聊聊Elasticsearch RestClient的DeadHostState

序

本文主要研究一下Elasticsearch RestClient的DeadHostState

DeadHostState

elasticsearch-7.0.1/client/rest/src/main/java/org/elasticsearch/client/DeadHostState.java

/**
 * Holds the state of a dead connection to a host. Keeps track of how many failed attempts were performed and
 * when the host should be retried (based on number of previous failed attempts).
 * Class is immutable, a new copy of it should be created each time the state has to be changed.
 */
final class DeadHostState implements Comparable<DeadHostState> {

    private static final long MIN_CONNECTION_TIMEOUT_NANOS = TimeUnit.MINUTES.toNanos(1);
    static final long MAX_CONNECTION_TIMEOUT_NANOS = TimeUnit.MINUTES.toNanos(30);

    private final int failedAttempts;
    private final long deadUntilNanos;
    private final TimeSupplier timeSupplier;

    /**
     * Build the initial dead state of a host. Useful when a working host stops functioning
     * and needs to be marked dead after its first failure. In such case the host will be retried after a minute or so.
     *
     * @param timeSupplier a way to supply the current time and allow for unit testing
     */
    DeadHostState(TimeSupplier timeSupplier) {
        this.failedAttempts = 1;
        this.deadUntilNanos = timeSupplier.nanoTime() + MIN_CONNECTION_TIMEOUT_NANOS;
        this.timeSupplier = timeSupplier;
    }

    /**
     * Build the dead state of a host given its previous dead state. Useful when a host has been failing before, hence
     * it already failed for one or more consecutive times. The more failed attempts we register the longer we wait
     * to retry that same host again. Minimum is 1 minute (for a node the only failed once created
     * through {@link #DeadHostState(TimeSupplier)}), maximum is 30 minutes (for a node that failed more than 10 consecutive times)
     *
     * @param previousDeadHostState the previous state of the host which allows us to increase the wait till the next retry attempt
     */
    DeadHostState(DeadHostState previousDeadHostState) {
        long timeoutNanos = (long)Math.min(MIN_CONNECTION_TIMEOUT_NANOS * 2 * Math.pow(2, previousDeadHostState.failedAttempts * 0.5 - 1),
                MAX_CONNECTION_TIMEOUT_NANOS);
        this.deadUntilNanos = previousDeadHostState.timeSupplier.nanoTime() + timeoutNanos;
        this.failedAttempts = previousDeadHostState.failedAttempts + 1;
        this.timeSupplier = previousDeadHostState.timeSupplier;
    }

    /**
     * Indicates whether it's time to retry to failed host or not.
     *
     * @return true if the host should be retried, false otherwise
     */
    boolean shallBeRetried() {
        return timeSupplier.nanoTime() - deadUntilNanos > 0;
    }

    /**
     * Returns the timestamp (nanos) till the host is supposed to stay dead without being retried.
     * After that the host should be retried.
     */
    long getDeadUntilNanos() {
        return deadUntilNanos;
    }

    int getFailedAttempts() {
        return failedAttempts;
    }

    @Override
    public int compareTo(DeadHostState other) {
        if (timeSupplier != other.timeSupplier) {
            throw new IllegalArgumentException("can't compare DeadHostStates with different clocks ["
                    + timeSupplier + " != " + other.timeSupplier + "]");
        }
        return Long.compare(deadUntilNanos, other.deadUntilNanos);
    }

    @Override
    public String toString() {
        return "DeadHostState{" +
                "failedAttempts=" + failedAttempts +
                ", deadUntilNanos=" + deadUntilNanos +
                ", timeSupplier=" + timeSupplier +
                '}';
    }

    /**
     * Time supplier that makes timing aspects pluggable to ease testing
     */
    interface TimeSupplier {
        TimeSupplier DEFAULT = new TimeSupplier() {
            @Override
            public long nanoTime() {
                return System.nanoTime();
            }

            @Override
            public String toString() {
                return "nanoTime";
            }
        };

        long nanoTime();
    }
}

• DeadHostState有failedAttempts、deadUntilNanos、timeSupplier三個屬性，它提供了兩類構造器，一類是根據timeSupplier來構造，一類是根據previousDeadHostState來構造
• 根據previousDeadHostState來構造時會從新計算deadUntilNanos，遞增failedAttempts；其中deadUntilNanos爲timeSupplier.nanoTime() + timeoutNanos，而timeoutNanos的計算公式爲(long)Math.min(MIN_CONNECTION_TIMEOUT_NANOS * 2 * Math.pow(2, previousDeadHostState.failedAttempts * 0.5 - 1)
• DeadHostState提供了shallBeRetried方法，其判斷邏輯爲timeSupplier.nanoTime() - deadUntilNanos > 0；DeadHostState實現了Comparable的compareTo方法，它是根據deadUntilNanos來進行比較

RestClient

elasticsearch-7.0.1/client/rest/src/main/java/org/elasticsearch/client/RestClient.java

public class RestClient implements Closeable {
	//......

    /**
     * Select nodes to try and sorts them so that the first one will be tried initially, then the following ones
     * if the previous attempt failed and so on. Package private for testing.
     */
    static Iterable<Node> selectNodes(NodeTuple<List<Node>> nodeTuple, Map<HttpHost, DeadHostState> blacklist,
                                      AtomicInteger lastNodeIndex, NodeSelector nodeSelector) throws IOException {
        /*
         * Sort the nodes into living and dead lists.
         */
        List<Node> livingNodes = new ArrayList<>(Math.max(0, nodeTuple.nodes.size() - blacklist.size()));
        List<DeadNode> deadNodes = new ArrayList<>(blacklist.size());
        for (Node node : nodeTuple.nodes) {
            DeadHostState deadness = blacklist.get(node.getHost());
            if (deadness == null) {
                livingNodes.add(node);
                continue;
            }
            if (deadness.shallBeRetried()) {
                livingNodes.add(node);
                continue;
            }
            deadNodes.add(new DeadNode(node, deadness));
        }

        if (false == livingNodes.isEmpty()) {
            /*
             * Normal state: there is at least one living node. If the
             * selector is ok with any over the living nodes then use them
             * for the request.
             */
            List<Node> selectedLivingNodes = new ArrayList<>(livingNodes);
            nodeSelector.select(selectedLivingNodes);
            if (false == selectedLivingNodes.isEmpty()) {
                /*
                 * Rotate the list using a global counter as the distance so subsequent
                 * requests will try the nodes in a different order.
                 */
                Collections.rotate(selectedLivingNodes, lastNodeIndex.getAndIncrement());
                return selectedLivingNodes;
            }
        }

        /*
         * Last resort: there are no good nodes to use, either because
         * the selector rejected all the living nodes or because there aren't
         * any living ones. Either way, we want to revive a single dead node
         * that the NodeSelectors are OK with. We do this by passing the dead
         * nodes through the NodeSelector so it can have its say in which nodes
         * are ok. If the selector is ok with any of the nodes then we will take
         * the one in the list that has the lowest revival time and try it.
         */
        if (false == deadNodes.isEmpty()) {
            final List<DeadNode> selectedDeadNodes = new ArrayList<>(deadNodes);
            /*
             * We'd like NodeSelectors to remove items directly from deadNodes
             * so we can find the minimum after it is filtered without having
             * to compare many things. This saves us a sort on the unfiltered
             * list.
             */
            nodeSelector.select(new Iterable<Node>() {
                @Override
                public Iterator<Node> iterator() {
                    return new DeadNodeIteratorAdapter(selectedDeadNodes.iterator());
                }
            });
            if (false == selectedDeadNodes.isEmpty()) {
                return singletonList(Collections.min(selectedDeadNodes).node);
            }
        }
        throw new IOException("NodeSelector [" + nodeSelector + "] rejected all nodes, "
                + "living " + livingNodes + " and dead " + deadNodes);
    }

    /**
     * Called after each successful request call.
     * Receives as an argument the host that was used for the successful request.
     */
    private void onResponse(Node node) {
        DeadHostState removedHost = this.blacklist.remove(node.getHost());
        if (logger.isDebugEnabled() && removedHost != null) {
            logger.debug("removed [" + node + "] from blacklist");
        }
    }

    /**
     * Called after each failed attempt.
     * Receives as an argument the host that was used for the failed attempt.
     */
    private void onFailure(Node node) {
        while(true) {
            DeadHostState previousDeadHostState =
                blacklist.putIfAbsent(node.getHost(), new DeadHostState(TimeSupplier.DEFAULT));
            if (previousDeadHostState == null) {
                if (logger.isDebugEnabled()) {
                    logger.debug("added [" + node + "] to blacklist");
                }
                break;
            }
            if (blacklist.replace(node.getHost(), previousDeadHostState,
                    new DeadHostState(previousDeadHostState))) {
                if (logger.isDebugEnabled()) {
                    logger.debug("updated [" + node + "] already in blacklist");
                }
                break;
            }
        }
        failureListener.onFailure(node);
    }

    //......
}

• RestClient的selectNodes方法首先將不在blacklist中的node，或者在blacklist中可是shallBeRetried返回爲true的node歸爲livingNodes，以後經過nodeSelector來從這些livingNodes中選擇一個node
• RestClient的onResponse方法會將該node的host從blacklist中移除
• RestClient的onFailure方法會往blacklist建立或更新host對應的DeadHostState，若是以前該host沒有DeadHostState則使用TimeSupplier.DEFAULT建立一個新的並放入blacklist，若是該host已經有DeadHostState則使用該DeadHostState建立新的DeadHostState而後更新到blacklist中

小結

• DeadHostState有failedAttempts、deadUntilNanos、timeSupplier三個屬性，它提供了兩類構造器，一類是根據timeSupplier來構造，一類是根據previousDeadHostState來構造；DeadHostState提供了shallBeRetried方法，其判斷邏輯爲timeSupplier.nanoTime() - deadUntilNanos > 0
• 根據previousDeadHostState來構造DeadHostState時會從新計算deadUntilNanos，遞增failedAttempts；其中deadUntilNanos爲timeSupplier.nanoTime() + timeoutNanos，而timeoutNanos的計算公式爲(long)Math.min(MIN_CONNECTION_TIMEOUT_NANOS * 2 * Math.pow(2, previousDeadHostState.failedAttempts * 0.5 - 1)
• RestClient的selectNodes方法首先將不在blacklist中的node，或者在blacklist中可是shallBeRetried返回爲true的node歸爲livingNodes，以後經過nodeSelector來從這些livingNodes中選擇一個node；RestClient的onResponse方法會將該node的host從blacklist中移除；RestClient的onFailure方法會往blacklist建立或更新host對應的DeadHostState，若是以前該host沒有DeadHostState則使用TimeSupplier.DEFAULT建立一個新的並放入blacklist，若是該host已經有DeadHostState則使用該DeadHostState建立新的DeadHostState而後更新到blacklist中

doc

• DeadHostState