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Re: [libvirt] [PATCH 00/10] new virNetworkUpdate API

On Mon, Sep 17, 2012 at 05:48:45AM -0400, Laine Stump wrote:
> This patchset implements a new API function called virNetworkUpdate
> which enables updating certain parts of a libvirt network's definition
> without the need to destroy/re-start the network. This is especially
> useful, for example, to add/remove hosts from the dhcp static hosts
> table, or change portgroup settings.
> This was previously discussed in this thread:
>  https://www.redhat.com/archives/libvir-list/2012-August/msg01535.html
> continuing here in September:
>  https://www.redhat.com/archives/libvir-list/2012-September/msg00328.html
> with the final form here:
>  https://www.redhat.com/archives/libvir-list/2012-September/msg00465.html
> In short, the single function has a "section" specifier which tells
> the part of the network definition to be updated, a "parentIndex" that
> gives the index of the *parent* element containing this section (when
> there are multiples - in particular in the case of the <ip> element),
> and a fully formed XML element which will be added as-is in the case
> of VIR_NETWORK_UPDATE_ADD_* (after checking for a duplicate), used to
> search for the specific element to delete in case of
> VIR_NETWORK_UPDATE_DELETE, and used both to find the existing element
> and replace its current contents in the case of VIR_UPDATE_EXISTING
> (this implies that you can't change the change the attribute used for
> indexing, e.g. the name of a portgroup, or mac address of a dhcp host
> entry).
> An example of use: to add a dhcp host entry to network "net", you would do this:
>    virNetworkUpdate(net, VIR_NETWORK_SECTION_IP_DHCP_HOST, -1,
>                     "<host mac='00:11:22:33:44:55' ip=''/>",
>                     VIR_NETWORK_UPDATE_AFFECT_LIVE
>                     | VIR_NETWORK_UPDATE_AFFECT_CONFIG
>                     | VIR_NETWORK_UPDATE_ADD_LAST);
> To delete that same entry:
>    virNetworkUpdate(net, VIR_NETWORK_SECTION_IP_DHCP_HOST, -1,
>                     "<host mac='00:11:22:33:44:55'/>",
>                     VIR_NETWORK_UPDATE_AFFECT_LIVE
>                     | VIR_NETWORK_UPDATE_AFFECT_CONFIG
>                     | VIR_NETWORK_UPDATE_DELETE);
> If you wanted to force any of these to affect the dhcp host list in
> the 3rd <ip> element of the network, you would replace "-1" with "2".
> Another example: to modify the portgroup named "engineering" (e.g. to
> increase the inbound average bandwidth from 1000 to 2000):
>     virNetworkUpdate(net, VIR_NETWORK_SECTION_PORTGROUP, -1,
>                      "<portgroup name='engineering' default='yes'>"
>                      "  <virtualport type='802.1Qbh'>"
>                      "    <parameters profileid='test'/>"
>                      "  </virtualport>"
>                      "  <bandwidth>"
>                      "    <inbound average='2000' peak='5000' burst='5120'/>"
>                      "    <outbound average='1000' peak='5000' burst='5120'/>"
>                      "  </bandwidth>"
>                      "</portgroup>",
>                      | VIR_NETWORK_UPDATE_CONFIG)
> (note that parentIndex is irrelevant for PORTGROUP, since they are in
> the toplevel of <network>, so there aren't multiple instances of
> parents. In such cases, the caller *must* set parentIndex to -1 or 0 -
> any other value indicates that they don't understand the purpose/usage
> of parentIndex, so it must result in an error. Also note that the
> above function would fail if it couldn't find an existing portgroup
> with name='engineering' (i.e. it wouldn't automatically add a new one).)

I've been trying to think about how this might all map into the
LibvirtGObject/LibvirtGConfig APIs, and the thing I'm struggling
with is the parentIndex parameter.

First of all, in the GConfig API I won't be exposing the virNetworkUpdate
API as it is. To be typesafe, there will be separate APIs for each possible
operation. eg


Consider how the <network> schema will eventually map into objects,

<network>                == GVirConfigNetwork
  <forward mode='bridge'/>
  <bridge name='virbr5' stp='on' delay='0' />
  <mac address='52:54:00:38:81:4D'/>
  <domain name='example.com'/>
  <forward mode='private'/>
    <interface dev="eth20"/>       == GVirConfigNetworkInterface
    <interface dev="eth21"/>       == GVirConfigNetworkInterface
    <interface dev="eth22"/>       == GVirConfigNetworkInterface
    <interface dev="eth23"/>       == GVirConfigNetworkInterface
    <interface dev="eth24"/>       == GVirConfigNetworkInterface
  <portgroup name='engineering' default='yes'> == GVirConfigNetworkPortGroup
    <virtualport type='802.1Qbh'>
      <parameters profileid='test'/>
      <inbound average='1000' peak='5000' burst='5120'/>
      <outbound average='1000' peak='5000' burst='5120'/>
  <portgroup name='sales'>                     == GVirConfigNetworkPortGroup
    <virtualport type='802.1Qbh'>
      <parameters profileid='salestest'/>
      <inbound average='500' peak='2000' burst='2560'/>
      <outbound average='128' peak='256' burst='256'/>
  <dns>                                         == GVirConfigNetworkDNS
    <txt name="example" value="example value" /> == GVirConfigNetworkDNSEntry
    <srv service='name' protocol='tcp' domain='test-domain-name' target='.' port='1024' priority='10' weight='10'/> == GVirConfigNetworkDNSEntry
    <host ip=''>  == GVirConfigNetworkDNSEntry
  <ip address='' netmask=''>  == GVirConfigNetworkAddress
    <dhcp>  == GVirConfigNetworkDHCP
      <range start='' end='' />
      <host mac='52:54:3e:77:e2:ed' name='X.example.com' ip='' />   == GVirConfigNetworkDHCPHost
      <host mac='52:54:3e:77:e2:ef' name='Y.example.com' ip='' />
      <host mac='52:54:34:77:e2:f0' name='Z.example.com' ip='' />
      <host mac='52:54:3e:77:e2:f1' name='A.example.com' ip='' />
  <ip address='' netmask=''/>  == GVirConfigNetworkAddress

So for example we get the config object using

    GVirNetwork *net = gvir_connection_get_network_by_name("default");
    GVirConfigNetwork *conf = gvir_network_get_config(net);

Now we want to remove all portgroups. This is easy enough - I'd have
an API like

    GList *groups = gvir_config_network_get_portgroups(conf);

    while (groups) {
       GVirConfigNetworkPortgroup *group = groups->data;

       gvir_network_remove_portgroup(net, group);

       data = data->next;

As you say, the concept of parentIndex doesn't make sense for portgroups,
so I'll just ignore it in the API I expose.

Likewise adding/removing <interface> from <forward>, we just ignore the
parentIndex.  Modify doesn't make sense for this part of the schema
since there are no attributes to change, beyond the interface name.

DNS entries are reasonably easy to deal with add/remove, since again
parentIndex is irrelevant, there only being one <dns> block.

I'm a little fuzzy on whether a modify action is practical for DNS
entries, since the thing you'd want to change is probably also the
thing the API would want to use as the unique identifier. The only
way around this I see is to pass in both the original and new XML
for the DNS entry being modified. The original XML is used to lookup
which entry is being mdified, and then replace with the new XML.
Perhaps this doesn't matter, and add+remove is sufficient for DNS.

The updating of DHCP entries is the interesting / hard one that causes
the fun with parentIndex.

It is possible to come up with a mapping to GObject, 

    GList *addrs = gvir_config_network_get_addresses(conf);
    int idx = 0;

    while (addrs) {
       GVirConfigNetworkAddress *addr = addrs->data;

       GVirConfigNetworkDHCP *dhcp = gvir_config_network_address_get_dhcp(addr);

       GList *hosts = gvir_config_network_dhcp_get_hosts(dhcp);
       while (hosts) {
            GVirConfigNeworkDHCPHost *host = hosts->data;

            gvir_network_remove_dhcp_host(net, idx, host);

       data = data->next;

What I don't like is that the user has to maintain this 'idx' counter
value. It doesn't hurt in this example, but consider if you were just
passed a single "GVirConfigNetworkAddress" object, and wanted to add a
host entry to it. You have no idea what the parentIndex this corresponds
to.  This isn't fatal, but it is slightly unpleasant. I don't have any
better idea though, so I guess we'll just go with what you designed.

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