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Adding Function to XAPI

Add message to API

The file idl/ is an description of the API, from which the marshalling and handler code is generated.

In this file, the create_obj function is used to define a class which may contain fields and support operations (known as "messages"). For example, the identifier host is defined using create_obj to encapsulate the operations which can be performed on a host.

In order to add a function to the API, we need to add a message to an existing class. This entails adding a function in idl/ to describe the new message and adding it to the class's list of messages.

The function to describe the new message will look something like the following:

let host_price_of = call ~flags:[`Session]
  ~params:[Ref _host, "host", "The host containing the price information";
           String, "item", "The item whose price is queried"]
  ~result:(Float, "The price of the item")
  ~doc:"Returns the price of a named item."

By convention, the name of the function is formed from the name of the class and the name of the message: host and price_of, in the example. An entry for host_price_of is added to the messages of the host class:

let host =
    create_obj ...
      ~messages: [...

The parameters passed to call are all optional (except ~name and ~in_product_since).

The ~flags parameter is used to set conditions for the use of the message. For example, `Session is used to indicate that the call must be made in the presence of an existing session.

The value of the ~in_product_since parameter is a string taken from idl/ indicating the [[[XenServer]]] release in which this message was first introduced.

The ~params parameter describes a list of the formal parameters of the message. Each parameter is described by a triple. The first component of the triple is the type (from type ty in idl/; the second is the name of the parameter; the third is a human-readable description of the parameter. The first triple in the list is conventionally the instance of the class on which the message will operate. In the example, this is a reference to the host.

Similarly, the ~result describes the message's return type, although this is permitted to merely be a single value rather than a list of values. If no ~result is specified, the default is unit.

Compiling api.hg will cause the code corresponding to this message to be generated and output in autogen/ In the example above, a section handling an incoming call host.price_of appeared in autogen/ However, after this was generated, the rest of the build failed because this call expects a price_of function in the Host object.

Expected values in parameter ~in_product_since

In the example above, the value of the parameter ~in_product_since informs that the message host_price_of was added during the rel_orlando release cycle. If a new release cycle is required, then it needs to be added in the file idl/ The patch below shows how the new rel_george release identifier was added. Any class, message, etc. added during the rel_george release cycle should contain ~in_product_since:rel_george entries. (obs: the release and upgrade infrastructure can handle only one new rel_* identifier -- in this case, rel_george -- in each release)

--- a/ocaml/idl/ Tue Nov 11 15:17:48 2008 +0000
+++ b/ocaml/idl/ Tue Nov 11 15:53:29 2008 +0000
@@ -27,14 +27,13 @@
 (* useful constants for product vsn tracking *)
 let oss_since_303 = Some "3.0.3"
+ let rel_george = "george"
 let rel_orlando = "orlando"
 let rel_orlando_update_1 = "orlando-update-1"
 let rel_symc = "symc"
 let rel_miami = "miami"
 let rel_rio = "rio"
-let release_order = [engp:rel_rio; rel_miami; rel_symc; rel_orlando; rel_orlando_update_1]
+let release_order = [engp:rel_rio; rel_miami; rel_symc; rel_orlando; rel_orlando_update_1; rel_george] 

Update expose_get_all_messages_for list

Add your new class _name to expose_get_all_messages_for list, at the bottom of, in order to have automatically generated get_all and get_all_records functions attached to it.

Update the RBAC field containing the roles expected to use the new API call

After the RBAC integration, Xapi provides by default a set of static roles associated to the most common subject tasks.

The api calls associated with each role are defined by a new ~allowed_roles parameter in each api call, which specifies the list of static roles that should be able to execute the call. The possible roles for this list is one of the following names, defined in

  • role_pool_admin
  • role_pool_operator
  • role_vm_power_admin
  • role_vm_admin
  • role_vm_operator
  • role_read_only

So, for instance,

~allowed_roles:[role_pool_admin,role_pool_operator] (* this is not the recommended usage, see example below *)

would be a valid list (though it is not the recommended way of using allowed_roles, see below), meaning that subjects belonging to either role_pool_admin or role_pool_operator can execute the api call.

The RBAC requirements define a policy where the roles in the list above are supposed to be totally-ordered by the set of api-calls associated with each of them. That means that any api-call allowed to role_pool_operator should also be in role_pool_admin; any api-call allowed to role_vm_power_admin should also be in role_pool_operator and also in role_pool_admin; and so on. provides shortcuts for expressing these totally-ordered set of roles policy associated with each api-call:

  • _R_POOL_ADMIN, equivalent to [role_pool_admin]
  • _R_POOL_OP, equivalent to [role_pool_admin,role_pool_operator]
  • _R_VM_POWER_ADMIN, equivalent to [role_pool_admin,role_pool_operator,role_vm_power_admin]
  • _R_VM_ADMIN, equivalent to [role_pool_admin,role_pool_operator,role_vm_power_admin,role_vm_admin]
  • _R_VM_OP, equivalent to [role_pool_admin,role_pool_operator,role_vm_power_admin,role_vm_admin,role_vm_op]
  • _R_READ_ONLY, equivalent to [role_pool_admin,role_pool_operator,role_vm_power_admin,role_vm_admin,role_vm_op,role_read_only]

The ~allowed_roles parameter should use one of the shortcuts in the list above, instead of directly using a list of roles, because the shortcuts above make sure that the roles in the list are in a total order regarding the api-calls permission sets. Creating an api-call with e.g. allowed_roles:[role_pool_admin,role_vm_admin] would be wrong, because that would mean that a pool_operator cannot execute the api-call that a vm_admin can, breaking the total-order policy expected in the RBAC 1.0 implementation. In the future, this requirement might be relaxed.

So, the example above should instead be used as:

~allowed_roles:_R_POOL_OP  (* recommended usage via pre-defined totally-ordered role lists *)

and so on.

How to determine the correct role of a new api-call:

  • if only xapi should execute the api-call, ie. it is an internal call: _R_POOL_ADMIN
  • if it is related to subject, role, external-authentication: _R_POOL_ADMIN
  • if it is related to accessing Dom0 (via console, ssh, whatever): _R_POOL_ADMIN
  • if it is related to the pool object: R_POOL_OP
  • if it is related to the host object, licenses, backups, physical devices: _R_POOL_OP
  • if it is related to managing VM memory, snapshot/checkpoint, migration: _R_VM_POWER_ADMIN
  • if it is related to creating, destroying, cloning, importing/exporting VMs: _R_VM_ADMIN
  • if it is related to starting, stopping, pausing etc VMs or otherwise accessing/manipulating VMs: _R_VM_OP
  • if it is related to being able to login, manipulate own tasks and read values only: _R_READ_ONLY

Update message forwarding

The "message forwarding" layer describes the policy of whether an incoming API call should be forwarded to another host (such as another member of the pool) or processed on the host which receives the call. This policy may be non-trivial to describe and so cannot be auto-generated from the data model. For more details on message forwarding, see the Service dispatch and message forwarding page.

In xapi/, add a function to the relevant module to describe this policy. In the running example, we add the following function to the Host module:

let price_of ~__context ~host ~item =
      info "Host.price_of for item %s" item;
      let local_fn = Local.Host.price_of ~host ~item in
      do_op_on ~local_fn ~__context ~host
        (fun session_id rpc -> Client.Host.price_of rpc session_id host item)

After the ~__context parameter, the parameters of this new function should match the parameters we specified for the message. In this case, that is the host and the item to query the price of.

The do_op_on function takes a function to execute locally and a function to execute remotely and performs one of these operations depending on whether the given host is the local host.

The local function references Local.Host.price_of, which is a function we will write in the next step.

Implement the function

Now we write the function to perform the logic behind the new API call. For a host-based call, this will reside in xapi/ For other classes, other files with similar names are used.

We add the following function to xapi/

let price_of ~__context ~host ~item =
  if item = "fish" then 3.14 else 0.00

Congratulations, you've added a function to the API!

Add the operation to the CLI

Edit xapi/ Add a block to the definition of cmdtable_data as in the following example:

      reqd=["host-uuid"; "item"];
      help="Find out the price of an item on a certain host.";
      implementation= No_fd Cli_operations.host_price_of;

Include here the following:

  • The names of required (reqd) and optional (optn) parameters.
  • A description to be displayed when calling xe help <cmd> in the help field.
  • The implementation should use With_fd if any communication with the client is necessary (for example, showing the user a warning, sending the contents of a file, etc.) Otherwise, No_fd can be used as above.
  • The flags field can be used to set special options:
    • Vm_selectors: adds a "vm" parameter for the name of a VM (rather than a UUID)
    • Host_selectors: adds a "host" parameter for the name of a host (rather than a UUID)
    • Standard: includes the command in the list of common commands displayed by xe help
    • Neverforward:
    • Hidden:
    • Deprecated of string list:

Now we must implement Cli_operations.host_price_of. This is done in xapi/ This function typically extracts the parameters and forwards them to the internal implementation of the function. Other arbitrary code is permitted. For example:

let host_price_of printer rpc session_id params =
  let host = Client.Host.get_by_uuid rpc session_id (List.assoc "host-uuid" params) in
  let item = List.assoc "item" params in
  let price = string_of_float (Client.Host.price_of rpc session_id host item) in
  printer (Cli_printer.PList [price])

Tab Completion in the CLI

The CLI features tab completion for many of its commands' parameters. Tab completion is implemented in the file ocaml/xe-cli/bash-completion, which is installed on the host as /etc/bash_completion.d/cli, and is done on a parameter-name rather than on a command-name basis. The main portion of the bash-completion file is a case statement that contains a section for each of the parameters that benefit from completion. There is also an entry that catches all parameter names ending at -uuid, and performs an automatic lookup of suitable UUIDs. The host-uuid parameter of our new host-price-of command therefore automatically gains completion capabilities.

Executing the CLI operation

Recompile xapi with the changes described above and install it on a test machine.

Execute the following command to see if the function exists:

xe help host-price-of

Invoke the function itself with the following command:

xe host-price-of host-uuid=<tab> item=fish

and you should find out the price of fish.