聊聊HttpClient的ConnectionBackoffStrategy

序

本文主要研究一下HttpClient的ConnectionBackoffStrategy

ConnectionBackoffStrategy

org/apache/http/client/ConnectionBackoffStrategy.java

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/**
 * When managing a dynamic number of connections for a given route, this
 * strategy assesses whether a given request execution outcome should
 * result in a backoff signal or not, based on either examining the
 * {@code Throwable} that resulted or by examining the resulting
 * response (e.g. for its status code).
 *
 * @since 4.2
 *
 */
public interface ConnectionBackoffStrategy {

    /**
     * Determines whether seeing the given {@code Throwable} as
     * a result of request execution should result in a backoff
     * signal.
     * @param t the {@code Throwable} that happened
     * @return {@code true} if a backoff signal should be
     *   given
     */
    boolean shouldBackoff(Throwable t);

    /**
     * Determines whether receiving the given {@link HttpResponse} as
     * a result of request execution should result in a backoff
     * signal. Implementations MUST restrict themselves to examining
     * the response header and MUST NOT consume any of the response
     * body, if any.
     * @param resp the {@code HttpResponse} that was received
     * @return {@code true} if a backoff signal should be
     *   given
     */
    boolean shouldBackoff(HttpResponse resp);
}

ConnectionBackoffStrategy定义了shouldBackoff方法，它根据异常或者response来进行判断

NullBackoffStrategy

org/apache/http/impl/client/NullBackoffStrategy.java

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public class NullBackoffStrategy implements ConnectionBackoffStrategy {

    @Override
    public boolean shouldBackoff(final Throwable t) {
        return false;
    }

    @Override
    public boolean shouldBackoff(final HttpResponse resp) {
        return false;
    }
}

NullBackoffStrategy实现了ConnectionBackoffStrategy，shouldBackoff方法返回false

DefaultBackoffStrategy

org/apache/http/impl/client/DefaultBackoffStrategy.java

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public class DefaultBackoffStrategy implements ConnectionBackoffStrategy {

    @Override
    public boolean shouldBackoff(final Throwable t) {
        return t instanceof SocketTimeoutException || t instanceof ConnectException;
    }

    @Override
    public boolean shouldBackoff(final HttpResponse resp) {
        return resp.getStatusLine().getStatusCode() == 429 ||
            resp.getStatusLine().getStatusCode() == HttpStatus.SC_SERVICE_UNAVAILABLE;
    }

}

DefaultBackoffStrategy在SocketTimeoutException或者ConnectException的时候返回true，或者在response code为429或者503的时候返回true

BackoffStrategyExec

org/apache/http/impl/execchain/BackoffStrategyExec.java

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@Contract(threading = ThreadingBehavior.IMMUTABLE_CONDITIONAL)
public class BackoffStrategyExec implements ClientExecChain {

    private final ClientExecChain requestExecutor;
    private final ConnectionBackoffStrategy connectionBackoffStrategy;
    private final BackoffManager backoffManager;

    public BackoffStrategyExec(
            final ClientExecChain requestExecutor,
            final ConnectionBackoffStrategy connectionBackoffStrategy,
            final BackoffManager backoffManager) {
        super();
        Args.notNull(requestExecutor, "HTTP client request executor");
        Args.notNull(connectionBackoffStrategy, "Connection backoff strategy");
        Args.notNull(backoffManager, "Backoff manager");
        this.requestExecutor = requestExecutor;
        this.connectionBackoffStrategy = connectionBackoffStrategy;
        this.backoffManager = backoffManager;
    }

    @Override
    public CloseableHttpResponse execute(
            final HttpRoute route,
            final HttpRequestWrapper request,
            final HttpClientContext context,
            final HttpExecutionAware execAware) throws IOException, HttpException {
        Args.notNull(route, "HTTP route");
        Args.notNull(request, "HTTP request");
        Args.notNull(context, "HTTP context");
        CloseableHttpResponse out = null;
        try {
            out = this.requestExecutor.execute(route, request, context, execAware);
        } catch (final Exception ex) {
            if (out != null) {
                out.close();
            }
            if (this.connectionBackoffStrategy.shouldBackoff(ex)) {
                this.backoffManager.backOff(route);
            }
            if (ex instanceof RuntimeException) {
                throw (RuntimeException) ex;
            }
            if (ex instanceof HttpException) {
                throw (HttpException) ex;
            }
            if (ex instanceof IOException) {
                throw (IOException) ex;
            }
            throw new UndeclaredThrowableException(ex);
        }
        if (this.connectionBackoffStrategy.shouldBackoff(out)) {
            this.backoffManager.backOff(route);
        } else {
            this.backoffManager.probe(route);
        }
        return out;
    }

}

BackoffStrategyExec实现了ClientExecChain接口，其execute执行requestExecutor.execute，捕获到异常的时候通过connectionBackoffStrategy.shouldBackoff(ex)来决定是否需要backOff，是的话执行backoffManager.backOff(route)；若没有异常则通过connectionBackoffStrategy.shouldBackoff(out)根据response来判断是否需要backOff，是的化执行backoffManager.backOff(route)

小结

HttpClient的DefaultBackoffStrategy在SocketTimeoutException或者ConnectException的时候返回true，或者在response code为429或者503的时候返回true；BackoffStrategyExec则通过connectionBackoffStrategy与backoffManager来配合执行backOff。这个backOff的目的就是动态调整每个route的connection大小(MaxPerRoute)。