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.NET Core Distributed SQL Server Cache


Introduction:

A caching that can be shared or consumed by the multiple apps servers can be called a Distributed Cache. Distributed Cache is an external service, which increases the performance and scalability of applications. The advantages of Distributed Cache are like:
  • No effect on the cache server on an application crash.
  • No need to use local memory.
  • Consistency in serving cached data around multiple applications.
In General, Distributed Cache can be achieved by using Redi's Cache, SQL Cache, Third Party Cache Libraries. Here we will learn about .NET Core Distributed SQL Server Cache.

Create A .NET Core WebAPI Project:

Create a sample .NET Core Web API application to test the Distributed Cache. You can use either Visual Studio 2019 or Visual Studio Code editor, here I'm using Visual Studio Code. Click here to know basics steps to use Visual Studio Code for .NET Core applications

SQL Table For Cache:

To store the cache data we need a table. So .NET Core has defined a structure of the table like how the table should be and what the table should store data, in simple words a templated predefined cache table.

To create a SQL cache table .NET Core provided CLI command as follows.
dotnet sql-cache create "your_database_connection_string" dbo your_cache_table_name
If having any issue to create the SQL cache table using the above CLI command, we can create the cache table manually, but the column names and column type should match as .NET core prescribed by default. So run the following SQL script query to create a cache table.
CREATE TABLE [dbo].[MyCache]
(
    [Id] NVARCHAR(900) NOT NULL PRIMARY KEY, 
    [Value] VARBINARY(MAX) NOT NULL, 
    [ExpiresAtTime] DATETIMEOFFSET NOT NULL, 
    [SlidingExpirationInSeconds] BIGINT NULL, 
    [AbsoluteExpiration] DATETIMEOFFSET NULL
)
'Id' Column - is a primary column of type String. In this column, we need to store a 'key' value which helps query the table.
'Value' Column - its type is Binary. In these columns, we need to store data to be cached and stores the data in binary format.
'SlidingExpirationInSeconds' Column - its type is BigInt. In this column, we need to store time in seconds. If the record is not consumed by any application or client which means the record is idle for specified seconds in the 'SlidingExpirationInSeconds' then the record will be in expired state. Which means no client can consume this record.
'ExpiresAtTime' Column - its type is DateTime. In this column, we need to store the idle expiration DateTime. This means its value will be created by adding the total number of seconds in the 'SlidingExpirationInSeconds' column to the current DateTime.
'AbsoluteExpiration' Column - its type is DateTime. In this column, we need to store the absolute expiration of DateTime means a max lifetime of the record. 'ExpiresAtTime' column must be within the limit of the 'AbsoluteExpiration' column value.

Install SQL Server Cache NuGet:

Microsft provided an extended NuGet package for SQL Server Cache. Use the below command to install the package.
Package Manager:
Install-Package Microsoft.Extensions.Caching.SqlServer -Version 3.1.3
.NET CLI:
dotnet add package Microsoft.Extensions.Caching.SqlServer --version 3.1.3

Configure AddDistributedSqlServerCache Service:

Configure the cache database connection string to the app.setting.json file.
app.setting.json:
"ConnectionStrings":{
   "MyWorldDbConnection":"Your_databse_connectionString"
}
Configure AddDistributedSqlServerCache Service provider in Startup.cs file.
Startup.cs:
services.AddDistributedSqlServerCache(options => {
 options.ConnectionString = Configuration.GetConnectionString("MyWorldDbConnection");
 options.SchemaName = "dbo";
 options.TableName = "MyCache";
});
  • ConnectionString mapped to the value in the app.setting.json file. 
  • "dbo" is a SQL database schema. 
  • TableName mapped to the cache table which we created at the initial step.

IDistributedCache Interface:

IDistributedCache Interface is implemented from 'Microsoft.Extensions.Caching.Distributed' library. IDistributeCache is not specific to SQL Server Cache, but it was implemented like generic which supports all kinds of Distributed Cache like Redis, SQL Server Cache, etc. IDistributeCache supports the default dependency inject of .NET Core, so it can be easily injected in the application where ever needed. IDistributedCache Interface holds a set of methods that helps to work with cache, some of the methods like as below.
  • Get, GetAsync - fetches the cached data by the key passed to the method.
  • Set, SetAsync - saves the data to the cache table with a specific key.
  • Remove, RemoveAsync - removes the cached data with specific to key passed.
Create an API controller and inject the IDistributedCache interface into its constructor as below.
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Caching.Distributed;
namespace SqlDistributedCache.API.Controllers
{
    [ApiController]
    [Route("[controller]")]
    public class TestController: ControllerBase
    {
        private readonly IDistributedCache _distributedCache;
        public TestController(IDistributedCache distributedCache)
        {
            _distributedCache = distributedCache;
        }
    }
}

Save Data To SQL Cache Table:

IDistributedCache interface provided a method like SetAsync, SetStringAsync to store cache data.

SetAsync - this method expects data to be stored as Byte[]. So on using SetAsync overloaded methods we need explicitly convert the data to Byte[].

SetStringAsync - this is an extended method implemented by 'Microsoft.Extensions.Caching.Distributed'. So on using this method it simply expects the stored data to be String format. Internally it will convert that String data to Byte[] while saving to cache table.
Note:
IDistributedCache interface also provides non async methods like Set, SetString , but async methods recomended.
Here in the sample, we explore using SetStringAsync. let's look more about the SetStringAsync overloaded methods as below.

  • SetStringAsync(string key,string value, DistributedCacheEntryOptions options, CancellationToken token = default) in this overloaded method we pass 'key', 'value'(data to be catches), 'DistributeCacheEntryOptions' provides an explicit way of defining the expiration Date Time into our cache table
  • SetStringAsync(string key, string value, CancellationToken token = default) in this overload method we simply pass 'key' and data as string format to 'value' variable. Here we don't have an option to specify DateTime explicitly. So on using this method expiration Date Time will be assigned with default values that are implicitly or explicitly configured in 'AddDistributedSqlServerCache' service.
Let test cache saving using SetStringAsync method without 'DistributedCacheEntryOptions' parameter as follow
TestController.cs:
[Route("add-cache-no-time-options")]
[HttpGet]
public async Task<IActionResult> AddCacheNoTimeOptions()
{
 string key = "test1";
 string value = "naveen";
 await _distributedCache.SetStringAsync(key, value);
 return Ok("success");
}
Now test this API route and check record added in the cache table.

  • In the 'Value' column the data to be cached is stored in Binary format.
  • In the 'Id' column stores key value.
  • Since we didn't specify any expiration options explicitly, then the default options set by the 'AddDistributedSqlServerCache' service will be stored in 'ExpiresAtTime', 'SlidingExpirationInSeconds' columns.
  • The default value for 'SlidingExpirationInSeconds' 20 minutes which will be saved as seconds(1200 seconds).
  • The default value for 'ExpiresAtTime' is the addition of current date time value with 'SlidingExpirationInSeconds' column.
Let's test the cache saving using the SetStringAsync method with the 'DistributedCacheEntryOptions' parameter which means specifying expiration values explicitly as follows.
[Route("add-cache")]
[HttpGet]
public async Task<IActionResult> AddCache()
{
 string key = "test2";
 string value = "Naveen Bommindi";
 DistributedCacheEntryOptions options = new DistributedCacheEntryOptions
 {
  SlidingExpiration = TimeSpan.FromMinutes(1),
  AbsoluteExpirationRelativeToNow = TimeSpan.FromHours(1)
 };
 await _distributedCache.SetStringAsync(key, value, options);
 return Ok("success");
}
  • 'DistributedCacheEntryOptions' type helps to explicitly specify the expiration time values. 
  • Here property like 'SlidingExpiration' to 1 minute, this explains after being idle for 1 minute this record state will be expired. This 'SlidingExpiration' property value will be saved to the 'SlidingExpirationInSeconds' column in the cache table. 
  • Based on 'SlidingExpirationInSeconds' value, the column  'ExpireAtTime' value will be assigned(current date-time plus 'SlidingExpirationInSeconds' column value). 
  • Here property like 'AbsoluteExpirationRelativeToNow' is like the main expiration date-time, it will save the value in the 'AbsoluteExpiration' column. So the value in 'ExpireAtTime' value should be within the limits of the 'AbsoluteExpiration' column. 
  • By any chance the value in 'ExpireAtTime' is greater than 'AbsoluteExpiration' value then the record will be an expired state because 'AbsoluteExpiration' value takes priority.
Run application and access this new endpoint where expirations specified explicitly and then check the cache table as below.

Fetch Cached Data From SQL Cache Table:

IDistributedCache interface provided methods like GetAsync, GetStringAsync to fetch data.

GetAsync - this method expects an input parameter, which is 'key' to query the cache database. This GetAsync method return Byte[] array of data where we need to explicitly convert that result to a string before serving the results to API clients.

GetStringAsync - this method expects an input parameter, which is 'key' to query the cache database. This GetStringAsync method returns the string directly to the API clients. Basically, cache data is in Byte[] format while retrieving from the database but this method implicitly converts those Byte[] of data to the string.
Note:
IDistributedCache interface also provides non async methods like Get, GetString , but async methods recomended.
Let's implement an endpoint to fetch the cached data as follow.
TestController.cs:
[Route("get-cache")]
[HttpGet]
public async Task<IActionResult> GetCache()
{
 string name = await _distributedCache.GetStringAsync("test2");
 return Ok(name);
}
Run the application and access the endpoint then look the result as follows.

Delete Cached Data:

IDistributedCache interface provided a method like RemoveAsync to delete the cached record from the cache table.

RemoveAsync - this method expects a 'key' as the input parameter, based on the 'key' value removes the record from the cache table.
Note:
IDistributedCache interface also provides non async method like Remove, but async methods recomended.
To test, create a sample endpoint to delete the cache data as below
TestController.cs:
[Route("delete-cache")]
[HttpGet]
public async Task<IActionResult> DeleteCache(string key)
{
 await _distributedCache.RemoveAsync(key);
 return Ok();
}

Configure SlidingExpriation Time Globally:

'AddDistributedSqlServerCache' service by default will configure SlidingExpiration time to 20 minutes. This time will get overridden if we use 'DistributedCacheEntryOptions' parameter in SetStringAsync or SetAsync methods.

To configure SlidingExpiration time globally and explicitly do as below.
services.AddDistributedSqlServerCache(options => {
 options.DefaultSlidingExpiration = TimeSpan.FromMinutes(10);
});
Here global default SlidingExpiration changed to 10 minutes.

Deleting Expired Items From Cache Table: 

'AddDistributedSqlServerCache' service by default configure with the option of deleting the expired cache records. It has an option like 'ExpiredItemsDeletionInterval' where it value by default assigned with 30 minutes. So if you want we can change it explicitly by assigning our own interval time.

If we are creating a cache item by using the SetStringAsync method without the 'DistributedCacheEntryOptions' parameter in this in our table 'AbsoluteExpiratin' column will be null.
So in this case after expiration on the 'ExpireAtTime' column, it will wait for 30  minutes and then deletes the record from the cache table.

If we are creating a cache item by using the SetStringAsync method with the 'DistributedCacheEntryOptions' parameter, in this case, we will 'AbsoluteExpriation' column with value. So after expiration on the 'AbsoluteExpiration' column, it will wait for 30 minutes and then deletes the record from the cache table.

Set 'ExpiredItemsDeletionInterval' as below
Startup.cs:
services.AddDistributedSqlServerCache(options => {
 options.ExpiredItemsDeletionInterval = TimeSpan.FromMinutes(6);
});
Here set deletion takes place after 6 minutes.

For 'ExpiredItemsDeletionInterval' there is constraint like its time should not be less than 5 minutes. If you try to set less the 5minutes application gives an error below.

Sample Code Caching Real Data:

For understanding purpose, created an endpoint for each IDistributedCache interface method. In real-time all methods mostly used inside of single endpoint based on its functionality.
GadgetController.cs:
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Caching.Distributed;
using SqlDistributedCache.API.Data;
using System.Threading.Tasks;
using Newtonsoft.Json;
using System.Linq;
using System.Collections.Generic;
using SqlDistributedCache.API.Entities;
using Microsoft.EntityFrameworkCore;
using System;

namespace SqlDistributedCache.API.Controllers
{
    [ApiController]
    [Route("[controller]")]
    public class GadgetController : ControllerBase
    {
        private readonly IDistributedCache _distributedCache;
        private readonly MyWorldDbContext _myWorldDbContext;
        public GadgetController(
            IDistributedCache distributedCache,
            MyWorldDbContext myWorldDbContext)
        {
            _distributedCache = distributedCache;
            _myWorldDbContext = myWorldDbContext;
        }

        [Route("get")]
        [HttpGet]
        public async Task<IActionResult> Get()
        {
            var cachedGadgets = await _distributedCache.GetStringAsync("myGadgets");
            if (!string.IsNullOrEmpty(cachedGadgets))
            {
                List<Gadgets> result = JsonConvert.DeserializeObject<List<Gadgets>>(cachedGadgets);
                return Ok(new {IsCache = true,result});
            }
            else
            {
                List<Gadgets> result = await _myWorldDbContext.Gadgets.ToListAsync();
                cachedGadgets = JsonConvert.SerializeObject(result);
                await _distributedCache.SetStringAsync("myGadgets", cachedGadgets, new DistributedCacheEntryOptions
                {
                    SlidingExpiration = TimeSpan.FromMinutes(30),
                    AbsoluteExpirationRelativeToNow = TimeSpan.FromMinutes(30)
                });
                return Ok(new {IsCache = false,result});
            }
        }

    }
}
For complete implementation check the below GitHub link.
Note:
It is recomended to use seperate database to have cache tables.

Wrapping Up:

Hopefully, this article will help to understand the .NET Core Distributed SQL Server Cache. I love to have your feedback, suggestions, and better techniques in the comment section.

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