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写在前面 周末无意中发现了之前写的一些笔记，不管是技术还是感悟，看起来一些笔记还是比较透意思的，不知道当时是看到了一段文字记下来还是看了某个文章写了读后感，可以再做整理看一看。 TCP/IP 理解互联网就要理解网络，理解网络就要理解TCP/IP。 TCP/IP模型是一系列网络协议的总称，目的就是使计算机之间可以进行信息交换。 协议简单分四层： 从下到上的四层，分别为连接层（Link Layer）、网络层（Internet Layer）、传输层（Transport Layer）、应用层（Application Layer）。 1）连接层负责建立电路连接，是整个网络的物理基础，典型的协议包括以太网、ADSL等等； 2）网络层负责分配地址和传送二进制数据，主要协议是IP协议； 3）传输层负责传送文本数据，主要协议是TCP协议； 4）应用层负责传送各种最终形态的数据，是直接与用户打交道的层，典型协议是HTTP、FTP等。 科学家在上世纪70年代设计互联网的原始目的，就是为了传输文本。所有协议最初都是为了这个目标而设计的，互联网架构的核心就是文本对话。 比如通过telnet命令建立一个应用层协议，它的作用是在两台主机间，建立一个TCP连接，也就是打开两台主机间文本传输的一个通道。 还有我们使用的HTTP协议也是基于TCP/IP，如果要使用其他"应用层协议"与主机进行对话，你只要改变端口就行了

I’m very much new in software development. Personally I think layered architecture is a great way to reduce the complexities that arise in the process of software development in object oriented approach and, not to mention, to keep your code organized. Now, I’ve run into some problems to be introduced with DDD (Domain Driven Design). Ofcourse, beginner level ones. Here it is – Let's say, I want to build an application to save “person” related data in database and display person details in a wpf datagrid (DDD is definitely not for the apps of such scale, but just to keep things simple for an

开发人员基本都知道，我们的数据存在数据库中（目前最多的是mysql和oracle，由于作者更擅长mysql，所以这里默认数据库为mysql），服务器通过sql语句将查询数据的请求传入到mysql数据库。数据库拿到sql语句以后。都是进行了哪些操作呢？这里向大家介绍下我的个人的理解，欢迎大家评论区批评指正。 流程概述 mysql得到sql语句后，大概流程如下： 1.sql的解析器：负责解析和转发sql 2.预处理器：对解析后的sql树进行验证 3.查询优化器：得到一个执行计划 4.查询执行引擎：得到数据结果集 5.将数据放回给调用端。 流程图如下所示： 分发器及缓存阶段 首先，如果系统的缓存功能开启着的话，sql语句进入mysql后，sql进行判断，是否为select关键字。如果是，那么先去查询缓存中进行查询，如果在查询缓存中可以命中sql语句，那么直接返回查询缓存中的查询语句对应的value值（在缓存中，把查询语句做一个hash运算，结果作为key值，查询的结果集为value）。 如果命中缓存的话，查询速度是相当快的。但是查询缓存也有它相应的缺点。 首先，开启缓存的话，服务器会消耗大量的内存空间；其次，缓存有的时候并不适用；最后，有的情况下，开启缓存也不会将对应的sql语句写入缓存。 缓存不适用的情况： 缓存的锁的力度比较大，而且对于动态sql的支持度不够。

H All, I am Using IText for adding text layers on PDF. Now i want to edit the existing layers on the PDF, layers are also created by IText only. Seems IText dont have such methods to suppport. I thought of other way around is remove existing Layer and place new layer on its place. Seems remove also not supported by IText. Any way todo these? Thanks in adavance. mkl As it turned out in comments, the layers in question are what iText indeed calls layers but what actually in PDF lingo is called optional content groups. There indeed is a utility class for removing such layers in the iText Xtra

When we add any UI or container in WinForms, the later added component comes over the earlier added components, we can say it is in a higher layer. How to change that layer order or component order after adding components? Is it when you load components dynamically in code or in the designer? If it is in the designer you can use the Format -> Order -> Send to Back and Format -> Order -> Bring to Front commands. Send to Back and Bring to Front are also available in the context menu when you right-click a control, as well as in the "Layout" toolbar that should appear automatically when you work

I have a small paint program that I am working on. I am using SetPixel on a bitmap to do that drawing of lines. When the brush size gets large, like 25 pixels across there is a noticeable performance drop. I am wondering if there is a faster way to draw to a bitmap. Here is a bit of the background of the project: I am using bitmaps so that I can utilise layers, like in Photoshop or The GIMP. Lines are being drawn manually because this will eventually use graphics tablet pressure to alter the size of the line over its length. The lines should eventually be anti-aliaced/smoothed along the edges.