Entity Framework——讀寫分離
阿新 • • 發佈：2017-11-19
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1 實現
CustomDbContext擴展了DbContext，其構造函數帶有形式參nameOrConnectionString，可以在使用CustomDbContext時指定數據庫連接字符串。
DbContextFactory包含兩個屬性MasterDbContext和SlaveDbContext，MasterDbContext為主庫上下文，SlaveDbContext為從庫上下文。DbContextFactory還包含了一個方法：UpdateSlaves用於實現對SlaveDbContext的更新，因為SlaveDbContext是從多個配置的從庫隨機取出一個，因此定時檢測不可用從庫，將其從從庫集合中剔除。
JobScheduler為定時任務規劃器，使用Quartz實現。quartz.config和quartz_jobs.xml為定時任務配置文件。
為了使定時任務工作，在WebApiApplication類的Application_Start()函數應添加：
JobScheduler jobScheduler = new JobScheduler(); jobScheduler.log4netPath = AppSettings.Log4netPathForWeb; jobScheduler.OnStart();
關鍵代碼
/// <summary> /// 自定義上下文/// </summary> [DbConfigurationType(typeof(MySqlEFConfiguration))] public class CustomDbContext:DbContext { public CustomDbContext(string nameOrConnectionString) : base(nameOrConnectionString) { Database.SetInitializer<CustomDbContext>(null); } ...... public DbSet<Collection> Collections { get; set; } public DbSet<CollectionUser> CollectionUsers { get; set; } ...... protected override void OnModelCreating(DbModelBuilder modelBuilder) { base.OnModelCreating(modelBuilder); EntityConfiguration.Set(modelBuilder); } } public class EntityConfiguration { public static void Set(DbModelBuilder modelBuilder) { modelBuilder.Entity<Collection>().Property(c => c.FileName) .IsUnicode(false) .IsRequired() .HasMaxLength(50); modelBuilder.Entity<Collection>().Property(c => c.TableName) .IsUnicode(false) .IsRequired() .HasMaxLength(50); modelBuilder.Entity<Collection>().Property(c => c.Title) .IsUnicode(false) .IsRequired() .HasMaxLength(200); modelBuilder.Entity<Collection>().Property(c => c.Author) .IsUnicode(false) .IsOptional() .HasMaxLength(50); modelBuilder.Entity<Collection>().Property(c => c.PublicationName) .IsUnicode(false) .IsOptional() .HasMaxLength(200); modelBuilder.Entity<Collection>().Property(c => c.DiscNo) .IsUnicode(false) .IsOptional() .HasMaxLength(50); modelBuilder.Entity<Collection>().Property(c => c.ResourceType) .IsUnicode(false) .IsOptional() .HasMaxLength(50); modelBuilder.Entity<Collection>().Property(c => c.PublisherUnit) .IsUnicode(false) .IsOptional() .HasMaxLength(200); modelBuilder.Entity<Collection>().Property(c => c.Year) .IsUnicode(false) .IsOptional() .HasMaxLength(10); modelBuilder.Entity<Collection>().Property(c => c.Period) .IsUnicode(false) .IsOptional() .HasMaxLength(10); modelBuilder.Entity<Collection>().Property(c => c.PublicationDate) .IsOptional(); modelBuilder.Entity<Collection>().Property(c => c.Downloads) .IsOptional(); modelBuilder.Entity<Collection>().Property(c => c.CitationNumber) .IsOptional(); } } /// <summary> /// db上下文工廠 /// </summary> public class DbContextFactory { private static List<string> allSlaves = GetAllSlaves(); private DbContextFactory() { } /// <summary> /// 主 /// </summary> public static CustomDbContext MasterDbContext { get { return new CustomDbContext("name=Master"); } } /// <summary> /// 從 /// </summary> public static CustomDbContext SlaveDbContext { get { Random rm = new Random(); if (allSlaves.Count > 0) { int i = rm.Next(allSlaves.Count); string name = string.Format("name={0}", allSlaves.ElementAt(i)); return new CustomDbContext(name); } else { return MasterDbContext; } } } /// <summary> /// 獲得所有可用連接 /// </summary> /// <returns></returns> private static List<string> GetAllSlaves() { List<string> connNames = new List<string>(); var conns = ConfigurationManager.ConnectionStrings; if (conns == null) { throw new Exception("ConfigurationManager.ConnectionStrings 是空值，請檢查Web.config"); } var masterConn = conns["Master"]; if (masterConn == null) { throw new Exception("名稱為Master的連接配置不存在，請檢查Web.config"); } //conn中必然包含master，還有一個默認的LocalSqlServer int connCount = conns.Count - 1; if (connCount == 0) { throw new Exception("連接配置中只包含Master，不包含任何Slave，請檢查Web.config，並配置Slave"); } for (int i = 0; i < connCount; i++) { string connName = string.Format("Slave{0}", i); var conn = ConfigurationManager.ConnectionStrings[connName]; if (conn == null) { string msg = string.Format("{0}不存在，請檢查配置Web.config", connName); throw new Exception(msg); } //檢測是否可連接 bool canConn = CanConnect(connName); if (canConn) { connNames.Add(connName); } } return connNames; } public static void UpdateSlaves() { allSlaves = GetAllSlaves(); if (allSlaves.Count == 0) { allSlaves.Add("Master"); } } private static bool CanConnect(string connName) { bool ret = false; DbConnection dbConnection = null; try { string connStr = ConfigurationManager.ConnectionStrings[connName].ToString(); MySqlConnectionFactory factory = new MySqlConnectionFactory(); dbConnection = factory.CreateConnection(connStr); dbConnection.Open();//打不開會拋異常 ret = true; } catch (Exception ex) { } finally { if (dbConnection != null && dbConnection.State == System.Data.ConnectionState.Open) dbConnection.Close(); } return ret; } } public class JobScheduler { /// <summary> /// log4net配置文件位置 /// </summary> public string log4netPath { get; set; } private IScheduler scheduler; public JobScheduler() { } public void OnStart() { //構造函數自動加載Quartz.config,並通過quartz.plugin.xml.fileNames加載~/quartz_jobs.xml try { if (string.IsNullOrWhiteSpace(log4netPath)) { log4netPath = AppSettings.Log4netPathForApp; } //加載日誌 LogConfigLoading.Load(log4netPath); ISchedulerFactory sf = new StdSchedulerFactory(); scheduler = sf.GetScheduler(); scheduler.Start(); } catch (Exception ex) { LogHelper.LogError(ex, "JobScheduler"); } } public void OnStop() { if (scheduler != null && scheduler.IsStarted) { scheduler.Shutdown(false); } } public void OnPause() { if (scheduler != null && scheduler.IsStarted) { scheduler.PauseAll(); } } public void OnContinue() { if (scheduler != null) { bool isJobGroupPaused = false; var groups = scheduler.GetJobGroupNames(); foreach (var group in groups) { isJobGroupPaused = scheduler.IsJobGroupPaused(group); if (!isJobGroupPaused) { break; } } if (isJobGroupPaused) { scheduler.ResumeAll(); } } } } public class DbMonitorJob : IJob { public void Execute(IJobExecutionContext context) { DbContextFactory.UpdateSlaves(); } }
定時器配置文件quartz_jobs.xml
<?xml version="1.0" encoding="UTF-8"?> <!-- This file contains job definitions in schema version 2.0 format --> <job-scheduling-data xmlns= "http://quartznet.sourceforge.net/JobSchedulingData" xmlns:xsi ="http://www.w3.org/2001/XMLSchema-instance " version ="2.0 "> <processing-directives> <overwrite-existing-data>true</overwrite-existing-data> </processing-directives> <schedule> <job> <name>DbMonitorJob</name> <group>DbMonitorJobGroup</group> <description>更新可用從庫</description> <job-type>HY_WebApi.TaskScheduler.Jobs.DbMonitorJob, HY_WebApi.TaskScheduler</job-type> <durable>true</durable> <recover>false</recover> </job> <trigger> <simple> <name>DbMonitorJobTrigger</name> <group>DbMonitorJobTriggerGroup</group> <description>更新可用從庫</description> <job-name>DbMonitorJob</job-name> <job-group>DbMonitorJobGroup</job-group> <misfire-instruction>SmartPolicy</misfire-instruction> <repeat-count>-1</repeat-count> <repeat-interval>10000</repeat-interval> </simple> </trigger> </schedule> </job-scheduling-data>
定時任務配置文件quartz.config
# You can configure your scheduler in either <quartz> configuration section # or in quartz properties file # Configuration section has precedence quartz.scheduler.instanceName = ServerScheduler # configure thread pool info quartz.threadPool.type = Quartz.Simpl.SimpleThreadPool, Quartz quartz.threadPool.threadCount = 1 quartz.threadPool.threadPriority = Normal # job initialization plugin handles our xml reading, without it defaults are used quartz.plugin.xml.type = Quartz.Plugin.Xml.XMLSchedulingDataProcessorPlugin, Quartz quartz.plugin.xml.fileNames = ~\quartz_jobs.xml # export this server to remoting context quartz.scheduler.exporter.type = Quartz.Simpl.RemotingSchedulerExporter, Quartz quartz.scheduler.exporter.port = 555 quartz.scheduler.exporter.bindName = QuartzScheduler quartz.scheduler.exporter.channelType = tcp quartz.scheduler.exporter.channelName = httpQuartz
web項目配置文件Web.config
<configuration> <connectionStrings> <clear/><!--清除默認的連接字符串，務必加上！！！--> <add name="Master" connectionString="Database=hy_webapi_n;Data Source=192.168.107.65;User Id=root;Password=cnki2016;CharSet=utf8;port=3306" providerName="MySql.Data.MySqlClient" /> <add name="Slave0" connectionString="Database=hy_webapi_n;Data Source=192.168.107.62;User Id=root;Password=cnki2016;CharSet=utf8;port=3306" providerName="MySql.Data.MySqlClient" /> <add name="Slave1" connectionString="Database=hy_webapi_n;Data Source=192.168.107.63;User Id=root;Password=cnki2016;CharSet=utf8;port=3306" providerName="MySql.Data.MySqlClient" /> </connectionStrings> ...... </configuration>
加載定時器
public class WebApiApplication : System.Web.HttpApplication { protected void Application_Start() { GlobalConfiguration.Configure(WebApiConfig.Register); JobScheduler jobScheduler = new JobScheduler(); jobScheduler.log4netPath = AppSettings.Log4netPathForWeb; jobScheduler.OnStart(); } }
2 代碼分析
最核心的部分是DbContextFactory。下面詳細分析其設計與實現。
獲得web.config配置文件中的連接名稱
使用靜態私鑰變量allSlaves來表示從庫集合，這樣做的好處是：靜態私有變量只在使用前初始化一次，當第一次被allSlaves使用時初始化一次，即調用GetAllSlaves()方法獲得所有可用的從庫。當第二次使用allSlaves時，即當SlaveDbContext屬性第二次被調用時，不在計算allSlaves。大部分時間都花費在測試數據庫是否可用，因此不在重復計算allSlaves節省了時間。直接的效果就是由於檢測數據庫是否可用的影響可以忽略不計。
不可使用單例模式
由於檢測數據庫是否可用相對耗費時間的比例較大，於是想到通過單例模式來實現DbContextFactory，這樣會導致系統報錯：The operation cannot be completed because the DbContext has been disposed.其原因就在於使用DbContext時，慎重使用單例模式，全局的DbContext會引起第二次調用出錯，即第一次調用後DbContext資源即被釋放。
類似於單例模式的實現，即全局的DbContext，也是不可取的。
基於上述考慮設計實現SlaveDbContext，在每次被調用時，都會返回一個新的實例。
多從庫隨機選擇
當配置了多個從庫時，應隨機從從庫集合中選擇一個。於是使用偽隨機數生成器Random。
所有從庫不可用時切換到主庫
當所有從庫都不可用時，SlaveDbContext值為MasterDbContext。這裏還應該增加一個額外的監測服務，當有從庫不可用時自動報警，供系統維護人員查看。
註意先寫後讀的操作
對於這種操作，若主從同步延遲稍大，那麽會造成操作失敗，解決的辦法是：只操作主庫。保守的做法就是只操作主庫，一般主從分部在內網的兩臺機器上，網絡通信延遲一旦較大時，就會造成數據無法同步的假象。
Entity Framework——讀寫分離