Message flow¶
When a selector receives some OP_READ event, the associated IoProcessor is awaken and starts to process this event. It all starts with reading the channel, then pushing the read data through the filter chain down to the IoHandler (aka, the application). If the application decides to write back a response, it goes through the chain again, andultimately the data to be pushed into the channel is pushed into a queue (writeRequestQueue), then the stack is unfolled, down to the _IoProcessor, which pops the data from the queue and writes it into the channel.
Here is the representation of such a flow, where the SslFilter has been included (we will come back to it later)
+-------------------+
| IoAcceptor |
| +---------+ |
| | Select | |
| +----+----+ |
| | |
+---------+---------+
. flush/
+---------+ . +---------+ wakeup
| Channel <-----read------+ . +-----write-----> Channel | +----+
+---------+ | | | +---------+ | |
+---+--v--+---+ +----v----+---+
| | writeRequestQueue | |
| IoProcessor | +-+-+-+-+-+-+-+-+ | IoProcessor |
| <- pop -+ | | | | | | | <- put- | |
+---+-----^---+ +-+-+-+-+-+-+-+-+ +---------^---+
| . . |
fireMessageReceived | . . | write
| . . |
+---v---------+ +---v-----+---+
| HeadFilter | | HeadFilter <---------------filterWrite---------------+
+---+-----^---+ +-------------+ |
| . . +-----+------+
MessageReceived | . . +----------flushScheduledEvents--------> SslHandler |
| . . | +-----+------+
+---v---------+ +---v------+--+ |
| SslFilter | | SslFilter +----put----+ |
+---+-----^---+ +---------^---+ | |
| . . | | filterWriteQueue |
MessageReceived | . . | filterWrite | +-+-+-+-+-+-+-+-+ |
| . . | +--> | | | | | | | <----pop---+
+---v---------+ +---v---------+ +-+-+-+-+-+-+-+-+
| <filters> | | <filters> |
+---+-----v---+ +-------------+
| . . |
MessageReceived | . . | filterWrite
| . . |
+---v---------+ +---v-----+---+
| TailFilter | | TailFilter |
+---+-----^---+ +---------^---+
| . . |
MessageReceived | . . | fireFilterWrite
| . . |
+---v---------+ . |
| IoHandler | . |
+---+-----^---+ . |
| . . |
| ......................................... |
| |
+-------------------sessionWrite--------------------+
For instance, if we consider an application which filter chain contains the MdcInjectionFilter, SslFilter, CompressionFilter, ProtocolCodecFilter, LoggingFilter, we end uo with a stack of 77 calls, which needs to be emptied before the response can eventually be written. Here is the stack (in reverse order, the latest call on top of it):
NioProcessor(AbstractPollingIoProcessor<S>).flush(S) line: 449 NioProcessor(AbstractPollingIoProcessor<S>).write(S, WriteRequest) line: 436 NioProcessor(AbstractPollingIoProcessor<S>).write(IoSession, WriteRequest) line: 68 SimpleIoProcessorPool<S>.write(S, WriteRequest) line: 278 SimpleIoProcessorPool<S>.write(IoSession, WriteRequest) line: 80 DefaultIoFilterChain$HeadFilter.filterWrite(IoFilter$NextFilter, IoSession, WriteRequest) line: 914 DefaultIoFilterChain.callPreviousFilterWrite(IoFilterChain$Entry, IoSession, WriteRequest) line: 753 DefaultIoFilterChain.access$1500(DefaultIoFilterChain, IoFilterChain$Entry, IoSession, WriteRequest) line: 49 DefaultIoFilterChain$EntryImpl$1.filterWrite(IoSession, WriteRequest) line: 1146 IoFilterEvent.fire() line: 131 MdcInjectionFilter.filter(IoFilterEvent) line: 162 MdcInjectionFilter(CommonEventFilter).filterWrite(IoFilter$NextFilter, IoSession, WriteRequest) line: 68 DefaultIoFilterChain.callPreviousFilterWrite(IoFilterChain$Entry, IoSession, WriteRequest) line: 753 DefaultIoFilterChain.access$1500(DefaultIoFilterChain, IoFilterChain$Entry, IoSession, WriteRequest) line: 49 DefaultIoFilterChain$EntryImpl$1.filterWrite(IoSession, WriteRequest) line: 1146 SslHandler.flushScheduledEvents() line: 330 SslFilter.filterWrite(IoFilter$NextFilter, IoSession, WriteRequest) line: 671 DefaultIoFilterChain.callPreviousFilterWrite(IoFilterChain$Entry, IoSession, WriteRequest) line: 753 DefaultIoFilterChain.access$1500(DefaultIoFilterChain, IoFilterChain$Entry, IoSession, WriteRequest) line: 49 DefaultIoFilterChain$EntryImpl$1.filterWrite(IoSession, WriteRequest) line: 1146 CompressionFilter.filterWrite(IoFilter$NextFilter, IoSession, WriteRequest) line: 152 DefaultIoFilterChain.callPreviousFilterWrite(IoFilterChain$Entry, IoSession, WriteRequest) line: 753 DefaultIoFilterChain.access$1500(DefaultIoFilterChain, IoFilterChain$Entry, IoSession, WriteRequest) line: 49 DefaultIoFilterChain$EntryImpl$1.filterWrite(IoSession, WriteRequest) line: 1146 ProtocolCodecFilter.filterWrite(IoFilter$NextFilter, IoSession, WriteRequest) line: 340 DefaultIoFilterChain.callPreviousFilterWrite(IoFilterChain$Entry, IoSession, WriteRequest) line: 753 DefaultIoFilterChain.access$1500(DefaultIoFilterChain, IoFilterChain$Entry, IoSession, WriteRequest) line: 49 DefaultIoFilterChain$EntryImpl$1.filterWrite(IoSession, WriteRequest) line: 1146 LoggingFilter(IoFilterAdapter).filterWrite(IoFilter$NextFilter, IoSession, WriteRequest) line: 138 DefaultIoFilterChain.callPreviousFilterWrite(IoFilterChain$Entry, IoSession, WriteRequest) line: 753 DefaultIoFilterChain.access$1500(DefaultIoFilterChain, IoFilterChain$Entry, IoSession, WriteRequest) line: 49 DefaultIoFilterChain$EntryImpl$1.filterWrite(IoSession, WriteRequest) line: 1146 DefaultIoFilterChain$TailFilter(IoFilterAdapter).filterWrite(IoFilter$NextFilter, IoSession, WriteRequest) line: 138 DefaultIoFilterChain.callPreviousFilterWrite(IoFilterChain$Entry, IoSession, WriteRequest) line: 753 DefaultIoFilterChain.fireFilterWrite(WriteRequest) line: 746 NioSocketSession(AbstractIoSession).write(Object, SocketAddress) line: 570 NioSocketSession(AbstractIoSession).write(Object) line: 515 ChatProtocolHandler.messageReceived(IoSession, Object) line: 106 DefaultIoFilterChain$TailFilter.messageReceived(IoFilter$NextFilter, IoSession, Object) line: 1015 DefaultIoFilterChain.callNextMessageReceived(IoFilterChain$Entry, IoSession, Object) line: 650 DefaultIoFilterChain.access$1300(DefaultIoFilterChain, IoFilterChain$Entry, IoSession, Object) line: 49 DefaultIoFilterChain$EntryImpl$1.messageReceived(IoSession, Object) line: 1128 LoggingFilter.messageReceived(IoFilter$NextFilter, IoSession, Object) line: 208 DefaultIoFilterChain.callNextMessageReceived(IoFilterChain$Entry, IoSession, Object) line: 650 DefaultIoFilterChain.access$1300(DefaultIoFilterChain, IoFilterChain$Entry, IoSession, Object) line: 49 DefaultIoFilterChain$EntryImpl$1.messageReceived(IoSession, Object) line: 1128 ProtocolCodecFilter$ProtocolDecoderOutputImpl.flush(IoFilter$NextFilter, IoSession) line: 413 ProtocolCodecFilter.messageReceived(IoFilter$NextFilter, IoSession, Object) line: 257 DefaultIoFilterChain.callNextMessageReceived(IoFilterChain$Entry, IoSession, Object) line: 650 DefaultIoFilterChain.access$1300(DefaultIoFilterChain, IoFilterChain$Entry, IoSession, Object) line: 49 DefaultIoFilterChain$EntryImpl$1.messageReceived(IoSession, Object) line: 1128 CompressionFilter.messageReceived(IoFilter$NextFilter, IoSession, Object) line: 169 DefaultIoFilterChain.callNextMessageReceived(IoFilterChain$Entry, IoSession, Object) line: 650 DefaultIoFilterChain.access$1300(DefaultIoFilterChain, IoFilterChain$Entry, IoSession, Object) line: 49 DefaultIoFilterChain$EntryImpl$1.messageReceived(IoSession, Object) line: 1128 SslHandler.flushScheduledEvents() line: 335 SslFilter.messageReceived(IoFilter$NextFilter, IoSession, Object) line: 553 DefaultIoFilterChain.callNextMessageReceived(IoFilterChain$Entry, IoSession, Object) line: 650 DefaultIoFilterChain.access$1300(DefaultIoFilterChain, IoFilterChain$Entry, IoSession, Object) line: 49 DefaultIoFilterChain$EntryImpl$1.messageReceived(IoSession, Object) line: 1128 IoFilterEvent.fire() line: 105 MdcInjectionFilter.filter(IoFilterEvent) line: 162 MdcInjectionFilter(CommonEventFilter).messageReceived(IoFilter$NextFilter, IoSession, Object) line: 84 DefaultIoFilterChain.callNextMessageReceived(IoFilterChain$Entry, IoSession, Object) line: 650 DefaultIoFilterChain.access$1300(DefaultIoFilterChain, IoFilterChain$Entry, IoSession, Object) line: 49 DefaultIoFilterChain$EntryImpl$1.messageReceived(IoSession, Object) line: 1128 DefaultIoFilterChain$HeadFilter(IoFilterAdapter).messageReceived(IoFilter$NextFilter, IoSession, Object) line: 122 DefaultIoFilterChain.callNextMessageReceived(IoFilterChain$Entry, IoSession, Object) line: 650 DefaultIoFilterChain.fireMessageReceived(Object) line: 643 NioProcessor(AbstractPollingIoProcessor<S>).read(S) line: 539 AbstractPollingIoProcessor<S>.access$1200(AbstractPollingIoProcessor, AbstractIoSession) line: 68 AbstractPollingIoProcessor$Processor.process(S) line: 1223 AbstractPollingIoProcessor$Processor.process() line: 1212 AbstractPollingIoProcessor$Processor.run() line: 683 NamePreservingRunnable.run() line: 64 ThreadPoolExecutor.runWorker(ThreadPoolExecutor$Worker) line: 1149 ThreadPoolExecutor$Worker.run() line: 624 Thread.run() line: 748
Writing the data is done when we are back in the AbstractPollingIoProcessor$Processor.run() call. Here is the stack for that action :
NioProcessor.write(NioSession, IoBuffer, int) line: 384 NioProcessor.write(AbstractIoSession, IoBuffer, int) line: 47 AbstractPollingIoProcessor$Processor.writeBuffer(S, WriteRequest, boolean, int, long) line: 1107 AbstractPollingIoProcessor$Processor.flushNow(S, long) line: 994 AbstractPollingIoProcessor$Processor.flush(long) line: 921 AbstractPollingIoProcessor$Processor.run() line: 688 NamePreservingRunnable.run() line: 64 ThreadPoolExecutor.runWorker(ThreadPoolExecutor$Worker) line: 1149 ThreadPoolExecutor$Worker.run() line: 624 Thread.run() line: 748
The SslFilter implementation¶
As we can see,the SslFilter never calls the next filter. It delegates that to the SslHandler class. The trick is that it uses a queue to store encrypted messages, which will wait until the message is fully encrypted before flushing them. The rational is that a message can be pretty big, and teh SSL/TLS protocol just allows limited encrypted data blocks (around 16k), so it keeps each block of encrypted data into the queue temporarily.
Room for improvements¶
Reducing the stack size¶
This is clearly sub-optimal. First of all, the stack is crazy big, there is no reason for that. The idea was intially to allow a session to dynamically update the chain, by adding or removing a filter while a session is active. This is a fine idea, as some protocols need to do that: typically, LDAP allows a user to send a startTls extended request which tells the server to establish a secured communication. In this very case, we add the SslFilter in the session's filter chain.
Now, we can do that more efficiently. A session's filter chain is created when the session itself is created. Adding a new filter in this chain is done by the application, and at this point, no other thread uses this session (each session is associated with one single IoProcessor instance's thread), unless an ExecutorFilter has been added to the chain. In this case, things get complicated... Having an ExecutorFilter in the chain means any event can be processed concurrently, in different threads. Although they are sharing the same filter chain. We have to protect this chain against concurrent modifications, and we must also guarantee that any change made in the chain will not be an issue for the application. Typically, adding a SslFilter in a chain while a message is being written is problematic: we have to wait for any pending messages to be sent to the remote peer, which may have already initiated a handshake (although it's the client responsability to handle this situation), or we decide to dicth any of those messages (the right thing to do, IMHO) we have to deal with any message that are somewhere in the chain before the SslFilter. Again, the strategy would be to discard them * and we have to deal with any message somwhere in the chain after the SslFilter and before they are pushed into the session write queue, for the exact same reason.
For messages received before the addition of the SslFilter, but still in the chain, we should ditch them for the exact same reason.
Bottom line, this is a application issue, and should be handled by the application, because MINA can't deal with all the possible. starties in place of the application.
That being said, we can use the IoProcessor as a controller. It would b eits responsability to propagate events from one filter to the other, using such a loop:
for each filter in the chain
do
propagate the event to the filter with the previous result
store the result
done
What we call the 'result' is just the transformed data. Typically, when calling the ProtocolCodecFilter.messageReceived event, the result will be the decoded message, not the IoBuffer (or whatever) input. This will be passed to the next filter (remember that the IoFilter.messageReceived method takes an Object as a parameter).