[Perf] Access tokens are listed too many times in AcquireTokenSilent / AcquireTokenForClient

[pmaytak]

This issue impacts scenarios of large token cache, which is common for client credential flows (300k+ tokens). MSAL copies access tokens from memory into a list several times, which is a perf impact (TBD - numbers)

Technical details:

InMemoryTokenCacheAccessor.GetAllAccessTokens() iterates through the whole AT collection and returns a List. When TokenCacheNotificationArgs.HasTokens property is set, it calls TokenCache.ITokenCacheInternal.HasTokensNoLocks() which eventually calls InMemoryTokenCacheAccessor.GetAllAccessTokens(). CacheSessionManager.RefreshCacheForReadOperationsAsync sets that property twice.

So the AT dictionary ends up being iterated 3 times for a read, 2 times for a save. This can have a perf hit when cache has a large number of tokens.

Possible enhancement: Update GetAllAccessTokens() to not create a list or reduce the number of calls to GetAllAccessTokens().

microsoft-authentication-library-for-dotnet/src/client/Microsoft.Identity.Client/PlatformsCommon/Shared/InMemoryTokenCacheAccessor.cs

Lines 155 to 165 in 24cb466

	public IEnumerable<MsalAccessTokenCacheItem> GetAllAccessTokens()
	{
	// perf: do not call ConcurrentDictionary.Values as it takes a lock
	List<MsalAccessTokenCacheItem> ats = new List<MsalAccessTokenCacheItem>();
	foreach (var kvp in _accessTokenCacheDictionary)
	{
	ats.Add(kvp.Value);
	}
	return ats;
	}

microsoft-authentication-library-for-dotnet/src/client/Microsoft.Identity.Client/TokenCache.ITokenCacheInternal.cs

Lines 818 to 822 in 24cb466

	bool ITokenCacheInternal.HasTokensNoLocks()
	{
	return _accessor.GetAllRefreshTokens().Any() ||
	_accessor.GetAllAccessTokens().Any(at => !IsAtExpired(at));
	}

microsoft-authentication-library-for-dotnet/src/client/Microsoft.Identity.Client/Cache/CacheSessionManager.cs

Lines 106 to 129 in 24cb466

	try
	{
	var args = new TokenCacheNotificationArgs(
	TokenCacheInternal,
	_requestParams.ClientId,
	_requestParams.Account,
	hasStateChanged: false,
	TokenCacheInternal.IsApplicationCache,
	hasTokens: TokenCacheInternal.HasTokensNoLocks(),
	suggestedCacheKey: key);
	await TokenCacheInternal.OnBeforeAccessAsync(args).ConfigureAwait(false);
	}
	finally
	{
	var args = new TokenCacheNotificationArgs(
	TokenCacheInternal,
	_requestParams.ClientId,
	_requestParams.Account,
	hasStateChanged: false,
	TokenCacheInternal.IsApplicationCache,
	hasTokens: TokenCacheInternal.HasTokensNoLocks(),
	suggestedCacheKey: key);
	await TokenCacheInternal.OnAfterAccessAsync(args).ConfigureAwait(false);
	}

[neha-bhargava]

Fixed in Msal release 4.24.0