次的HTTP請求都會斷開上次的TCP鏈接，而後從新3次握手新建一個TCP鏈接。

鏈接池

HttpComponentsClientHttpRequestFactory默認使用HTTP鏈接池PoolingHttpClientConnectionManager，其默認參數maxTotal爲10即鏈接池最大HTTP鏈接數量爲10，maxPerRoute爲2即鏈接池中相同目標IP和端口號的HTTP鏈接數量最多爲2。

HttpComponentsClientHttpRequestFactory httpClientFactory = new HttpComponentsClientHttpRequestFactory();
httpClientFactory.setConnectTimeout(2000);
httpClientFactory.setReadTimeout(10000);

RestTemplate restTemplate = new RestTemplate(httpClientFactory);
for(int i=0;i<100;i++) {
	ResponseEntity<String> responseEntity = restTemplate.getForEntity("https://wakzz.cn", String.class);
	System.out.println(responseEntity.getBody());
}
複製代碼

或者用戶自定義PoolingHttpClientConnectionManager的參數：

PoolingHttpClientConnectionManager poolingConnectionManager = new PoolingHttpClientConnectionManager();
poolingConnectionManager.setMaxTotal(100);
poolingConnectionManager.setDefaultMaxPerRoute(10);

HttpClient httpClient = HttpClientBuilder.create()
		.setConnectionManager(poolingConnectionManager)
		.build();

HttpComponentsClientHttpRequestFactory httpClientFactory = new HttpComponentsClientHttpRequestFactory();
httpClientFactory.setConnectTimeout(2000);
httpClientFactory.setReadTimeout(10000);
httpClientFactory.setHttpClient(httpClient);

RestTemplate restTemplate = new RestTemplate(httpClientFactory);
for(int i=0;i<100;i++) {
	ResponseEntity<String> responseEntity = restTemplate.getForEntity("https://wakzz.cn", String.class);
	System.out.println(responseEntity.getBody());
}
複製代碼

經過抓包以下圖，每次的HTTP請求都複用的同一個TCP鏈接，無需在創建TCP鏈接上花費多餘的性能消耗。

鏈接池鏈接狀態：

源碼分析

HttpComponentsClientHttpRequestFactory缺省值狀況下在org.apache.http.impl.client.HttpClientBuilder#build建立HTTP鏈接池：

final PoolingHttpClientConnectionManager poolingmgr = new PoolingHttpClientConnectionManager(
        RegistryBuilder.<ConnectionSocketFactory>create()
            .register("http", PlainConnectionSocketFactory.getSocketFactory())
            .register("https", sslSocketFactoryCopy)
            .build(),
        null,
        null,
        dnsResolver,
        connTimeToLive,
        connTimeToLiveTimeUnit != null ? connTimeToLiveTimeUnit : TimeUnit.MILLISECONDS);
if (defaultSocketConfig != null) {
    poolingmgr.setDefaultSocketConfig(defaultSocketConfig);
}
if (defaultConnectionConfig != null) {
    poolingmgr.setDefaultConnectionConfig(defaultConnectionConfig);
}
if (systemProperties) {
    String s = System.getProperty("http.keepAlive", "true");
    if ("true".equalsIgnoreCase(s)) {
        s = System.getProperty("http.maxConnections", "5");
        final int max = Integer.parseInt(s);
        poolingmgr.setDefaultMaxPerRoute(max);
        poolingmgr.setMaxTotal(2 * max);
    }
}
if (maxConnTotal > 0) {
    poolingmgr.setMaxTotal(maxConnTotal);
}
if (maxConnPerRoute > 0) {
    poolingmgr.setDefaultMaxPerRoute(maxConnPerRoute);
}
複製代碼

org.apache.http.impl.execchain.MainClientExec#execute從鏈接池獲取HTTP鏈接：

final ConnectionRequest connRequest = connManager.requestConnection(route, userToken);
if (execAware != null) {
    if (execAware.isAborted()) {
        connRequest.cancel();
        throw new RequestAbortedException("Request aborted");
    } else {
        execAware.setCancellable(connRequest);
    }
}

final RequestConfig config = context.getRequestConfig();

final HttpClientConnection managedConn;
try {
    final int timeout = config.getConnectionRequestTimeout();
    managedConn = connRequest.get(timeout > 0 ? timeout : 0, TimeUnit.MILLISECONDS);
} catch(final InterruptedException interrupted) {
    Thread.currentThread().interrupt();
    throw new RequestAbortedException("Request aborted", interrupted);
} catch(final ExecutionException ex) {
    Throwable cause = ex.getCause();
    if (cause == null) {
        cause = ex;
    }
    throw new RequestAbortedException("Request execution failed", cause);
}
複製代碼

每次發起HTTP請求時經過路由route（目標IP和端口號）從鏈接池獲取連接，默認timeout爲0即從鏈接池無限等待獲取鏈接不超時。