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  • 手动刷新缓存

    千次阅读 2018-09-07 11:24:35
    缓存一般都是如下的结构 if(cache.get(targetKey) == null){ cache.put(targetKey,targetObject); } return cache.get(targetKey); 先从缓存里面取数据,缓存里面没有,即没有命中缓存,就去请求新的数据,...

    缓存一般都是如下的结构

    if(cache.get(targetKey) == null){
                cache.put(targetKey,targetObject);
            }
            return cache.get(targetKey);

    先从缓存里面取数据,缓存里面没有,即没有命中缓存,就去请求新的数据,然后保存到缓存;如果缓存有数据,就直接返回。缓存失效了怎么办?

    清空缓存,让用户重试,此时再执行到这段代码,就会重新申请数据,因为缓存为空。

    所以可以预留一个清空缓存的接口来做refresh

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  • 缓存更新机制

    千次阅读 2018-07-16 10:22:46
    看到好些人在写更新缓存数据代码时,先删除缓存,然后再更新数据库,而后续的操作会把数据再装载的缓存中。然而,这个是逻辑是错误的。试想,两个并发操作,一个是更新操作,另一个是查询操作,更新操作删除缓存后,...

    看到好些人在写更新缓存数据代码时,先删除缓存,然后再更新数据库,而后续的操作会把数据再装载的缓存中。然而,这个是逻辑是错误的。试想,两个并发操作,一个是更新操作,另一个是查询操作,更新操作删除缓存后,查询操作没有命中缓存,先把老数据读出来后放到缓存中,然后更新操作更新了数据库。于是,在缓存中的数据还是老的数据,导致缓存中的数据是脏的,而且还一直这样脏下去了。

           我不知道为什么这么多人用的都是这个逻辑,当我在微博上发了这个贴以后,我发现好些人给了好多非常复杂和诡异的方案,所以,我想写这篇文章说一下几个缓存更新的Design Pattern(让我们多一些套路吧)。
           这里,我们先不讨论更新缓存和更新数据这两个事是一个事务的事,或是会有失败的可能,我们先假设更新数据库和更新缓存都可以成功的情况(我们先把成功的代码逻辑先写对)。
           更新缓存的的Design Pattern有四种:Cache aside, Read through, Write through, Write behind caching,我们下面一一来看一下这四种Pattern。


    Cache Aside Pattern
           这是最常用最常用的pattern了。其具体逻辑如下:
           1.失效:应用程序先从cache取数据,没有得到,则从数据库中取数据,成功后,放到缓存中。
           2.命中:应用程序从cache中取数据,取到后返回。

           3.更新:先把数据存到数据库中,成功后,再让缓存失效。

          

           注意,我们的更新是先更新数据库,成功后,让缓存失效。那么,这种方式是否可以没有文章前面提到过的那个问题呢?我们可以脑补一下。
           一个是查询操作,一个是更新操作的并发,首先,没有了删除cache数据的操作了,而是先更新了数据库中的数据,此时,缓存依然有效,所以,并发的查询操作拿的是没有更新的数据,但是,更新操作马上让缓存的失效了,后续的查询操作再把数据从数据库中拉出来。而不会像文章开头的那个逻辑产生的问题,后续的查询操作一直都在取老的数据。
           这是标准的design pattern,包括Facebook的论文《Scaling Memcache at Facebook》也使用了这个策略。为什么不是写完数据库后更新缓存?你可以看一下Quora上的这个问答《Why does Facebook use delete to remove the key-value pair in Memcached instead of updating the Memcached during write request to the backend?》,主要是怕两个并发的写操作导致脏数据。
           那么,是不是Cache Aside这个就不会有并发问题了?不是的,比如,一个是读操作,但是没有命中缓存,然后就到数据库中取数据,此时来了一个写操作,写完数据库后,让缓存失效,然后,之前的那个读操作再把老的数据放进去,所以,会造成脏数据。

           但,这个case理论上会出现,不过,实际上出现的概率可能非常低,因为这个条件需要发生在读缓存时缓存失效,而且并发着有一个写操作。而实际上数据库的写操作会比读操作慢得多,而且还要锁表,而读操作必需在写操作前进入数据库操作,而又要晚于写操作更新缓存,所有的这些条件都具备的概率基本并不大。
           所以,这也就是Quora上的那个答案里说的,要么通过2PC或是Paxos协议保证一致性,要么就是拼命的降低并发时脏数据的概率,而Facebook使用了这个降低概率的玩法,因为2PC太慢,而Paxos太复杂。当然,最好还是为缓存设置上过期时间。


    Read/Write Through Pattern
           我们可以看到,在上面的Cache Aside套路中,我们的应用代码需要维护两个数据存储,一个是缓存(Cache),一个是数据库(Repository)。所以,应用程序比较啰嗦。而Read/Write Through套路是把更新数据库(Repository)的操作由缓存自己代理了,所以,对于应用层来说,就简单很多了。可以理解为,应用认为后端就是一个单一的存储,而存储自己维护自己的Cache。

        1.Read Through
            Read Through 套路就是在查询操作中更新缓存,也就是说,当缓存失效的时候(过期或LRU换出),Cache Aside是由调用方负责把数据加载入缓存,而Read Through则用缓存服务自己来加载,从而对应用方是透明的。

        2.Write Through
           Write Through 套路和Read Through相仿,不过是在更新数据时发生。当有数据更新的时候,如果没有命中缓存,直接更新数据库,然后返回。如果命中了缓存,则更新缓存,然后再由Cache自己更新数据库(这是一个同步操作)
           下图自来Wikipedia的Cache词条。其中的Memory你可以理解为就是我们例子里的数据库。

                

    Write Behind Caching Pattern

            Write Behind 又叫 Write Back。一些了解Linux操作系统内核的同学对write back应该非常熟悉,这不就是Linux文件系统的Page Cache的算法吗?是的,你看基础这玩意全都是相通的。所以,基础很重要,我已经不是一次说过基础很重要这事了。
            Write Back套路,一句说就是,在更新数据的时候,只更新缓存,不更新数据库,而我们的缓存会异步地批量更新数据库。这个设计的好处就是让数据的I/O操作飞快无比(因为直接操作内存嘛 ),因为异步,write backg还可以合并对同一个数据的多次操作,所以性能的提高是相当可观的。
            但是,其带来的问题是,数据不是强一致性的,而且可能会丢失(我们知道Unix/Linux非正常关机会导致数据丢失,就是因为这个事)。在软件设计上,我们基本上不可能做出一个没有缺陷的设计,就像算法设计中的时间换空间,空间换时间一个道理,有时候,强一致性和高性能,高可用和高性性是有冲突的。软件设计从来都是取舍Trade-Off。
           另外,Write Back实现逻辑比较复杂,因为他需要track有哪数据是被更新了的,需要刷到持久层上。操作系统的write back会在仅当这个cache需要失效的时候,才会被真正持久起来,比如,内存不够了,或是进程退出了等情况,这又叫lazy write。
           在wikipedia上有一张write back的流程图,基本逻辑如下:

              

    再多唠叨一些
           1)上面讲的这些Design Pattern,其实并不是软件架构里的mysql数据库和memcache/redis的更新策略,这些东西都是计算机体系结构里的设计,比如CPU的缓存,硬盘文件系统中的缓存,硬盘上的缓存,数据库中的缓存。基本上来说,这些缓存更新的设计模式都是非常老古董的,而且历经长时间考验的策略,所以这也就是,工程学上所谓的Best Practice,遵从就好了。
           2)有时候,我们觉得能做宏观的系统架构的人一定是很有经验的,其实,宏观系统架构中的很多设计都来源于这些微观的东西。比如,云计算中的很多虚拟化技术的原理,和传统的虚拟内存不是很像么?Unix下的那些I/O模型,也放大到了架构里的同步异步的模型,还有Unix发明的管道不就是数据流式计算架构吗?TCP的好些设计也用在不同系统间的通讯中,仔细看看这些微观层面,你会发现有很多设计都非常精妙……所以,请允许我在这里放句观点鲜明的话——如果你要做好架构,首先你得把计算机体系结构以及很多老古董的基础技术吃透了。
           3)在软件开发或设计中,我非常建议在之前先去参考一下已有的设计和思路,看看相应的guideline,best practice或design pattern,吃透了已有的这些东西,再决定是否要重新发明轮子。千万不要似是而非地,想当然的做软件设计。
           4)上面,我们没有考虑缓存(Cache)和持久层(Repository)的整体事务的问题。比如,更新Cache成功,更新数据库失败了怎么吗?或是反过来。关于这个事,如果你需要强一致性,你需要使用“两阶段提交协议”——prepare, commit/rollback,比如Java 7 的XAResource,还有MySQL 5.7的 XA Transaction,有些cache也支持XA,比如EhCache。当然,XA这样的强一致性的玩法会导致性能下降,关于分布式的事务的相关话题,你可以看看《分布式系统的事务处理》一文。

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  • 刷新页面缓存动作

    2017-08-25 09:08:54
    css样式 html,body{width:100%;height:100%;} html,body,div{margin:0;padding:0;}  #bodyLoading{position:absolute;top:0;left:0;width:100%;height:100%;background:#fff;z-index:999999;...#load

    css样式

    <style>
    html,body{width:100%;height:100%;}
    html,body,div{margin:0;padding:0;}
       #bodyLoading{position:absolute;top:0;left:0;width:100%;height:100%;background:#fff;z-index:999999;opacity:1;}
    #loading-center-absolute{position:absolute;left:50%;top:50%;height:150px;width:150px;margin-top:-75px;margin-left:-75px;-moz-border-radius:50% 50% 50% 50%;-webkit-border-radius:50% 50% 50% 50%;border-radius:50% 50% 50% 50%;z-index:99999999999999;}
    .object{width:15px;height:15px;background-color:#6cf;position:absolute;-moz-border-radius:50% 50% 50% 50%;-webkit-border-radius:50% 50% 50% 50%;border-radius:50% 50% 50% 50%;-webkit-animation:animate .8s infinite;animation:animate .8s infinite}
    #object_one{top:19px;left:19px}
    #object_two{top:0;left:65px;-webkit-animation-delay:.1s;animation-delay:.1s}
    #object_three{top:19px;left:111px;-webkit-animation-delay:.2s;animation-delay:.2s}
    #object_four{top:65px;left:130px;-webkit-animation-delay:.3s;animation-delay:.3s}
    #object_five{top:111px;left:111px;-webkit-animation-delay:.4s;animation-delay:.4s}
    #object_six{top:130px;left:65px;-webkit-animation-delay:.5s;animation-delay:.5s}
    #object_seven{top:111px;left:19px;-webkit-animation-delay:.6s;animation-delay:.6s}
    #object_eight{top:65px;left:0;-webkit-animation-delay:.7s;animation-delay:.7s}
    @-webkit-keyframes animate{25%{-ms-transform:scale(1.5);-webkit-transform:scale(1.5);transform:scale(1.5)}
    75%{-ms-transform:scale(0);-webkit-transform:scale(0);transform:scale(0)}
    }
    @keyframes animate{50%{-ms-transform:scale(1.5,1.5);-webkit-transform:scale(1.5,1.5);transform:scale(1.5,1.5)}
    100%{-ms-transform:scale(1,1);-webkit-transform:scale(1,1);transform:scale(1,1)}
    }
    input[type=button]:disabled{background:#f9f6f6;border:none;color:#c7c7c7;cursor:not-allowed}
    </style>


    html内容

    <div id="bodyLoading"></div>
      <div id="loading-center-absolute">
    <div class="object" id="object_one"></div>
    <div class="object" id="object_two"></div>
    <div class="object" id="object_three"></div>
    <div class="object" id="object_four"></div>
    <div class="object" id="object_five"></div>
    <div class="object" id="object_six"></div>
    <div class="object" id="object_seven"></div>
    <div class="object" id="object_eight"></div>
    </div>


    js

    $(function(){
     $('#bodyLoading').remove();
     $('#loading-center-absolute').fadeOut(300);
     });

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  • 刷新DNS缓存

    千次阅读 2012-10-23 10:15:19
    答案就是在本地DNS有解析缓存,电脑第一次访问后,在一定的时间内就将其缓存下来,下次访问该域名时电脑通过查找本地DNS缓存,就可以直接知道IP了,而不用再进行域名解析了,这就提高了效率,这就是DNS缓存,而域名...

    在日常工作中,常有人问到域名解析了不是即时生效的嘛,怎么还是原来的呢?答案就是在本地DNS有解析缓存,电脑第一次访问后,在一定的时间内就将其缓存下来,下次访问该域名时电脑通过查找本地DNS缓存,就可以直接知道IP了,而不用再进行域名解析了,这就提高了效率,这就是DNS缓存,而域名更改过解析后,虽然DNS服务器上已经更新,但本地还有DNS缓存,造成还是老的IP,可以通过下面的方法来解决:

    刷新DNS缓存的方法一

    首先进入命令提示符下(开始——运行——cmd);

    先运行:ipconfig /displaydns这个命令,查看一下本机已经缓存了那些的dns信息的,然后输入下面的命令

    ipconfig /flushdns

    这时本机的dns缓存信息已经清空了,我们可以再次输入第一次输入的命令来看一下,

    ipconfig /displaydns

    刷新DNS缓存的方法二

    直接禁用网卡再启用网卡,这样也可以




    How To Flush DNS


    When surfing the Internet, a problem that can arise for consumers is getting a number of DNS errors or 404 errors in the web browser despite a working Internet connection. If this is the case, then learning how to flush DNS for your computer’s operating system (OS) is key to restoring the proper quality of service for your computer’s web browsing capability. The act of flushing the DNS removes the local resolution for the website name saved or cached on your computer which can result in a slower initial revisit to the webpage; however, it will eliminate browsing errors encountered from the entry being corrupted.

    What is DNS?

    The Domain Name System (DNS) is a database system which translates the plain language web address or URL into an IP address. The system was created since it’s easier to remember a domain name such as www.tech-faq.com than 123.456.789.10. Typically, every local computer network will have at least one server that handles DNS requests or queries (called a name server) which performs a caching or quick-lookup function to minimize the time it takes to resolve domain look-ups. If the local name server doesn’t have a domain name stored, it will request the identity from the next name server it knows about and so forth.

    History of DNS

    Before DNS was invented in 1982, the idea of using a name vice a computer host’s numerical address on the network dates to the ARPANET. Prior to 1982, every computer on the network would have a HOSTS.TXT file that would be retrieved from a computer at SRI. The field would map the names to numerical addresses. The hosts file is still in use on modern operating systems to map 127.0.0.1 to the “localhost.” The rapid growth of computer networks; however, made the hand edited hosts file almost unusable. As a result, Paul Mockapetris invented the Domain Name System in 1983 and wrote the first implementation. The original implementation was subsequently published as an IETF standard and has been superseded since that time. The first Unix implementation of DNS was completed in 1984 by Douglas Terry, David Riggle, Mark Painter, and Songnian Zhou referred to as BIND (Berkeley Internet Name Domain) server. BIND would later be ported to Windows NT in the early 1990s.

    DNS Cache Poisoning

    Similar to other fundamental Internet constructs, the Domain Name System was not originally designed to be secure. DNS cache poisoning is the term used to describe a data integrity or security compromise in DNS. The vulnerability occurs when data gets introduced to DNS cache that did not come from or originate with an authoritative DNS source. This can occur from a malicious attack or from a bad or misconfiguration of DNS cache. Once a DNS server has received the non-authenticated information and caches or saves to the database, it is then considered poisoned when the bad information starts to be supplied to client machines of the server. At the minimum, a poisoned server will send end-users to incorrect websites. During malicious attacks, they may be directed to malicious web pages designed to infect the client computer or pretend to be legitimate websites designed to steal user’s information and/or money.

    Preventing DNS Cache Poisoning

    Many of the traditional DNS cache poisoning attacks are prevented by making DNS servers trust the information passed to them by other DNS servers less. This is accomplished by ignoring DNSrecords that are returned to the requesting server not related to the query. The most current version of BIND addresses poisoning attacks in this manner. Additionally, implementing source port randomization for DNS requests (makes requests harder to spoof) along with using cryptographically-secure random numbers to select the source port and nonce significantly reduce the probability of DNS attacks. Secure DNS or DNSSEC is the current initiative being slowly deployed to fully solve the DNS cache poisoning issue by using electronic signatures and public key certificates to validate the authenticity of data.

    Common Problems with DNS

    There are wide arrays of DNS issues that can arise at the network administrator or power user level. For the end-user; however, the majority of DNS problems arise from either bad configuration entries or the local computer’s DNS storage requiring flushing. Independent of the type of operating system, many home computer users will input the DNS Server for their respective Internet Service Provider (ISP) incorrectly resulting in a failed Internet connection. Each ISP will have a slightly different configuration process; however, the IP address of the DNS server for your home network to use will be provided on registration for service. Many times the ISP will use the address for their actual DNS server, where others it will be the same as the Gateway IP for the service (also the IP address of the router).

    How to Flush DNS in Windows Using the Command Prompt

    Step 1 – Close all open web browsers and applications on your computer.

    Step 2 – Open the Windows command prompt by selecting the “Start” menu and entering “cmd” in the search text field box followed by clicking the “enter” key.

    Step 3 – At the command prompt, enter “ipconfig /flushdns” followed by pressing the “enter” key. After a moment, Windows will display a message similar to: “Successfully flushed the DNS ResolverCache.” Once the message is displayed, the DNS will be flushed removing all incorrect entries.

    Step 4 – View the DNS resolver cache by entering, “ipconfig /displaydns” at the command prompt followed by clicking the “Enter” key.

    How to Turn Off DNS Caching in Windows

    If clearing DNS cache does not solve frequent DNS errors on the computer running Microsoft Windows, client-side DNS caching can be disabled. When DNS caching on the client disabled, your computer will still be “usable,” but just not as efficient or “fast” as it is with the service turned on and working properly.

    Step 1 – Open the Windows command prompt by selecting the “Start” menu button and entering “cmd” in the search text field followed by pressing the “enter” key on your computer.

    Step 2 –  Enter “net stop dnscache” or “sc servername stop dnscache” at the command prompt followed by pressing the “enter” key on your computer. DNS caching will be disabled until the next time the computer is restarted or rebooted. In order to make the change permanent, the DNS Client Service will need to be changed to disabled using the Microsoft Service Controller or

    Services tool.

    Steps to Stop DNS Client Services in Windows Using the Services Tool

    Step 1 – Open the Windows command prompt. Then, input services.msc at the command prompt and press the enter key.

    Step 2 – Locate the “DNS Client” application and double click the program icon.

    Step 3 – Select the “Stop” menu button. Conversely, the application can be restarted in the same location of the services user interface.

    Changing DNS Cache Settings in Windows

    An alternative method that can be used to modify Windows DNS caching on the client-side is to change two of the registry entries in the registry associated with the service.

    Step 1 – Select the “Start” menu button and enter “regedit” in the search text field followed by pressing the “enter” key.

    Step 2 – Click the “Edit” and “Find”  and enter “DNSCache” or use the menu on the left hand side of the editor to locate: HKEY_LOCAL_MACHINESYSTEM -> CurrentControlSet -> Services -> Dnscache -> Parameters.

    Step 3 – The MaxCacheTTL is the maximum amount of time that Windows will cache a DNS lookup. The default value at the time of this writing is 86,400 seconds. You can change this entry to 1 to force windows to clear the cache every second. This can result in a negative performance drop on your computer.

    Step 4 – Another registry key setting that can be changed is MaxNegativeCacheTTL which is the maximum amount of time that a failed DNS result will be cached. This is normally set to 900 seconds, but if you change it to 0, Windows will not store failed look-ups.

    *Note, the path to the DNS cache registry key will be slightly different based on the version of Windows installed on your computer. Modifying the registry should be done with caution and not by those who lack significant computer experience.

    How to Flush DNS in Mac OS X

    If you are an Internet or web developer or do a fair amount of administrator tasks on your Mac, then the requirement to flush DNS cache will arise. Depending on what version of the OS is installed on your computer (Leopard vs Tiger), there will be a slightly different command to flush DNS.

    Step 1 – If Mozilla Firefox is installed on your computer, exit the application if it is open.

    Step 2 – Open the terminal on your computer.

    Step 3 – On a computer running Lion (Mac OS X 10.5, 10.6, or 10.7) enter the following command followed by pressing the “return” key:

    dscacheutil –flushcache

    Step 4 – In Mac OS X 10.4 Tiger, enter the following command followed by pressing the “return” key:

    lookupd –flushcache

    How to Flush DNS in Linux

    In many builds of Linux the nscd daemon is used to manage the client DNS cache. For builds that use this method, restarting the nscd daemon is the primary means to flush DNS cache. Other builds of Linux may run BIND or dnsmasq as the primary name service.

    Steps to Flush NSCD DNS Cache

    NSCD is used to speed up consecutive access to the same data and improve overall system performance.

    Step 1 – Enter “$sudo /etc/init.d/nscd restart and press the “enter” or “return” key.

    Step 2 – Once nscd restarts the DNS will be flushed. Alternatively, you can enter “# service nscd restart.”

    Steps to Flush DNSMASQ DNS Cache

    DNMASQ is used as a lightweight DHCP, TFTP, and DNS server. It was primarily designed to provide DHCP and DNS services to a LAN, accepts DNS queries, and answers them. It is also installed on a number of routers to cache DNS queries or look-ups.

    Step 1 – Enter “$ sudo /etc/init.d/dnsmasq restart”

    Step 2 – Once dnsmasq restarts the DNS will be flushed.

    Steps to Flush BIND Server DNS Cache

    Similar to nscd and dnsmasq, to flush DNS cache in BIND simply requires a restart to clear the cache.

    Step 1 – Enter “# /etc/init.d/named restart” followed by pressing the “enter” or “return” key.

    Step 2 – Once BIND completes restart the DNS will be flushed.

    How to Disable Firefox DNS Cache

    Unlike other popular web browsers, Firefox runs its own DNS cache separate from the client services on your computer. As a result, if you commonly switch between a VPN, home ISP, or other connections, undesired results may occur if you access services which require use of your VPN, etc. For this reason, when you take actions to fully flush DNS on your computer, FireFox should be closed. If DNS issues persist on your computer and FireFox is your preferred browser, then the answer may lay in disabling DNS Cache in the browser specifically.

    Step 1 – Launch Mozilla Firefox on your computer.

    Step 2 – Install the Firefox DNS cache plugin.

    Step 3 – Right click the “Home” menu button on Firefox and then select the “Customize” menu option.

    Step 4 – Locate the “DNS Cache” menu button and then drag it beside the “Home” button on Firefox followed by clicking the “Done” button.

    Step 5 – Click the “DNS Cache” menu button whenever you want to have Firefox DNS cache disabled.

    Step 6 – Alternatively, you can change the FireFox DNS settings to have the cache expire after 0 seconds. To do so, enter “about:config” in the location bar followed by pressing the “enter” or “return” key. Right click at a blank location on the subsequently displayed record and select “New” and “Integer.”

    Step 7 – Enter “network.dnsCacheExpiration” as the preference name with “0” as the integer value. If “network.dnsCacheExpiration” already exists, modify the integer value to “0.” If you want to restore FireFox defaults, simply change the value back to the default of  “3600.” With 0 entered, DNS cache in FireFox will automatically expire on your computer.

    Video on How to Flush DNS

    The following video provides highlights on how to flush DNS in Microsoft Windows, MAC OS X, and Linux.


    展开全文
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