Asterisk round-robin memory trunk group

Uniform round-robin (circular) distribution of calls through several trunks. The trunk group is defined in the local asterisk database. Trunks can be identified by any alphanumeric key in the same family. Add and delete in any order.

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MariaDB [realtimedb]> SELECT * FROM dp_trunks ORDER BY exten, priority;
+----+---------+--------+----------+--------+--------------------------------------+
| id | context | exten  | priority | app    | appdata                              |
+----+---------+--------+----------+--------+--------------------------------------+
| 14 | trunks  | DELETE |        1 | SET    | DB_DELETE(LAST/NEXT)=ignored         |
| 16 | trunks  | DELETE |        2 | RETURN |                                      |
|  1 | trunks  | _X.    |        1 | gotoif | ${DB_EXISTS(LAST/NEXT)}?3            |
| 20 | trunks  | _X.    |        2 | SET    | DB(LAST/NEXT)=1                      |
|  4 | trunks  | _X.    |        3 | SET    | arr=${DB_KEYS(TRUNKGROUP)}           |
| 17 | trunks  | _X.    |        4 | SET    | MAX=${FIELDQTY(arr,\,)}                      |
| 18 | trunks  | _X.    |        5 | SET    | i=${DB(LAST/NEXT)}                   |
| 11 | trunks  | _X.    |        6 | SET    | START=${CUT(arr,\,,${i})}            |
|  2 | trunks  | _X.    |        7 | SET    | trunkname=${DB(TRUNKGROUP/${START})} |
|  3 | trunks  | _X.    |        8 | dial   | SIP/${trunkname}/${EXTEN},10,gF      |
| 12 | trunks  | _X.    |        9 | SET    | i=$[${i} + 1]                        |
| 13 | trunks  | _X.    |       10 | gotoif | $[${i}=${MAX}]?DELETE,1              |
| 19 | trunks  | _X.    |       11 | SET    | DB(LAST/NEXT)=${i}                   |
| 15 | trunks  | _X.    |       12 | RETURN |                                      |
+----+---------+--------+----------+--------+--------------------------------------+

function 'FIELDQTY', function 'DB', Asterisk function 'CUT', function 'POP'

*CLI> database show TRUNKGROUP
/TRUNKGROUP/1                           : trunkname1               
/TRUNKGROUP/2                           : trunkname2               
/TRUNKGROUP/5                           : trunkname3               
/TRUNKGROUP/8                           : trunkname4
/TRUNKGROUP/99                          : empty                
5 results found.

Call the trunk group:

 exten => _X.,1,GoSub(sub-context,${EXTEN},1)