企业级工作流解决方案（十三）--集成Abp和ng-alain--数据库读写分离

　　说到程序里面数据库管理，无非就是两件事情，一是数据库操作，对于数据库的操作，各种程序语言都有封装，也就是所谓的ORM框架，.net 方向一般用得比较多和就是.net framework和dapper，abp里还集成了NHibernate，另外就是连接字符串的管理，简单的应用直接用一个数据库连接字符串就可以了，但是对于大型的应用，比如有多租户概念的系统，比如有一些分库分表需求的设计系统，那么连接字符串的管理将是非常复杂和核心的内容。

　　对于读写分离，大家应该比较熟悉，数据库层面，大型的关系型数据库都支持，这里的读写分离是指代码层面，针对DBA已经做好的数据库读写分离来管理数据库连接字符串。

　　Abp基本框架提供了最基础的数据库连接字符串管理，zero项目实现了多租户的数据库连接管理，即把每个租户的连接字符串存储在租户里面，对于每一个Uow操作，都会找租户的连接字符串，如果找到，就使用，没有找到，向上层找默认的连接字符串。代码如下：

/// <summary>
    /// Implements <see cref="IDbPerTenantConnectionStringResolver"/> to dynamically resolve
    /// connection string for a multi tenant application.
    /// </summary>
    public class DbPerTenantConnectionStringResolver : DefaultConnectionStringResolver, IDbPerTenantConnectionStringResolver
    {
        /// <summary>
        /// Reference to the session.
        /// </summary>
        public IAbpSession AbpSession { get; set; }

        private readonly ICurrentUnitOfWorkProvider _currentUnitOfWorkProvider;
        private readonly ITenantCache _tenantCache;

        /// <summary>
        /// Initializes a new instance of the <see cref="DbPerTenantConnectionStringResolver"/> class.
        /// </summary>
        public DbPerTenantConnectionStringResolver(
            IAbpStartupConfiguration configuration,
            ICurrentUnitOfWorkProvider currentUnitOfWorkProvider,
            ITenantCache tenantCache)
            : base(configuration)
        {
            _currentUnitOfWorkProvider = currentUnitOfWorkProvider;
            _tenantCache = tenantCache;

            AbpSession = NullAbpSession.Instance;
        }

        public override string GetNameOrConnectionString(ConnectionStringResolveArgs args)
        {
            if (args.MultiTenancySide == MultiTenancySides.Host)
            {
                return GetNameOrConnectionString(new DbPerTenantConnectionStringResolveArgs(null, args));
            }

            return GetNameOrConnectionString(new DbPerTenantConnectionStringResolveArgs(GetCurrentTenantId(), args));
        }

        public virtual string GetNameOrConnectionString(DbPerTenantConnectionStringResolveArgs args)
        {
            if (args.TenantId == null)
            {
                //Requested for host
                return base.GetNameOrConnectionString(args);
            }

            var tenantCacheItem = _tenantCache.Get(args.TenantId.Value);
            if (tenantCacheItem.ConnectionString.IsNullOrEmpty())
            {
                //Tenant has not dedicated database
                return base.GetNameOrConnectionString(args);
            }

            return tenantCacheItem.ConnectionString;
        }

        protected virtual int? GetCurrentTenantId()
        {
            return _currentUnitOfWorkProvider.Current != null
                ? _currentUnitOfWorkProvider.Current.GetTenantId()
                : AbpSession.TenantId;
        }
    }

　　那么我们要改造的地方其他就可以参照这个来管理连接字符串

　　还有一个问题，就是我们怎么让框架知道我们使用的是读库还是写库呢？

　　Abp里面，公开给用户控制Uow的，就是UnitOfWorkAttribute装饰器，增加一个读库还是写库的标识IsReadDb，在UnitOfWorkOptions类里面也要加对应的属性，那么我们在构造UnitOfWorkOptions类的时候，可以把属性装饰器里面的IsReadDb属性赋值给UnitOfWorkOptions，再获取DbContext方法的时候，把此参数传入Uow连接字符串管理，在连接字符串管理里面，判断此参数的值，确定数据库字符串选择。

　　主要代码：

/// <summary>
    /// Unit of work options.
    /// </summary>
    public class UnitOfWorkOptions
    {
        // ......

        /// <summary>
        /// 自定义：设置是否是读库
        /// </summary>
        public bool IsReadDb { get; set; }
}

[AttributeUsage(AttributeTargets.Method | AttributeTargets.Class | AttributeTargets.Interface)]
    public class UnitOfWorkAttribute : Attribute
    {
        // ......

        /// <summary>
        /// 自定义：设置是否是读库
        /// </summary>
        public bool IsReadDb { get; set; }
        public UnitOfWorkOptions CreateOptions()
        {
            return new UnitOfWorkOptions
            {
                IsTransactional = IsTransactional,
                IsolationLevel = IsolationLevel,
                Timeout = Timeout,
                Scope = Scope,
                IsReadDb = IsReadDb
            };
        }
    }

　　在获取DbContext方法的时候，传递数据库读写参数

public static class UnitOfWorkExtensions
    {
        public static TDbContext GetDbContext<TDbContext>(this IActiveUnitOfWork unitOfWork, MultiTenancySides? multiTenancySide = null, string name = null)
            where TDbContext : DbContext
        {
            if (unitOfWork == null)
            {
                throw new ArgumentNullException("unitOfWork");
            }

            if (!(unitOfWork is EfCoreUnitOfWork))
            {
                throw new ArgumentException("unitOfWork is not type of " + typeof(EfCoreUnitOfWork).FullName, "unitOfWork");
            }

            return (unitOfWork as EfCoreUnitOfWork).GetOrCreateDbContext<TDbContext>(multiTenancySide, name, unitOfWork.Options.IsReadDb);
        }
}

public virtual TDbContext GetOrCreateDbContext<TDbContext>(MultiTenancySides? multiTenancySide = null, string name = null,bool isReadDb = false)
            where TDbContext : DbContext
        {
            var concreteDbContextType = _dbContextTypeMatcher.GetConcreteType(typeof(TDbContext));

            var connectionStringResolveArgs = new ConnectionStringResolveArgs(multiTenancySide);
            connectionStringResolveArgs["DbContextType"] = typeof(TDbContext);
            connectionStringResolveArgs["DbContextConcreteType"] = concreteDbContextType;
            connectionStringResolveArgs["IsReadDb"] = isReadDb;
            var connectionString = ResolveConnectionString(connectionStringResolveArgs);
}

　　最终，参照Abp连接字符串的管理，代码如下：

public class DbPerTenantConnectionStringResolver : DefaultConnectionStringResolver, IDbPerTenantConnectionStringResolver
    {
        /// <summary>
        /// Reference to the session.
        /// </summary>
        public IAbpSession AbpSession { get; set; }

        private readonly ICurrentUnitOfWorkProvider _currentUnitOfWorkProvider;

        /// <summary>
        /// Initializes a new instance of the <see cref="DbPerTenantConnectionStringResolver"/> class.
        /// </summary>
        public DbPerTenantConnectionStringResolver(
            IAbpStartupConfiguration configuration,
            ICurrentUnitOfWorkProvider currentUnitOfWorkProvider)
            : base(
                  configuration)
        {
            _currentUnitOfWorkProvider = currentUnitOfWorkProvider;
            AbpSession = NullAbpSession.Instance;
        }

        public override string GetNameOrConnectionString(ConnectionStringResolveArgs args)
        {
            if (args.MultiTenancySide == MultiTenancySides.Host)
            {
                return GetNameOrConnectionString(new DbPerTenantConnectionStringResolveArgs(null, args));
            }

            return GetNameOrConnectionString(new DbPerTenantConnectionStringResolveArgs(GetCurrentTenantId(), args));
        }

        public virtual string GetNameOrConnectionString(DbPerTenantConnectionStringResolveArgs args)
        {
            if (args.TenantId == null)
            {
                //Requested for host
                return base.GetNameOrConnectionString(args);
            }

            var tenantCacheItem = Rpc.Call<Sys_TenantQueryDto>("CommonService.CommonServiceServiceAppService.GetTenantInfo", args.TenantId);
            if(tenantCacheItem == null)
            {
                return base.GetNameOrConnectionString(args);
            }

            if(Convert.ToBoolean(args["IsReadDb"]))
            {
                if(!string.IsNullOrEmpty(tenantCacheItem.ReadConnectionString))
                {
                    return tenantCacheItem.ReadConnectionString;
                }
                else
                {
                    return base.GetNameOrConnectionString(args);
                }
            }
            else
            {
                if (!string.IsNullOrEmpty(tenantCacheItem.ConnectionString))
                {
                    return tenantCacheItem.ConnectionString;
                }
                else
                {
                    return base.GetNameOrConnectionString(args);
                }
            }
        }

        protected virtual int? GetCurrentTenantId()
        {
            return _currentUnitOfWorkProvider.Current != null
                ? _currentUnitOfWorkProvider.Current.GetTenantId()
                : AbpSession.TenantId;
        }
    }

　　使用的时候，在类或者方法上，增加Uow属性装饰器上定义参数即可

　　补充说明：可以参照这种方式，自定义的扩展，比如每一个DbContext自定义连接字符串，我们可以在自己的租户管理表中添加属性，自定义数据库连接字符串选择逻辑。