Vanguard (microkernel)
Vanguard was an experimental microkernel developed at Apple Computer in the early 1990s. Based on the V-System, Vanguard introduced standardized object identifiers and a unique "message chaining" system for improved performance. Vanguard was not used in any of Apple's commercial products, and development ended in 1997 when Apple's research group (ATG) was dismantled.
Basic concepts
Vanguard was generally very similar to the V-System, but added support for true object oriented programming of the operating system. This meant that kernel and server interfaces were exported as objects, which could be inherited and extended in new code. This change has no real effect on the system itself, it is primarily a change in the source code that makes programming easier.
For instance, Vanguard had an I/O class which was supported by a number of different servers, networking and file servers for instance, which new applications could interact with by importing the I/O interface and calling methods. This also made writing new servers much easier, both because they had a standard to program to, as well as being able to share code more easily.
V messaging semantics
A key concept to almost all microkernels is breaking down a single large kernel into a set of communicating servers. Instead of having a single large program in control of the entire hardware side of the computer system, these sorts of duties are handed out to smaller programs that are given rights to control different parts of the machine. For instance, a particular server might be given control of the networking hardware, while another has the task of managing the hard drives. Another server would handle the file system, calling both of these lower-level servers. User applications ask for services by sending messages to these servers, using some form of inter-process communications (IPC), as opposed to asking the kernel to do this work via a syscall or trap.
Under V the IPC system appears to be conceptually modeled on remote procedure calls (RPC) from the client application's perspective. The client application imported an interface definition file containing information about the calls supported by the kernel, or other applications, and then used this definition to package up requests. When called, the kernel would immediately take over, examine the results, and pass the information off to the right handler, potentially within the kernel itself. Any results were then handed back through the kernel to the client.
In general terms, the operation of the system as it appears to the client application is very similar to working with a normal monolithic kernel. Although the results passed back might come from a third party handler, this was essentially invisible to the client. Servers handling these requests operated in a similar fashion to the clients, opening connections with the kernel to pass data. However, servers generally spawned new threads as required to handle longer-lasting requests. When these were handled and the responses posted, the thread could be de-allocated and the servers could go into a "receive mode" awaiting further requests.
In contrast, most microkernel systems are based on a model of asynchronous communications, as opposed to synchronous procedure calls. The canonical microkernel system, Mach, modeled messages as I/O, which has several important side-effects. Primary among these is that the normal task schedulers under Unix-like systems will normally block a client that is waiting on an I/O request, so in this way the actions of pausing and restarting applications waiting on messages was already built into the underlying system. The downside to this approach is that the scheduler is fairly "heavyweight", and calling it was a serious performance bottleneck and led to extensive development efforts to improve its performance. Under the V-System model the message passing overhead is reduced because the process scheduler does not have to be consulted, there is no question as to who should next be run – it's the server being called. The downside to the V approach is that it requires more work on the server side if the response may take some time to process.
Chaining
One major addition to the IPC system under Vanguard, as opposed to V, was the concept of message chains, allowing a single message to be sent between several interacting servers in a single round-trip. In theory, chaining could improve the performance of common multi-step operations.
Consider the case where a client application wishes to read a file. Normally this would require one message to the kernel to find the file server, then three additional messages to the file server itself - one to resolve the file name into an object id, another to open that id, then finally another to read the file. Using Vanguard's chaining, a single message could be constructed by the client that contained all of these requests. The message would be sent to the kernel, and then passed off to the file server who would handle all three requests before finally returning data.
Much of the performance problem normally associated with microkernel systems are due to the context switches as messages are passed back and forth between applications. In the example above running on a V system, there would have to be a total of eight context switches; two for each request as the client switched to and from the kernel. In Vanguard using a chain would reduce this to only three switches; one out of the client into the kernel, another from the kernel to the file server, and finally from the server back to the client. In some cases the overhead of a context switch is greater than the time it takes to actually run the request, so Vanguard's chaining mechanism could result in real-world performance improvements.
Object naming
V had also introduced a simple distributed name service. The name service stored "well known" character names representing various objects in a distributed V system, for instance "2nd floor laser printer". Applications could ask the name server for objects by name, and would be handed back an identifier that would allow them to interact with that object. The name service was not itself a separate server, and was managed by code in the kernel itself. Contrast this with the full-blown name server under the Spring operating system, which not only knew about objects inside the system, but was also used by other servers on the system to translate their private names – file names and IP addresses for instance.
Under the V-System, objects in servers were referred to via an ad hoc private key of some sort, say a 32-bit integer. Clients would pass these keys into the servers in order to maintain a conversation about a specific task. For instance, an application might ask the kernel for the "file system" and be handed a 32-bit key representing a program id, and then use that key to send a message to the file system asking it to open the file "my addresses", which would result in a 64-bit key being handed back. The keys in this example are proprietary to the servers, there was no common key format being used across the system.
This sort of name resolving was so common under V that the authors decided to make these keys first-class citizens under Vanguard. Instead of using whatever object ID's the servers just happened to use, under Vanguard all servers were expected to understand and return a globally unique 128-bit key, the first 64-bits containing a server identifier, the second identifying an object in that server. The server id was maintained in the kernel, allowing it to hand off the message over the network if the server being referenced was on a remote machine. To the client this was invisible. It is not clear if the id's were handed out randomly to avoid "guessing" by ill-intentioned software.