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Transcript of "The Promise System"

Presented by Mark S Miller

Initially transcribed by David Jones

0:00:26 MM The promise system exists to solve several different problems, in trying to do a client server system. The interface to the Xanadu server is conceptually not simply like your are sending messages, sort of like the one-bit object???, but rather there is a whole bunch of little objects that you send messages to. Sort of all the objects on the surface of the Xanadu server, and in response to a message to one object, one frequently gets another object is created or made accessible to which you then get a pointer to or capability to.

0:01:17 MM This talk as we will be providing is pitched to those who know C++ but are not assumed to know particularly anything else.

0:01:27 MM So in exchange for a message to one object, you may get hold of another object, and then you have various message that you can send to it. This protocol consisting of a bunch of different classes and messages that define the interface to the server.

0:01:50 MM Initially we came up with a fairly standard Remote Procedure Call idea for how you stretch this across a wire so that the front-end application in interacting with the Xanadu server can be interacting with local C++ objects which really mirror the remote C++ objects and every message sent to the local object, under remote procedure call terminology, gets marshalled into a network packet, sent across, unmarshalled over there, into a data structure ????? which can be sent to the local C++ object, whatever the return from that local message, that return gets marshalled, sent back across this network packet, gets unmarshalled, and returned to the guy in the front-end who originally invoked the message. This was all very clean, but it had several problems, the major problem being that it made you very very sensitive to network latency.

0:03:10 ?? You could only have one invocation?? in the pipe at once?

0:03:13 MM Right.

0:04:15 MM A common observation about local area networks, is local area networks for most things that you do with them are not really a problem with lack of bandwidth but rather latency, they really have plenty of bandwidth when you compare it with what is happening inside a computer, and the other thing is that the result of paying this network latency cost is that it forced you into a terrible engineering trade-off with respect to the semantic level of the interface to the server.

0:05:00 MM Over here [left] we have the server that is semantically defined in terms of a bunch of simple composable primitives and then on top of this we have [centre] a lot of typical idioms of things that we compose out of those primitives and over here [right] we have a client that is sitting on top of that composition. What we would like to do for flexibility and semantic cleanliness is to say that this stuff is in the server machine and this stuff is all part of the client. This is basically all just library code that the client links in, because that way if the clients wants to try variations on the idiom there is nothing special about this idioms. The problem is that the client makes one request at this level and that might turn into a whole lot of traffic at this level, so from a network latency problem given the restriction of RPC what you end up getting forced into that. And before we started the promise system we were worried about precisely that trade off. The promise system essentially lets us stick with this both semantically and implementationally without paying the penalty.

0:06:34 MM Now, how does it work?

0:06:49 MM What we did was we sacrificed transparency. The class hierarchy on the server side that defines the semantic surface of the server, gets transformed into a parallel class hierarchy that exists on the client side. The parallelism keeps intact the kind of semantic transparency, the details at the C++ level, a client who is talking to this is definitely not compilation compatible with a client that is built to talk to that.

0:07:42 MM What happens is, over here we have a client application and inside his machine he's talking to an XuFoo object. When he sends the x message, to the XuFoo object, then the XuFoo object has a return which is a XuInt value, so what happens is, the x message gets marshalled and we have a network packet or a network part of a network packet with this x message in it, that thing gets streamed out to the server machine immediately - there is a stream going from client to server. So when you send the x message to this guy, then what happens is, this gets put on the stream, that will eventually make it here but instead of waiting for the x - within the server there is the actual foo object - so instead of waiting for this message to make it here for the foo object to receive it and the result to come back, since you know that the result is going to be an XuInt value, what happens locally is this guy essentially creates an object of the right type, which is a promise for the return value. And returns that as the return value of that invocation immediately.

0:09:54 MM Now the thing is, the int value is an interesting case because an int value is something that represents an integer value and you don't know what the value is yet. Well, what's over here is the primitive class hierarchy that is defined by the promise system itself.

0:10:21 ?? Internal to its working?

0:10:23 MM It is also visible to the client, so these are all visible class hierarchy. In fact there are some other classes that are internal. When you take a class protocol and do the promise transformation to it, the interface that you are presenting to the front-end client that uses your API necessarily include these classes. The blue one is not really here, just the black classes here. All of the other classes like XuFoo hang off of Promise. Generally they hang off of Promise but they can also be subclasses of some of these other classes, and also ????????. The first useful approximation is that the classes resulting from this transformation specific to your kind of server hang off of Promise, and for all of those all of the input arguments and all of the output arguments are simply other promises. So for them, you don't have to wait for a round trip before you can do everything that they can do. So with an XuFoo there are two things that you can do with it, which is m and x, and you can do m and x to your hearts content without waiting for a round trip. However some of the things that they return to you, such as an XuInt value such that some of the operations on these primitive values like a value message to an XuInt value which returns to you a raw integer. And that is something you can only do once the promise is fulfilled.

0:12:20 MM So what is going on here is, as we make requests each request gets put on the output stream immediately, so we've got this sequence of requests on this output stream, and the output stream can just sort of opportunistically be going out there, it does not have to get flushed immediately onto the wire. And we have a stream of return results coming back, so for a message, lets say that returns an XuFoo, which I don't have an example of here, the corresponding result packet over here would simply say whether or not that request succeeded or not. If it succeeded you don't need any other information, if it failed then you need all the exceptional information. That was by the way where all of the hard problems dealing with the promise system were. Were what happens if the result of invoking x over here is that x over here raises an exception instead of returning a value. You've already returned a promise for the return value and the computation has gone forward. So coming up with a good abstraction so that this guy can deal with the fact that he has gone forward with a promise for a return value that turns out never to appear here, is where all the interesting design was.

0:13:54 MM A promise can be in one of three states. A promise is born not-ready, then when the result comes back the promise either makes a transition to being fulfilled or broken. So if the request was successful then you have a fulfilled promise, if the request was not successful then the promise turns into a broken promise.

0:16:12 ?? And this is something that happens on the client end?

0:16:16 MM Right. So once the result packet from the x request over here comes back this XuInt value either turns into a fulfilled int value or it turns into a broken int value. Now if it is fulfilled, in the case of an int value the packet that comes back actually contains the integer value. So a fulfilled int value actually can now respond to the value message with no problem, and similarity for a float value.

MM 0:16:54 Broken promises by the way actually have two varieties of broken, there is directly and indirectly broken. In the case where the operation X raised an exception then you have a directly broken promise, and with a directly broken promise you can inquire about the exception that caused it to be broken, and that is the problem. You can ask what was the problem? Why did you break your promise to me? An indirectly broken promise results from making a request where either the recipient of the request or one of the arguments of the request was itself a broken promise, because what can happen here is that I've got this int value back now what I can do with this int value is I can now make a further request of the int value, so I can take two int values and add them together and get a new int value, but it turns out that if the original x message results in a broken promise then this int value becomes a broken promise and therefore there is no integer back over here for me to ask to be added to another integer. So broken promises are never really represented by objects at all on the server, it's the comm layer that either one side of the wire or the other depending on how much information exists locally or not, can tell that this request involves a broken promise and therefore the promise that results from the broken request is indirectly broken and the broken promise that caused this guy to be indirectly broken is this guys excuse. Why did you break your promise to me? Oh don't blame me, it's his fault! And then you can track down the excuse chain to find the directly broken promises problem. You can have a chain of indirectly broken promises and that chain always has to end in a directly broken promise that has a problem.

0:19:30 ?? And that chain exists over on C?

0:19:34 MM Yes, in the heap on C. That chain does not take up any server storage.

0:20:06 MM The result is that we can now have all of these primitive things that we can use to compose together and something that's this idiom that involves doing the foo operation on this and taking the result and applying this operation to that, it just becomes a stream of these small requests going out here. This guy is able to just process these requests one after the other and it's really just like sending a program across the wire. We really are in some sense getting a lot of the kinds of benefits you get with postscript???? type window system where you can send these little procedures over where the procedure then executes locally and you get the result of the procedure in some sense coming back but without ever sending over something that defines the procedure, that would take you into a whole level of complexity that you wouldn't want to get into. Another way you can think of this is in terms of floating point processors do, register renaming, so every time you make one of these requests to any promise object you always get a new promise object, because obviously the promises cant know that the promise turns out to be a promise for an object that it replied with earlier, and there is an internal numbering system where these things are numbered so you can think of the requests going out of here kind of like an instruction stream to one of these infinite register machines. The games being played by being able to delay the responses getting sent back are very much the kinds of things you get with funny instruction scheduling.

0:22:11 ?? The program as it is in some sense that is shipped over to the server becomes a data flow program.

0:22:21 MM In what sense? The semantics are defined such that the server is obligated to execute these things in sequence. And the result chain also has to be in the same sequence. And in fact part of the compactness of the protocol results from that assumption.

0:22:48 MM The place where this bottoms out again is raw data. We talked about raw int values and float values, and the other place where we get raw data are arrays. There are four kinds of arrays. This is probably a level of detail not worth talking about...

0:23:18 MM I gave you the rational for why we created the promise system, and it turns out that the major benefit of the promise system was not with respect to network latency, it was with respect interactive performance in the context of server latency. No matter how blindingly fast our server is, it is a server that a bunch of users are going to be beating on simultaneously and sometimes the server itself is going to be slow. In a direct manipulation user interface you want the part of the user interface that is the direct manipulation part to always be speedy. When you click on something, when you do various operations, you like to feel like the part that is immediate, the part that is driven by the mouse action always happens immediately. And to a first approximation, and there are some exceptions by we are moving in this direction, but to a first approximation the way we achieve this is all of the direct manipulation parts of our front-end simply result in requests being streamed out and local manipulation happens, ie the direct manipulation part, never waits for a round trip.

0:24:42 MM There is an operation that I didn't mention that is called force. The force operation, which is in the front-end library, forces a flush of this stream, and actually places a special force marker in the stream over here. So the stream gets flushed, such that the server can see it all now. When the server sees the forced marker it then flushes the result stream, so that this guy can see it all. If you just flushed the stream without the force marker then there would be nothing forcing the result stream to get flushed, and you could still hang. And finally the force operation in the front-end library waits for the result stream to catch up with the last request.

0:25:34 MM The goal here is that a really responsive front-end should never force other than for operations where it is acceptable to put up an hour glass. Like save or something. Even if it typically takes a very small amount of time, that operation is when you are dependent on the server coming back in. Like I said, we are not at that goal yet, but we are moving in that direction.

0:26:08 MM So for example, say you have a document on the screen, and you bring up a link pane, what happens is that the process of bringing up the link pane causes a bunch of messages to get streamed out over here but with no force. The bringing up of the pane is just all local user interface stuff, it doesn't depend on any information back from the server. But displaying links in the link pane depends on information back from the server, so what you do is drive this off of various kinds of detector hooks. Detector hooks are basically call-back objects, so there is one very important kind of detector hook which is a ready hook which simply waits for a given promise to be ready. A lot of the things that happen in the front-end happen as a result of making some requests and then posting a ready hook that simply says "execute this object and send the standard message to this ready hook object when the result stream has caught up with where the request stream is right now." What happens is while you are interacting, user interface events; mouse motion and keyboard typing take priority over processing results back from the back-end, so while you are doing things the front-end is responding to you directly and sending requests out to the back-end and then when you are not doing things, processing the result stream that in turn triggers ready hooks that have been posted and now asynchronously you get things filling in on the screen and other things happening that needed the data. The things that didn't need the data are the direct manipulation things, and the things that did need the data can wait until the user is idle. Or of course try and slip it in. Idle is relative performance term, the faster the computer is the smaller amount of time that constitutes idle.

0:28:38 ?? All of these promises once made, you try to fill them. There is no concept of trying to get away with not doing one.

0:28:52 MM There is. I didn't mention, but over here at the top of the class hierarchy is a class Counted???. All of these promise objects as well as the various detector objects, are reference counted. For class xuFoo, the pointers to it are actually class objects of type xuFooP. So this is like saying xuFoo* at, except by using xuFooP at, the object f is an object that acts like a pointer to an xuFoo except that xuFoos keep a reference count of how many xuFooP point at it. The result is that lets say you have the following piece of code.

0:30:37 MM So over here we've got variables which are external to this block g and i. Inside the block is send a zip message to a g and get back a promise for a foo, then we send to the foo message x with that very foo as argument, and we get back a promise for an integer, and then at the close of the curlys this variable goes out of scope, its destructor gets invoked and the ref counter of the foo goes away and what gets put out on the output stream is a packet saying this promise is now no longer needed. The server does not have any logic in it to do anything with those packets, it just ignores them, but they are in the protocol so that if the computation of the intermediate is not necessary in order to compute the things that are necessary then you know that you can skip it. If we use the garbage collector instead of ref counts over here then there would be no good guarantee about when you would find out that you didn't need f, and therefore there would be a delay before you could tell the server that you didn't it, and that would probably be a sufficient delay to ruin the effect. It is this ability that enables all the query optimisation kinds of things that the database folks do, even though we are not doing any of that right now. This is the thing that allows a sent over procedure to have a local variable so that the server can act like an optimising compiler for the procedure and optimise out the existence one of the variables.

0:33:14 ?? This allows several processes to be going on at once in C, all using this interface. Is there any restriction about the various processes on the client and having their side-effects take place in some order.

0:33:42 MM The promise library itself assumes it is only going to be invoked in a single threaded fashion. So as long as you put mutual exclusion around access to the promise library it doesn't care if there is any further multi-threading in the client. That would be the case independent of what we get, we didn't have to do anything of course to enable that. The expectation is that if you want multi-threads accessing the server you can either do mutual-exclusion around your local promise library as a whole, or you can have multiple independent instantiations of the promise library each of which is maintaining its own pair of threads to the server.

MM 0:33:45 Now let me talk a little about generating this. We started off by generating these from an annotated version of this. We stopped doing that. What we now have is a file that sort of looks like suggestively like C++ but it is in our own defined syntax which basically contains a definition in a rule sub-language, an entire server protocol, as well as assigning numbers to the individual classes and numbers to the individual requests. That way assignment of numbers is standardised by the file, the file is the thing that defines the protocol that goes over the wire. And then that file gets processed both to create the promise library classes that do the marshalling, all the helper code that does the marshalling, as well as to create not foo itself but the code within the server that does the unmarshalling and invokes it. If the types as they appear within the client, the text in the received file does not match the types as they exist in the classes that it specifies then you get a type checking error when you try to compile and link them together.

0:36:38 MM Another interesting thing, speaking of types is, we talk about the way we do casts in the promise system. One of the things that you have to face in C++ all the time.

0:37:23 MM Let me introduce a notation that I like for talking about static and dynamic types. I was influenced by David Arrells???? notation for what he calls hiraps???? which are a mixture of Venn-diagrams and directed graphs. He makes a compelling case that Venn-diagrams are much better off with labelled boxes rather than just putting the label inside the box. In any case, this represents that bar and zap are subclasses of foo, and we are going to represent a pointer in a double way. Actual objects are these shaded circular things in this convention. Pointer has both a static part and a dynamic part. The static part refers to a box and the dynamic part refers to an object. The different places that you can exist within a box defines different behaviours that you can have, and the box say you can have any behaviour within this allowable space of behaviours. You can have any state, lets call it a state rather than a behaviour. You can have any state within this allowable space of states, so for example if zap is a leaf class the different places in zap define different states according to the values of your instance variables. So from the point of view of foo, which is not a leaf class, part of the state of an object which is a kind of foo is what class it has, ie the vtable pointers is really an instance variable which is part of its state. Within that class the other instance variables determine in a more fine grained way where it is in the state space. The static part of a pointer points at the box, the dynamic part points at an object and the correctness constraint on the type-checking system is that the place that the dynamic pointer points at has to be within the box that the static pointer points at. This is a complete aside from the promise system, the place where this diagramming turned out to make an essential clear explanation is within the server code, we have a notion of type safe become, where an object at runtime can change its class and continue to be the same object. We figured out how to do that and maintain type safety. What happens is you have to pre-declare in the class hierarchy which classes have instance that may become other classes. For example if instances of zap may become instances of bar then there is a declaration in zap that may become zap bar. The correctness constraint is that if this declaration appears, then there must also be a declaration as follows. Not-a-type zap. In general if you have got a class hierarchy and this guy may become that guy, then this guy may not be a type and that guy may not be a type. Basically the guy who may become some other guy, he may not be a type, and all his ancestors until you get to the common super type with the guy he may become may not be types. What it means to not be a type, and I've diagrammed this with a dotted box. These two declarations result in zap being this dotted box, it's dotted because the object can escape. See the solid box says that the object can dynamically change its behaviour but only within the cage, the box is also a cage. The dotted box is not a cage. Cages may close to keep things in, but they don't keep things out. So this object may now transition to being a bar and the result of this declaration is that because of the way in which we declare pointers, the set of macros that we use by convention to declare pointers, if you try to declare a pointer as a type pointer-to-zap you get a compile time error. You find out that zap cannot have pointers that are statically declared as pointers to zap. However what you can do is you can use our cast macro to cast. Lets say that f is a member function that is declared only in zap. You may not declare a pointer of type pointer-to-zap and therefore there is no situation in which you can say q foo because there is no pointer type for q to be declared as. But what you can do is momentarily do this cast, this is a safe cast, that checks that at this instant this object is within this cage. And during that instant that allows the member function f to be applied to it. However you cant divide this expression up into something that assigns the variable and then uses the variable because there is no variable you can use to preserve the effect of the cast, which is good because it is only an instantaneous property.

0:45:26 MM Now the reason I mention all of this. All this may become stuff is completely irrelevant to the promise system and completely irrelevant to the front-end programmer. The front-end programmer is programming in C++ and he is not programming in Xperanto????. Let me introduce you to some language names here by the way. So Xperanto or Xpidgin??? are the two names, we have been using the name Xpidgin but we are likely to transfer over to Xperanto. We had this language that has a semantics that can be specified, and part of my task over the next 13 months is to specify it, but this language exists in some sense by convention on top of both Smalltalk and C++. So like the pidgin language, it doesn't have a fixed grammar, and it is also a compromise between existing languages, and also like the pidgin language you can tell by which grammatical constructs that someone uses what their native language is. So Xperanto has two ways that it can be written, and those are referred to as Xtalk and X++ which is the Xperanto language as written in the Smalltalk notation versus the Xperanto language as written in the C++ notation. But all that is completely internal to the server.

0:47:50 MM I'll leave this notation up here for talking about what tasks look like over the promise system. Also it is the case that none of the object that are on the semantic surface of the server are objects that may change there class dynamically. So all that class changing issue is also not an issue with respect to the promise system.

0:48:33 MM If in our original class we've got a member function that returns a foo star then in our transformed promise for a foo class then we have something that returns a promise for a foo as a static type. However we are calling our dynamic picture its the case that when we invoke this member function on our promise for foo since we don't what the dynamic type of the actual object that gets created on the server that the promise that we are getting is bound to then the only thing that we have to go on is the declared static type so we actually go ahead and create a promise that will be bound to, lets say it is a bar. So now lets say this particular invocation of the x message is actually going to create or make accessible a var object from the back-end, however since statically all we know is it is some kind of foo we get back a promise for a foo, and then we get that back immediately it's only when the result packet comes back that it really is in some sense bound to this particular object. Lets say what we want to do is the equivalent of the thing I was doing earlier with the cast macro. The way we write that in the client library is lets say b gets xubar::cast, we use a static member function. Lets say zip is a message that only xubars can respond to. What we have done here is we've taken f and we've optimistically casted it to a bar. What that does is it sends another packet out on the request stream saying "try to cast f down to a bar" and it immediately creates a promise of type bar which is a promise for the result of the cast. There is still no round trips needed. Now when we zip on the result of that cast operator and get back whatever the result of the zip is. And we can do all of that immediately without waiting for a round trip. Now lets say that in fact f is not a kind of bar, its a kind of zap. Well then what happens is the cast operation results in b being a broken promise and then the result of the zip operation returns will be an indirectly broken promise where its excuse is b, and b's problem is case violation. So we can go ahead and optimistically try things out, and then the whole optimistic chain gets broken as a result of our bad assumptions, and we can even go ahead and optimistically try out several chains, and we actually use this in the code. In straight line code we can cast f to a bar and do some things if it turns out to be a bar and also cast f to a zap and do some things in the case it turns out to be a zap. What will happen is the entire sequence goes out to the engine, we're not using the term server or backend anymore, and depending on what the dynamic type is of f one or the other of those two parts will be rejected and result in indirectly broken promises.

0:53:19 ?? But perhaps the other wont be.

0:53:23 MM If f is necessarily either a zap or a bar, then one of these two will be successful. If f maybe a bar or maybe a zap or maybe something else then you might end up with both of them failing.

0:53:47 MM So there is two techniques here you can use. One is that you go ahead and try to be exhaustive, and other is if one case really stands out as the typical case you can just optimistically go ahead and do the typical case inline and then only wait for the round trip and the recovery code to bother with the atypical case.

0:54:17 MM To me it's very interesting and weird to the degree to which there are analogies between what we went through with the promise system and the kind of things one does with instruction scheduling. Optimising for the typical case and letting the atypical case be both out of line and be something that loses all latency optimisations.

0:54:40 ?? And the attempt to deal with exceptions is a big pain in instruction scheduling.

0:54:47 MM In fact the broken promises were directly inspired by the handling of NANDs. The way we refer to broken promises while we were designing them, the other phrase we used was they are Not An Object (NAO). So when they propagate through they result in things being derivedly not numbers.

1:00:13 MM The promise system allows a certain degree of asynchronity, but because of the imposition of sequence, the degree of asynchronous is quite limited. The results have to come back in the same sequence as the requests. The results in some sense are just the result of the request. The reason we need detectors is because Xanadu is an interpersonal medium some of the things you want to be informed about are the results of things other people are doing, not the result of requesting the ????.

1:01:23 MM Part of detectors are so that you can get asynchronous notifications and be able to react to things other people have done, so basically you can post a persistent query is the model that we use, and as there come to be more answers to that query you continue to get informed of further answers. The other reason for detectors is that even when they are just a result of things you've done, in order to allow the backend to engage in further latency optimisations, this is a further optimisation that we have not engaged in yet but we know we want to, we have all the hooks there but haven't jumped into the water yet, is there are certain parts of the semantics that we have very carefully defined as this particular class of results of this operation happens eventually. You are guaranteed that it will eventually happen but it does not need to happen immediately and you are not guaranteed when it happens. In order to find out that the result happened out of band you have a detector.

1:02:58 MM The way in which it works as far as the class hierarchy is concerned is that we actually have a parallel set of detector classes and detector hook classes. The super classes are Dectector and Hook, we will leave off the Xu's, and under that we might have FooDector and FooHook, and BarDector and BarHook, etc. The hook classes are really the ones that are significant to the user of the library, the detector classes themselves are much more internal to the library, they are basically the object that the hook gets posted on. And what happens in terms of managing the protocol is that the way you post a detector is you make a request whose return value is a detector, so instead of getting a promise back you get a detector object back, and then you post hooks onto the detector object, and that works well with the other mechanisms of the protocol in terms of assigning a unique number to the detector so that then the asynchronous notifications can have that number to use, and it works well with respect to the whole ref counting system. So that when you drop the detector object then that's the notification object to the back-end that we are no longer interested in messages that come back to the detector because they might be crossing each other. There is all the logic to be able to continue to accept messages for a detector that no longer exists and just drop them on the fly, as opposed to some kind of weird angle??? reference ???.

1:05:34 ?? And you eventually get to the point where you don't have to do that anymore?

1:05:40 MM Right, when the reply stream catches up with the place in the request stream where the detector was dropped we know then that we no longer have to keep track of what we needed to keep track of in order to be able throw away those messages.

1:05:56 ?? So you can take it off the list of hints which you can drop on the fly?

1:06:02 MM So what we actually do is leave the detector object there in a funny state until we catch up to the place where we dropped it. How the mechanism works I may have gotten wrong, but I believe that is how it works. In any case whether the user uses the hook classes is the hook classes themselves that are defined are abstract classes and the notification messages, the messages to the hook classes that correspond to the notification messages in the protocol, the asynchronous notifications, those are all pure-virtual functions in the hook classes, so the way in which the user uses the hook classes is by creating subclasses of them that contain the action that the users application program wants to take in response to a given notification. So those classes exist purely to be subclassed by the user in order to add these actions to them.

1:07:20 MM At the level of the promise system I think I've pretty much said what there is to say. The notifications come in, there is code that unmarshalls the notification as they come in, then goes to the detector and all the hooks posted on that detector, of which right now because of the way the client library is set up there is necessarily one, and the result of that is that it invokes whatever action the user had that hook object do.

1:08:22 MM I just described to you the hook invocation mechanism as it existed a while ago, the bug that we find with it was that by invoking the hook action immediately, as soon as you received that reply packet, we ended up essentially with some scheduling problems. It was very much the kind of problem you get into when dealing with multiple threads, even though really there is not multiple threads happening on either side of the wire here. The reason being that some operations in the front-end would cause a force that would force the reply packet back immediately, some of those operations themselves would happen within the code that was the action code of a hook. So while processing one hook one could in the midst of processing one asynchronous notification cause the reception of another asynchronous notification and get nested and in so doing and one could always carefully code the hook actions so that was never any problem but because - I don't remember how we got into trouble with that, and I can't justify why it is right now, so suffice to say we did get into trouble and what we ended up doing instead is the asynchronous notifications are reified and delayed and then what you do is the front-end at the top of its event loop, when it is not processing user actions that's where it processes the delayed detector invocations.

1:10:42 ?? From other detectors that you were processing before?

1:10:50 MM Right, basically as you are processing one detector event if it causes other detector events to be noticed they get queued up and it is only once you have finished and gone back to the top of your event loop, if there is no user action waiting then you go to the next one. This has the additional advantage of once again you are able to give user events greater responsiveness in that if there is a user event waiting you are only going to go through one asynchronous notification before you get back to the user.

1:11:24 ?? Let me ask a different view if you will, who wrote all the code for this?

1:11:33 MM In some sense all of us, but for the promise library itself dean, robby and myself between us we wrote 95% of the promise library code.

1:11:53 ?? Are you distinguishing between the server and client layer or are you including all of it?

1:12:09 MM The server side of the promise system was written I think almost entirely by dean. The client side all three of us had a major hand in it.

1:12:16 ?? Client side is a set of C++ functions which one can operate on that live on this machine for example. How much effort is there?

1:12:30 MM Getting it done the first time was interesting. We had a first design which involved a lot more forcing and a different set of abstractions for how you deal with exceptions. It is still the case that it is a complicated piece of engineering. Even with all the learning it would take a long time. How long did it take? Less than a man year. Between three of us, it might very well be nine man months. One thing in general I should point, I am a very good qualitative thinker and very bad quantitative thinker, so robby could give you a more informed answer.

1:13:58 ?? Suppose we said our front end will not be Windows but will be a Next station?

1:14:08 MM This thing is highly portable, the client library is used by the server regression tester called Arnold, the thing that exercises the server. Arnold has run successfully on Suns, on the SGI, and on the PC. So we certainly believe that the client library itself is highly portable, given a C++ compiler. The one interesting thing to port, interesting for some platforms is the comm aspect. The place where it interfaces with TCP/IP sockets. All the interesting stuff in the promise system simply depends on having there be some kind of communication system that gives you reliable byte streams with flush. Given something that gives you reliable byte streams with flush I think the rest of it is just highly portable and depends on hardly anything except C++.

1:15:20 ?? Who should I ask about moving this comms mechanism to NeXT Step?

1:15:28 MM Well I suppose Robby would be the right person to ask. But NeXTStep, especially since it's Unix based and has TCP/IP in it, it should be trivial. I would be surprised if it took more than two man weeks to get the thing working and would be surprised if that the vast majority of that were not just the details of the TCP/IP system. As far as the rest of the promise system is concerned, I would be surprised if it took as much as a week.