Re: [vdsm] [RFC] An alternative way to provide a supported interface -- libvdsm
On Fri, Jul 06, 2012 at 03:53:08PM +0300, Itamar Heim wrote: On 07/06/2012 01:15 AM, Robert Middleswarth wrote: On 07/05/2012 04:45 PM, Adam Litke wrote: On Thu, Jul 05, 2012 at 03:47:42PM -0400, Saggi Mizrahi wrote: - Original Message - From: Adam Litke a...@us.ibm.com To: Saggi Mizrahi smizr...@redhat.com Cc: Anthony Liguori anth...@codemonkey.ws, VDSM Project Development vdsm-devel@lists.fedorahosted.org Sent: Thursday, July 5, 2012 2:34:50 PM Subject: Re: [RFC] An alternative way to provide a supported interface -- libvdsm On Wed, Jun 27, 2012 at 02:50:02PM -0400, Saggi Mizrahi wrote: The idea of having a supported C API was something I was thinking about doing (But I'd rather use gobject introspection and not schema generation) But the problem is not having a C API is using the current XML RPC API as it's base I want to disect this a bit to find out exactly where there might be agreement and disagreement. C API is a good thing to implement - Agreed. I also want to use gobject introspection but I don't agree that using glib precludes the use of a formalized schema. My proposal is that we write a schema definition and generate the glib C code from that schema. I agree that the _current_ xmlrpc API makes a pretty bad base from which to start a supportable API. XMLRPC is a perfectly reasonable remote/wire protocol and I think we should continue using it as a base for the next generation API. Using a schema will ensure that the new API is well-structured. There major problems with XML-RPC (and to some extent with REST as well) are high call overhead and no two way communication (push events). Basing on XML-RPC means that we will never be able to solve these issues. I am not sure I am ready to conceed that XML-RPC is too slow for our needs. Can you provide some more detail around this point and possibly suggest an alternative that has even lower overhead without sacrificing the ubiquity and usability of XML-RPC? As far as the two-way communication point, what are the options besides AMQP/ZeroMQ? Aren't these even worse from an overhead perspective than XML-RPC? Regarding two-way communication: you can write AMQP brokers based on the C API and run one on each vdsm host. Assuming the C API supports events, what else would you need? I personally think that using something like AMQP for inter-node communication and engine - node would be optimal. With a rest interface that just send messages though something like AMQP. I would also not dismiss AMQP so soon we want a bug with more than a single listener at engine side (engine, history db, maybe event correlation service). collectd as a means for statistics already supports it as well. I'm for having REST as well, but not sure as main one for a consumer like ovirt engine. I agree that a message bus could be a very useful model of communication between ovirt-engine components and multiple vdsm instances. But the complexities and dependencies of AMQP do not make it suitable for use as a low-level API. AMQP will repel new adopters. Why not establish a libvdsm that is more minimalist and can be easily used by everyone? Then AMQP brokers can be built on top of the stable API with ease. All AMQP should require of the low-level API are standard function calls and an events mechanism. Thanks Robert The current XML-RPC API contains a lot of decencies and inefficiencies and we would like to retire it as soon as we possibly can. Engine would like us to move to a message based API and 3rd parties want something simple like REST so it looks like no one actually wants to use XML-RPC. Not even us. I am proposing that AMQP brokers and REST APIs could be written against the public API. In fact, they need not even live in the vdsm tree anymore if that is what we choose. Core vdsm would only be responsible for providing libvdsm and whatever language bindings we want to support. If we take the libvdsm route, the only reason to even have a REST bridge is only to support OSes other then Linux which is something I'm not sure we care about at the moment. That might be true regarding the current in-tree implementation. However, I can almost guarantee that someone wanting to write a web GUI on top of standalone vdsm would want a REST API to talk to. But libvdsm makes this use case of no concern to the core vdsm developers. I do think that having C supportability in our API is a good idea, but the current API should not be used as the base. Let's _start_ with a schema document that describes today's API and then clean it up. I think that will work better than starting from scratch. Once my schema is written I will post it and we can 'patch' it as a community until we arrive at a 1.0 version we are all happy with. +1 Ok. Redoubling my efforts to get this done. Describing the output of list(True) takes awhile :)
Re: [vdsm] [RFC] An alternative way to provide a supported interface -- libvdsm
I don't think AMQP is a good low level supported protocol as it's a very complex protocol to set up and support. Also brokers are known to have their differences in standard implementation which means supporting them all is a mess. It looks like the most accepted route is the libvirt route of having a c library abstracting away client server communication and having more advanced consumers build protocol specific bridges that may have different support standards. On a more personal note, I think brokerless messaging is the way to go in ovirt because, unlike traditional clustering, worker nodes are not interchangeable so direct communication is the way to go, rendering brokers pretty much useless. - Original Message - From: Adam Litke a...@us.ibm.com To: Itamar Heim ih...@redhat.com Cc: vdsm-devel@lists.fedorahosted.org Sent: Monday, July 9, 2012 9:56:17 AM Subject: Re: [vdsm] [RFC] An alternative way to provide a supported interface -- libvdsm On Fri, Jul 06, 2012 at 03:53:08PM +0300, Itamar Heim wrote: On 07/06/2012 01:15 AM, Robert Middleswarth wrote: On 07/05/2012 04:45 PM, Adam Litke wrote: On Thu, Jul 05, 2012 at 03:47:42PM -0400, Saggi Mizrahi wrote: - Original Message - From: Adam Litke a...@us.ibm.com To: Saggi Mizrahi smizr...@redhat.com Cc: Anthony Liguori anth...@codemonkey.ws, VDSM Project Development vdsm-devel@lists.fedorahosted.org Sent: Thursday, July 5, 2012 2:34:50 PM Subject: Re: [RFC] An alternative way to provide a supported interface -- libvdsm On Wed, Jun 27, 2012 at 02:50:02PM -0400, Saggi Mizrahi wrote: The idea of having a supported C API was something I was thinking about doing (But I'd rather use gobject introspection and not schema generation) But the problem is not having a C API is using the current XML RPC API as it's base I want to disect this a bit to find out exactly where there might be agreement and disagreement. C API is a good thing to implement - Agreed. I also want to use gobject introspection but I don't agree that using glib precludes the use of a formalized schema. My proposal is that we write a schema definition and generate the glib C code from that schema. I agree that the _current_ xmlrpc API makes a pretty bad base from which to start a supportable API. XMLRPC is a perfectly reasonable remote/wire protocol and I think we should continue using it as a base for the next generation API. Using a schema will ensure that the new API is well-structured. There major problems with XML-RPC (and to some extent with REST as well) are high call overhead and no two way communication (push events). Basing on XML-RPC means that we will never be able to solve these issues. I am not sure I am ready to conceed that XML-RPC is too slow for our needs. Can you provide some more detail around this point and possibly suggest an alternative that has even lower overhead without sacrificing the ubiquity and usability of XML-RPC? As far as the two-way communication point, what are the options besides AMQP/ZeroMQ? Aren't these even worse from an overhead perspective than XML-RPC? Regarding two-way communication: you can write AMQP brokers based on the C API and run one on each vdsm host. Assuming the C API supports events, what else would you need? I personally think that using something like AMQP for inter-node communication and engine - node would be optimal. With a rest interface that just send messages though something like AMQP. I would also not dismiss AMQP so soon we want a bug with more than a single listener at engine side (engine, history db, maybe event correlation service). collectd as a means for statistics already supports it as well. I'm for having REST as well, but not sure as main one for a consumer like ovirt engine. I agree that a message bus could be a very useful model of communication between ovirt-engine components and multiple vdsm instances. But the complexities and dependencies of AMQP do not make it suitable for use as a low-level API. AMQP will repel new adopters. Why not establish a libvdsm that is more minimalist and can be easily used by everyone? Then AMQP brokers can be built on top of the stable API with ease. All AMQP should require of the low-level API are standard function calls and an events mechanism. Thanks Robert The current XML-RPC API contains a lot of decencies and inefficiencies and we would like to retire it as soon as we possibly can. Engine would like us to move to a message based API and 3rd parties want something simple like REST so it looks like no one actually wants to use XML-RPC. Not even us. I am proposing that AMQP brokers and REST APIs could be written against the public API. In fact, they need not even live in the vdsm tree anymore if that is what we choose.
Re: [vdsm] [RFC] An alternative way to provide a supported interface -- libvdsm
On 07/09/2012 05:56 PM, Saggi Mizrahi wrote: I don't think AMQP is a good low level supported protocol as it's a very complex protocol to set up and support. Also brokers are known to have their differences in standard implementation which means supporting them all is a mess. It looks like the most accepted route is the libvirt route of having a c library abstracting away client server communication and having more advanced consumers build protocol specific bridges that may have different support standards. On a more personal note, I think brokerless messaging is the way to go in ovirt because, unlike traditional clustering, worker nodes are not interchangeable so direct communication is the way to go, rendering brokers pretty much useless. but brokerless doesn't let multiple consumers which a bus provides? - Original Message - From: Adam Litke a...@us.ibm.com To: Itamar Heim ih...@redhat.com Cc: vdsm-devel@lists.fedorahosted.org Sent: Monday, July 9, 2012 9:56:17 AM Subject: Re: [vdsm] [RFC] An alternative way to provide a supported interface -- libvdsm On Fri, Jul 06, 2012 at 03:53:08PM +0300, Itamar Heim wrote: On 07/06/2012 01:15 AM, Robert Middleswarth wrote: On 07/05/2012 04:45 PM, Adam Litke wrote: On Thu, Jul 05, 2012 at 03:47:42PM -0400, Saggi Mizrahi wrote: - Original Message - From: Adam Litke a...@us.ibm.com To: Saggi Mizrahi smizr...@redhat.com Cc: Anthony Liguori anth...@codemonkey.ws, VDSM Project Development vdsm-devel@lists.fedorahosted.org Sent: Thursday, July 5, 2012 2:34:50 PM Subject: Re: [RFC] An alternative way to provide a supported interface -- libvdsm On Wed, Jun 27, 2012 at 02:50:02PM -0400, Saggi Mizrahi wrote: The idea of having a supported C API was something I was thinking about doing (But I'd rather use gobject introspection and not schema generation) But the problem is not having a C API is using the current XML RPC API as it's base I want to disect this a bit to find out exactly where there might be agreement and disagreement. C API is a good thing to implement - Agreed. I also want to use gobject introspection but I don't agree that using glib precludes the use of a formalized schema. My proposal is that we write a schema definition and generate the glib C code from that schema. I agree that the _current_ xmlrpc API makes a pretty bad base from which to start a supportable API. XMLRPC is a perfectly reasonable remote/wire protocol and I think we should continue using it as a base for the next generation API. Using a schema will ensure that the new API is well-structured. There major problems with XML-RPC (and to some extent with REST as well) are high call overhead and no two way communication (push events). Basing on XML-RPC means that we will never be able to solve these issues. I am not sure I am ready to conceed that XML-RPC is too slow for our needs. Can you provide some more detail around this point and possibly suggest an alternative that has even lower overhead without sacrificing the ubiquity and usability of XML-RPC? As far as the two-way communication point, what are the options besides AMQP/ZeroMQ? Aren't these even worse from an overhead perspective than XML-RPC? Regarding two-way communication: you can write AMQP brokers based on the C API and run one on each vdsm host. Assuming the C API supports events, what else would you need? I personally think that using something like AMQP for inter-node communication and engine - node would be optimal. With a rest interface that just send messages though something like AMQP. I would also not dismiss AMQP so soon we want a bug with more than a single listener at engine side (engine, history db, maybe event correlation service). collectd as a means for statistics already supports it as well. I'm for having REST as well, but not sure as main one for a consumer like ovirt engine. I agree that a message bus could be a very useful model of communication between ovirt-engine components and multiple vdsm instances. But the complexities and dependencies of AMQP do not make it suitable for use as a low-level API. AMQP will repel new adopters. Why not establish a libvdsm that is more minimalist and can be easily used by everyone? Then AMQP brokers can be built on top of the stable API with ease. All AMQP should require of the low-level API are standard function calls and an events mechanism. Thanks Robert The current XML-RPC API contains a lot of decencies and inefficiencies and we would like to retire it as soon as we possibly can. Engine would like us to move to a message based API and 3rd parties want something simple like REST so it looks like no one actually wants to use XML-RPC. Not even us. I am proposing that AMQP brokers and REST APIs could be written against the public API. In fact, they need not even live in the vdsm tree anymore if that is what we choose. Core vdsm would only be responsible for providing libvdsm and whatever language bindings
Re: [vdsm] [RFC] An alternative way to provide a supported interface -- libvdsm
- Original Message - From: Itamar Heim ih...@redhat.com To: Saggi Mizrahi smizr...@redhat.com Cc: Adam Litke a...@us.ibm.com, vdsm-devel@lists.fedorahosted.org Sent: Monday, July 9, 2012 11:03:43 AM Subject: Re: [vdsm] [RFC] An alternative way to provide a supported interface -- libvdsm On 07/09/2012 05:56 PM, Saggi Mizrahi wrote: I don't think AMQP is a good low level supported protocol as it's a very complex protocol to set up and support. Also brokers are known to have their differences in standard implementation which means supporting them all is a mess. It looks like the most accepted route is the libvirt route of having a c library abstracting away client server communication and having more advanced consumers build protocol specific bridges that may have different support standards. On a more personal note, I think brokerless messaging is the way to go in ovirt because, unlike traditional clustering, worker nodes are not interchangeable so direct communication is the way to go, rendering brokers pretty much useless. but brokerless doesn't let multiple consumers which a bus provides? All consumers can connect to the host and *some* events can be broadcasted to all connected clients. The real question is weather you want to depend on AMQP's routing \ message storing Also, if you find it preferable to have a centralized host (single point of failure) to get all events from all hosts for the price of some clients (I assume read only clients) not needing to know the locations of all worker nodes. But IMHO we already have something like that, it's called the ovirt-engine, and it could send aggregated events about the cluster (maybe with some extra enginy data). The question is what does mandating a broker gives us something that an AMQP bridge wouldn't. The only thing I can think of is vdsm can assume unmoderated vdsm to vdsm communication bypassing the engine. This means that VDSM can have some clustered behavior that requires no engine intervention. Further more, the engine can send a request and let the nodes decide who is performing the operation among themselves. Essentially: [ engine ] [ engine ] | | VS | [vdsm][vdsm] [ broker ] | | [vdsm][vdsm] *All links are two way links This has dire consequences on API usability and supportability. So we need to converge on that. There needs to be a good reason why the aforementioned logic code can't sit on a another ovirt specific entity (lets call it ovirt-dynamo) that uses VDSM's supported API but it's own APIs (or more likely messaging algorithms) are unsupported. [engine ] ||| | [ broker ] | ||| | [vdsm]-[dynamo] : [dynamo]-[vdsm] Host A : Host B *All links are two way links - Original Message - From: Adam Litke a...@us.ibm.com To: Itamar Heim ih...@redhat.com Cc: vdsm-devel@lists.fedorahosted.org Sent: Monday, July 9, 2012 9:56:17 AM Subject: Re: [vdsm] [RFC] An alternative way to provide a supported interface -- libvdsm On Fri, Jul 06, 2012 at 03:53:08PM +0300, Itamar Heim wrote: On 07/06/2012 01:15 AM, Robert Middleswarth wrote: On 07/05/2012 04:45 PM, Adam Litke wrote: On Thu, Jul 05, 2012 at 03:47:42PM -0400, Saggi Mizrahi wrote: - Original Message - From: Adam Litke a...@us.ibm.com To: Saggi Mizrahi smizr...@redhat.com Cc: Anthony Liguori anth...@codemonkey.ws, VDSM Project Development vdsm-devel@lists.fedorahosted.org Sent: Thursday, July 5, 2012 2:34:50 PM Subject: Re: [RFC] An alternative way to provide a supported interface -- libvdsm On Wed, Jun 27, 2012 at 02:50:02PM -0400, Saggi Mizrahi wrote: The idea of having a supported C API was something I was thinking about doing (But I'd rather use gobject introspection and not schema generation) But the problem is not having a C API is using the current XML RPC API as it's base I want to disect this a bit to find out exactly where there might be agreement and disagreement. C API is a good thing to implement - Agreed. I also want to use gobject introspection but I don't agree that using glib precludes the use of a formalized schema. My proposal is that we write a schema definition and generate the glib C code from that schema. I agree that the _current_ xmlrpc API makes a pretty bad base from which to start a supportable API. XMLRPC is a perfectly reasonable remote/wire protocol and I think we should continue using it as a base for the next generation API. Using a schema will ensure that the new API is well-structured. There major problems with XML-RPC (and to some extent with REST as well) are high call overhead and no two way communication (push
