order

This paper mainly studies the sentinel connection of lettuce

RedisClient.connectSentinel

lettuce-core-5.0.4.RELEASE-sources.jar!/io/lettuce/core/RedisClient.java

    private <K, V> StatefulRedisSentinelConnection<K, V> connectSentinel(RedisCodec<K, V> codec, RedisURI redisURI,
            Duration timeout) {
        assertNotNull(codec);
        checkValidRedisURI(redisURI);

        ConnectionBuilder connectionBuilder = ConnectionBuilder.connectionBuilder();
        connectionBuilder.clientOptions(ClientOptions.copyOf(getOptions()));
        connectionBuilder.clientResources(clientResources);

        DefaultEndpoint endpoint = new DefaultEndpoint(clientOptions);

        StatefulRedisSentinelConnectionImpl<K, V> connection = newStatefulRedisSentinelConnection(endpoint, codec, timeout);

        logger.debug("Trying to get a Redis Sentinel connection for one of: " + redisURI.getSentinels());

        connectionBuilder.endpoint(endpoint).commandHandler(() -> new CommandHandler(clientOptions, clientResources, endpoint))
                .connection(connection);
        connectionBuilder(getSocketAddressSupplier(redisURI), connectionBuilder, redisURI);

        if (clientOptions.isPingBeforeActivateConnection()) {
            connectionBuilder.enablePingBeforeConnect();
        }

        if (redisURI.getSentinels().isEmpty() && (isNotEmpty(redisURI.getHost()) || !isEmpty(redisURI.getSocket()))) {
            channelType(connectionBuilder, redisURI);
            try {
                getConnection(initializeChannelAsync(connectionBuilder));
            } catch (RuntimeException e) {
                connection.close();
                throw e;
            }
        } else {

            boolean connected = false;
            boolean first = true;
            Exception causingException = null;
            validateUrisAreOfSameConnectionType(redisURI.getSentinels());

            for (RedisURI uri : redisURI.getSentinels()) {
                if (first) {
                    channelType(connectionBuilder, uri);
                    first = false;
                }
                connectionBuilder.socketAddressSupplier(getSocketAddressSupplier(uri));

                if (logger.isDebugEnabled()) {
                    SocketAddress socketAddress = SocketAddressResolver.resolve(uri, clientResources.dnsResolver());
                    logger.debug("Connecting to Redis Sentinel, address: " + socketAddress);
                }
                try {
                    getConnection(initializeChannelAsync(connectionBuilder));
                    connected = true;
                    break;
                } catch (Exception e) {
                    logger.warn("Cannot connect Redis Sentinel at " + uri + ": " + e.toString());
                    causingException = e;
                }
            }

            if (!connected) {
                connection.close();
                throw new RedisConnectionException("Cannot connect to a Redis Sentinel: " + redisURI.getSentinels(),
                        causingException);
            }
        }

        if (LettuceStrings.isNotEmpty(redisURI.getClientName())) {
            connection.setClientName(redisURI.getClientName());
        }

        return connection;
    }

• The connectSentinel method will traverse sentinel, take the master one by one to get the connection, and continue to get the next sentinel if the connection fails or throws an exception
• If all sentinel exceptions are thrown after traversing, the redisconnectionexception ("cannot connect to a redis sentinel:" + redisuri. Getsentinels(), causingexception) will be thrown finally
• The initializeChannelAsync method of AbstractRedisClient will be called here

AbstractRedisClient.initializeChannelAsync

lettuce-core-5.0.4.RELEASE-sources.jar!/io/lettuce/core/AbstractRedisClient.java

    /**
     * Connect and initialize a channel from {@link ConnectionBuilder}.
     *
     * @param connectionBuilder must not be {@literal null}.
     * @return the {@link ConnectionFuture} to synchronize the connection process.
     * @since 4.4
     */
    @SuppressWarnings("unchecked")
    protected <K, V, T extends RedisChannelHandler<K, V>> ConnectionFuture<T> initializeChannelAsync(
            ConnectionBuilder connectionBuilder) {

        SocketAddress redisAddress = connectionBuilder.socketAddress();

        if (clientResources.eventExecutorGroup().isShuttingDown()) {
            throw new IllegalStateException("Cannot connect, Event executor group is terminated.");
        }

        logger.debug("Connecting to Redis at {}", redisAddress);

        CompletableFuture<Channel> channelReadyFuture = new CompletableFuture<>();
        Bootstrap redisBootstrap = connectionBuilder.bootstrap();

        RedisChannelInitializer initializer = connectionBuilder.build();
        redisBootstrap.handler(initializer);

        clientResources.nettyCustomizer().afterBootstrapInitialized(redisBootstrap);
        CompletableFuture<Boolean> initFuture = initializer.channelInitialized();
        ChannelFuture connectFuture = redisBootstrap.connect(redisAddress);

        connectFuture.addListener(future -> {

            if (!future.isSuccess()) {

                logger.debug("Connecting to Redis at {}: {}", redisAddress, future.cause());
                connectionBuilder.endpoint().initialState();
                channelReadyFuture.completeExceptionally(future.cause());
                return;
            }

            initFuture.whenComplete((success, throwable) -> {

                if (throwable == null) {
                    logger.debug("Connecting to Redis at {}: Success", redisAddress);
                    RedisChannelHandler<?, ?> connection = connectionBuilder.connection();
                    connection.registerCloseables(closeableResources, connection);
                    channelReadyFuture.complete(connectFuture.channel());
                    return;
                }

                logger.debug("Connecting to Redis at {}, initialization: {}", redisAddress, throwable);
                connectionBuilder.endpoint().initialState();
                Throwable failure;

                if (throwable instanceof RedisConnectionException) {
                    failure = throwable;
                } else if (throwable instanceof TimeoutException) {
                    failure = new RedisConnectionException("Could not initialize channel within "
                            + connectionBuilder.getTimeout(), throwable);
                } else {
                    failure = throwable;
                }
                channelReadyFuture.completeExceptionally(failure);

                CompletableFuture<Boolean> response = new CompletableFuture<>();
                response.completeExceptionally(failure);

            });
        });

        return new DefaultConnectionFuture<T>(redisAddress, channelReadyFuture.thenApply(channel -> (T) connectionBuilder
                .connection()));
    }

• Here, when initializeChannelAsync, the connectionBuilder.socketAddress() method will be called, and then the getSocketAddress method of RedisClient will be called

RedisClient.getSocketAddress

lettuce-core-5.0.4.RELEASE-sources.jar!/io/lettuce/core/RedisClient.java

    protected SocketAddress getSocketAddress(RedisURI redisURI) throws InterruptedException, TimeoutException,
            ExecutionException {
        SocketAddress redisAddress;

        if (redisURI.getSentinelMasterId() != null && !redisURI.getSentinels().isEmpty()) {
            logger.debug("Connecting to Redis using Sentinels {}, MasterId {}", redisURI.getSentinels(),
                    redisURI.getSentinelMasterId());
            redisAddress = lookupRedis(redisURI);

            if (redisAddress == null) {
                throw new RedisConnectionException("Cannot provide redisAddress using sentinel for masterId "
                        + redisURI.getSentinelMasterId());
            }

        } else {
            redisAddress = SocketAddressResolver.resolve(redisURI, clientResources.dnsResolver());
        }
        return redisAddress;
    }

    private SocketAddress lookupRedis(RedisURI sentinelUri) throws InterruptedException, TimeoutException, ExecutionException {
        try (StatefulRedisSentinelConnection<String, String> connection = connectSentinel(sentinelUri)) {
            return connection.async().getMasterAddrByName(sentinelUri.getSentinelMasterId())
                    .get(timeout.toNanos(), TimeUnit.NANOSECONDS);
        }
    }

• The getSocketAddress method calls the lookupRedis method, while the lookupRedis method calls the getMasterAddrByName method to obtain the ip address of the master through sentinel

Summary

• The sentinel of redis is similar to the service discovery center of a master. If the master fails, a new master can be obtained through sentinel to achieve failover.
• Sentinel deploys multiple to achieve high availability. If a sentinel is hung, the client uses the next sentinel to obtain the master address

doc

• lettuce Redis-Sentinel