Module 10: CDB Subscribers

1©2015 Tail-f Systems all rights reserved February 13, 2015

CDB Subscribers


Triggers when

anything in

scope changes

Subscribe to Configuration Changes

subsock = socket(PF_INET, SOCK_STREAM, 0);

cdb_connect(subsock, CDB_SUBSCRIPTION_SOCKET, &confd, size);

cdb_subscribe(subsock, 3, dhcpd__ns, &spoint, "/dhcp");

cdb_subscribe_done(subsock);

/ dhcp/

aaa/

voip/

defaultLe…

maxLeas…

SubNets/ subNet/

mask

range

lowAddr

highAddr

logFacility

net

Managed Objects typically have several subscription points


Enter Event Loop

• Event loop needs to monitor subscription socket

• Typically using poll()/select()

while(1) {

set[0].fd = subsock;

set[0].events = POLLIN;

set[0].revents = 0;

poll(set, 1, -1); // Monitor one socket, wait forever

if (set[0].revents & POLLIN) {

cdb_read_subscription_socket(subsock, sub_points, &reslen);

// React on changes here…

// sub_points[] contains triggering subscription points,

// reslen how many

}

cdb_sync_subscription_socket(subsock, CDB_DONE_PRIORITY);

}


React to Configuration Changes

• Triggered subscription means something in scope changed, but will not tell us what (nor to what value)

• To find out more, read all or start a diff walk

/ dhcp/

aaa/

voip/

defaultLe…

maxLeas…

SubNets/ subNet/

mask

range

lowAddr

highAddr

logFacility

net?

?



Left extreme:

On any change, read all configuration• Linux daemon style

•Simple to program

•Simple to test

•Maximum code reuse

•Robust

Right extreme:

On any change, look at every changed element• NET-SNMP agent style

• In some cases very much more efficient



Left extreme:

void startup() {

init_obj_A();

init_obj_B();

init_obj_C();

}

void handle_change() {

if(trigger_A) init_obj_A();

if(trigger_B) init_obj_B();

if(trigger_C) init_obj_C();

}

Right extreme:

void startup() {

init_obj_A();

init_obj_B();

init_obj_C();

}


cdb_diff_iterate(…iterator…);

}

void iterator(…keypath…) {

if(keypath is field /dhcp/logFacility)

...

if(keypath is field /…) …



No need to be extreme

Use balanced approach

Walk the diff when it makes sense, but not down to individual leaf elements

Use multiple subscription points

• Simple to program with high degree of reuse

• Simple to test

• Robust

• Efficient



void startup() {

init_obj_A();

n = cdb_num_instances(“/B”);

for(i=0; i<n; i++)

init_obj_B(i);

init_obj_C();

}


if(trigger_A) init_obj_A();

if(trigger_B)

cdb_diff_iterate(…iterator_B…)

if(trigger_C) init_obj_C();

}

void iterator_B(…keypath…) {

table_row = extract_row(keypath);

init_obj_B(table_row);

return ITER_CONTINUE;

}


Walking the Diff in a Balanced Way

Find the right level of granularity for your subscriptions and diff walks

/ dhcp/

aaa/

voip/

defaultLe…

maxLeas…

SubNets/ subNet/

mask

range

lowAddr

highAddr

logFacility

net



Event loop with balanced approach

while(1) {



set[0].revents = 0;




for(i=0; i<reslen; i++) {

if(…) cdb_diff_iterate(subsock, sub_points[i], iter, ITER_WANT_PREV, 0);

else if(…) init_A();

}

}


}



An iterator function might look like this

static enum cdb_iter_ret iter(confd_hkeypath_t *kp, enum cdb_iter_op op,

confd_value_t *oldv, confd_value_t *newv, void *state)

{

switch (op) {

case MOP_CREATED: {

confd_value_t *ctag = &kp->v[1][0];

switch (CONFD_GET_XMLTAG(ctag)) {

case root_RFHead: { // an rfhead was created

// keypath is /root/NodeB/RFHead{$key}

// 3 2 1 0

confd_value_t *hkey = &kp->v[0][0];

read_head(cdbsock, hkey);




• Each item seen by the iterator function is one of

• MOP_CREATED – container or leaf created

• MOP_DELETED – container or leaf deleted

• MOP_MODIFIED – container where a child element has been created/deleted/set

• MOP_VALUE_SET – leaf that has been set

• MOP_MOVED_AFTER – list element that was moved

• Each time iterator function returns

• ITER_RECURSE – examine changes to child elements

• ITER_CONTINUE – skip changes to child elements

• ITER_UP – skip changes to child and sibling elements

• ITER_SUSPEND – suspend the iteration temporarily

• ITER_STOP – stop iteration now


int main(…) {

cdb_subscribe(subsock, 3, root__ns,

&headpoint,"/root/NodeB/RFHead”);

…


cdb_diff_iterate(…, iter, …);

}

}

static enum cdb_iter_ret iter(…) {

…

switch (op) {

case MOP_CREATED: {

fprintf(stderr, "Create: %s\n",…);

…


Diff Walk Example

container root {

container NodeB {

list RFHead {

key dn;

leaf dn { type int64; }

leaf SECTORID_ID {

type string; default "N/A";

}

list Child {

key cdn;

leaf cdn { type int64; }

leaf childAttr {

type string; default "N/A";

}

}

}

}

}


Create Object with Attribute

% set root NodeB RFHead 2 SECTOR_ID 2

% commit

Create: /root/NodeB/RFHead{2}

ITER_CONTINUE


Modify Attribute of Object

% set root NodeB RFHead 2 SECTOR_ID 5

% commit

Modified /root/NodeB/RFHead{2}

ITER_RECURSE

Value Set: /root/NodeB/RFHead{2}/SECTORID_ID --> (5)

ITER_CONTINUE


Create Object with Child Object

% set root NodeB RFHead 3 Child 1 childAttr 1

% commit


ITER_CONTINUE


Create additional Child Object



% commit


ITER_RECURSE

Create: /root/NodeB/RFHead{3}/Child{2}

ITER_CONTINUE


ITER_CONTINUE


Set Attribute of Child Object


% commit


ITER_RECURSE

Modified /root/NodeB/RFHead{3}/Child{2}

ITER_RECURSE

Value Set: /root/NodeB/RFHead{3}/Child{2}/childAttr --> (32)

ITER_CONTINUE


Delete Child Object

% delete root NodeB RFHead 3 Child 1

% commit


ITER_RECURSE

Delete: /root/NodeB/RFHead{3}/Child{1}

ITER_CONTINUE


Delete Everything

% delete root

% commit

Delete: /root/NodeB/RFHead{2}

ITER_CONTINUE

Delete: /root/NodeB/RFHead{3}

ITER_CONTINUE


Hashed Key Path (hkeypath)

• To make it easy to program (not necessarily learn)

• To make it efficient

• Encoded as two dimensional array

/root/NodeB/RFHead{1} would be encoded

• kp[0][0] contains ‘1’

• kp[1][0] contains ‘RFHead’

[0][] [1][] [2][] [3][] [4][]

[][0] INT32‘1’

XMLTAG ‘RFHead’ XMLTAG‘NodeB’

XMLTAG‘root’

NOEXISTS

[][1] NOEXISTS NOEXISTS NOEXISTS NOEXISTS NOEXISTS



• The second index is for multiple keys

/routes/route{12.60.0.0 16}/enabled would be encoded

• kp[1][0] contains ’12.60.0.0’

• kp[1][1] contains ‘16’

• There may be up to 8 keys in any table

• I.e. second index ranges from 0 up to possibly 7

[0][] [1][] [2][] [3][] [4][]

[][0]

XMLTAG‘enabled’

IPV4‘12.60.0.0’

XMLTAG‘route’

XMLTAG‘routes’

NOEXISTS

[][1]

NOEXISTS INT8’16’

NOEXISTS NOEXISTS NOEXISTS



• Sequence of XMLTAG hash values

• An XMLTAG is encoded as a 31-bit integer

• Not as strings: variable length, inefficient with string compare

• Hash function ensures same string always gets same value

• Collisions are rare (we have never seen one to date)

• Use idValue attribute in data model in case of collision

• This allows efficient and easy switch-case statements

switch (CONFD_GET_XMLTAG(ctag)) {

case root_RFHead: { // an rfhead was created

• Header file contains all the hash-constants

• Header file generated from data model using confdc


Ensuring Changes happen in Order

• Consider transaction A

• Add interface eth5Add route 55.66.0.0/18 over interface eth5

• Transactions are all-at-once, there is no internal ordering

• Transaction A is therefore equivalent to

• Add route 55.66.0.0/24 over interface eth5Add interface eth5



• Consider transaction A

• Add interface eth5Add route 55.66.0.0/18 over interface eth5

• Still, in our code, the interface subsystem must be informed of this change before the routing subsystem

• The routing subsystem will call the interface subsystem referencing eth5

• This dependency information is encoded in

cdb_subscribe(subsock, 20, dhcpd__ns, &spoint, ”/interfaces");

cdb_subscribe(subsock, 30, dhcpd__ns, &spoint, ”/routes");




• Consider then transaction B

• Delete interface eth5Delete route 55.66.0.0/24 over interface eth5

• In this case the ordering must be reversed

• Again, the routing subsystem will call the interface subsystem and reference eth5

• This is handled by adding more subscription points

// This subscription handles additions and modification of interfaces

cdb_subscribe(subsock, 20, dhcpd__ns, &spoint1, ”/interfaces");

cdb_subscribe(subsock, 30, dhcpd__ns, &spoint2, ”/routes");

// This subscription handles deletes of interfaces

cdb_subscribe(subsock, 40, dhcpd__ns, &spoint3, ”/interfaces");




Event loop with balanced approachwhile(1) {



set[0].revents = 0;




for(i=0; i<reslen; i++) {

if(…) cdb_diff_iterate(subsock, sub_points[i], iter, ITER_WANT_PREV, 0);

else if(…) init_A();

}

}


}


Application Initialization / Start Order

• Read, Subscribe

• Protect using read session or operators locked out

• Potentially more efficient than trigger method

• Subscribe, Each application triggers its own subscriptions

• Must be able to handle all configuration changes anyway

• Subscribe, Startup process triggers all subscriptions

• Good at system start when all processes start at the same time

• Subscribe, System process triggers restarting app subscriptions

• Can handle all scenarios


Yang Validation

Application Validation

PreparePhase

Notify Applications

Commit Phase

Point of No Return

After the Point of No Return the transaction cannot be aborted.

• Make sure you validate properly before Point of No Return

• If an application fails after this point, treat it the same regardless of it happening 2 ms/minutes/weeks after commit

Operator types/clicks

commit

Operator gets OK and next prompt

Point of No Return


Yang Validation

Application Validation

PreparePhase

Notify Applications

Commit Phase

Validators and Two Phase Subscribers

Resource reservation can be handled by aTwo Phase Subscriber

• Called during Prepare Phase

• Will get an abortedcallback if transactionaborted

• Uses cdb_subscribe() mechanismwith an extra flag

• Don’t activate, just check in prepare

MO

Changes that

might happen

Changes that did

happen


ConfD CDB Subscription Handling Examples

• examples.confd/intro/1-2-3-start-query-model

• examples.confd/cdb_subscription/iter

• examples.confd/cdb_subscription/trigger

• examples.confd/cdb_subscription/twophase


Date post:	18-Jul-2015
Category:	Technology
Upload:	tail-f-systems
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Module 10: CDB Subscribers

Technology