Implement thread-safe cache for GroupDocs.Viewer Leave feedback
This page describes how to develop a thread-sage cache using the C# lock statement and the ConcurrentDictionary<,> class.
A method is thread-safe if multiple threads can call it without breaking the functionality. Achieving thread safety is a complex task, so general-purpose classes are usually not thread-safe. The most common way to achieve thread safety is to lock the resource for exclusive use by a single thread at any given time.
You need to develop a web application where multiple users can simultaneously view the same file. The web application uses GroupDocs.Viewer on the server side. You have to ensure that multiple threads can safely read and write to the cache.
In GroupDocs.Viewer, you can use caching to improve the performance if the same document is processed multiple times (read more about caching here.) The FileCache class is an implementation of the ICache interface that uses a local disk to store the cache files. The FileCache is not thread safe, so you need to make it so.
The FileCache class uses a local disk to read and write output files. You need to implement thread safe reading and writing to disk. To do this, use the list to store the key or the file ID and associated object you need to lock. The simplest way is to use the ConcurrentDictionary<,> class of the .NET Framework 4.0. The ConcurrentDictionary is a thread safe implementation of a dictionary of key-value pairs. Implement the ThreadSafeCache class that wraps around not thread safe class that implements the ICache interface.
using GroupDocs.Viewer.Caching;
using System.Collections.Generic;
internal class ThreadSafeCache : ICache
{
private readonly ICache _cache;
private readonly IKeyLockerStore _keyLockerStore;
public ThreadSafeCache(ICache cache, IKeyLockerStore keyLockerStore)
{
_cache = cache;
_keyLockerStore = keyLockerStore;
}
public void Set(string key, object value)
{
lock (_keyLockerStore.GetLockerFor(key))
{
_cache.Set(key, value);
}
}
public bool TryGetValue<TEntry>(string key, out TEntry value)
{
lock (_keyLockerStore.GetLockerFor(key))
{
return _cache.TryGetValue(key, out value);
}
}
public IEnumerable<string> GetKeys(string filter)
{
lock (_keyLockerStore.GetLockerFor("get_keys"))
{
return _cache.GetKeys(filter);
}
}
}
Imports System.Runtime.InteropServices
Imports GroupDocs.Viewer.Caching
Class ThreadSafeCache
Implements ICache
Private ReadOnly _cache As ICache
Private ReadOnly _keyLockerStore As IKeyLockerStore
Public Sub New(cache As ICache, keyLockerStore As IKeyLockerStore)
_cache = cache
_keyLockerStore = keyLockerStore
End Sub
Public Function TryGetValue(Of TEntry)(key As String, <Out> ByRef value As TEntry) As Boolean Implements ICache.TryGetValue
SyncLock _keyLockerStore.GetLockerFor(key)
Return _cache.TryGetValue(key, value)
End SyncLock
End Function
Public Function ICache_GetKeys(filter As String) As IEnumerable(Of String) Implements ICache.GetKeys
SyncLock _keyLockerStore.GetLockerFor("get_keys")
Return _cache.GetKeys(filter)
End SyncLock
End Function
Public Sub ICache_Set(key As String, value As Object) Implements ICache.[Set]
SyncLock _keyLockerStore.GetLockerFor(key)
_cache.Set(key, value)
End SyncLock
End Sub
End Class
All the ThreadSafeCache class methods use locks to make calls thread safe. The ConcurrentDictionaryKeyLockerStore class uses ConcurrentDictionary to create the locker object or to retrieve it if it already exists. It also creates a unique key that identifies a cached file.
using System.Collections.Concurrent;
// ...
interface IKeyLockerStore
{
object GetLockerFor(string key);
}
class ConcurrentDictionaryKeyLockerStore : IKeyLockerStore
{
private readonly ConcurrentDictionary<string, object> _keyLockerMap;
private readonly string _uniqueKeyPrefix;
public ConcurrentDictionaryKeyLockerStore(ConcurrentDictionary<string, object> keyLockerMap, string uniqueKeyPrefix)
{
_keyLockerMap = keyLockerMap;
_uniqueKeyPrefix = uniqueKeyPrefix;
}
public object GetLockerFor(string key)
{
string uniqueKey = GetUniqueKey(key);
return _keyLockerMap.GetOrAdd(uniqueKey, k => new object());
}
private string GetUniqueKey(string key)
{
return $"{_uniqueKeyPrefix}_{key}";
}
}
Imports System.Collections.Concurrent
' ...
Interface IKeyLockerStore
Function GetLockerFor(key As String) As Object
End Interface
Class ConcurrentDictionaryKeyLockerStore
Implements IKeyLockerStore
Private ReadOnly _keyLockerMap As ConcurrentDictionary(Of String, Object)
Private ReadOnly _uniqueKeyPrefix As String
Public Sub New(keyLockerMap As ConcurrentDictionary(Of String, Object), uniqueKeyPrefix As String)
_keyLockerMap = keyLockerMap
_uniqueKeyPrefix = uniqueKeyPrefix
End Sub
Public Function GetLockerFor(key As String) As Object Implements IKeyLockerStore.GetLockerFor
Dim uniqueKey As String = Me.GetUniqueKey(key)
Return _keyLockerMap.GetOrAdd(uniqueKey, Function(k) New Object())
End Function
Private Function GetUniqueKey(key As String) As String
Return $"{_uniqueKeyPrefix}_{key}"
End Function
End Class
using System.IO;
using System.Collections.Generic;
using System.Collections.Concurrent;
using GroupDocs.Viewer;
using GroupDocs.Viewer.Caching;
using GroupDocs.Viewer.Interfaces;
using GroupDocs.Viewer.Options;
namespace ThreadSaveCacheExample
{
static class Program
{
private static readonly ConcurrentDictionary<string, object> KeyLockerMap = new ConcurrentDictionary<string, object>();
static void Main()
{
string fileName = "sample.pdf";
string cacheFolder = fileName.Replace('.', '_');
string cachePath = Path.Combine("cache", cacheFolder);
string uniqueKeyPrefix = cachePath;
ICache fileCache = new FileCache(cachePath);
IKeyLockerStore keyLockerStore = new ConcurrentDictionaryKeyLockerStore(KeyLockerMap, uniqueKeyPrefix);
ICache threadSafeCache = new ThreadSafeCache(fileCache, keyLockerStore);
ViewerSettings viewerSettings = new ViewerSettings(threadSafeCache);
List<MemoryStream> pages = new List<MemoryStream>();
using (Viewer viewer = new Viewer(fileName, viewerSettings))
{
IPageStreamFactory pageStreamFactory = new MemoryPageStreamFactory(pages);
ViewOptions viewOptions = HtmlViewOptions.ForEmbeddedResources(pageStreamFactory);
viewer.View(viewOptions);
}
}
}
class ThreadSafeCache : ICache
{
private readonly ICache _cache;
private readonly IKeyLockerStore _keyLockerStore;
public ThreadSafeCache(ICache cache, IKeyLockerStore keyLockerStore)
{
_cache = cache;
_keyLockerStore = keyLockerStore;
}
public void Set(string key, object value)
{
lock (_keyLockerStore.GetLockerFor(key))
{
_cache.Set(key, value);
}
}
public bool TryGetValue<TEntry>(string key, out TEntry value)
{
lock (_keyLockerStore.GetLockerFor(key))
{
return _cache.TryGetValue(key, out value);
}
}
public IEnumerable<string> GetKeys(string filter)
{
lock (_keyLockerStore.GetLockerFor("get_keys"))
{
return _cache.GetKeys(filter);
}
}
}
interface IKeyLockerStore
{
object GetLockerFor(string key);
}
class ConcurrentDictionaryKeyLockerStore : IKeyLockerStore
{
private readonly ConcurrentDictionary<string, object> _keyLockerMap;
private readonly string _uniqueKeyPrefix;
public ConcurrentDictionaryKeyLockerStore(ConcurrentDictionary<string, object> keyLockerMap, string uniqueKeyPrefix)
{
_keyLockerMap = keyLockerMap;
_uniqueKeyPrefix = uniqueKeyPrefix;
}
public object GetLockerFor(string key)
{
string uniqueKey = GetUniqueKey(key);
return _keyLockerMap.GetOrAdd(uniqueKey, k => new object());
}
private string GetUniqueKey(string key)
{
return $"{_uniqueKeyPrefix}_{key}";
}
}
class MemoryPageStreamFactory : IPageStreamFactory
{
private readonly List<MemoryStream> _pages;
public MemoryPageStreamFactory(List<MemoryStream> pages)
{
_pages = pages;
}
public Stream CreatePageStream(int pageNumber)
{
MemoryStream pageStream = new MemoryStream();
_pages.Add(pageStream);
return pageStream;
}
public void ReleasePageStream(int pageNumber, Stream pageStream)
{
//Do not release page stream as we'll need to keep the stream open
}
}
}
Imports System.IO
Imports System.Collections.Concurrent
Imports GroupDocs.Viewer
Imports GroupDocs.Viewer.Caching
Imports GroupDocs.Viewer.Interfaces
Imports GroupDocs.Viewer.Options
Imports System.Runtime.InteropServices
Module Program
Private ReadOnly KeyLockerMap As ConcurrentDictionary(Of String, Object) = New ConcurrentDictionary(Of String, Object)()
Public Sub Main()
Dim fileName As String = "resume.pdf"
Dim cacheFolder As String = fileName.Replace("."c, "_"c)
Dim cachePath As String = Path.Combine("cache", cacheFolder)
Dim uniqueKeyPrefix As String = cachePath
Dim fileCache As ICache = New FileCache(cachePath)
Dim keyLockerStore As IKeyLockerStore = New ConcurrentDictionaryKeyLockerStore(KeyLockerMap, uniqueKeyPrefix)
Dim threadSafeCache As ICache = New ThreadSafeCache(fileCache, keyLockerStore)
Dim viewerSettings As ViewerSettings = New ViewerSettings(threadSafeCache)
Dim pages As List(Of MemoryStream) = New List(Of MemoryStream)()
Using viewer As Viewer = New Viewer(fileName, viewerSettings)
Dim pageStreamFactory As IPageStreamFactory = New MemoryPageStreamFactory(pages)
Dim viewOptions As ViewOptions = HtmlViewOptions.ForEmbeddedResources(pageStreamFactory)
viewer.View(viewOptions)
End Using
End Sub
End Module
Class ThreadSafeCache
Implements ICache
Private ReadOnly _cache As ICache
Private ReadOnly _keyLockerStore As IKeyLockerStore
Public Sub New(cache As ICache, keyLockerStore As IKeyLockerStore)
_cache = cache
_keyLockerStore = keyLockerStore
End Sub
Public Function TryGetValue(Of TEntry)(key As String, <Out> ByRef value As TEntry) As Boolean Implements ICache.TryGetValue
SyncLock _keyLockerStore.GetLockerFor(key)
Return _cache.TryGetValue(key, value)
End SyncLock
End Function
Public Function ICache_GetKeys(filter As String) As IEnumerable(Of String) Implements ICache.GetKeys
SyncLock _keyLockerStore.GetLockerFor("get_keys")
Return _cache.GetKeys(filter)
End SyncLock
End Function
Public Sub ICache_Set(key As String, value As Object) Implements ICache.[Set]
SyncLock _keyLockerStore.GetLockerFor(key)
_cache.Set(key, value)
End SyncLock
End Sub
End Class
Interface IKeyLockerStore
Function GetLockerFor(key As String) As Object
End Interface
Class ConcurrentDictionaryKeyLockerStore
Implements IKeyLockerStore
Private ReadOnly _keyLockerMap As ConcurrentDictionary(Of String, Object)
Private ReadOnly _uniqueKeyPrefix As String
Public Sub New(keyLockerMap As ConcurrentDictionary(Of String, Object), uniqueKeyPrefix As String)
_keyLockerMap = keyLockerMap
_uniqueKeyPrefix = uniqueKeyPrefix
End Sub
Public Function GetLockerFor(key As String) As Object Implements IKeyLockerStore.GetLockerFor
Dim uniqueKey As String = Me.GetUniqueKey(key)
Return _keyLockerMap.GetOrAdd(uniqueKey, Function(k) New Object())
End Function
Private Function GetUniqueKey(key As String) As String
Return $"{_uniqueKeyPrefix}_{key}"
End Function
End Class
Class MemoryPageStreamFactory
Implements IPageStreamFactory
Private ReadOnly _pages As List(Of MemoryStream)
Public Sub New(pages As List(Of MemoryStream))
_pages = pages
End Sub
Public Sub IPageStreamFactory_ReleasePageStream(pageNumber As Integer, pageStream As Stream) Implements IPageStreamFactory.ReleasePageStream
'Do not release page stream as we'll need to keep the stream open
End Sub
Public Function IPageStreamFactory_CreatePageStream(pageNumber As Integer) As Stream Implements IPageStreamFactory.CreatePageStream
Dim pageStream As MemoryStream = New MemoryStream()
_pages.Add(pageStream)
Return pageStream
End Function
End Class
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