User:Which Linden/Office Hours/2007 Nov 15

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Transcript of Which Linden's office hours:

[11:00] Tillie Ariantho: there we are. :)
[11:01] Which Linden: Hey folks!
[11:01] Masaw Umaga: hey!
[11:01] Which Linden: I invited Donovan to come by since our topic today (suggested by Zha) is to talk about Pantheon and what it's for.
[11:01] Morgaine Dinova: 'Morning :-)
[11:02] Which Linden: Good morning, Morgaine.
[11:02] Morgaine Dinova: I don't see Zha
[11:02] Tillie Ariantho: She's fetching a diagram or something.
[11:03] Donovan Linden: woah, we're going to talk about pantheon? :)
[11:03] Which Linden: Yeah, weren't you there?
[11:03] Saijanai Kuhn: what ist he topic?
[11:03] Which Linden: The cool tool Pantheon
[11:03] Donovan Linden: i should have drawn some diagrams or something
[11:04] Which Linden: Sorry, yeah, I kinda forgot what the topic was until just like 30 minutes ago
[11:04] Donovan Linden: hehe
[11:04] Tillie Ariantho: :-p
[11:04] Which Linden: Actually now I have to doube check
[11:04] Saijanai Kuhn: is OK, we moved our meeting here so we could get feedback from you if you had time
[11:04] Which Linden: Whats' your meeting about?
[11:04] Saijanai Kuhn: but your meeting comes frist
[11:05] Which Linden: Yeah, it was definitely Pantheon
[11:05] Tillie Ariantho: Any of the Lindens here have an idea about why the "missing gesture from database" can be something like the default standing animation? ,)
[11:05] Tillie Ariantho: Happens to me quite often that after a login it isnt found and I am 'walking' with the 'flying' anim...
[11:06] Donovan Linden: ok well let's get started I guess
[11:06] Tillie Ariantho: .)
[11:06] Which Linden: You know, brokenness.
[11:06] Which Linden: Yeah, Donovan, what *is* Pantheon??
[11:06] Saijanai Kuhn: This is the first "formal" meeting of the client VAG. https://wiki.secondlife.com/wiki/Multi-Process_Client_VAG_--_draft
[11:06] Donovan Linden: saijanai: I saw that on the mailing list, it looks very nice
[11:06] Morgaine Dinova: Not this, hehe. After Which's lol
[11:06] Donovan Linden: ok, so how many people here have heard of comet, in the same context as ajax
[11:07] Tillie Ariantho: not me.
[11:07] Masaw Umaga: no
[11:07] Emma Nowhere: me
[11:07] Wyn Galbraith wants to sit by the bush
[11:07] Tillie Ariantho: I only know a bit ajax. .)
[11:07] Wut Burt: aha
[11:07] Tao Takashi: Hi
[11:07] Wut Burt: is that not unlike what google chat uses?
[11:07] Donovan Linden: ok. comet is like ajax, except instead of polling the server for changes, the client always keeps a connection open to the server
[11:08] Donovan Linden: it is probably what google chat uses, I haven't looked
[11:08] Which Linden: I think it is
[11:08] Wut Burt: blocked socket, basically
[11:08] Tillie Ariantho: Is that good? Keeping open connections?
[11:08] Donovan Linden: the server keeps these sockets open, but doesn't respond to the http request in them, until there is an event on the server
[11:08] Morgaine Dinova: If it's just event messaging, it's good.
[11:08] Tillie Ariantho: We sometimes have problems with too many open connections between web and app server... so it sounds not so good...
[11:08] Emma Nowhere: depends, some unix kernals have to be recompiled to handle it
[11:08] Donovan Linden: when there is an event on the server, the server sends it over the socket
[11:09] Squirrel Wood: /ao off
[11:09] Donovan Linden: our networking library, eventlet, uses non-blocking io and coroutines to scale to 10,000 connections
[11:09] Tillie Ariantho: Sounds a bit like MQSeries. :)
[11:09] Seeping Blister: sure is.
[11:09] Donovan Linden: any comet server is likely to have to use non-blocking sockets on the server side
[11:10] Tillie Ariantho: That network lib is used on a sim server?
[11:10] Donovan Linden: but there are http servers that can handle the load for many languages
[11:10] Which Linden: 10000 threads = THE PAIN
[11:10] Morgaine Dinova: Donovan: I like the sound of that. Everything I do is event-driven.
[11:10] Donovan Linden: tillie it is
[11:10] Wut Burt: Which: :)
[11:10] Donovan Linden: ok, so that's what comet is. mulib, my "rest services library", has two comet implementations
[11:10] Morgaine Dinova: Not 10,000 threads, hehe, 10,000 coroutines :-)
[11:10] Tillie Ariantho: Hope it is not for agent server and such... 10.000 fixed connections there sounds bad. .)
[11:11] Donovan Linden: tillie: the agent domain hosts should be able to handle thousands of simultaneous logged in agents without a problem
[11:11] Donovan Linden: once we actually write them
[11:11] Saijanai Kuhn: login will be an interesting issue though
[11:12] Tillie Ariantho: Yes, but if the connections stay open 10.000 will be able to connect and the others... not. That I meant.
[11:12] Donovan Linden: anyway, back to mulib. mulib.eventrouter is a comet server I did in about december of 06
[11:12] Which Linden: We won't have any central servers any more, so we'll be able to add more servers, Tillie.
[11:12] Donovan Linden: tillie: I can't imagine we would have 10,000 simultaneous users on an agent host
[11:12] Wyn Galbraith: In the old days when you had to walk 10 miles in 10 feet of snow to get to the computer.
[11:12] Tillie Ariantho: Okay. :) I am quiet. .)
[11:12] otakup0pe Neumann: the central db is dead long live the central db.
[11:13] Which Linden: so, eventrouter, how does that use comet?
[11:13] Donovan Linden: eventrouter is an interesting experiment because it is completely asynchronous
[11:13] Wyn Galbraith: Wouldn't that be decentral?
[11:13] Donovan Linden: eventrouter exposes python methods that can be called from javascript (ajax)
[11:14] Donovan Linden: but it also exposes javascript functions that can be called from python
[11:14] Donovan Linden: neither of these calls have a return value
[11:14] Morgaine Dinova: Donovan: Are you documenting the on-the-wire protocol too?
[11:14] Tillie Ariantho: .D
[11:14] Donovan Linden: the wire protocol is json
[11:14] Morgaine Dinova: Oh, I thought Zero didn't like JSON
[11:15] Donovan Linden: one other interesting thing about eventrouter is that it gives you a unique server-side python object, the Target, for each browser which is connected
[11:15] Emma Nowhere: any chance you could do like Yahoo does with it's webservices and let you specify the return type as xml or json?
[11:15] Donovan Linden: so it is easy to create pages which look completely different to different users, which is slightly more difficult with pantheon
[11:16] Donovan Linden: emma: yes, the head of mulib does content-type negotitation, and internally we can switch between json and llsd
[11:16] Morgaine Dinova: That's nice
[11:16] Which Linden: The javascript client doesn't have an llsd parser though, does it?
[11:16] Emma Nowhere: json is awesome for ajax work, but if you're building server to server stuff, or even invoking from LSL, it's not as good
[11:16] Donovan Linden: it does negotiation based on the Accept and Content-Type headers
[11:16] Donovan Linden: which: right, which is why the server supports generating json
[11:16] Which Linden: I see, so a non-javascript client could speak only llsd with the server
[11:17] Donovan Linden: yup, or you can plug in your own parsers/generators for whatever format you want, based on mime type
[11:17] Donovan Linden: ok, so eventrouter is a very simple asynchronous message bus
[11:17] Donovan Linden: pantheon is a little different of a beast
[11:17] Donovan Linden: pantheon is also a comet server, but it is also a pure REST server
[11:18] Donovan Linden: instead of the server having apis to send events to the clients, the event is generated by performing a PUT or DELETE
[11:19] Donovan Linden: clients indicate which subpaths they are "observing" and the server takes care of handing all the events to those subpaths to that client
[11:19] Donovan Linden: so in this case, clients can have different views of the same page, but only by maintaining client state
[11:19] Donovan Linden: for example, this tree item is revealed, or that "pane" is open
[11:19] Emma Nowhere: so, when i connect via rest, i can get all the events since my last connect?
[11:19] Donovan Linden: or tab or whatever you want to imagine for us
[11:20] Donovan Linden: s/us/ui/
[11:20] Donovan Linden: yes, when the pantheon javascript or lsl client connects, it has no etags to give to the server
[11:20] Donovan Linden: every time the client sees a new event, it gets the etag for the resource the event occurred on
[11:21] Donovan Linden: when the clients ask for new events, they simply say "here are the paths I am interested in, and here are the latest etags I have for them:
[11:21] Donovan Linden: "
[11:21] Donovan Linden: what this means is that it is very easy to bridge between systems: lsl can get events in one representation,
[11:21] Donovan Linden: json can generate and watch events using the json representation
[11:22] Morgaine Dinova: So the server is building a forest, and the etags point into somewhere in the trees. How does past history get culled from the forest?
[11:22] Donovan Linden: and scripts can use llsd, or insert your favorite format here
[11:22] Donovan Linden: for the culling question, let me bring up the code
[11:22] Donovan Linden: the etag implementation I just described has been happening in the collaborative-editor branch and hasn't merged into release yet
[11:23] Which Linden: So the culling has to do with event eliding right?
[11:23] otakup0pe Neumann: i just want to add that the collaborative editor is pretty hot.
[11:23] Which Linden: If you're looking at /counter and somebody changed it from 1 to 2 to 3 to 4
[11:23] Morgaine Dinova: Yeah, getting rid of old stuff not accessible through etags any longer
[11:23] Which Linden: You only need to know that it's currenltly at 4
[11:23] Donovan Linden: http://svn.secondlife.com/trac/mulib/browser/branches/collaborative-editor/mulib/pantheon.py
[11:24] Donovan Linden: the todo in the docstring needs to be changed, that code is using etags
[11:24] Donovan Linden: each path on the server keeps it's current etag, and also every etag that it ever was
[11:25] Donovan Linden: so when an old client comes in with an old event, no matter how old, the client gets the "latest" event for that path
[11:25] Donovan Linden: because we're using put, and we're assuming idempotency (each put replaces the last completely)
[11:25] Donovan Linden: oh, I guess I should mention the difference between resource events and container events
[11:26] Tillie Ariantho: Oh ah, I see Donovan is a non-documenter, I dont see comments in there. .)
[11:26] Donovan Linden: clients can choose to subscribe to a specific thing, such as "first_name"
[11:26] Donovan Linden: well, pantheon has just been a weekend creative project for me, so I tend to stay heads-down until the design is finished
[11:26] Donovan Linden: the page I pasted is horribly messy with commented out print statements
[11:27] Donovan Linden: ok, back to kinds of events
[11:27] Donovan Linden: if a client subscribes to first_name, and someone does a PUT /first_name, the client will get the event
[11:27] Donovan Linden: but there are also container events, because sometimes you don't know the names of all the possible resources you are interested in
[11:28] Donovan Linden: so a client might subscribe to people/*
[11:28] Donovan Linden: and if someone does a PUT to /people/fred or /people/bob, the client will get the event
[11:28] Which Linden: How does the client know it's a container event?
[11:28] Which Linden: Does it need to know?
[11:29] Donovan Linden: which I assume the callback it registers will know
[11:29] Donovan Linden: a callback registered for people/* will not currently be triggered by a PUT /people
[11:29] Which Linden: Oh, right, because it has to explictly register for container/*
[11:29] Donovan Linden: so client can register both callbacks for people/* and people if they want
[11:29] Morgaine Dinova: What is the semantic of GET /people/* ? Get all current subscribers, or get a container tag that will explode to all subscribers at time of use?
[11:30] Donovan Linden: so pantheon is great because clients get to decide which events they see
[11:30] Donovan Linden: morgaine: GET /people/* would 404
[11:30] Donovan Linden: GET /people/ would get you a json map
[11:30] Morgaine Dinova: kk
[11:30] Donovan Linden: with fred and bob keys
[11:30] Donovan Linden: with whatever body was associated with the appropriate put
[11:31] Donovan Linden: ok, I think I'm about done blabbin
[11:31] Morgaine Dinova: So the map is for time of query, not for time of subsequent PUT
[11:31] Donovan Linden: morgaine: right, it's pure rest, you get a representation from that time
[11:31] Donovan Linden: but then you have the etag, and can use that to request a comet event when that container changes
[11:31] Morgaine Dinova: Donovan: it's not blabbin ... you're wonderfully clear :-)))
[11:32] Donovan Linden: good :) thanks
[11:32] Which Linden: Yeah man, nice exposition
[11:32] Which Linden: So how does the client register for events?
[11:32] Donovan Linden: http://svn.secondlife.com/trac/mulib/browser/branches/collaborative-editor/mulib/pantheon.js
[11:32] Which Linden: (i.e. what's the REST api for that)
[11:32] Donovan Linden: the javascript api is observe(path, callback)
[11:33] Donovan Linden: the wire encoding can be negotiated as I mentioned
[11:33] Donovan Linden: the format of the body is a map with an observe key
[11:33] Donovan Linden: wait let me double check
[11:34] Emma Nowhere: Donovan, is that JS designed for people to use on their own pages?
[11:34] Morgaine Dinova: Well I've understood only part of this, but going to give it a long hard think. The concept sounds great.
[11:34] Donovan Linden: a map with client-id and observe keys
[11:34] Saijanai Kuhn: so this is kind of a like a baby form of a Tobject (Saijanai is a bit slow)?
[11:34] Donovan Linden: emma: sure, if there is a pantheon server running somewhere
[11:34] Which Linden: This is? sendContent': serializeJSON({ 96 'client-id': _client_id, 97 'observe': watching})}
[11:34] Donovan Linden: saijanai: I don't know what Tobject is
[11:34] Which Linden: I think that's teh line that sends it
[11:34] Which Linden: (the 96 and 97 are newlines)
[11:34] Donovan Linden: the observe key is a list of maps with a path key and an optional etag key
[11:35] Saijanai Kuhn: Croquet's base object. All things in a croquet world are TObjects, and broadcast themselves to other Croquet clients
[11:35] Donovan Linden: the response is a map with event (PUT or DELETE), path, and body keys
[11:35] Donovan Linden: although you may choose to structure your code so it ignores the body and does a GET on the resource, to play well with cachine
[11:35] Donovan Linden: caching
[11:36] Donovan Linden: saijanai: oh, interesting
[11:36] Donovan Linden: so yeah, it's a way of getting event notifications between machines, but it's also REST, which I think is fabulous :)
[11:36] Donovan Linden: I'm very happy with the way pantheon turned out, although I still think of it as a research project
[11:37] Which Linden: Yeah, it's very cool. Very "pure" in a sense
[11:37] Donovan Linden: one of the best things about it is that the clients and servers can speak different languages, and the pantheon server itself doesn't have to be any more complicated than a Panthon() as the root Resource
[11:37] Which Linden: I had a question from before: what happens if the client sends down an etag that isn't in the server's list?
[11:37] Donovan Linden: as long as you trust everyone equally to make changes
[11:37] Donovan Linden: which: I dunno, I think it just gives it the latest
[11:38] Donovan Linden: which: that might happen if a client was connected across a server restart
[11:38] Which Linden: Right, or if the client is behaving badl
[11:38] Which Linden: OR something
[11:38] Donovan Linden: it would be interesting to implement retry logic in the errback
[11:38] Donovan Linden: pantheon has the advantage over eventrouter that it can actually survive system restarts
[11:38] Which Linden: EVentrouter can't?
[11:39] Donovan Linden: well, the applications that are developed on it often can't
[11:39] Morgaine Dinova: Can the client subscribe to a notification to be received on container growth?
[11:39] Donovan Linden: because they assume they have some server-side state for each client
[11:39] Which Linden: Oh, right, because each client is assigned a target-id that it never re-requests
[11:39] Donovan Linden: morgaine: yes, that is what the container events (people/*) are for
[11:39] Donovan Linden: they trigger on growth and shrinkage (PUT and DELETE)
[11:40] Donovan Linden: as far as granting different clients different access rights to pantheon...
[11:40] Which Linden: So if you have a list [] and it gets a new list inside: [ [ ] ] you get the entire inner list in the event?
[11:40] Donovan Linden: which: I htink that is correct
[11:40] Which Linden: Might be bad if you PUT a huge list, though I guess that would be a fool thing to do
[11:41] Donovan Linden: I haven't quite fleshed out security yet, but I'm pretty sure it's as simple as having a non-generic-pantheon server side service that gives out capabilities
[11:41] Saijanai Kuhn: so you can be sure soneone will do it
[11:41] Donovan Linden: the "public" pantheon can then be wrapped to not accept PUT and DELETE
[11:41] Donovan Linden: and the only PUT and DELETEs that will trigger events will happen through those capabilities
[11:41] Donovan Linden: but that part isn't developed yet
[11:42] Which Linden: So can you use POST to make changes?
[11:42] Which Linden: To the REST structure?
[11:42] Donovan Linden: sure, you can implement whatever server-side python you want with standard mulib
[11:42] Morgaine Dinova: Good, I just want to be sure that the client can receive the membership notifications automatically ... having to query for subscription list before every container put wouldn't be great.
[11:42] Donovan Linden: if you change the pantheon's rest store, you have to call a method to notify clients
[11:43] Donovan Linden: morgaine: play with it to see what's going over the wire
[11:43] Morgaine Dinova: kk
[11:43] Donovan Linden: for exmaple, run mud and try the hello_pantheon example while sniffing the wire
[11:43] Donovan Linden: or, firebug is also great for seeing what happens
[11:43] Donovan Linden: actually I think firebug is pretty much a necessity :)
[11:44] Which Linden: Ha ha, yeah it rules
[11:44] Which Linden: Though we did just write some code that only worked if you had Firebug installed :-/
[11:44] Donovan Linden: oh, I suppose I should mention the very hacky lsl client
[11:44] Morgaine Dinova: lol Which :P
[11:44] Donovan Linden: http://svn.secondlife.com/trac/mulib/browser/branches/collaborative-editor/mulib/cube.lsl
[11:44] Donovan Linden: here is an lsl client
[11:45] Donovan Linden: it uses a very weird format involving newlines and pipes and base 64 encoding
[11:45] Donovan Linden: I would love to know if there is a more standard format I could use
[11:45] Donovan Linden: because I have to parse/generate this format in mulib
[11:45] Donovan Linden: for the content type text/plain! because that's what lsl sends
[11:46] Donovan Linden: I suppose I should convert the input format to the same format.. right now it's json for the input, weird format for the output
[11:46] Which Linden: It would be cool if we could send llsd+xml natively from within LSL
[11:46] Donovan Linden: sure would, but lsl isn't nearly rich enough, type wise
[11:47] Which Linden: Right, we only have lists
[11:47] Wut Burt: ish :)
[11:47] Donovan Linden: homogenous lists?
[11:47] Donovan Linden: or are they heterogenous?
[11:48] Which Linden: No one seems to know....
[11:48] Morgaine Dinova: lol
[11:48] Donovan Linden: oh well :)
[11:48] Which Linden: I think I heard homogeneous, but I've never uesd one myself (lame, I know)
[11:48] Morgaine Dinova: Sai/Wut, looks like we'll need a Pantheon front end too :-)
[11:49] Donovan Linden: so for anyone who hasn't seen my blog post about the cube.lsl example:
[11:49] Donovan Linden: http://ulaluma.com/pyx/archives/2007/09/rest_to_lsl_via_python_and_comet.html#comments
[11:49] Which Linden: So do you envision Pantheon being used for any infrastructure in Second Life?
[11:49] Donovan Linden: (warning: plays a movie)
[11:49] Which Linden: The movie is quite cool, BTW
[11:49] Donovan Linden: all right I'm done blabbin again
[11:50] Morgaine Dinova: I can't get flash-encap'd movies on this box, 64-bit browser
[11:50] Which Linden: Cool
[11:50] Donovan Linden: which: I think we could use it for infrastructure
[11:50] Donovan Linden: but currently I have no plans for that
[11:51] Donovan Linden: I have been thinking of a new event queue design which works more like pantheon
[11:51] Morgaine Dinova: I'll ssh URL over to another box
[11:51] Donovan Linden: and allows selecting events from more than the simulator
[11:51] Which Linden: Agent presence was listed as a possibility -- i.e. you subscribe to each of your friends. Seems like it would lead to meltdown though
[11:51] Donovan Linden: no, I don't think it would lead to meltdown
[11:51] Donovan Linden: I think that is perfectly reasonable
[11:51] Which Linden: Oh, I guess each viewer/sim would have only one connection per agent to the presence servers
[11:52] Donovan Linden: I think in the long run, the viewer will have a comet connection open to both it's agent host and it's simulator
[11:52] Donovan Linden: the agent host and the simulator would then be able to select from another set of backend hosts to fulfill the client's request for event notifications
[11:53] Donovan Linden: if we end up going with the presence-publish-subscribe design I did a while back,
[11:53] Donovan Linden: we will probably have a hybrid architecture, where GETs against a server cause lazy subscriptions to the actual back-end event source
[11:53] Morgaine Dinova: Well you don't want to expand distributions lists by querying group membership at messaging time, that scales very badly. Really need the membership-change event to create relatively static lightweight exploders. That's one problem with current IM.
[11:53] Donovan Linden: and if there aren't GETs for that resource after a timeout, the subscription is dropped
[11:53] Which Linden: Morgaine: True
[11:54] Donovan Linden: morgaine: yes, there has been an internal design written up for chat that includes flooding all the agent hosts
[11:54] Morgaine Dinova: The exploders can live on the clients, for simple IM. But harder when messaging is server-side.
[11:55] Which Linden: So, any more questions?
[11:55] Donovan Linden: well, seems like everybody is in awe :)
[11:55] Which Linden: Ha ha, yeah. Or jonesing for the Client VAG.
[11:55] Morgaine Dinova: I'm in total awe. Now comes the hard work of figuring it out. :-)
[11:56] Emma Nowhere: nope, reading all the code you posted url's to
[11:56] Donovan Linden: so I'll lurk for the vag meeting but I'll be afk
[11:56] Morgaine Dinova: Cool
[11:56] Which Linden: OK, let's wrap up the office hours and do the VAG.