liblogging: liblogging - a slim library for reliable syslog

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# liblogging - a slim library for reliable syslog

## Introduction

RFC 3195 offers reliable connections between syslog clients and collectors (servers, daemons). This is highly desirable in todays networking world. However, RFC 3195 was initially not implemented because many implementors thought it was too complex. The reason for this is that 3195 is a BEEP profile (see RFC 3080 and 3081). BEEP itself is a very powerful, yet relatively complex protocol. Full BEEP libraries are hard to find and often place restrictions on the developer, especially in regard to threading models and such.

Liblogging takes a different approach. It is a library specifcally created to talk syslog, NOT general BEEP. It provides just the bare functionality needed for syslog, nothing else. Consequently it is very slim. The authors hope that this will enable developers of even low-powered devices to adopt the library and provide reliable syslog logging.

This library has been named liblogging because it may eventually contain code for additional logging methods in the future (like "plain" UDP syslog, 3195 COOKED and syslog-sign). But this is no promise ;)

If you are interested in "just" using the library, please go straigt to srAPI::h.

Continue to read only if you would like to learn the inner workings or port the lib to a different environment.

This library compiles under a variety of platforms. It is originally developed and maintened under Win32. It is frequently ported and tested on Red Hat Linux. These are the platforms (in this order) we have the most confidience in the library on. Others may (though unlikely) show unexpected behaviour.

This documentation file here is NOT up to date. However, it will provide you a good overview of the overall design and many specifics. To get the totally right details, please see the respective modules' documentation. Please note that the SEQ handling described below for the listener part is NOT YET implemented - but it is still on the list. As such, the server API should be considered unstable. If you base a server on liblogging, please be prepared to update your server to make it work with future versions of liblogging. Of course we try to keep the impact as low as possible, but that's not a guarantee!

The client side API is stable and changes should not be necessary for future releases (except, of course, you would like to use new features).

## Design Goals

The overall mission of liblogging is to provide a small, easy to use library that will work as an enabler for reliable syslog.

There are a number of design goals

- self-contained
  no additional libraries shall be needed
- low ressource consumption
  memory and CPU usage should be kept to a minimum so that the lib can also be used on low-powered devices
- thread-model agnostic
  no specific threading model is needed for this library. Everything can be done from a single thread, BUT the library is thread-safe so that it can be called from multiple threads concurrently.
- extensible
  Even though the initial design includes a very limited feature set, liblogging should be able to be extended easily.
- portability That's a hard one. liblogging is developed under Win32. While we take reasonable care to make it portable, we only know this once ports have actually been done.

## Architecture

### Level of BEEP Compliance

In order to achieve our primary goals, we do NOT provide a full BEEP stack. Effectively, you could even argue that we do not provide a BEEP stack but a "BEEP pretender". It implements just what it needs and uses e.g. canned greetings instead of actual ones. It often does only accept some very specific answers and will probably answer with a BEEP ERR if others are provided.

All in all, RFC3195 provides a very limited need for BEEP features and we only provide this set. So do NOT expect a small, full BEEP lib - you will be severely disappointed. If you are interested in basic syslog functionality, this lib is for you. If you want more, you should continue searching. liblogging will probably provide more headache as it is worth in this case.

BEEP peers can either be initiators or listeners. Initiators are what traditionally is called "clients" - they talk to a server in order to acomplish some work. "Listeners" are traditionally called servers.

This library has different approaches for initiators and listeners. Keep in mind that the lib shall not impose any specific threading model onto its host. As such, we must be able to run on a single thread, even if there are potentially multiple activities going on (like channel 0 and channel 1 operations).

#### Initiators

Initiators create a BEEP session to the remote peer. The session object also provides channel 0 management (this is part of the session).

Upon creating a session, the session object sends this peers greeting and reads (and stores) the remote peers greeting. Other than that, nothing happens. This peers greeting will always be empty as we do not advertise any listener functionality at this stage.

Finally, the initiator profile calls the channel setup function. It provides a pointer to its profile object to the session object. The session object in turn takes the URI from the profile object and initiates a channel with the remote peer. The peers response is read and interpreted. If it is positive, a new channel object is created and passed onto the initiator profile.

One the channel has been created, it is up to the initiator profile to control data exchange. To do so, the channel object provides send and receive methods. As of this release, the send and receive full messages only and DO NOT provide support for fragmentation.

It is important to keep in mind that even though a "simple" syslog raw application will not run more than one channel, there are potentially messages being exchanged to do NOT belong to the actual syslog data exchanged. Most importantly, these are the RFC 3081 SEQ frames, which must be paid attention to. If they are not honered, the session will stall as soon as more than a few bytes of data are being sent. Other than that, there may be some control messages on channel 0.

In order to support these asynchronous channels, we need to provide for an intelligent TCP stream reader. We have decided to implement it in a way that will also facilitate later additions of true multi-channel BEEP sessions.

The corenerstone is a session management method called readFrame(). It receives a pointer to the channel object. Its duty, from the channel object point of view, is to obtain the next frame from the physical stream. From the BEEP sesseion point of view, its duty is to read & buffer all incoming data and also do session management and SEQ frame processing. This is done as follows:

WHEN RECEIVING DATA, the initiator profile calls the message object and asks it to receive a message. The message object then calls the frame object and asks it to receive the frame. Please note that in this version of the library, fragmentation is not supported and as such there is a one-to-one relationship between messages and frames. Should that be changed in later versions of the library, the message object can easily be upgraded to receive multiple frames up until the final frame has been received. Back to the frame object: once the frame object's receive method is called, itself calls back into the session object and instructs the session object to gather the data. This is very important, because now the session object is capable of handling intermixed control messages as well as SEQa. This provides a kind of multi-tasking without real multi-threading. When the session object receives control, it first queries the socket (via select) if any data is ready to be received. If so, the session object receives all data (in method DoReceive() until no more data is ready to be received. To actually receive the data, it calls back into another method of the frame object (let's call this method ActualRecvFram() for now). We will describe this method in a short while. Each frame returned from ActualRecvFram() will be examined by the session object. If it is a SEQ frame, the associated channel will be called to process the SEQ immediately. If it is a data frame, it is buffered in an in-memory queue. The queue is on a per-channel basis. Once all messages are processed, the session object looks at the requesting channels data queue. If there is at least one entry, that entry is passed back to the channel object. if there is still no entry, the session object re-itereates its processing, starting with the polling of the incoming data. In order to avoid a tight loop, the session object must Sleep() some milliseconds (at least 10) if there are no frames ready in consequtive calls. Using this mode of operation, the session objects receive frame method will not only provide the next data frame to its uppper layer but will also be able to act on SEQ frames as well as other channel's data. It is also possible that a specifically configured channel/profile may ask the session object to call a callback as soon as data is received. this mode will probably be used in listeneres and provide full multiprocessing. It is not thought out yet.

Back the the ActualRecvFram() method of the frame object: it will first receive the 3-byte header and then use the processing that is required by the header type. E.g. SEQ has a very different processing. Once this is done, it will read the channel number and once the channel is known, ActualRecvFrame() will call back into the session object to obtain a handle to this channel's object. This handle will then be used to do seqno verification. The channel object will be updated by ActualRecvFram().

WHEN SENDING DATA, The initiator profile constructs a message and asks the message object to send it. The message object then constructs the necessary frames (in this release only one!) and then asks the frame object to send this frame. The frame object hands the request down to a send method in the session object. The session object then first checks if there is unread data in the TCP stream. If so, it reads this data off the session. This process is the same as outlined in "when receiving data". In fact, the same method (DoReceive()) is to be called. It takes care of the necessary housekeeping and also processes all SEQ frame not processed so far. This is a very important step, as an on-the-wire SEQ frame may be necessary to be read before the actual to-be-sent frame can be send. Once DoReceive() returns, the session object sends the frame with the help of the channel object. The channel object must check if the current frame can be fully send (in respect to the current Window size). If it can't, it must flag an error condition and NOT send the frame. It should then return immediately to the session object. If it can send the frame, it must do so and then return with an OK return status to the session object. The session object must check the return value. If the return value indicates the current frame could not be sent due to the Window being insufficient, the session object must again do a DoReceive(). However, this time it can be done in a blocking manner as no further processing is possible without receiving the obviously outstanding SEQ. Once this is done, the session object re- iterates its processing and will be able to send the frame during either this stage or a later one.

If the channel object returned with either another error or an OK condition, the session object can also terminate and pass the return status to its upper layer (and so forth until the profile object is reached).

#### Listeners

Listeners provide server functionality. They are implemented via the sbLstn Object. Please note that some of the description below may refer to this as part of the session object. It is NO LONGER, this was changed. The writing below will be updated once the implementation is fully done (if not forgotten ;)).

Not every potential liblogging user will need to implement listeners. In fact, we assume most will use it for implemnting clients, only. In order to keep the library footprint small, all session management methods needed by the listener code only has been moved to beepsessionLstn.c. This way, it will be a separate object module and only be linked in to those applications actually needing it. The prototypes, however, are in beepsession.h as they logically belong there.

If somebody for some reason would like to disable the listener functionality at all, simply remove the beepsessionLstn.c file from the build. Be sure to do the same for all listner profiles (and any other profiles you don't need). YOU CAN NOT USE LISTENER PROFILES IF YOU HAVE REMOVED beepsessionLstn.c (for obvious reasons). Trying so will result in linker errors.

Channel 0 protocol is not handled via profiles. It is done in BEEPSession handler, only.

##### Overview

Listeners are quite challenge for liblogging's goal to be independent of any threading model. This release will be build around select(), that is it will be running on a single thread for all of its network receive needs. If that model is too simplistic for a caller, the caller may go ahead and use the provided events to call back into some of his multiple threads, doing whatever threading logic he needs to do. I know this is a bit tricky, but it is the current state of affairs. Later versions of the library may provide an alternate mean of threading in which the caller is assumed to create a new thread for each incoming session (not channel). If this happens, of course the currently existing mode will also remain. So don't panic, no code to change in that event ;-).

Anyhow, keep in mind that the select() method is very powerful and - together with proper other processing - can be used to build high-performance servers.

While not enforcing this model, we recommend that profiles using this library (and apps using the provided profiles) just pass the data off to some queue (whereever it may be) and process that data asynchronously. Methods have been build into liblogging to do this very efficiently.

OK, having said this, the current approach is:

At the core of the listener functionality is a method called sbLstnRun() (or a similar name). That method *is* the actual listener. In concept, it just does the following:

1. It assembles a list of all active sessions (NOT channels!) AND the session-initiation listener socket.
2. It calls select() on all of them, asking select() to return only when new data is available (or a new connect came in). Please note that this is in fact a bit more complicated (see the handling of SEQ below), but we keep it simple here to convey the idea.
3. When select returns, sbLstnRun() checks if it is a new connection request or data received.
   - If it is a new connection request, it is passed on to the session object to create a new session including channel 0.
   - If it is new data, that data is passed onto the session object. It will validate it and act on channel 0 management messages. If it is profile data, it passes the data onto the registered event hook of the profile in question.

These three steps are repeated until an terminate processing request is issued to sbLstnRun(). This can be done by using a large timeout (10 seconds?) on the select() wait. If the timeout expires and there was no network activity, a simple variable is checked. If it is set to e.g. 0, the loop will be terminated, otherwise it will be continued. The variable may be best called "RunIndicator" or such.

If we look at this model, a profile would use the following steps to set up a server:

- Create the profile object and populate the required profile event pointers.
- Add the profile to the pool of the BEEP listners supported profiles.

These two steps should be done for each profile supported. When finished with this

- The application must call sbLstnRun()

From now on, the server is active and will call back into the profiles as requested. The server will automatically

- accept new incoming connections
- provide a greeting message specifying the actual listener profiles
- do channel 0 management
- call the respective profile hooks for incoming data and other events (see below)

The sbLstnRun() can only be stopped by calling sbSessTerminteListen(). This, in turn, would be most probably called in response to an external event. This could be either a signal (like SIGUSR1 or SIGINT) or the result of a profile action. It could also be set to auto-terminate after the last open session has been terminated and there are no other active sessions (though this would only be useful for the most simplistic listenere). Most apropriately, the sbSessTerminteListen() method should be called from a different thread inside the server process. Anyhow, we must not have multiple threads running to run multiple channels concurrently. All can be done on a single thread. The sample-server.c example program is such a simple, single-threaded server. It terminates as soon as there are no more open sessions (OR SHOULD IT TERMINATE ON SIGINT???? - let's see what ports easier...).

Please note that sbLstnRun() will construct the greeting message dynamically on the list of profiles available AT THE TIME OF THE CONNECT. In a multi-threading caller, profiles may be added and removed while sbLstnRun() is active. This is a valid action. (

Todo:
  The linked list of profiles is thread-safe). When a profile is added, it is immediatly available for the next connection. If a profile is removed, (

  implement this later!) the profile is immediately no longer be advertised in the greeting BUT it is actually removed only after the last channel using it has been closed.

It is valid, though useless, to call sbLstnRun() whithout any profile being set. This is to support multi-threaed application which set profiles later. Of course, an empty profile list means no channel can be created at all (but the session will be accepted and channel 0 be established even in this case).

In order to perform its work, the session object keeps a linked list of all active sessions. This is used to feed select(). The current listen channel is managed by sbLstnRun() itself and NOT part of the linked list.

##### Sending Data

Well, believe it or not, I forgot that a server must not only receive but send data ;) So this paragraph here is a little disconnected from the rest of the sbLstnRun() description.

In order to work successfully with select(), you must make sure that your application never blocks when using sockets. This obviously includes responses send. You can (unfortunately ;)) not assume that a send call will always be non-blocking - even with small amounts of data, transmit queues can be build. As such, we need to find a model which enables us to asynchronously send data that must be sent as a response.

Fortunately, this is relatively easy to acomplish. The key is that all IO (including sending) is handled by the listener process itself (the one that controls the select()). As such, we enhance the session object to contain a queue of to-be send data as well as a state variable. The session (of a listener) object is now a state machine. Data is send depending on the current state. Whenever data is to be sent, it is first checked if any send operation is in progress. If not, it is tried to send the data. If that succeeds, no further action is taken. If it does not succeed, however, the to be send data is enqueued on the session object.

Method sbLstnRun() checks if sessions have outstanding send requests. If so, it includes their sockets in the select() call. If select returns with these sockets, it is tried to send as many data as possible. Data successfully send is removed from the queue. Sending stops if either the queue is empty or the send would block. In the later case, the rest of the queue will be tried to send after the next select call (and so on).

A frame can be sent if

- the socket is ready for sending
- the remaining window is large enough

If either of the two is not true, sending needs to be deferred. If a frame is already partly send, no other frame on the same session can be send before the currently "active" frame has completely been sent.

When receiving data: The frame needs to be constructed on the fly. The IO handler, when receivin, has two different states:

- in header
- in body

While in header, it is looking for the terminating CRLF. There should also a saveguard (e.g. 4K) so that it can not be overrun. While it is state = in_header, no processing takes place. Each received byte is added to the received header line. As soon as CRLF are received, the header line is parsed and a protocol error is then detected (as such, no errors are caught). When in body, messages are received until the specified message limit. Then, the status changes to "in trailer" which then will verify new incoming messages against the trailer.

##### Threading Model

As can be seen, no specific threading model is needed, we can also operate a multi-tasking server on a single thread. HOWEVER, we recommend the following scheme to users of this library:

- Have at least on thread for managing your application. That thread should call in to add and/or remove profiles to the listener.
- Have one other thread whoms sole purpose is to call sbLstnRun() and provide a worker for that as well as the profile callbacks.
- Have at least one other thread that will actually process the data received by the profiles.

It is recommended that the profiles (on the sbLstnRun() thread) just do profile handling and put the received data into some queue that is then processed by the third thread pool. They also need to keep track of acknowledgments and thus the queue size. They should not carry out any other action and return as quickly as possible to sbLstnRun(). THIS IS A CRITICAL REQUIREMENT - if the profile callbacks do too lengthy operations (or even waits), they can thrash the whole multithreaded application.

Of course, if the applications is single-threaded, the profile callbacks can do (and must do) whatever processing is needed, no matter how long it will take. ;-)

We strongly encourage implementors to use the above model with at least 3 threads for production software.

There are a number of "events" that a listener profile can implement. Some MUST be implemented.

- OnFramRecv [MANDATORY] Called whenever a complete frame is received.
- OnChanCreate
- OnChanPeerCloseRequest
- OnChanClosed

All of these methods receive a pointer to the current channel object. No other data is passed to the caller. The channel object, however, contains a pointer to the current profile object, where the caller in turn can include a pointer to more relevant data.

Please note that there is no "peer connect" event. The peer will never connect to a profile, but to a session. The session (channel 0) is then used to open up a channel. Consequently, the OnChanCreate() event is the first event that a profile can receive.

The profile is responsible for following proper order of messages. We think that a profile will most probably be implemented as a state machine, but this is left to the profile implementor.

A profile MUST support the following callbacks:

- SendSEQUpTo This is used by the BEEP stack to ask the profile up to which message ID a SEQ frame should be sent. The profile must return the ID of the last frame that it has processed or queued (at the choice of the profile).

As a hint to profile developers, SendSEQUpTo() can be used to allow an in-memory queue to grow up to a specifc size and if it outgrows this size to slow down the receiver. If a profile is implemented in that way, it allows for fast buffering of burst traffic but prevents exhaustion of system ressources when too much data comes in too quickly. As a general guideline, we recommend that a profile initially buffers incoming data and acknowledges all received packets immediately (when the BEEP stack calls SendSEQUpTo()). If the queue threshold is reached, it then should always return the same ID it returned previously. That way, no new SEQ will be sent to the remote peer and the remote peer will stop sending new data once the window size is reached. Please note that the profile will probably receive multiple calls to SendSEQUpTo() during this process as the remote peer will continue to send data until the end of the window. As such, some more message will need to be enqueued and OnMesgReceive() will be called some more times. Once the queue has been emptied beyond the threshold, the client can again answer with the most recent ID, which will make the BEEP stack send a SEQ frame for that ID and the remote peer will continue to send data. Now the same process begins again, eventually the server will again hit the threshold, stop acknowledging messages and thus slowing down the sender. If using the logic described here, the queue will never grow above queue threshold + size of window.

Of course, the above described is only a recommendation to the profile implementor and the profile may acknowledge each message immediately, not paying attention to any queue. Or it may defer acknowledging the entry until some action has been carried out. Whatever it does, it must make sure that it suffciently often acknowledges the messages as when the sender hits the window size AND does not receive an ACK, the sender will not send any more packets. Such a connection would be stalled and no way to recover it. It is the profile implementor's task to avoid this. Nobody will save you from this ;)

Of course, the above description also includes much information on how the BEEP stack should operate. If we look into the specifics there, we see there is need for SendSEQUpTo() to be called frequently. At least, it must be called once AFTER each OnMesgRecv(). If the caller always immediately acknowledges the msgno, things are straightforward: we can send a SEQ frame and communication can carry on. However, if the caller defers acknowledgment, we may run into an issue. Again, everything is easy if the caller acknowledges the msgno before the end of the window is reached - a SEQ frame will be sent and communication continues. However, if the caller is not ready to acknoledge the msgno after the last OnMesgRecv() in this window, we need to ask him at other occasions for the acknowledgement. The reason is that OnMesgRecv() will never be called once the window has been used up - no new messages can arrive in this stage. We assume, this situation will often occur during traffic bursts (assuming a "good" profile implementation). So we must handle this efficiently.

A key point is that we must try conserve system ressources while on the other hand providing a fast response time once the profile is ready to accept new data. As we need to call the profile, there is some polling involved. We have several ways to talk to the profile.

First of all, we can use our sbLstnRun() method to call back to any "stalled" profiles as soon as there is ANY network activity (meaning new connections or frames received - could even include frames send from another profile/channel). This is a natural way, as the method already was awoken and there is little overhead in doing the SendSEQUpTo().

Unfortunately, this works only well if we have frequent other traffic. In a scenario with just one channel active, that doesn't help anything at all - there is no network activity. In this case, we need to work with timeouts (driving an actual polling cycle). The problem with a timeout obviously is that if we use a too short period, we eat up too many system ressources and using a too long period, we loose performance and response time. Past experience shows that a fixed timeout period can most probably be only set inappropriately ;)

We are trying to avoid this with the following algorithm: The timeout period is dynamically adjusted. We assume that the longer an acknowledgement is outstanding, the less likely is a quick acknowledgment. We also assume that even when an acknowledgment would happen very quickly (but after an overall long wait period), it would not really matter if we have some more delay in sending the SEQ because the work queue for the profile would probably still being very full (given a "good" profile implementation according to our recommendation). We take advantage of this approach. The timeout period is initialized with a 10ms timeout. If it expires, SendSEQUpTo() is called for all profiles with missing acknowledgments. If none of them returns a new acknowledgment, the timeout value is doubled. Then a new wait is done for at most this timeout period. After it returns, SendSEQUpTo() is called again ... and so on. The timeout value is always double until it reaches a maxmimum timout value, which could be 640 ms or such. The initial and maximum timeout values could be configurable parameters. Once the maximum timeout value is reached, processing continues as described, only the timeout value is never increased. If any of the profiles returns a new acknowledgement, the corresponding SEQ is sent and the algorithm is completely restarted again. Restarted means it begins to look if there are outstanding acknowledgments and then it decides wheter to use timeout or not - and if so, it starts from the initial timeout value.

Note well: this algorithm is ONLY applied if we have at least one channel who is "stalled" in a way that the profile must acknowledge outstanding msgnos. If all profiles/channels have acknowledge the last received message, the algorithm is NOT used and a "normal" blocking wait for network activity is done. The author assumes that this is the normal case and as such any impact from a still-improper timeout value is minimalistic.

### Progamming Language Used

While it is tempting to use C++, we have decided to stick with C in order to make the lib available to as many platforms as possible.

### Objects

WARNING: this section is not totally up to date!

We use an object-oriented approach even though the lib is written in plain C. Objects are simulated by instance structures. All non-static class members receive a "this" point (pointer to the instance data structure) passed to as the first parameter. The constructor will create the instance data structures.

The following classes are known as of now:

- sbSock sockets.h
- BEEP "pretender classes":
  - sbSess beepsession.h
  - sbChan beepchannel.h
  - sbFram beepframe.h
  - sbMesg beepmessage.h
  - sbProf beepprofile.h
- srAPI srAPI.h

Each class is contained in the respective source file. Class names either start with sr (syslog reliable) or sb (slim beep). They are followed by a 4-character abbreviation. This serves also as a namespace - the 6 character class name is prepended to all method names.

## Naming Conventions

As this is plain C, we do not have a real namespace. To avoid conflicts, all globals start with prefixes, followed by the actual function name. The function/method/data object name begin with a capital letter. The prefix is mentioned in the respective header.

## Authors

This project was initiated by Rainer Gerhards, who is also the developer of the initial version. Rainer can be contacted at rgerhards@adiscon.com (Watch for the Anti-Spam reply and follow the instructions!)

## Restrictions

- tuning profiles are not supported
- only 2 channels (channel 0 and 1 syslog channel) are supported
- at this time, only the RAW profile is supported

The file architecture.c does NOT contain any code - it is solely used to provide this documentation.

Author:
  Rainer Gerhards <rgerhards@adiscon.com>

Date:
  2003-08-04 0.1.0 Initial version created

  2003-08-05 0.1.1 version - now compiles under Linux (confirmed with Red Hat 8.0). There is also a makefile who does the trick. This is an interim release - it already includes some linked list code, which so far is used nowhere. This is in preparation of the 0.2.0 release which will include an XML parser and more robust BEEP protocol error handling.

  2003-08-13 Description for listener operations added (begins 0.3.0, what probably later will become 1.0.0).

## Legal State

Copyright 2002-2003 Rainer Gerhards and Adiscon GmbH. All Rights Reserved.

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