Merge lp:~wallyworld/juju-core/unit-agent-knows-assigned-machine into lp:~go-bot/juju-core/trunk

[Ian Booth]

Unit agents know their assigned machine

The unit agent conf file has a new value called
MACHINE_TAG added when the unit is deployed. The
value is the tag of the machine on which the
unit is deployed.

To allow new tools to run on older deployments
without the updated agent conf file, a new Uniter
API is provided which allows the unit agent to
look up the assigned machine over the API.
The API will be removed when it is possible
to have all current unit agent config files
upgraded.

This functionality is used by a downstream
branch which reworks the uniter upgrade worker.

https://codereview.appspot.com/60930043/

[Ian Booth (wallyworld)]

Reviewers: mp+205291_code.launchpad.net,

Message:
Please take a look.

Description:
Unit agents know their assigned machine

The unit agent conf file has a new value called
MACHINE_TAG added when the unit is deployed. The
value is the tag of the machine on which the
unit is deployed.

To allow new tools to run on older deployments
without the updated agent conf file, a new Uniter
API is provided which allows the unit agent to
look up the assigned machine over the API.
The API will be removed when it is possible
to have all current unit agent config files
upgraded.

This functionality is used by a downstream
branch which reworks the uniter upgrade worker.

https://code.launchpad.net/~wallyworld/juju-core/unit-agent-knows-assigned-machine/+merge/205291

Requires:
https://code.launchpad.net/~wallyworld/juju-core/machiner-access-for-uniter/+merge/205290

(do not edit description out of merge proposal)

Please review this at https://codereview.appspot.com/60930043/

Affected files (+126, -22 lines):
   A [revision details]
   M agent/agent.go
   M state/api/uniter/unit.go
   M state/api/uniter/unit_test.go
   M state/api/uniter/uniter.go
   M state/apiserver/uniter/uniter.go
   M state/apiserver/uniter/uniter_test.go
   M worker/deployer/simple.go
   M worker/deployer/simple_test.go

[Dimiter Naydenov (dimitern)]

Very good direction, but I have some concerns about backward
compatibility with <1.18 API servers.

https://codereview.appspot.com/60930043/diff/1/state/api/uniter/unit.go
File state/api/uniter/unit.go (right):

https://codereview.appspot.com/60930043/diff/1/state/api/uniter/unit.go#newcode456
state/api/uniter/unit.go:456: err := u.st.caller.Call(uniter, "",
"GetAssignedMachine", args, &results)
We need to be careful here for CodeNotImplemented errors, if this is run
against an older API server. What's the appropriate action to take when
we can't read it from the API server?

https://codereview.appspot.com/60930043/diff/1/state/apiserver/uniter/uniter.go
File state/apiserver/uniter/uniter.go (right):

https://codereview.appspot.com/60930043/diff/1/state/apiserver/uniter/uniter.go#newcode1004
state/apiserver/uniter/uniter.go:1004: if canAccess(entity.Tag) {
To reduce the code nesting, you could define a:

func (u *UniterAPI) getOneAssignedMachine(tag string) (string, error) {
     unit, err := u.getUnit(tag)
     if err != nil {
         return "", err
     }
     machineId, err := unit.AssignedMachineId()
     if err != nil {
         return "", err
     }
     return machineId, nil
}

And then reduce the following block to:

machineId, err := u.getOneAssignedMachine(entity.Tag)
if err == nil {
     reslt.Results[i].Result = names.MachineTag(machineId)
}

https://codereview.appspot.com/60930043/diff/1/state/apiserver/uniter/uniter_test.go
File state/apiserver/uniter/uniter_test.go (right):

https://codereview.appspot.com/60930043/diff/1/state/apiserver/uniter/uniter_test.go#newcode1441
state/apiserver/uniter/uniter_test.go:1441: {Tag: "unit-mysql-0"},
I'd like to see a subsequent test in this method that first unassigns
wordpress/0 and then calls GetAssignedMachine again with args =
params.Entities{Entities: []params.Entity{{Tag: "unit-wordpress-0"}}},
so that the returned error is checked to match
apiservertesting.NotAssignedError(..).

https://codereview.appspot.com/60930043/

[lp:~wallyworld/juju-core/unit-agent-knows-assigned-machine]

2306. By Ian Booth on 2014-02-11

Merge trunk, resolve conflict

2307. By Ian Booth on 2014-02-11

Merged machiner-access-for-uniter into unit-agent-knows-assigned-machine.

2308. By Ian Booth on 2014-02-11

Add test

[Ian Booth (wallyworld)]

Please take a look.

https://codereview.appspot.com/60930043/diff/1/state/api/uniter/unit.go
File state/api/uniter/unit.go (right):

https://codereview.appspot.com/60930043/diff/1/state/api/uniter/unit.go#newcode456
state/api/uniter/unit.go:456: err := u.st.caller.Call(uniter, "",
"GetAssignedMachine", args, &results)
On 2014/02/10 08:55:04, dimitern wrote:
> We need to be careful here for CodeNotImplemented errors, if this is
run against
> an older API server. What's the appropriate action to take when we
can't read it
> from the API server?

When we upgrade, is the API server upgraded first? If so then,
everything is ok. If not, then I don't think there's much choice other
than to let the error propagate and cause the caller (the unit agent) to
be restarted so it can try again. This call needs to succeed before the
new, upgraded unit agent can come up. So I think letting it keep
retrying is ok here?

https://codereview.appspot.com/60930043/diff/1/state/apiserver/uniter/uniter.go
File state/apiserver/uniter/uniter.go (right):

https://codereview.appspot.com/60930043/diff/1/state/apiserver/uniter/uniter.go#newcode1004
state/apiserver/uniter/uniter.go:1004: if canAccess(entity.Tag) {
On 2014/02/10 08:55:04, dimitern wrote:
> To reduce the code nesting, you could define a:

> func (u *UniterAPI) getOneAssignedMachine(tag string) (string, error)
{
> unit, err := u.getUnit(tag)
> if err != nil {
> return "", err
> }
> machineId, err := unit.AssignedMachineId()
> if err != nil {
> return "", err
> }
> return machineId, nil
> }

> And then reduce the following block to:

> machineId, err := u.getOneAssignedMachine(entity.Tag)
> if err == nil {
> reslt.Results[i].Result = names.MachineTag(machineId)
> }

Done.

https://codereview.appspot.com/60930043/diff/1/state/apiserver/uniter/uniter_test.go
File state/apiserver/uniter/uniter_test.go (right):

https://codereview.appspot.com/60930043/diff/1/state/apiserver/uniter/uniter_test.go#newcode1441
state/apiserver/uniter/uniter_test.go:1441: {Tag: "unit-mysql-0"},
On 2014/02/10 08:55:04, dimitern wrote:
> I'd like to see a subsequent test in this method that first unassigns
> wordpress/0 and then calls GetAssignedMachine again with args =
> params.Entities{Entities: []params.Entity{{Tag: "unit-wordpress-0"}}},
so that
> the returned error is checked to match
apiservertesting.NotAssignedError(..).

Done.

https://codereview.appspot.com/60930043/

[Dimiter Naydenov (dimitern)]

LGTM

https://codereview.appspot.com/60930043/diff/20001/agent/agent.go
File agent/agent.go (right):

https://codereview.appspot.com/60930043/diff/20001/agent/agent.go#newcode34
agent/agent.go:34: BootstrapJobs = "BOOTSTRAP_JOBS"
Where is this coming from? Update the CL description?

https://codereview.appspot.com/60930043/

[Ian Booth (wallyworld)]

https://codereview.appspot.com/60930043/diff/20001/agent/agent.go
File agent/agent.go (right):

https://codereview.appspot.com/60930043/diff/20001/agent/agent.go#newcode34
agent/agent.go:34: BootstrapJobs = "BOOTSTRAP_JOBS"
On 2014/02/11 09:52:19, dimitern wrote:
> Where is this coming from? Update the CL description?

I merged trunk and shiteveld can't handle such merges properly so it
gets confused. Launchpad has the correct diff.

https://codereview.appspot.com/60930043/

[William Reade (fwereade)]

On 2014/02/11 10:28:03, wallyworld wrote:
> https://codereview.appspot.com/60930043/diff/20001/agent/agent.go
> File agent/agent.go (right):

https://codereview.appspot.com/60930043/diff/20001/agent/agent.go#newcode34
> agent/agent.go:34: BootstrapJobs = "BOOTSTRAP_JOBS"
> On 2014/02/11 09:52:19, dimitern wrote:
> > Where is this coming from? Update the CL description?

> I merged trunk and shiteveld can't handle such merges properly so it
gets
> confused. Launchpad has the correct diff.

as prereq, not lgtm

https://codereview.appspot.com/60930043/

Unmerged revisions

2308. By Ian Booth on 2014-02-11

Add test

2307. By Ian Booth on 2014-02-11

Merged machiner-access-for-uniter into unit-agent-knows-assigned-machine.

2306. By Ian Booth on 2014-02-11

Merge trunk, resolve conflict

2305. By Ian Booth on 2014-02-07

Unit agents can find out their assigned machine

2304. By Ian Booth on 2014-02-07

Unit agents have read access to machines on which their units are deployed

=== modified file 'agent/agent.go'
--- agent/agent.go	2014-02-05 08:01:22 +0000
+++ agent/agent.go	2014-02-11 02:38:51 +0000
@@ -32,6 +32,7 @@
 	MongoServiceName = "MONGO_SERVICE_NAME"
 	RsyslogConfPath  = "RSYSLOG_CONF_PATH"
 	BootstrapJobs    = "BOOTSTRAP_JOBS"
+	MachineTag       = "MACHINE_TAG"
 )
 
 // The Config interface is the sole way that the agent gets access to the

=== modified file 'state/api/uniter/unit.go'
--- state/api/uniter/unit.go	2013-09-26 17:23:34 +0000
+++ state/api/uniter/unit.go	2014-02-11 02:38:51 +0000
@@ -62,7 +62,7 @@
 			{Tag: u.tag, Status: status, Info: info, Data: data},
 		},
 	}
-	err := u.st.caller.Call("Uniter", "", "SetStatus", args, &result)
+	err := u.st.caller.Call(uniter, "", "SetStatus", args, &result)
 	if err != nil {
 		return err
 	}
@@ -76,7 +76,7 @@
 	args := params.Entities{
 		Entities: []params.Entity{{Tag: u.tag}},
 	}
-	err := u.st.caller.Call("Uniter", "", "EnsureDead", args, &result)
+	err := u.st.caller.Call(uniter, "", "EnsureDead", args, &result)
 	if err != nil {
 		return err
 	}
@@ -89,7 +89,7 @@
 	args := params.Entities{
 		Entities: []params.Entity{{Tag: u.tag}},
 	}
-	err := u.st.caller.Call("Uniter", "", "Watch", args, &results)
+	err := u.st.caller.Call(uniter, "", "Watch", args, &results)
 	if err != nil {
 		return nil, err
 	}
@@ -129,7 +129,7 @@
 	args := params.Entities{
 		Entities: []params.Entity{{Tag: u.tag}},
 	}
-	err := u.st.caller.Call("Uniter", "", "ConfigSettings", args, &results)
+	err := u.st.caller.Call(uniter, "", "ConfigSettings", args, &results)
 	if err != nil {
 		return nil, err
 	}
@@ -163,7 +163,7 @@
 	args := params.Entities{
 		Entities: []params.Entity{{Tag: u.tag}},
 	}
-	err := u.st.caller.Call("Uniter", "", "Destroy", args, &result)
+	err := u.st.caller.Call(uniter, "", "Destroy", args, &result)
 	if err != nil {
 		return err
 	}
@@ -176,7 +176,7 @@
 	args := params.Entities{
 		Entities: []params.Entity{{Tag: u.tag}},
 	}
-	err := u.st.caller.Call("Uniter", "", "DestroyAllSubordinates", args, &result)
+	err := u.st.caller.Call(uniter, "", "DestroyAllSubordinates", args, &result)
 	if err != nil {
 		return err
 	}
@@ -192,7 +192,7 @@
 	args := params.Entities{
 		Entities: []params.Entity{{Tag: u.tag}},
 	}
-	err := u.st.caller.Call("Uniter", "", "Resolved", args, &results)
+	err := u.st.caller.Call(uniter, "", "Resolved", args, &results)
 	if err != nil {
 		return "", err
 	}
@@ -216,7 +216,7 @@
 	args := params.Entities{
 		Entities: []params.Entity{{Tag: u.tag}},
 	}
-	err := u.st.caller.Call("Uniter", "", "GetPrincipal", args, &results)
+	err := u.st.caller.Call(uniter, "", "GetPrincipal", args, &results)
 	if err != nil {
 		return false, err
 	}
@@ -237,7 +237,7 @@
 	args := params.Entities{
 		Entities: []params.Entity{{Tag: u.tag}},
 	}
-	err := u.st.caller.Call("Uniter", "", "HasSubordinates", args, &results)
+	err := u.st.caller.Call(uniter, "", "HasSubordinates", args, &results)
 	if err != nil {
 		return false, err
 	}
@@ -264,7 +264,7 @@
 	args := params.Entities{
 		Entities: []params.Entity{{Tag: u.tag}},
 	}
-	err := u.st.caller.Call("Uniter", "", "PublicAddress", args, &results)
+	err := u.st.caller.Call(uniter, "", "PublicAddress", args, &results)
 	if err != nil {
 		return "", err
 	}
@@ -289,7 +289,7 @@
 			{Tag: u.tag, Address: address},
 		},
 	}
-	err := u.st.caller.Call("Uniter", "", "SetPublicAddress", args, &result)
+	err := u.st.caller.Call(uniter, "", "SetPublicAddress", args, &result)
 	if err != nil {
 		return err
 	}
@@ -309,7 +309,7 @@
 	args := params.Entities{
 		Entities: []params.Entity{{Tag: u.tag}},
 	}
-	err := u.st.caller.Call("Uniter", "", "PrivateAddress", args, &results)
+	err := u.st.caller.Call(uniter, "", "PrivateAddress", args, &results)
 	if err != nil {
 		return "", err
 	}
@@ -334,7 +334,7 @@
 			{Tag: u.tag, Address: address},
 		},
 	}
-	err := u.st.caller.Call("Uniter", "", "SetPrivateAddress", args, &result)
+	err := u.st.caller.Call(uniter, "", "SetPrivateAddress", args, &result)
 	if err != nil {
 		return err
 	}
@@ -353,7 +353,7 @@
 			{Tag: u.tag, Protocol: protocol, Port: number},
 		},
 	}
-	err := u.st.caller.Call("Uniter", "", "OpenPort", args, &result)
+	err := u.st.caller.Call(uniter, "", "OpenPort", args, &result)
 	if err != nil {
 		return err
 	}
@@ -372,7 +372,7 @@
 			{Tag: u.tag, Protocol: protocol, Port: number},
 		},
 	}
-	err := u.st.caller.Call("Uniter", "", "ClosePort", args, &result)
+	err := u.st.caller.Call(uniter, "", "ClosePort", args, &result)
 	if err != nil {
 		return err
 	}
@@ -390,7 +390,7 @@
 	args := params.Entities{
 		Entities: []params.Entity{{Tag: u.tag}},
 	}
-	err := u.st.caller.Call("Uniter", "", "CharmURL", args, &results)
+	err := u.st.caller.Call(uniter, "", "CharmURL", args, &results)
 	if err != nil {
 		return nil, err
 	}
@@ -423,7 +423,7 @@
 			{Tag: u.tag, CharmURL: curl.String()},
 		},
 	}
-	err := u.st.caller.Call("Uniter", "", "SetCharmURL", args, &result)
+	err := u.st.caller.Call(uniter, "", "SetCharmURL", args, &result)
 	if err != nil {
 		return err
 	}
@@ -436,13 +436,37 @@
 	args := params.Entities{
 		Entities: []params.Entity{{Tag: u.tag}},
 	}
-	err := u.st.caller.Call("Uniter", "", "ClearResolved", args, &result)
+	err := u.st.caller.Call(uniter, "", "ClearResolved", args, &result)
 	if err != nil {
 		return err
 	}
 	return result.OneError()
 }
 
+// AssignedMachine returns the tag of this unit's assigned machine (if
+// any), or a CodeNotAssigned error.
+// TODO(wallyworld) - this API is needed for backwards compatibility since for > 1.18 the
+// assigned machine is recorded in the unit agent config file. For < 1.18, this API must
+// be used instead. Once upgrade infrastructure is in place, this API can be removed.
+func (u *Unit) AssignedMachine() (string, error) {
+	var results params.StringResults
+	args := params.Entities{
+		Entities: []params.Entity{{Tag: u.tag}},
+	}
+	err := u.st.caller.Call(uniter, "", "GetAssignedMachine", args, &results)
+	if err != nil {
+		return "", err
+	}
+	if len(results.Results) != 1 {
+		return "", fmt.Errorf("expected one result, got %d", len(results.Results))
+	}
+	result := results.Results[0]
+	if result.Error != nil {
+		return "", result.Error
+	}
+	return result.Result, nil
+}
+
 // WatchConfigSettings returns a watcher for observing changes to the
 // unit's service configuration settings. The unit must have a charm URL
 // set before this method is called, and the returned watcher will be
@@ -452,7 +476,7 @@
 	args := params.Entities{
 		Entities: []params.Entity{{Tag: u.tag}},
 	}
-	err := u.st.caller.Call("Uniter", "", "WatchConfigSettings", args, &results)
+	err := u.st.caller.Call(uniter, "", "WatchConfigSettings", args, &results)
 	if err != nil {
 		return nil, err
 	}

=== modified file 'state/api/uniter/unit_test.go'
--- state/api/uniter/unit_test.go	2013-09-27 15:40:55 +0000
+++ state/api/uniter/unit_test.go	2014-02-11 02:38:51 +0000
@@ -14,6 +14,7 @@
 	"launchpad.net/juju-core/state/api/uniter"
 	statetesting "launchpad.net/juju-core/state/testing"
 	jc "launchpad.net/juju-core/testing/checkers"
+	"launchpad.net/juju-core/utils"
 )
 
 type unitSuite struct {
@@ -177,6 +178,29 @@
 	c.Assert(mode, gc.Equals, params.ResolvedNone)
 }
 
+func (s *unitSuite) TestAssignedMachine(c *gc.C) {
+	tag, err := s.apiUnit.AssignedMachine()
+	c.Assert(err, gc.IsNil)
+	c.Assert(tag, gc.Equals, s.wordpressMachine.Tag())
+}
+
+func (s *unitSuite) TestAssignedMachineUnassignedUnit(c *gc.C) {
+	unassignedUnit, err := s.wordpressService.AddUnit()
+	c.Assert(err, gc.IsNil)
+	password, err := utils.RandomPassword()
+	c.Assert(err, gc.IsNil)
+	err = unassignedUnit.SetPassword(password)
+	c.Assert(err, gc.IsNil)
+	st := s.OpenAPIAs(c, unassignedUnit.Tag(), password)
+	uniter := st.Uniter()
+	c.Assert(uniter, gc.NotNil)
+
+	apiUnit, err := uniter.Unit(unassignedUnit.Tag())
+	c.Assert(err, gc.IsNil)
+	_, err = apiUnit.AssignedMachine()
+	c.Assert(err, jc.Satisfies, params.IsCodeNotAssigned)
+}
+
 func (s *unitSuite) TestIsPrincipal(c *gc.C) {
 	ok, err := s.apiUnit.IsPrincipal()
 	c.Assert(err, gc.IsNil)

=== modified file 'state/api/uniter/uniter.go'
--- state/api/uniter/uniter.go	2014-02-03 11:45:15 +0000
+++ state/api/uniter/uniter.go	2014-02-11 02:38:51 +0000
@@ -156,7 +156,7 @@
 // Environment returns the environment entity.
 func (st *State) Environment() (*Environment, error) {
 	var result params.EnvironmentResult
-	err := st.caller.Call("Uniter", "", "CurrentEnvironment", nil, &result)
+	err := st.caller.Call(uniter, "", "CurrentEnvironment", nil, &result)
 	if params.IsCodeNotImplemented(err) {
 		// Fall back to using the 1.16 API.
 		return st.environment1dot16()
@@ -177,7 +177,7 @@
 // using an older API server that does not support CurrentEnvironment API call.
 func (st *State) environment1dot16() (*Environment, error) {
 	var result params.StringResult
-	err := st.caller.Call("Uniter", "", "CurrentEnvironUUID", nil, &result)
+	err := st.caller.Call(uniter, "", "CurrentEnvironUUID", nil, &result)
 	if err != nil {
 		return nil, err
 	}

=== modified file 'state/apiserver/uniter/uniter.go'
--- state/apiserver/uniter/uniter.go	2014-02-03 11:45:15 +0000
+++ state/apiserver/uniter/uniter.go	2014-02-11 02:38:51 +0000
@@ -986,3 +986,41 @@
 		Result: service.GetOwnerTag(),
 	}, nil
 }
+
+// GetAssignedMachine returns the assigned machine tag (if any) for each given unit.
+// TODO(wallyworld) - this API is needed for backwards compatibility since for > 1.18 the
+// assigned machine is recorded in the unit agent config file. For < 1.18, this API must
+// be used instead. Once upgrade infrastructure is in place, this API can be removed.
+func (u *UniterAPI) GetAssignedMachine(args params.Entities) (params.StringResults, error) {
+	result := params.StringResults{
+		Results: make([]params.StringResult, len(args.Entities)),
+	}
+	canAccess, err := u.accessUnit()
+	if err != nil {
+		return params.StringResults{}, err
+	}
+	for i, entity := range args.Entities {
+		err := common.ErrPerm
+		if canAccess(entity.Tag) {
+			var machineId string
+			machineId, err = u.getOneAssignedMachine(entity.Tag)
+			if err == nil {
+				result.Results[i].Result = names.MachineTag(machineId)
+			}
+		}
+		result.Results[i].Error = common.ServerError(err)
+	}
+	return result, nil
+}
+
+func (u *UniterAPI) getOneAssignedMachine(tag string) (string, error) {
+	unit, err := u.getUnit(tag)
+	if err != nil {
+		return "", err
+	}
+	machineId, err := unit.AssignedMachineId()
+	if err != nil {
+		return "", err
+	}
+	return machineId, nil
+}

=== modified file 'state/apiserver/uniter/uniter_test.go'
--- state/apiserver/uniter/uniter_test.go	2014-02-03 11:45:15 +0000
+++ state/apiserver/uniter/uniter_test.go	2014-02-11 02:38:51 +0000
@@ -1435,3 +1435,35 @@
 		Result: "user-admin",
 	})
 }
+
+func (s *uniterSuite) TestGetAssignedMachine(c *gc.C) {
+	args := params.Entities{Entities: []params.Entity{
+		{Tag: "unit-mysql-0"},
+		{Tag: "unit-wordpress-0"},
+		{Tag: "unit-foo-42"},
+	}}
+	result, err := s.uniter.GetAssignedMachine(args)
+	c.Assert(err, gc.IsNil)
+	c.Assert(result, gc.DeepEquals, params.StringResults{
+		Results: []params.StringResult{
+			{Error: apiservertesting.ErrUnauthorized},
+			{Result: "machine-0", Error: nil},
+			{Error: apiservertesting.ErrUnauthorized},
+		},
+	})
+}
+
+func (s *uniterSuite) TestGetAssignedMachineUnassignedUnit(c *gc.C) {
+	err := s.wordpressUnit.UnassignFromMachine()
+	c.Assert(err, gc.IsNil)
+	args := params.Entities{Entities: []params.Entity{
+		{Tag: "unit-wordpress-0"},
+	}}
+	result, err := s.uniter.GetAssignedMachine(args)
+	c.Assert(err, gc.IsNil)
+	c.Assert(result, gc.DeepEquals, params.StringResults{
+		Results: []params.StringResult{
+			{Error: apiservertesting.NotAssignedError("wordpress/0")},
+		},
+	})
+}

=== modified file 'worker/deployer/simple.go'
--- worker/deployer/simple.go	2014-01-22 22:48:54 +0000
+++ worker/deployer/simple.go	2014-02-11 02:38:51 +0000
@@ -96,6 +96,7 @@
 	logger.Debugf("API addresses: %q", result.APIAddresses)
 	containerType := ctx.agentConfig.Value(agent.ContainerType)
 	namespace := ctx.agentConfig.Value(agent.Namespace)
+	machineTag := ctx.AgentConfig().Tag()
 	conf, err := agent.NewAgentConfig(
 		agent.AgentConfigParams{
 			DataDir:  dataDir,
@@ -109,6 +110,7 @@
 			Values: map[string]string{
 				agent.ContainerType: containerType,
 				agent.Namespace:     namespace,
+				agent.MachineTag:    machineTag,
 			},
 		})
 	if err != nil {

=== modified file 'worker/deployer/simple_test.go'
--- worker/deployer/simple_test.go	2013-11-21 03:22:49 +0000
+++ worker/deployer/simple_test.go	2014-02-11 02:38:51 +0000
@@ -188,8 +188,11 @@
 	c.Assert(fis, gc.HasLen, count)
 }
 
+// The machine to which units are deployed.
+const machineTag = "machine-tag"
+
 func (fix *SimpleToolsFixture) getContext(c *gc.C) *deployer.SimpleContext {
-	config := agentConfig("machine-tag", fix.dataDir)
+	config := agentConfig(machineTag, fix.dataDir)
 	return deployer.NewTestSimpleContext(config, fix.initDir, fix.logDir, fix.syslogConfigDir)
 }
 
@@ -257,6 +260,7 @@
 	c.Assert(err, gc.IsNil)
 	c.Assert(conf.Tag(), gc.Equals, tag)
 	c.Assert(conf.DataDir(), gc.Equals, fix.dataDir)
+	c.Assert(conf.Value("MACHINE_TAG"), gc.Equals, machineTag)
 
 	jujudData, err := ioutil.ReadFile(jujudPath)
 	c.Assert(err, gc.IsNil)