Kicker

Introduction

Kickers constitutes a declarative notification mechanism for triggering actions on certain stimuli like a database change or a received notification. These different stimuli and their kickers are defined separately as data-kicker and notification-kicker respectively.

Common to all types of kickers is that they are declarative. Kickers are modeled in YANG and Kicker instances stored as configuration data in CDB.

Immediately after a transaction which defines a new kicker is committed the kicker will be active. The same holds for removal. This also implies that the amount of programming for a kicker is a matter of implementing the action to be invoked.

The data-kicker replicates much of the functionality otherwise attained by a CDB subscriber. Without the extra coding in registration and runtime daemon that comes with a CDB subscriber. The data-kicker works for all data providers.

The notification-kicker reacts on notifications received by NSO using a defined notification subscription under /ncs:devices/device/notifications/subscription. This simplifies handling of southbound emitted notifications. Traditionally these where choosen to be stored in CDB as operational data and a separate CDB subscriber was used to act on the received notifications. With the use of notification-kicker the CDB subscriber can be removed and there is no longer any need to store the received notification in CDB.

Kicker action invocation

An action as defined by YANG contains an input parameter definition and an output parameter definition. However a kicker that invokes an action treats the input parameters in a specific way.

The kicker mechanism first checks if the input parameters matches those in the kicker:action-input-params YANG grouping defined in the tailf-kicker.yang file. If so the action will be invoked with the input parameters:

id: The id (name) of the invoking kicker.
monitor: The path of the current monitor triggering the kicker
tid: The transaction id to a synthetic transaction containing the changes that lead to the triggering of the kicker.

The "synthetic" transaction implies that this is a copy of the original transaction that lead to the kicker triggering. It only contains the data tree under the monitor. The original transaction is already committed and this data might no longer reflect the "running" datastore. Its useful in that the action implementation can attach and diff-iterate over this transaction and retrieve the certain changes that lead to the kicker invocation.

If the kicker mechanism finds an action that do not match the above input parameters it will invoke the action with an empty parameter list. This implies that an kicker action must either match the above kicker:action-input-params grouping precisely or accept an empty incoming parameter list. Otherwise the action invocation will fail.

Data Kicker Concepts

For a Data Kicker the following principles hold:

Kicker are triggered by changes in the sub-tree indicated by the monitor. potentially triggers a Kicker.
Actions are invoked during the commit phase. Hence an aborted transaction never trigger kickers.
No distinction is made between configuration and operational data.
No distinction is made between CRUD types, i.e. create, delete, update. All changes potentially trigger kickers.
Kickers may have constraints that suppress invocations. Changes in the sub-tree indicated by monitor is a necessary but perhaps not a sufficient condition for the action to be invoked.

Generalized Monitors

For a Data Kicker it is the monitor that specifies which subtree under which a change should invoke the kicker. The monitor leaf is of type node-instance-identifier which means that predicates for keys are optional, i.e. keys may be omitted and then represent all instances for that key.

The resulting evaluation of the monitor defines a node-set. Each node in this node-set will be root context for any further xpath evaluations necessary before invoking the kicker action.

The following example shows the strengths of using xpath to define the kickers. Say that we have a situation described by the following YANG model snippet:

module example {
  namespace "http://tail-f.com/ns/test/example";
  prefix example;

  ...

  container sys {
    list ifc {
      key name;
      max-elements 64;
      leaf name {
        type interfaceName;
      }
      leaf description {
        type string;
      }
      leaf enabled {
        type boolean;
        default true;
      }
      container hw {
        leaf speed {
          type interfaceSpeed;
        }
        leaf duplex {
          type interfaceDuplex;
        }
        leaf mtu {
          type mtuSize;
        }
        leaf mac {
          type string;
        }
      }
      list ip {
        key address;
        max-elements 1024;
        leaf address {
          type inet:ipv4-address;
        }
        leaf prefix-length {
          type prefixLengthIPv4;
          mandatory true;
        }
        leaf broadcast {
          type inet:ipv4-address;
        }
      }

      tailf:action local_me {
        tailf:actionpoint kick-me-point;
        input {
        }
        output {
        }
      }
    }

    tailf:action kick_me {
      tailf:actionpoint kick-me-point;
      input {
      }
      output {
      }
    }

    tailf:action iter_me {
      tailf:actionpoint kick-me-point;
      input {
        uses kicker:action-input-params;
      }
      output {
      }
    }

  }
}

Then we can define a kicker for monitoring a specific element in the list and calling the correlated local_me action:

admin@ncs(config)# kickers data-kicker e1 \
> monitor /sys/ifc[name='port-0'] \
>kick-node /sys/ifc[name='port-0']\
> action-name local_me

admin(config-data-kicker-e1)# commit
Commit complete
admin(config-data-kicker-e1)# top
admin@ncs(config)#  show full-configuration kickers
kickers data-kicker e1
 monitor     /sys/ifc[name='port-0']
 kick-node   /sys/ifc[name='port-0']
 action-name local_me
!

On the other hand we can define a kicker for monitoring all elements of the list and and call the correlated local_me action for each element:

admin@ncs(config)# kickers data-kicker e2 \
> monitor /sys/ifc \
>kick-node . \
> action-name local_me

admin(config-data-kicker-e2)# commit
Commit complete
admin(config-data-kicker-e2)# top
admin@ncs(config)#  show full-configuration kickers
kickers data-kicker e2
 monitor     /sys/ifc
 kick-node   .
 action-name local_me
!

Here the "." in the kick-node refer to the current node in the node-set defined by the monitor.

Kicker Constraints/Filters

A Data Kicker may be constrained by adding conditions that suppress invocations. The leaf trigger-expression contains a boolean XPath expression that is evaluated twice, before and after the change-set of the commit has been applied to the database(s).

The Xpath expression has to evaluated twice in order to detect the change caused by the transaction.

The two boolean results together with the leaf trigger-type controls if the Kicker should be triggered or not:

enter-and-leave: false -> true (i.e. positive flank) or true -> false (negative flank)
enter: false -> true

admin(config)# kickers data-kicker k1 monitor /sys/ifc \
> trigger-expr "hw/mtu > 800" \
> trigger-type enter \
> kick-node /sys \
> action-name kick_me
admin(config-data-kicker-k1)# commit
Commit complete
admin(config-data-kicker-k1)# top
admin@ncs%
admin@ncs% show kickers
kickers data-kicker k1
 monitor      /sys/ifc
 trigger-expr "hw/mtu > 800"
 trigger-type enter
 kick-node    /sys
 action-name  kick_me
!

Start by changing the MTU to 800:

admin(config)# sys ifc port-0 hw mtu 800
admin(config-ifc-port-0)# commit | debug kicker
 2017-02-15T16:35:36.039 kicker: k1 at /kicker_example:sys/kicker_example:ifc[kicker_example:name='port-0'] changed;
not invoking 'kick_me' trigger-expr false -> false
Commit complete.

Since the trigger-expression evaluates to false, the kicker is not triggered. Let's try again:

admin(config)# sys ifc port-0 hw mtu 801
admin(config-ifc-port-0)# commit | debug kicker
 2017-02-15T16:35:36.039 kicker: k1 at /kicker_example:sys/kicker_example:ifc[kicker_example:name='port-0'] changed;
invoking 'kick-me' trigger-expr false -> true
Commit complete.

Variable Bindings

A Data Kicker may be provided with a list of variables (named values). Each variable binding consists of a name and a XPath expression. The Xpath expressions are evaluated on-demand, i.e. when used in either of monitor or trigger-expression nodes.

admin@ncs(config)# set kickers data-kicker k3 monitor $PATH/c
                         kick-node /x/y[id='n1']
                         action-name kick-me
                         variable PATH value "/a/b[k1=3][k2='3']"
admin@ncs(config)#

In the example above PATH is defined and refered to by the monitor expression by using the expression $PATH.

Note

A monitor expression is not evaluated by the XPath engine. Hence no trace of the evaluation can be found in the the Xpath log.

Monitor expressions are expanded and installed in an internal data-structure at kicker creation/compile time. XPath may be used while defining kickers by referring to a named XPath expression.

A Simple Data Kicker Example

This example is part of the examples.ncs/web-server-farm/web-site-service example. It consists of an action and a README_KICKER file. For all kickers defined in this example the same action is used. This action is defined in the website-service package. The following is the yang snippet for the action definition from the website.yang file:

module web-site {
  namespace "http://examples.com/web-site";
  prefix wse;

  ...

  augment /ncs:services {

    ...

    container actions {
      tailf:action diffcheck {
        tailf:actionpoint diffcheck;
        input {
          uses kicker:action-input-params;
        }
        output {
        }
      }
    }
  }

}

The implementation of the action can be found in the WebSiteServiceRFS.java class file. Since it takes the kicker:action-input-params as input, the "Tid" for the synthetic transaction is available. This transaction is attached and diff-iterated. The result of the diff-iteration is printed in the ncs-java-vm.log:

class WebSiteServiceRFS {

    ....

    @ActionCallback(callPoint="diffcheck", callType=ActionCBType.ACTION)
    public ConfXMLParam[] diffcheck(DpActionTrans trans, ConfTag name,
                                   ConfObject[] kp, ConfXMLParam[] params)
    throws DpCallbackException {
        try {

            System.out.println("-------------------");
            System.out.println(params[0]);
            System.out.println(params[1]);
            System.out.println(params[2]);

            ConfUInt32 val = (ConfUInt32) params[2].getValue();
            int tid = (int)val.longValue();

            Socket s3 = new Socket("127.0.0.1", Conf.NCS_PORT);
            Maapi maapi3 = new Maapi(s3);
            maapi3.attach(tid, -1);

            maapi3.diffIterate(tid, new MaapiDiffIterate() {
                // Override the Default iterate function in the TestCase class
                public DiffIterateResultFlag iterate(ConfObject[] kp,
                                                     DiffIterateOperFlag op,
                                                     ConfObject oldValue,
                                                     ConfObject newValue,
                                                     Object initstate) {
                    System.out.println("path = " + new ConfPath(kp));
                    System.out.println("op = " + op);
                    System.out.println("newValue = " + newValue);
                    return DiffIterateResultFlag.ITER_RECURSE;

                }

            });


            maapi3.detach(tid);
            s3.close();


        return new ConfXMLParam[]{};

        } catch (Exception e) {
            throw new DpCallbackException("diffcheck failed", e);
        }
    }
}

We are now ready to start the website-service example and define our data-kicker. Do the following:

$ make all
$ ncs-netsim start
$ ncs
$ ncs_cli -C -u admin

admin@ncs# devices sync-from
sync-result {
    device lb0
    result true
}
sync-result {
    device www0
    result true
}
sync-result {
    device www1
    result true
}
sync-result {
    device www2
    result true
}

The kickers are defined under the hide-group "debug". To be able to show and declare kickers we need first to unhide this hide-group:

admin@ncs# config
admin@ncs(config)# unhide debug

We now define a data-kicker for the "profile" list under the by the service augmented container "/services/properties/wsp:web-site":

admin@ncs(config)# kickers data-kicker a1 \
> monitor /services/properties/wsp:web-site/profile \
> kick-node /services/wse:actions action-name diffcheck

admin@ncs(config-data-kicker-a1)# commit
admin@ncs(config-data-kicker-a1)# top
admin@ncs(config)# show full-configuration kickers data-kicker a1
kickers data-kicker a1
 monitor     /services/properties/wsp:web-site/profile
 kick-node   /services/wse:actions
 action-name diffcheck
!

We now commit a change in the profile list and we use the "debug kicker" pipe option to be able to follow the kicker invokation:

admin@ncs(config)# services properties web-site profile lean lb lb0
admin@ncs(config-profile-lean)# commit | debug kicker
 2017-02-15T16:35:36.039 kicker: a1 at /ncs:services/ncs:properties/wsp:web-site/wsp:profile[wsp:name='lean'] changed; invoking diffcheck
Commit complete.

admin@ncs(config-profile-lean)# top
admin@ncs(config)# exit

We can also check the result of the action by looking into the ncs-java-vm.log:

admin@ncs# file show logs/ncs-java-vm.log

In the end we will find the following printout from the diffcheck action:

-------------------
{[669406386|id], a1}
{[669406386|monitor], /ncs:services/properties/web-site/profile{lean}}
{[669406386|tid], 168}
path = /ncs:services/properties/wsp:web-site/profile{lean}
op = MOP_CREATED
newValue = null
path = /ncs:services/properties/wsp:web-site/profile{lean}/name
op = MOP_VALUE_SET
newValue = lean
path = /ncs:services/properties/wsp:web-site/profile{lean}/lb
op = MOP_VALUE_SET
newValue = lb0
[ok][2017-02-15 17:11:59]

Notification Kicker Concepts

For a Notification Kicker the following principles hold:

Notification Kickers are triggered by the arrival of notifications from any device subscription. These subscriptions are defined under the /devices/device/notification/subscription path.
Storing the received notifications in CDB is optional and not part of the notification kicker functionality.
The kicker invocations are serialized under a certain subscription i.e. kickers are invoked in the same sequence as notifications are received for the same subscription. This means that invocations are queued up and executed as quickly as the action permits.

Notification selector expression

The notification kicker is defined using a mandatory "selector-expr" which is an XPATH 1.0 expression. When the notification is received a synthetic transaction is started and the notification is written as if it would be stored under the path /devices/device/notification/received-notifications/data. Actually storing the notification in CDB is optional. The selector-expr is evaluated with the notification node as the current context and '/' as the root context. For example, if the device model defines a notification like this:

module device {
  ...
  notification mynotif {
    leaf message {
      type string;
    }
  }
  ...
}

the notification node 'mynotif' will be the current context for the selector-expr There are four predefined variable bindings used when evaluating this expression:

DEVICE: The name of the device emitting the current notification.
SUBSCRIPTION_NAME: The name of the current subscription from which the notification was received. the kicker
NOTIFICATION_NAME: The name of the current notification.
NOTIFICATION_NS: The namespace of the current notification.

The selector-expr technique for defining the notification kickers is very flexible. For instance a kicker can be defined:

To receive all notifications for a device.
To receive all notifications of a certain type for any device.
To receive a subset of notifications of a subset of devices by the use of specific subscriptions with the same name in several devices.

In addition to this usage of the predefined variable bindings it is possible to further drill down into the specific notification to trigger on certain leafs in the notification.

Variable Bindings

In addition to the four variable bindings mentioned above, a Notification Kicker may also be provided with a list of variables (named values). Each variable binding consists of a name and a XPath expression. The Xpath expression is evaluated when the selector-expr is run.

            admin@ncs(config)# set kickers notification-kicker k4
            selector-expr "$NOTIFICATION_NAME=linkUp and address[ip=$IP]"
            kick-node /x/y[id='n1']
            action-name kick-me
            variable IP value '192.168.128.55'
admin@ncs(config)#

In the example above PATH is defined and refered to by the monitor expression by using the expression $PATH.

Note

A monitor expression is not evaluated by the XPath engine. Hence no trace of the evaluation can be found in the the Xpath log.

Monitor expressions are expanded and installed in an internal data-structure at kicker creation/compile time. XPath may be used while defining kickers by referring to a named XPath expression.

A Simple Notification Kicker Example

In this example we use the same action and setup as in the Data kicker example above. The procedure for starting is also the same.

The website-service example has devices that has notifications generated on the stream "interface". We start with defining the notification kicker for a certain SUBSCRIPTION_NAME = "mysub". This subscription does not exist for the moment and the kicker will therefore not be triggered:

admin@ncs# config

admin@ncs(config)# kickers notification-kicker n1 \
> selector-expr "$SUBSCRIPTION_NAME = 'mysub'" \
> kick-node /services/wse:actions \
> action-name diffcheck

admin@ncs(config-notification-kicker-n1)# commit
admin@ncs(config-notification-kicker-n1)# top

admin@ncs(config)# show full-configuration kickers notification-kicker n1
kickers notification-kicker n1
 selector-expr "$SUBSCRIPTION_NAME = 'mysub'"
 kick-node     /services/wse:actions
 action-name   diffcheck
!

Now we define the "mysub" subscription on a device "www0" and refer to the notification stream "interface". As soon as this definition is committed the kicker will start triggering:

admin@ncs(config)# devices device www0 notifications subscription mysub \
> local-user admin stream interface
admin@ncs(config-subscription-mysub)# commit

admin@ncs(config-profile-lean)# top
admin@ncs(config)# exit

If we now inspect the ncs-java-vm.log we will see a number of notifications that are received. We also see that the transaction that is diff-iterated contains the notification as data under the path /devices/device/notifications/received-notifications/notification/data. This is a operational data list. However this transaction is synthetic and will not be committed. If the notification will be stored CDB is optional and not depending on the notification kicker functionality:

admin@ncs# file show logs/ncs-java-vm.log

-------------------
{[669406386|id], n1}
{[669406386|monitor], /ncs:devices/device{www0}/notifications.../data/linkUp}
{[669406386|tid], 758}
path = /ncs:devices/device{www0}
op = MOP_MODIFIED
newValue = null
path = /ncs:devices/device{www0}/notifications...
op = MOP_CREATED
newValue = null
path = /ncs:devices/device{www0}/notifications.../event-time
op = MOP_VALUE_SET
newValue = 2017-02-15T16:35:36.039204+00:00
path = /ncs:devices/device{www0}/notifications.../sequence-no
op = MOP_VALUE_SET
newValue = 0
path = /ncs:devices/device{www0}/notifications.../data/notif:linkUp
op = MOP_CREATED
newValue = null
path = /ncs:devices/device{www0}/notifications.../data/notif:linkUp/address{192.168.128.55}
op = MOP_CREATED
newValue = null
path = /ncs:devices/device{www0}/notifications.../data/notif:linkUp/address{192.168.128.55}/ip
op = MOP_VALUE_SET
newValue = 192.168.128.55
path = /ncs:devices/device{www0}/notifications.../data/notif:linkUp/address{192.168.128.55}/mask
op = MOP_VALUE_SET
newValue = 255.255.255.0
path = /ncs:devices/device{www0}/notifications.../data/notif:linkUp/ifName
op = MOP_VALUE_SET
newValue = eth2
path = /ncs:devices/device{www0}/notifications.../data/notif:linkUp/linkProperty{0}
op = MOP_CREATED
newValue = null
path = /ncs:devices/device{www0}/notifications.../data/notif:linkUp/linkProperty{0}/extensions{0}
op = MOP_CREATED
newValue = 4668
path = /ncs:devices/device{www0}/notifications.../data/notif:linkUp/linkProperty{0}/extensions{1}/name
op = MOP_VALUE_SET
newValue = 2
path = /ncs:devices/device{www0}/notifications.../data/notif:linkUp/linkProperty{0}/flags
op = MOP_VALUE_SET
newValue = 42
path = /ncs:devices/device{www0}/notifications.../data/notif:linkUp/linkProperty{0}/newlyAdded
op = MOP_CREATED
newValue = null

We end by removing the kicker and the subscription:

admin@ncs# config
admin@ncs(config)# no kickers notification-kicker
admin@ncs(config)# no devices device www0 notifications subscription
admin@ncs(config)# commit

Reactive FastMap with Kicker

This example illustrates how to write a reactive FastMap application using the Kicker. The idea is to let the service code create a kicker which redeploys a service when something happens.

Use Cases

Kickers are very often used as an implementation technique for Reactive FastMap services. Assume an NFV/ESC based application which:

Asks ESC to start a VM
Once the VM is ready, wants to configure the VM

Such an application would create a kicker with a monitor for /devices/device[name=<vmname>]/ready Once the VM is 'ready', the service would be redeployed and it can continue its Reactive FastMap execution further and provision config to the newly started VM. Prior to the kickers, it was common with CDB subscriber code that

Subscribed to some change
Read that change, and then redeployed some service which the CDB subscriber code knew was waiting for that change

Now, with kickers we can simply such code by having a CDB subscriber that simply

Subscribes to some change (for example a device notification listener)
Writes some operational data field somewhere

The RFM service code is then responsible for setting up a kicker with the monitor pointing to that field written by the CDB subscriber. Thus effectively decoupling the CDB subscriber code from the RFM service code making them independent of each other. Another advantage is that the error handling code when the redeploy fails is unified inside the NSO kicker implementation.

RFM Example

The example can be found in examples.ncs/getting-started/developing-with-ncs/21-kicker and uses two NSO packages. The router package introduced in ../0-router-network and a package called 'ppp-accounting' which is described in this section.

The example is a bit contrived, but since want to exemplify the usage of kickers, it's simplified and artificial.

$ ls ./packages
router ppp-accounting

To build the three packages, do

$ make all

To start the ncs-netsim network, follow the instructions in ../0-router-network, it's the same network here.

$ ncs-netsim start
DEVICE ex0 OK STARTED
DEVICE ex1 OK STARTED
DEVICE ex2 OK STARTED

All the code for this example resides in ./packages/ppp-accounting

To run the example we do:

$ ncs

This will start NSO, and NSO will load the two packages, load the data models defined by the two packages and start the Java code defined by the packages.

The service data model we have here looks like:

  list ppp-accounting {

    uses ncs:service-data;
    ncs:servicepoint kickerspnt;

    key "interface";
    leaf interface {
      type string;
    }
  }

  list ppp-accounting-data {
    description "This is helper data, created by the service code for
                 /ppp-accounting";

    key "interface";
    leaf interface {
      type string;
    }
    leaf accounting {
      description "populated externally";
      type string;
    }
  }

The purpose of the service /ppp-accounting is to set the accounting field in the provided ppp interface on all routers in our example network. The catch here is that the name of the 'accounting' field is not provided as in input parameter to the service, instead it is populated externally and read and used by the service code.

The FastMap code tries to read the field /ppp-accounting-data[interface=<if>]/accounting and if it doesn't exist, the code creates a kicker on that field and returns. If the 'accounting' field exists, it used and data is written into the /devices tree for our routers.

To run this we do:

$ make all
$ ncs-netsim start
$ ncs
$ ncs_cli -u admin
admin connected from 127.0.0.1 using console on mac
admin@ncs> request devices sync-from
sync-result {
    device ex0
    result true
}
sync-result {
    device ex1
    result true
}
sync-result {
    device ex2
    result true
}
[ok][2016-12-13 16:18:45]
admin@ncs> configure
Entering configuration mode private
[ok][2016-12-13 16:19:06]
[edit]
admin@ncs% set ppp-accounting ppp0
[ok][2016-12-13 16:20:01]

[edit]
admin@ncs% commit
Commit complete.
[ok][2016-12-13 16:20:04]

[edit]
admin@ncs% request ppp-accounting ppp0 get-modifications
cli {
    local-node {
        data
    }
}

We created the service, and verified that it didn't do anything. Looking at the code in packages/ppp-accounting/src/java/src/com/example/kicker/KickerServiceRFS.java we can see though that the code created a kicker.

Let's take a look at that:

admin@ncs% show kickers
---------------------------------^
syntax error: element does not exist
[error][2016-12-13 16:22:53]

The kicker data is hidden, and we cannot directly view it in the CLI. The src/ncs/yang/tailf-kicker.yang file says:

  container kickers {
    tailf:info "Kicker specific configuration";
    tailf:hidden debug;

    list data-kicker {
      key id;

      ......

To view the kickers data we must do two things:

Provide an entry in the ncs.conf file
Unhide in the CLI

See the section called “Unhide Kickers” for details.

And now the kicker container is visible:

admin@ncs%$ show kickers
data-kicker ncs-internal-side-effects {
    monitor     /ncs:side-effect-queue;
    kick-node   /ncs:side-effect-queue;
    action-name invoke;
}
data-kicker ppp-accounting-ppp0 {
    monitor     /ppp-accounting-data[interface='ppp0']/accounting;
    kick-node   /ppp-accounting[interface='ppp0'];
    action-name reactive-re-deploy;
}

There we can see our newly created kicker.

To trigger this kicker, which will then execute the redeploy on the /ppp-accounting[interface='ppp0'] service, all we need to do is to assign some data to the field that is monitored by the kicker.

admin@ncs%$ set ppp-accounting-data ppp0 accounting radius
[ok][2016-12-13 16:26:43]

[edit]
admin@ncs%$ commit
Commit complete.
[ok][2016-12-13 16:26:46]

[edit]

admin@ncs%$ request ppp-accounting ppp0 get-modifications
cli {
    local-node {
        data  devices {
                   device ex0 {
                       config {
                           r:sys {
                               interfaces {
                                   serial ppp0 {
                                       ppp {
              -                            accounting acme;
              +                            accounting radius;
                                       }
                                   }
                               }
                           }
                       }
                   }
                   device ex1 {
                       config {
                           r:sys {
                               interfaces {
                                   serial ppp0 {
                                       ppp {
              -                            accounting acme;
              +                            accounting radius;


             ..........

Note

Looking at the RFM java code we see that the /ppp-accounting-data help entry is created by a so called PRE_MODIFICATION hook. This is a common trick in RFM applications. We don't want that data to be part of the FastMap diffset. Usually the help entry is also used to contain various 'config false' fields pertaining to the service instance. If that data was part of FastMap diffset, the data would disappear with every redeploy turn, thus we use the PRE_MODIFICATION trick.

Debugging kickers

Kicker CLI Debug target

In order to find out why a Kicker kicked when it shouldn't or more commonly and annoying, why it didn't kick when it should, use the CLI pipe debug kicker.

Evaluation of potential Kicker invocations are reported in the CLI together with XPath evaluation results:

admin@ncs(config)# set sys ifc port-0 hw mtu 8000
admin@ncs(config)# commit | debug kicker
 2017-02-15T16:35:36.039 kicker: k1 at /kicker_example:sys/kicker_example:ifc[kicker_example:name='port-0'] changed;
not invoking 'kick-me' trigger-expr false -> false
Commit complete.
admin@ncs(config)#

Unhide Kickers

The top level container kickers is by default invisible due to a hidden attribute. In order to make kickers visible in the CLI, two steps are required. First the following XML snippet must be added to ncs.conf:

<hide-group>
  <name>debug</name>
</hide-group>

Now the unhide command may be used in the CLI session:

admin@ncs(config)# unhide debug
admin@ncs(config)#

XPath log

Detailed information from the XPath evaluator can be enabled and made available in the xpath log. Add the following snippet to ncs.conf.

<xpathTraceLog>
  <enabled>true</enabled>
  <filename>./xpath.trace</filename>
</xpathTraceLog>

Devel Log

Error information is written to the development log. The development log is meant to be used as support while developing the application. It is enabled in ncs.conf:

Example 144. Enabling the developer log

<developer-log>
  <enabled>true</enabled>
  <file>
    <name>./logs/devel.log</name>
     <enabled>true</enabled>
  </file>
</developer-log>
<developer-log-level>trace</developer-log-level>