ASP.NET web developers have three built in options to store session state, namely, in-process memory, SQL Server and State Server.
In-process memory offers the fastest performance, but is unsuitable for use in web server farms because the session data is stored in the memory of the ASP.NET worker process.
SQL Server is an out of process session state storage option that works with web server farms. It stores session data in a SQL Server database. It is the most reliable option but the least performing one. One major issue with this option is that quite often developers want to cache data retrieved from a database in session state, to reduce database lookups. SQL Server session state defeats this purpose, because there is little performance gain in caching data retrieved from a database, in a database.
State Server is an out of process session state storage option that works with web server farms. It stores session data in memory and delivers better performance than SQL Server. This seems like a good compromise between the in-process option and the SQL server options. It has some drawbacks, however.
Firstly, several web servers typically depend on one state server for session state which introduces a critical single point of failure.
Secondly, in a load balanced environment, the load balancer may redirect a user’s request to a web server that is different from the web server that served the user’s previous request. If the new web server communicates with a different state server, the user’s original session state will not be found and the web application may not work properly.
This problem occurs even in persistence-based (a.k.a. sticky) load balancers either erroneously or due to server failure.
Thirdly, an issue that many developers are unaware about is that the web server and state server communicate in plain text. An eavesdropper can easily get hold of session state data traveling on the network. This may not be a threat if all servers are running in an internal network but it is certainly cause for concern when web servers and state servers are spread across the internet.
The peer to peer ASP.NET state server presented in this write-up addresses the aforementioned problems while transparently replacing the Microsoft provided state server.
The idea behind the peer to peer state server is simple -- let state servers on a network securely communicate and pass session state data amongst each other as needed, as shown below.
This design improves scalability because web servers can share multiple state servers, eliminating a single point of failure. Furthermore, if a load balancer erroneously or intentionally redirects a user to a different web server attached to a separate state server, the user’s session state will be requested from the state server that served the user’s previous request.
Security is also improved as peers can be configured to encrypt session data while sharing session state. Data transfers between the web server and the state server remain unencrypted but eavesdropping attacks can be eliminated by keeping web servers and linked state servers in trusted networks or on the same computer.
The peer to peer state server is fully backward compatible with the Microsoft provided state server and comes with all the benefits mentioned earlier.
To compile and install the state server:
- Download the source file.
- Open up the solution in Visual Studio. (Visual Studio 2008 will open up a Conversion Wizard. Complete the Wizard.)
The state server comes in two flavors. One runs as a console application and the other one runs as a windows service. The
StateService project compiles as a windows service and can be installed and uninstalled with the install_service.bat and uninstall_service.bat files. The
ConsoleServer project runs the service as a console application, which is a lot easier to test and debug. Both projects share the same sources and function identically.
- Open up the properties window for the project you want to build.
- a. If using Visual Studio 2005, add
NET20 in the conditional compilation symbols field of the Build tab.
b. If using Visual Studio 2008, select .NET Framework 3.5 in the Target Framework field of the Application tab.
- Build the project.
- If you built the
StateService project, navigate to the output folder and run install_service.bat to install the service.
- If you are already running the Microsoft state service on your machine, stop it.
- If you built and installed the windows service, you can start Peer to Peer State Service in the Services list. If you built the console server, run ConsoleServer.exe or simply start debugging from Visual Studio.
- You can now test and run any web applications you have with the running state server.
To add peer servers:
- Copy the compiled executable file and the application configuration file to another computer on your network.
- Open up the configuration file and add a new peer in the
<Peers> section. For instance, to configure the state server to connect to another state server running on a computer named SV3 with a peer port number of 42425, you would add
<add key="MyPeer" value="pc2:42425" /> to the
- You can start the state server on the computer and it will link up with the other state server(s) on the network.
- It’s up to you to set up the network in any topology you like. For example, consider a network of three state servers as shown below, each state server on each machine would have the configuration shown below:
You can run multiple console server peers on the same computer but each console server must have a unique web server port and peer port setting.
How It Works
The Microsoft provided state server works as shown below:
The Peer to Peer State Server works exactly as illustrated above, except when the state server doesn't have the requested session state, in which case it requests the session state from the network before responding, as illustrated below:
If the requested session state is not transferred within a set time period, the state server assumes the session state does not exist on the network and proceeds to process the web server request without the session state. The
GetTransferMessage class represents the message that is broadcast on the network when a node is requesting a session. Peers maintain connection between themselves principally to forward this message. Session state transfers occur out-of-band of the peer network.
Various programming techniques are used to implement different aspects of the state server. Some of the notable ones are highlighted below.
The state server is written in C# 2.0, but targets the NET 3.5 framework so as to take advantage of the ReaderWriterLockSlim class. If the
NET20 symbol is defined, the server uses the slower ReaderWriterLock class instead and is able to target the .NET 2.0 framework.
In order to create a state server that can transparently replace the state server, I needed to obtain and understand the full specification of the communication protocol between the web server and the Microsoft provided state server. The steps taken to piece out the protocol are documented in reverse chronology at http://ahuwanya.net/blog/category/Peer-to-Peer-Session-State-Service.aspx.
The server is largely message driven. The messaging subsystem is illustrated below:
When the server receives data from a socket, the data is accumulated in an instance of the
HTTPPartialData class currently assigned to that socket. The
HTTPPartialData instance validates the data, determines if the accumulated data is a complete HTTP message and checks for errors in the accumulated data. If there is a data error (for example, if the data does not conform to HTTP), the entire accumulated data is discarded and the socket is closed. If the data is valid but not yet complete, the sockets waits for more data to arrive.
If the accumulated data is a complete HTTP message, the data is sent to a
MessageFactory object. The
MessageFactory object inspects the data to determine the appropriate
ServiceMessage child class instance to create. The
ServiceMessage child class is instantiated and its implementation of the
Process method is called to process the message.
A pessimistic concurrency mechanism is employed while accessing session state in the session dictionary, which is defined by the
SessionDictionary class. A piece of session state can only be read or modified by one thread at a time. A thread declares exclusive access to operate on a piece of session state by setting the
IsInUse property to
true. This is done by calling the atomic compare and swap
CompareExchangeInUse method (a wrapper to the .NET
Interlocked.CompareExchange method that operates on the
IsInUse property). Setting this property to true lets other threads know that another thread is working with that session state.
If another thread wants to access the same session state and attempts to declare exclusive access, the attempt will fail because another thread already has exclusive access. The thread will keep trying to acquire exclusive access, and will eventually acquire it when the other thread releases access. This works pretty well because most of the time, only one thread needs to access a session state, and also because most operations on a session state take a very short time to complete. The export (transfer) operation which takes a much longer time is handled with a slightly different mechanism and is discussed in the contention management section below.
TIMERS – OR THE LACK OF THEM
The code has a lot of objects that expire or time-out and on which certain actions must take place on expiration – objects like individual session state dictionary entries that expire or asynchronous messages that timeout. Instead of assigning a timer or a wait handle to track these objects – they are stored in instances of a special collection class called the
DateSortedDictionary. Objects in this dictionary are sorted in place by their assigned timestamps. Specially designated threads poll these date sorted dictionaries for expired items and perform related actions if an item is expired. This design significantly reduces the number of threads needed to keep track of expiring items.
Diags class is used to keep track of messages, log server activity and detect deadlocks. Methods on the
Diags class are conditional and will not compile into release configuration code.
VERBOSE symbol can be defined to view or log all activity taking place at the server. This is particularly useful with the console server which outputs this information to the console window. If the
VERBOSE symbol is not defined, only critical information or unexpected errors are displayed.
The Microsoft provided state server transmits and receives unencrypted data to and from the web server. This was most likely done for performance reasons. To be compatible with the Microsoft provided state server, the peer to peer state server transmits unencrypted data to the web server. However the peer to peer state server can be configured to transmit encrypted data between peers. This effectively thwarts network eavesdropping attacks if web server and associated state servers are installed on the same computer or on a trusted network.
For example, take the Web server – Microsoft State Server configuration shown below.
Two web servers connect across the public internet to access a state server.
Using peer to peer state servers, the network can be secured by having the web servers have their own local state servers that connects securely to the remote state server on their behalf as shown below:
The local state servers can be installed on the same machine as the web server for maximum security and minimum latency.
This approach can help secure geographically distributed web and state servers.
Peer state servers also mutually authenticate each other while connecting, to ensure that the other party is an authorized peer.
Connections between peers form logical networks which can be designed with common network topologies in mind.
Network A shown above is a ring network of peer state servers which are individually connected to web servers whereas Network B is a ring network of computers which have both state server and web server connected and running. Existing isolated Microsoft state server networks can be upgraded to form a larger peer to peer network by replacing the Microsoft state servers with peer to peer state servers and linking them up as shown in Network A. Network B benefits from the security counter measures mentioned earlier and is somewhat more scalable since any node on the network is a web server and a peer state server.
Both networks will still function if one node fails, unlike on a bus network, however as more nodes are added to the network, the longer it takes for a message to traverse the network.
Network C is a star network. An advantage of having a star network is that no matter how many new nodes are added to the ring network, it takes only two hops for a message to reach any node on the network.
Network D is a network of three star networks that form a larger star network. This network too will require a lesser number of hops for a message to traverse the network. Both networks suffer from the disadvantage that if the central node fails, the entire network fails.
By connecting the leaf nodes on Network D, Network E, a partial mesh network is formed. Network E is a clever combination of a ring network and a star network. If the central node fails, the network will still function and it also takes a fewer number of hops for a message to traverse the network than on a ring network.
As demonstrated, the topology of the peer to peer state server network is limited only by the imagination of the network designer.
There are a lot of scenarios that occur in the state server that are handled using traditional peer to peer processes such as the time to live header which is used to prevent messages from circulating perpetually on the network, and message identifiers used by peers to recognize messages that have been seen earlier, however, there are two particular scenarios that occur in this peer network that are not so common.
To ensure that session data is not lost during a server shutdown, the state server proceeds to transfer all its session state data to connected peers in a round-robin fashion when a server shut down is initiated.
A request for a session on a network can narrowly miss the node holding the session if it is being transferred to it as illustrated below.
As shown above, node 1 is seeking session A from the network just about the same time node 4 wants to transfer the session to node 2.
When the message from node 1 reaches node 2, node 2 forwards the message to node 3 because it doesn’t have the session.
When the message reaches node 3, the session transfer between nodes 4 and 2 begins and by the time the message reaches node 4, the transfer is complete and node 4 no longer has the session anymore and forwards the message to node 5.
Thus, the message traverses the network without reaching any node with the sought session, even though the session exists on the network.
The state server addresses this issue by having nodes that recently transferred a session rebroadcast the message as shown below:
Here, node 4 rebroadcasts the message so that it also travels back the way it came and eventually reaches node 2 which has the session.
Rebroadcasted messages are duplicates of the original message except that they have a different Broadcast ID header which peers use to know it’s a different broadcast.
As stated earlier, the state server uses a pessimistic concurrency model when accessing session state entries in the session dictionary. This works well because most requests take a short time to process. However, one particular request can take a much longer time to process, and can lead to resource starvation and performance degradation.
GetTransferMessage message broadcast is initiated by a peer when it needs to work with a session state it does not have. When the broadcast reaches a peer with the requested session state, the session state is transferred to the requesting peer.
Unlike other operations on a session state, a transfer can take a significant amount of time because the peer has to connect to the other peer, possibly authenticate, and transmit (a potentially large amount of) data. It’s important to note that any request from the web server can kick start a GetTransferMessage broadcast.
During a transfer, the session is marked as “in use” and other requests on that session will have to wait as usual. However, since it takes a much longer time, Threads waiting for a transfer operation to complete consume a lot of system resources. They can also timeout if the transfer takes too long or if the session is repeatedly transferred around the network due to flooded messages. A bad case is illustrated below:
In the diagram above, a user is flooding a web application with requests, which in turn is causing session requests to be transmitted to a state server.
Because all requests originate from one user, all session requests reference the same session id. A load balancer or state partitioner distributes these requests among the three state servers.
It is important to note that even though it is unlikely that a load balancer or state partitioner will distribute requests for a session among different state servers, a user can produce the scenario shown above by simply pressing and holding the browser refresh key on a web application that uses a poorly implemented state partitioner or a malfunctioning load balancer.
Also, an organized group of malicious users (or a botnet) can produce this scenario even on properly functioning state partitioners and load balancers.
Each state server has requests waiting to be processed. If the highly in-demand session is say, on state server 3, requests on that state server will be processed one by one very quickly.
State servers 1 and 2 issue broadcasts requesting a session transfer. The message eventually reaches state server 3 and the request is transferred to say state server 2. Requests on state server 3 that were not processed will wait until the transfer is complete.
After the session transfer to server 2 is complete, requests on server 2 are processed, whereas requests on server 3 issue broadcasts requesting the session.
A broadcast that originated from state server 1 reaches state server 2 and the session is transferred to state server 1. This goes on and on and the servers keep transferring the session amongst themselves while most of the requests wait, because even when a session is transferred, the state server is only able to process a few requests before it is transferred to another state server.
To make matters worse, if a state server receives a
GetTransferMessage message after it has recently transferred the session, it rebroadcasts the message (as explained earlier), which leads to even more
GetTransferMessage broadcasts on the network, more back-and-forth transfers and prolonged resource starvation.
The transfer process is relatively slow and since all requests have to wait to be processed one at a time by each state server, requests start to time out and the web server starts discarding requests. The state server is unaware that the web server has discarded those requests and still proceeds to process them.
These redundant requests, waiting for their turn, eat up valuable server processor cycles and degrade the quality of service.
If plenty of these requests arrive, they'll quickly use up all processor resources and the server comes to a grinding halt.
While it may be impossible to stop any group of users from flooding the state server with requests, the state server guards against contentious sessions with the following principle: any degradation of service due to a contentious session should mainly affect the user of that session, and achieves this goal with the following mechanisms:
- When a request is to be processed and the server notices the session is being transferred, the request stops being processed and is queued to be processed when the transfer is over. This prevents the request from eating up processor cycles while waiting, and frees up resources, so that other requests from other users can be processed. If the number of requests on a queue waiting for a session to transfer is too long, then all those messages are discarded because it means the session is contentious and the server shouldn't bother processing them.
- After the transfer is complete and a queued request is ready to be reprocessed and the server notices that the same session is been transferred again by another request, then the request will be discarded and not be processed, because it means the session is highly contentious.
- Before a request tries to query the network (by broadcasting) for a session, it checks if it is expecting a reply from a previous query for that session, and if so, the request is queued to a list of requests to be processed when the query is received. This reduces the number of
GetTransferMessage messages that will be generated on the network, which in turn reduces unnecessary rebroadcasts and lookups. If the number of requests on a queue waiting for a session to arrive is too long, then all those requests are discarded because it means the session is contentious.
- Finally, all incoming requests are queued up in their session id-specific queue and the message processor polls the incoming request queues in a round-robin manner and processes them one after the other, as shown below:
This means that all session requests are treated fairly, no single user can significantly disrupt the rate at which messages originating from other users are processed. Additionally, if the queue for a particular session id is too long, that queue is discarded because it means that session is contentious.
All these techniques employed by the state server can only adversely affect the web application of the offending user.
The peer to peer state server is fully backward compatible with the Microsoft provided state server and can transparently replace it. Peer state servers can transfer sessions to each other, improving the reliability of session state dependent web applications. Peer state servers also act as a security layer that protects session data on the network.
This project started out as a simple idea but quickly evolved into a complex task. Hopefully, this implementation and other ideas presented in this article will be valuable to developers interested in distributed systems. Due to the level of complexity, there will be bugs and kinks to work out. Contributions and bug reports will be appreciated.
- 24th August, 2009: Initial publication
Sunny has been developing software for the Microsoft-based platforms since the MS-DOS days. He has coded in C, VB (4 to 6) and C#. He enjoys designing and developing server-side .NET distributed applications.
He currently works for a Fortune 500 company. When he's not coding, he likes reading, hanging out with friends and sight-seeing.