---
title: "DB_VIRTUAL Module"
---

## Admin Guide


### Overview


#### The idea


A virtual DB will expose the same front DB api however, it will
backed by many real DB. This means that a virtual DB URL 
translates to many real DB URLs. This virtual layer also 
enables us to use the real dbs in multiple ways such as: 
parallel, failover(hotswap) and round-robin.

Therefore:
each virtual DB URL with associated real dbs and
a way to use(mode) it's real dbs must be specified.


#### Modes


The implemented modes are:


- FAILOVER - Use the first URL; if it fails, take the next URL and redo the operation.
- PARALLEL - Use all the URLs in the virtual DB URL set. Fails if all the URLs fail.
- ROUND (round-robin) - Use the next URL each time; if it fails, use the next one, redo operation.


When choosing the db virtual mode, be sure that there is a full
compatibility between the DB operations you want to do (inserts, 
updates, deletes,...) and the relation (if any) between the real
DB URLs you have in the set - can be completely independent, can be
nodes of the same cluster, or any other combination.


#### Capabilities


For each set (or new virtual DB URL), the capabilities are
automatically calculated based on the capabilities provided by the
real DB URLs from the set. A logical AND is done for each
cabability over all the URLs in the set. Shortly, in order for the
virtual URL to provide a certain capability, ALL its real URLs 
must provide that capability.


> [!NOTE]
> Starting with version 2.2 db_virtual supports
> async_raw_query and async_raw_resume functions currently
> implemented only by the mysql database engine.


#### Failures


```c
When an operation from a process on a real DB fails:
	it is marked (global and local CAN flag down)
	its connection closed

Later a timer process (probe):
foreach virtual db_url
	foreach real db_url
		if global CAN down
			try to connect
		if ok
			global CAN up
			close connection

Later each process:
	if local CAN down and global CAN up
		if db_max_consec_retrys *
			try to connect
	if ok
		local CAN up

```


> [!NOTE]
> There could be inconsistencies between the probe and each process so a retry limit is in order.
> It is reset and ignored by an MI command.


#### The timer process


The timer process(probe) is a process that tries to reconnect to failed dbs from time to time.
It is a separate process so that when it blocks (for a timeout on the connection) it doesn't matter.


### Dependencies


#### OpenSIPS Modules


The following modules must be loaded before this module:


- *At least one real DB module*.


#### External Libraries or Applications


The following libraries or applications must be installed before running
OpenSIPS with this module loaded:


- *None*.


### Exported Parameters


#### db_urls (str)


Multiple value parameter used for virtual DB URLs declaration.


```opensips title="Set db_urls parameter"
...

modparam("group","db_url","virtual://set1")
modparam("presence|presence_xml", "db_url","virtual://set2")

modparam("db_virtual", "db_urls", "define set1 PARALLEL")
modparam("db_virtual", "db_urls", "mysql://opensips:opensipsrw@localhost/testa")
modparam("db_virtual", "db_urls", "postgres://opensips:opensipsrw@localhost/opensips")

modparam("db_virtual", "db_urls", "define set2 FAILOVER")
modparam("db_virtual", "db_urls", "mysql://opensips:opensipsrw@localhost/testa")
...
```


#### db_probe_time (integer)


Time interval after which a registered timer process attempts to check
failed(as reported by other processes) connections to real dbs. The probe will connect and
disconnect to the failed real DB and announce others.


*Default value is 10 (10 sec).*


```opensips title="Set db_probe_time parameter"
...
modparam("db_virtual", "db_probe_time", 20)
...
```


#### db_max_consec_retrys (integer)


After the timer process has reported that it can connect to the real db,
other processes will try to reconnect to it. There are cases where although
the probe could connect some might fail. This parameter represents the number
of consecutive failed retries that a process will do before it gives up.
This value is reset and suppressed by a MI function (db_virtual:set).


*Default value is 10 (10 consecutive times).*


```opensips title="Set db_max_consec_retrys parameter"
...
modparam("db_virtual", "db_max_consec_retrys", 20)
...

```


### Exported MI Functions


#### db_virtual:get


Replaces obsolete MI command: *db_get*.


Return information about global state of the real dbs.


Name:
*db_virtual:get*


Parameters:


- None.


MI FIFO Command Format:


```bash
opensips-cli -x mi db_virtual:get

```


#### db_virtual:set


Replaces obsolete MI command: *db_set*.


Sets the permissions for real dbs access per set per db.


Sets the reconnect reset flag.


Name:
*db_virtual:set*


Parameters:


- set_index [int]
- db_url_index [int]
- may_use_db_flag [boolean]
- ignore_retries[boolean]\(optional\)


db_virtual:set 3 2 0 1 means:


- 3 - the fourth set (must exist)
- 2 - the third URL in the fourth set(must exist)
- 0 - processes are not allowed to use that URL
- 1 - reset and suppress db_max_consec_retrys


MI FIFO Command Format:


```bash
opensips-cli -x mi db_virtual:set 3 2 0 1

```
<!-- CONTRIBUTORS -->

### License

All documentation files (i.e. .md extension) are licensed under the Creative Common License 4.0
