QUORUM: Prepaid Internet at the University of Zululand
QUORUM (QUOta-based Resource Usage Manager) is a server that provides a simple message API with two basic message types: tallies (bill a particular account/session for an item) and queries (ask the server if a given account/session is still in credit).
We use the QUORUM server to manage the tallies of all URLs retrieved by the proxy as well as the current credit balances of user accounts and also to track user sessions, which correspond to periods of uninterrupted WWW usage by a user. The server keeps a cache of all active account and session tallies and credit balances. Updated items in these caches periodically are written to a database. The server has a WWW interface for displaying account balances, user sessions and other administrative and debugging information.
An early version of the server was written in C, using various CGI programs to provide the WWW interface. However, this approach was abandoned when we discovered Java servlets. The QUORUM server is written as a collection of Java servlets and JSPs, with JDBC for the back-end database connectivity. We currently run it under Caucho's Resin Servlet/JSP container. We use MySQL for the back-end database with Resin's built-in JDBC classes for MySQL access and database connection pooling.
Java servlets are a natural choice for implementing an application like QUORUM. The server contains several persistent shared data structures for user sessions, per-session tallies and caches of account tallies and credit balances. Since servlets are simply Java classes that are part of a container application that implements a WWW server, it is simple to create these persistent objects as part of the server, as well as background threads that check for idle sessions and periodically flush changed data to the accounting database.
When a user requests a URL, her browser sends the URL request to the Squid proxy. Squid then sends this URL request information to the URL redirector. The URL redirector sends a querySsn message to the QUORUM server to ask if the user has a current QUORUM session and if the user's account is still in credit. If everything is okay, then the URL redirector replies to Squid with a blank line telling Squid to fetch the requested URL.
If the user does not have a current session, or the querySsn response indicates that the user's account has no credit, the browser is redirected to a JSP, ssnInfo.jsp, in the QUORUM server that shows the user's current account usage and available credit and asks her to start a QUORUM session by clicking on a link to another JSP, beginSsn.jsp. If the user's account is in credit, this JSP creates a QUORUM session for the user.
The URL redirector actually sends a redirect to a servlet that sends another redirect to ssnInfo.jsp. The URL redirector passes the user name and IP address of the user as a parameter in the request to the servlet:
The servlet saves the ssn_id in the HTTP session associated with the client's browser. This lets all the other JSPs use an HTTP session variable to retrieve the user's account and session information.
Usage accounting depends on the information in Squid's access.log file. After a URL has been fetched by Squid, it appends a line to the access.log file, containing the URL fetched, the size of the object in bytes, the user ID of the requester and the IP address of the workstation making the request. A Python script, squidTallies.py, reads the tail of access.log and generates tallySsnItem request messages to the QUORUM server. The QUORUM server processes these tally messages and keeps running usage tallies for user accounts and sessions, as well as credit balances for all active user accounts.
Account balances, usage tallies and logs of user sessions are written to a MySQL database. A background thread in the QUORUM server periodically walks through the caches of session tallies, account tallies and credit balances and flushes any changed data to the database. This enables the server to keep reasonably up-to-date information in the database without overwhelming the database with multiple updates for every single URL fetched.
QUORUM request messages are simple text messages terminated by a newline. For example:
ref000001 querySsn email@example.com ccode=11000
is a request message asking the server if the user soren at IP address 18.104.22.168 has a current QUORUM session, and if the user is still in credit for “cost code” 11000. The first field of all request messages is a user reference that is echoed back to the client in the response message, which allows multithreaded clients to match responses to request messages.
As a J2EE application, you might expect a hipper message transport such as SOAP over HTTP. We considered encoding the requests as individual HTTP GET requests to servlets that would implement the various QUORUM API messages. This would have been a cleaner fit with the servlet framework, but our tests indicated that the HTTP overhead would be too large for our application. The peak demand from our student labs is 20-30 URLs per second. Since each URL request would correspond to two QUORUM request messages (one querySsn request and one tallySsnItem request), the server would need to handle 60 requests per second comfortably. We use a single QUORUM server to handle both our staff and student proxies, so we need to handle at least 100-150 requests per second. We could not achieve these rates using a separate HTTP request for each QUORUM request message, even making back-to-back requests over a single persistent HTTP connection.
Instead, a single servlet, PostMsg, handles all QUORUM request messages from clients. A client application makes an HTTP POST request and then sends QUORUM request messages on the TCP connection of the POST request, as if it was uploading a file or other data in the request. The PostMsg servlet reads request messages, one per line, from the ServletInputStream with the readLine() method of the input stream. It processes the request messages and sends responses back to the client on the ServletOutputStream. After sending the response, it calls the flushBuffer() method on the ServletHTTPResponse object to ensure the response is sent to the client. Using this technique, a single QUORUM server can handle several thousand requests per second on a single client connection.
There are two minor problems with this technique. First, the servlet container will close any HTTP connection that is idle for more than a set time, typically 30 seconds or so. This means that a client will be disconnected when it doesn't send any requests for a while. The client will then have to reconnect and send another HTTP POST request in order to send more QUORUM request messages. We wrote a Python class that provides a UNIX pipe for sending requests and reading responses. The class establishes the HTTP connection on demand and sends the HTTP POST request. An advantage of the connect-on-demand technique is that the message transport is very robust—idle connections don't hang around and there aren't problems with clients getting stuck, thinking they have a connection that the server thinks is closed.
The other problem is that HTTP/1.1 requires that a POST request has a Content-length header in both the request and the response. While we found that Tomcat ignored this restriction, Resin did not, and it closed the connection when the number of bytes sent by either client or server exceeded the Content-length value. Our workaround was to set a very large value in the Content-length header and arrange for the client to close the connection well before that number of bytes was sent.
Performance of the URL redirector is critical for the proxy as each URL request is delayed until the URL redirector responds. QUORUM's URL redirector keeps a cache of querySsn replies for all recent sessions. This makes the redirector very fast for URL requests once a session has been established and is in credit, as the URL redirector responds based on the most recent reply message in the cache. The response time in this case is typically two milliseconds or less. Even if there is a cached reply, the redirector always sends a querySsn request to the QUORUM server. The reply message is used to update the reply cache, so the cache always contains up-to-date information about the status of user sessions and credit balances.
When the redirector doesn't have a cached reply message, or if the cached reply is negative (user out of credit), then the redirector sends a querySsn request and blocks the reply to the proxy server until the response from the QUORUM server has been received. In this case, the redirector may take from 20 to 50 milliseconds to respond.
Squid can be configured to start up several copies of the URL redirector so that URL redirection requests can be processed in parallel. The QUORUM URL redirector is multithreaded so that Squid can make several simultaneous URL requests. Squid actually talks to several copies of a small stub program that forwards the URL redirector requests to the URL redirector process over unix-domain socket connections. This allows the URL redirector to share the same cache of querySsn replies for all the requests.
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