Ubuntu Manpage: ovsdb - Open vSwitch Database (Overview)

Provided by: openvswitch-common_3.3.0-1ubuntu3.1_amd64

NAME

       ovsdb - Open vSwitch Database (Overview)

DESCRIPTION

OVSDB, the Open vSwitch Database, is a network-accessible database system. Schemas in OVSDB specify the
tables in a database and their columns’ types and can include data, uniqueness, and referential integrity
constraints. OVSDB offers atomic, consistent, isolated, durable transactions. RFC 7047 specifies the
JSON-RPC based protocol that OVSDB clients and servers use to communicate.

The OVSDB protocol is well suited for state synchronization because it allows each client to monitor the
contents of a whole database or a subset of it. Whenever a monitored portion of the database changes,
the server tells the client what rows were added or modified (including the new contents) or deleted.
Thus, OVSDB clients can easily keep track of the newest contents of any part of the database.

While OVSDB is general-purpose and not particularly specialized for use with Open vSwitch, Open vSwitch
does use it for multiple purposes. The leading use of OVSDB is for configuring and monitoring
ovs-vswitchd(8), the Open vSwitch switch daemon, using the schema documented in ovs-vswitchd.conf.db(5).
The Open Virtual Network (OVN) project uses two OVSDB schemas, documented as part of that project.
Finally, Open vSwitch includes the “VTEP” schema, documented in vtep(5) that many third-party hardware
switches support for configuring VXLAN, although OVS itself does not directly use this schema.

The OVSDB protocol specification allows independent, interoperable implementations of OVSDB to be
developed. Open vSwitch includes an OVSDB server implementation named ovsdb-server(1), which supports
several protocol extensions documented in its manpage, and a basic command-line OVSDB client named
ovsdb-client(1), as well as OVSDB client libraries for C and for Python. Open vSwitch documentation
often speaks of these OVSDB implementations in Open vSwitch as simply “OVSDB,” even though that is
distinct from the OVSDB protocol; we make the distinction explicit only when it might otherwise be
unclear from the context.

In addition to these generic OVSDB server and client tools, Open vSwitch includes tools for working with
databases that have specific schemas: ovs-vsctl works with the ovs-vswitchd configuration database and
vtep-ctl works with the VTEP database.

RFC 7047 specifies the OVSDB protocol but it does not specify an on-disk storage format. Open vSwitch
includes ovsdb-tool(1) for working with its own on-disk database formats. The most notable feature of
this format is that ovsdb-tool(1) makes it easy for users to print the transactions that have changed a
database since the last time it was compacted. This feature is often useful for troubleshooting.

SCHEMAS

Schemas in OVSDB have a JSON format that is specified in RFC 7047. They are often stored in files with
an extension .ovsschema. An on-disk database in OVSDB includes a schema and data, embedding both into a
single file. The Open vSwitch utility ovsdb-tool has commands that work with schema files and with the
schemas embedded in database files.

An Open vSwitch schema has three important identifiers. The first is its name, which is also the name
used in JSON-RPC calls to identify a database based on that schema. For example, the schema used to
configure Open vSwitch has the name Open_vSwitch. Schema names begin with a letter or an underscore,
followed by any number of letters, underscores, or digits. The ovsdb-tool commands schema-name and
db-name extract the schema name from a schema or database file, respectively.

An OVSDB schema also has a version of the form x.y.z e.g. 1.2.3. Schemas managed within the Open vSwitch
project manage version numbering in the following way (but OVSDB does not mandate this approach).
Whenever we change the database schema in a non-backward compatible way (e.g. when we delete a column or
a table), we increment <x> and set <y> and <z> to 0. When we change the database schema in a backward
compatible way (e.g. when we add a new column), we increment <y> and set <z> to 0. When we change the
database schema cosmetically (e.g. we reindent its syntax), we increment <z>. The ovsdb-tool commands
schema-version and db-version extract the schema version from a schema or database file, respectively.

Very old OVSDB schemas do not have a version, but RFC 7047 mandates it.

An OVSDB schema optionally has a “checksum.” RFC 7047 does not specify the use of the checksum and
recommends that clients ignore it. Open vSwitch uses the checksum to remind developers to update the
version: at build time, if the schema’s embedded checksum, ignoring the checksum field itself, does not
match the schema’s content, then it fails the build with a recommendation to update the version and the
checksum. Thus, a developer who changes the schema, but does not update the version, receives an
automatic reminder. In practice this has been an effective way to ensure compliance with the version
number policy. The ovsdb-tool commands schema-cksum and db-cksum extract the schema checksum from a
schema or database file, respectively.

SERVICE MODELS

OVSDB supports four service models for databases: standalone, active-backup, relay and clustered. The
service models provide different compromises among consistency, availability, and partition tolerance.
They also differ in the number of servers required and in terms of performance. The standalone and
active-backup database service models share one on-disk format, and clustered databases use a different
format, but the OVSDB programs work with both formats. ovsdb(5) documents these file formats. Relay
databases have no on-disk storage.

RFC 7047, which specifies the OVSDB protocol, does not mandate or specify any particular service model.

The following sections describe the individual service models.

Standalone Database Service Model
A standalone database runs a single server. If the server stops running, the database becomes
inaccessible, and if the server’s storage is lost or corrupted, the database’s content is lost. This
service model is appropriate when the database controls a process or activity to which it is linked via
“fate-sharing.” For example, an OVSDB instance that controls an Open vSwitch virtual switch daemon,
ovs-vswitchd, is a standalone database because a server failure would take out both the database and the
virtual switch.

To set up a standalone database, use ovsdb-tool create to create a database file, then run ovsdb-server
to start the database service.

To configure a client, such as ovs-vswitchd or ovs-vsctl, to use a standalone database, configure the
server to listen on a “connection method” that the client can reach, then point the client to that
connection method. See Connection Methods below for information about connection methods.

Open vSwitch 3.3 introduced support for configuration files via --config-file command line option. The
configuration file for a server with a standalone database may look like this:

{
"remotes": { "<connection method>": {} },
"databases": { "<database file>": {} }
}

ovsdb-server will infer the service model from the database file itself. However, if additional
verification is desired, an optional "service-model": "standalone" can be provided for the database file
inside the inner curly braces. If the specified service-model will not match the content of the database
file, ovsdb-server will refuse to open this database.

Active-Backup Database Service Model
An active-backup database runs two servers (on different hosts). At any given time, one of the servers
is designated with the active role and the other the backup role. An active server behaves just like a
standalone server. A backup server makes an OVSDB connection to the active server and uses it to
continuously replicate its content as it changes in real time. OVSDB clients can connect to either
server but only the active server allows data modification or lock transactions.

Setup for an active-backup database starts from a working standalone database service, which is initially
the active server. On another node, to set up a backup server, create a database file with the same
schema as the active server. The initial contents of the database file do not matter, as long as the
schema is correct, so ovsdb-tool create will work, as will copying the database file from the active
server. Then use ovsdb-server --sync-from=<active> to start the backup server, where <active> is an
OVSDB connection method (see Connection Methods below) that connects to the active server. At that
point, the backup server will fetch a copy of the active database and keep it up-to-date until it is
killed.

Open vSwitch 3.3 introduced support for configuration files via --config-file command line option. The
configuration file for a backup server in this case may look like this:

{
"remotes": { "<connection method>": {} },
"databases": {
"<database file>": {
"service-model": "active-backup",
"backup": true,
"source": {
"<active>": {
"inactivity-probe": <integer>,
"max-backoff": <integer>
}
}
}
}
}

All the fields in the "<database file>" description above are required. Options for the "<active>"
connection method ("inactivity-probe", etc.) can be omitted.

When the active server in an active-backup server pair fails, an administrator can switch the backup
server to an active role with the ovs-appctl command ovsdb-server/disconnect-active-ovsdb-server.
Clients then have read/write access to the now-active server. When the --config-file is in use, the same
can be achieved by changing the "backup" value in the file and running ovsdb-server/reload command. Of
course, administrators are slow to respond compared to software, so in practice external management
software detects the active server’s failure and changes the backup server’s role. For example, the
“Integration Guide for Centralized Control” in the OVN documentation describes how to use Pacemaker for
this purpose in OVN.

Suppose an active server fails and its backup is promoted to active. If the failed server is revived, it
must be started as a backup server. Otherwise, if both servers are active, then they may start out of
sync, if the database changed while the server was down, and they will continue to diverge over time.
This also happens if the software managing the database servers cannot reach the active server and
therefore switches the backup to active, but other hosts can reach both servers. These “split-brain”
problems are unsolvable in general for server pairs.

Compared to a standalone server, the active-backup service model somewhat increases availability, at a
risk of split-brain. It adds generally insignificant performance overhead. On the other hand, the
clustered service model, discussed below, requires at least 3 servers and has greater performance
overhead, but it avoids the need for external management software and eliminates the possibility of
split-brain.

Open vSwitch 2.6 introduced support for the active-backup service model.

IMPORTANT:
There was a change of a database file format in version 2.15. To upgrade/downgrade the ovsdb-server
processes across this version follow the instructions described under Upgrading from version 2.14 and
earlier to 2.15 and later and Downgrading from version 2.15 and later to 2.14 and earlier.

Another change happened in version 3.2. To upgrade/downgrade the ovsdb-server processes across this
version follow the instructions described under Upgrading from version 3.1 and earlier to 3.2 and
later and Downgrading from version 3.2 and later to 3.1 and earlier.

Clustered Database Service Model
A clustered database runs across 3 or 5 or more database servers (the cluster) on different hosts.
Servers in a cluster automatically synchronize writes within the cluster. A 3-server cluster can remain
available in the face of at most 1 server failure; a 5-server cluster tolerates up to 2 failures.
Clusters larger than 5 servers will also work, with every 2 added servers allowing the cluster to
tolerate 1 more failure, but write performance decreases. The number of servers should be odd: a 4- or
6-server cluster cannot tolerate more failures than a 3- or 5-server cluster, respectively.

To set up a clustered database, first initialize it on a single node by running ovsdb-tool
create-cluster, then start ovsdb-server. Depending on its arguments, the create-cluster command can
create an empty database or copy a standalone database’s contents into the new database.

Open vSwitch 3.3 introduced support for configuration files via --config-file command line option. The
configuration file for a server with a clustered database may look like this:

{
"remotes": { "<connection method>": {} },
"databases": { "<database file>": {} }
}

ovsdb-server will infer the service model from the database file itself. However, if additional
verification is desired, an optional "service-model": "clustered" can be provided for the database file
inside the inner curly braces. If the specified service-model will not match the content of the database
file, ovsdb-server will refuse to open this database.

To configure a client to use a clustered database, first configure all of the servers to listen on a
connection method that the client can reach, then point the client to all of the servers’ connection
methods, comma-separated. See Connection Methods, below, for more detail.

Open vSwitch 2.9 introduced support for the clustered service model.

How to Maintain a Clustered Database
To add a server to a cluster, run ovsdb-tool join-cluster on the new server and start ovsdb-server. To
remove a running server from a cluster, use ovs-appctl to invoke the cluster/leave command. When a
server fails and cannot be recovered, e.g. because its hard disk crashed, or to otherwise remove a server
that is down from a cluster, use ovs-appctl to invoke cluster/kick to make the remaining servers kick it
out of the cluster.

The above methods for adding and removing servers only work for healthy clusters, that is, for clusters
with no more failures than their maximum tolerance. For example, in a 3-server cluster, the failure of 2
servers prevents servers joining or leaving the cluster (as well as database access). To prevent data
loss or inconsistency, the preferred solution to this problem is to bring up enough of the failed servers
to make the cluster healthy again, then if necessary remove any remaining failed servers and add new
ones. If this cannot be done, though, use ovs-appctl to invoke cluster/leave --force on a running
server. This command forces the server to which it is directed to leave its cluster and form a new
single-node cluster that contains only itself. The data in the new cluster may be inconsistent with the
former cluster: transactions not yet replicated to the server will be lost, and transactions not yet
applied to the cluster may be committed. Afterward, any servers in its former cluster will regard the
server to have failed.

Once a server leaves a cluster, it may never rejoin it. Instead, create a new server and join it to the
cluster.

The servers in a cluster synchronize data over a cluster management protocol that is specific to Open
vSwitch; it is not the same as the OVSDB protocol specified in RFC 7047. For this purpose, a server in a
cluster is tied to a particular IP address and TCP port, which is specified in the ovsdb-tool command
that creates or joins the cluster. The TCP port used for clustering must be different from that used for
OVSDB clients. To change the port or address of a server in a cluster, first remove it from the cluster,
then add it back with the new address.

To upgrade the ovsdb-server processes in a cluster from one version of Open vSwitch to another, upgrading
them one at a time will keep the cluster healthy during the upgrade process. (This is different from
upgrading a database schema, which is covered later under Upgrading or Downgrading a Database.)

Clustered OVSDB does not support the OVSDB “ephemeral columns” feature. ovsdb-tool and ovsdb-client
change ephemeral columns into persistent ones when they work with schemas for clustered databases.
Future versions of OVSDB might add support for this feature.

Upgrading from version 2.14 and earlier to 2.15 and later
There is a change of a database file format in version 2.15 that doesn’t allow older versions of
ovsdb-server to read the database file modified by the ovsdb-server version 2.15 or later. This also
affects runtime communications between servers in active-backup and cluster service models. To upgrade
the ovsdb-server processes from one version of Open vSwitch (2.14 or earlier) to another (2.15 or higher)
instructions below should be followed. (This is different from upgrading a database schema, which is
covered later under Upgrading or Downgrading a Database.)

In case of standalone service model no special handling during upgrade is required.

For the active-backup service model, administrator needs to update backup ovsdb-server first and the
active one after that, or shut down both servers and upgrade at the same time.

For the cluster service model recommended upgrade strategy is following:

1. Upgrade processes one at a time. Each ovsdb-server process after upgrade should be started with
--disable-file-column-diff command line argument.

2. When all ovsdb-server processes upgraded, use ovs-appctl to invoke ovsdb/file/column-diff-enable
command on each of them or restart all ovsdb-server processes one at a time without
--disable-file-column-diff command line option.

Downgrading from version 2.15 and later to 2.14 and earlier
Similar to upgrading covered under Upgrading from version 2.14 and earlier to 2.15 and later, downgrading
from the ovsdb-server version 2.15 and later to 2.14 and earlier requires additional steps. (This is
different from upgrading a database schema, which is covered later under Upgrading or Downgrading a
Database.)

For all service models it’s required to:

1. Stop all ovsdb-server processes (single process for standalone service model, all involved processes
for active-backup and cluster service models).

2. Compact all database files with ovsdb-tool compact command.

3. Downgrade and restart ovsdb-server processes.

Upgrading from version 3.1 and earlier to 3.2 and later
There is another change of a database file format in version 3.2 that doesn’t allow older versions of
ovsdb-server to read the database file modified by the ovsdb-server version 3.2 or later. This also
affects runtime communications between servers in cluster service models. To upgrade the ovsdb-server
processes from one version of Open vSwitch (3.1 or earlier) to another (3.2 or higher) instructions below
should be followed. (This is different from upgrading a database schema, which is covered later under
Upgrading or Downgrading a Database.)

In case of standalone or active-backup service model no special handling during upgrade is required.

For the cluster service model recommended upgrade strategy is following:

1. Upgrade processes one at a time. Each ovsdb-server process after upgrade should be started with
--disable-file-no-data-conversion command line argument.

2. When all ovsdb-server processes upgraded, use ovs-appctl to invoke
ovsdb/file/no-data-conversion-enable command on each of them or restart all ovsdb-server processes one
at a time without --disable-file-no-data-conversion command line option.

Downgrading from version 3.2 and later to 3.1 and earlier
Similar to upgrading covered under Upgrading from version 3.1 and earlier to 3.2 and later, downgrading
from the ovsdb-server version 3.2 and later to 3.1 and earlier requires additional steps. (This is
different from upgrading a database schema, which is covered later under Upgrading or Downgrading a
Database.)

For all service models it’s required to:

1. Compact all database files via ovsdb-server/compact command with ovs-appctl utility. This should be
done for each involved ovsdb-server process separately (single process for standalone service model,
all involved processes for active-backup and cluster service models).

2. Stop all ovsdb-server processes. Make sure that no database schema conversion operations were
performed between steps 1 and 2. For standalone and active-backup service models, the database
compaction can be performed after stopping all the processes instead with the ovsdb-tool compact
command.

3. Downgrade and restart ovsdb-server processes.

Understanding Cluster Consistency
To ensure consistency, clustered OVSDB uses the Raft algorithm described in Diego Ongaro’s Ph.D. thesis,
“Consensus: Bridging Theory and Practice”. In an operational Raft cluster, at any given time a single
server is the “leader” and the other nodes are “followers”. Only the leader processes transactions, but
a transaction is only committed when a majority of the servers confirm to the leader that they have
written it to persistent storage.

In most database systems, read and write access to the database happens through transactions. In such a
system, Raft allows a cluster to present a strongly consistent transactional interface. OVSDB uses
conventional transactions for writes, but clients often effectively do reads a different way, by asking
the server to “monitor” a database or a subset of one on the client’s behalf. Whenever monitored data
changes, the server automatically tells the client what changed, which allows the client to maintain an
accurate snapshot of the database in its memory. Of course, at any given time, the snapshot may be
somewhat dated since some of it could have changed without the change notification yet being received and
processed by the client.

Given this unconventional usage model, OVSDB also adopts an unconventional clustering model. Each server
in a cluster acts independently for the purpose of monitors and read-only transactions, without verifying
that data is up-to-date with the leader. Servers forward transactions that write to the database to the
leader for execution, ensuring consistency. This has the following consequences:

• Transactions that involve writes, against any server in the cluster, are linearizable if clients take
care to use correct prerequisites, which is the same condition required for linearizability in a
standalone OVSDB. (Actually, “at-least-once” consistency, because OVSDB does not have a session
mechanism to drop duplicate transactions if a connection drops after the server commits it but before
the client receives the result.)

• Read-only transactions can yield results based on a stale version of the database, if they are executed
against a follower. Transactions on the leader always yield fresh results. (With monitors, as
explained above, a client can always see stale data even without clustering, so clustering does not
change the consistency model for monitors.)

• Monitor-based (or read-heavy) workloads scale well across a cluster, because clustering OVSDB adds no
additional work or communication for reads and monitors.

• A write-heavy client should connect to the leader, to avoid the overhead of followers forwarding
transactions to the leader.

• When a client conducts a mix of read and write transactions across more than one server in a cluster,
it can see inconsistent results because a read transaction might read stale data whose updates have not
yet propagated from the leader. By default, utilities such as ovn-sbctl (in OVN) connect to the
cluster leader to avoid this issue.

The same might occur for transactions against a single follower except that the OVSDB server ensures
that the results of a write forwarded to the leader by a given server are visible at that server before
it replies to the requesting client.

• If a client uses a database on one server in a cluster, then another server in the cluster (perhaps
because the first server failed), the client could observe stale data. Clustered OVSDB clients,
however, can use a column in the _Server database to detect that data on a server is older than data
that the client previously read. The OVSDB client library in Open vSwitch uses this feature to avoid
servers with stale data.

Relay Service Model
A relay database is a way to scale out read-mostly access to the existing database working in any service
model including relay.

Relay database creates and maintains an OVSDB connection with another OVSDB server. It uses this
connection to maintain an in-memory copy of the remote database (a.k.a. the relay source) keeping the
copy up-to-date as the database content changes on the relay source in the real time.

The purpose of relay server is to scale out the number of database clients. Read-only transactions and
monitor requests are fully handled by the relay server itself. For the transactions that request
database modifications, relay works as a proxy between the client and the relay source, i.e. it forwards
transactions and replies between them.

Compared to the clustered and active-backup models, relay service model provides read and write access to
the database similarly to a clustered database (and even more scalable), but with generally insignificant
performance overhead of an active-backup model. At the same time it doesn’t increase availability that
needs to be covered by the service model of the relay source.

Relay database has no on-disk storage and therefore cannot be converted to any other service model.

If there is already a database started in any service model, to start a relay database server use
ovsdb-server relay:<DB_NAME>:<relay source>, where <DB_NAME> is the database name as specified in the
schema of the database that existing server runs, and <relay source> is an OVSDB connection method (see
Connection Methods below) that connects to the existing database server. <relay source> could contain a
comma-separated list of connection methods, e.g. to connect to any server of the clustered database.
Multiple relay servers could be started for the same relay source.

Open vSwitch 3.3 introduced support for configuration files via --config-file command line option. The
configuration file for a relay database server in this case may look like this:

{
"remotes": { "<connection method>": {} },
"databases": {
"<DB_NAME>": {
"service-model": "relay",
"source": {
"<relay source>": {
"inactivity-probe": <integer>,
"max-backoff": <integer>
}
}
}
}
}

Both the "service-model" and the "source" are required. Options for the "<relay source>" connection
method ("inactivity-probe", etc.) can be omitted.

Since the way relays handle read and write transactions is very similar to the clustered model where
“cluster” means “set of relay servers connected to the same relay source”, “follower” means “relay
server” and the “leader” means “relay source”, same consistency consequences as for the clustered model
applies to relay as well (See Understanding Cluster Consistency above).

Open vSwitch 2.16 introduced support for relay service model.

DATABASE REPLICATION

       OVSDB can layer replication on top of any of its service models.  Replication, in this context, means  to
       make,  and  keep  up-to-date,  a  read-only copy of the contents of a database (the replica).  One use of
       replication is to keep an up-to-date backup of a database.  A replica used solely for  backup  would  not
       need  to  support clients of its own.  A set of replicas that do serve clients could be used to scale out
       read access to the primary database, however Relay Service Model is more suitable for that purpose.

       A database replica is set up in the same way as a backup  server  in  an  active-backup  pair,  with  the
       difference that the replica is never promoted to an active role.

       A database can have multiple replicas.

       Open vSwitch 2.6 introduced support for database replication.

CONNECTION METHODS

       An  OVSDB connection method is a string that specifies how to make a JSON-RPC connection between an OVSDB
       client and server.  Connection methods are part of the Open  vSwitch  implementation  of  OVSDB  and  not
       specified  by  RFC  7047.   ovsdb-server  uses  connection  methods  to  specify how it should listen for
       connections from clients and ovsdb-client uses them to  specify  how  it  should  connect  to  a  server.
       Connections  in  the  opposite  direction,  where ovsdb-server connects to a client that is configured to
       listen for an incoming connection, are also possible.

       Connection methods are classified as active or passive.  An active connection method  makes  an  outgoing
       connection  to a remote host; a passive connection method listens for connections from remote hosts.  The
       most common arrangement is to configure an OVSDB server with passive connection methods and clients  with
       active ones, but the OVSDB implementation in Open vSwitch supports the opposite arrangement as well.

       OVSDB supports the following active connection methods:

       ssl:<host>:<port>
              The specified SSL or TLS <port> on the given <host>.

       tcp:<host>:<port>
              The specified TCP <port> on the given <host>.

       unix:<file>
              On Unix-like systems, connect to the Unix domain server socket named <file>.

              On Windows, connect to a local named pipe that is represented by a file created in the path <file>
              to mimic the behavior of a Unix domain socket.

       <method1>,<method2>,…,<methodN>
              For a clustered database service to be highly available, a client must be able to connect  to  any
              of  the  servers  in  the  cluster.   To do so, specify connection methods for each of the servers
              separated by commas (and optional spaces).

              In theory, if machines go up and down and IP addresses change in the right  way,  a  client  could
              talk  to  the wrong instance of a database.  To avoid this possibility, add cid:<uuid> to the list
              of methods, where <uuid> is the cluster  ID  of  the  desired  database  cluster,  as  printed  by
              ovsdb-tool db-cid.  This feature is optional.

       OVSDB supports the following passive connection methods:

       pssl:<port>[:<ip>]
              Listen  on the given TCP <port> for SSL or TLS connections.  By default, connections are not bound
              to a particular local IP address.  Specifying <ip> limits connections to those from the given IP.

       ptcp:<port>[:<ip>]
              Listen on the given TCP <port>.  By default, connections are not bound to a  particular  local  IP
              address.  Specifying <ip> limits connections to those from the given IP.

       punix:<file>
              On Unix-like systems, listens for connections on the Unix domain socket named <file>.

              On Windows, listens on a local named pipe, creating a named pipe <file> to mimic the behavior of a
              Unix domain socket. The ACLs of the named pipe include LocalSystem,  Administrators,  and  Creator
              Owner.

       All  IP-based  connection methods accept IPv4 and IPv6 addresses.  To specify an IPv6 address, wrap it in
       square brackets, e.g.  ssl:[::1]:6640.  Passive IP-based connection methods by default  listen  for  IPv4
       connections  only; use [::] as the address to accept both IPv4 and IPv6 connections, e.g. pssl:6640:[::].
       DNS names are also accepted if built with unbound library.  On Linux, use %<device> to designate a  scope
       for IPv6 link-level addresses, e.g. ssl:[fe80::1234%eth0]:6653.

       The  <port>  may  be  omitted  from  connection  methods  that use a port number.  The default <port> for
       TCP-based connection methods is 6640, e.g. pssl: is equivalent to pssl:6640.  In Open  vSwitch  prior  to
       version  2.4.0, the default port was 6632.  To avoid incompatibility between older and newer versions, we
       encourage users to specify a port number.

       The  ssl  and  pssl  connection  methods  requires  additional   configuration   through   --private-key,
       --certificate,  and  --ca-cert  command  line options.  Open vSwitch can be built without SSL support, in
       which case these connection methods are not supported.

DATABASE LIFE CYCLE

This section describes how to handle various events in the life cycle of a database using the Open
vSwitch implementation of OVSDB.

Creating a Database
Creating and starting up the service for a new database was covered separately for each database service
model in the Service Models section, above. A single ovsdb-server process may serve any number of
databases with different service models at the same time.

Backing Up and Restoring a Database
OVSDB is often used in contexts where the database contents are not particularly valuable. For example,
in many systems, the database for configuring ovs-vswitchd is essentially rebuilt from scratch at boot
time. It is not worthwhile to back up these databases.

When OVSDB is used for valuable data, a backup strategy is worth considering. One way is to use database
replication, discussed above in Database Replication which keeps an online, up-to-date copy of a
database, possibly on a remote system. This works with all OVSDB service models.

A more common backup strategy is to periodically take and store a snapshot. For the standalone and
active-backup service models, making a copy of the database file, e.g. using cp, effectively makes a
snapshot, and because OVSDB database files are append-only, it works even if the database is being
modified when the snapshot takes place. This approach does not work for clustered databases.

Another way to make a backup, which works with all OVSDB service models, is to use ovsdb-client backup,
which connects to a running database server and outputs an atomic snapshot of its schema and content, in
the same format used for standalone and active-backup databases.

Multiple options are also available when the time comes to restore a database from a backup. For the
standalone and active-backup service models, one option is to stop the database server or servers,
overwrite the database file with the backup (e.g. with cp), and then restart the servers. Another way,
which works with any service model, is to use ovsdb-client restore, which connects to a running database
server and replaces the data in one of its databases by a provided snapshot. The advantage of
ovsdb-client restore is that it causes zero downtime for the database and its server. It has the
downside that UUIDs of rows in the restored database will differ from those in the snapshot, because the
OVSDB protocol does not allow clients to specify row UUIDs.

None of these approaches saves and restores data in columns that the schema designates as ephemeral.
This is by design: the designer of a schema only marks a column as ephemeral if it is acceptable for its
data to be lost when a database server restarts.

Clustering and backup serve different purposes. Clustering increases availability, but it does not
protect against data loss if, for example, a malicious or malfunctioning OVSDB client deletes or tampers
with data.

Changing Database Service Model
Use ovsdb-tool create-cluster to create a clustered database from the contents of a standalone database.
Use ovsdb-client backup to create a standalone database from the contents of a running clustered
database. When the cluster is down and cannot be revived, ovsdb-client backup will not work.

Use ovsdb-tool cluster-to-standalone to convert clustered database to standalone database when the
cluster is down and cannot be revived.

Upgrading or Downgrading a Database
The evolution of a piece of software can require changes to the schemas of the databases that it uses.
For example, new features might require new tables or new columns in existing tables, or conceptual
changes might require a database to be reorganized in other ways. In some cases, the easiest way to deal
with a change in a database schema is to delete the existing database and start fresh with the new
schema, especially if the data in the database is easy to reconstruct. But in many other cases, it is
better to convert the database from one schema to another.

The OVSDB implementation in Open vSwitch has built-in support for some simple cases of converting a
database from one schema to another. This support can handle changes that add or remove database columns
or tables or that eliminate constraints (for example, changing a column that must have exactly one value
into one that has one or more values). It can also handle changes that add constraints or make them
stricter, but only if the existing data in the database satisfies the new constraints (for example,
changing a column that has one or more values into a column with exactly one value, if every row in the
column has exactly one value). The built-in conversion can cause data loss in obvious ways, for example
if the new schema removes tables or columns, or indirectly, for example by deleting unreferenced rows in
tables that the new schema marks for garbage collection.

Converting a database can lose data, so it is wise to make a backup beforehand.

To use OVSDB’s built-in support for schema conversion with a standalone or active-backup database, first
stop the database server or servers, then use ovsdb-tool convert to convert it to the new schema, and
then restart the database server.

OVSDB also supports online database schema conversion for any of its database service models. To convert
a database online, use ovsdb-client convert. The conversion is atomic, consistent, isolated, and
durable. ovsdb-server disconnects any clients connected when the conversion takes place (except clients
that use the set_db_change_aware Open vSwitch extension RPC). Upon reconnection, clients will discover
that the schema has changed.

Schema versions and checksums (see Schemas above) can give hints about whether a database needs to be
converted to a new schema. If there is any question, though, the needs-conversion command on ovsdb-tool
and ovsdb-client can provide a definitive answer.

Working with Database History
Both on-disk database formats that OVSDB supports are organized as a stream of transaction records. Each
record describes a change to the database as a list of rows that were inserted or deleted or modified,
along with the details. Therefore, in normal operation, a database file only grows, as each change
causes another record to be appended at the end. Usually, a user has no need to understand this file
structure. This section covers some exceptions.

Compacting Databases
If OVSDB database files were truly append-only, then over time they would grow without bound. To avoid
this problem, OVSDB can compact a database file, that is, replace it by a new version that contains only
the current database contents, as if it had been inserted by a single transaction. From time to time,
ovsdb-server automatically compacts a database that grows much larger than its minimum size.

Because ovsdb-server automatically compacts databases, it is usually not necessary to compact them
manually, but OVSDB still offers a few ways to do it. First, ovsdb-tool compact can compact a standalone
or active-backup database that is not currently being served by ovsdb-server (or otherwise locked for
writing by another process). To compact any database that is currently being served by ovsdb-server, use
ovs-appctl to send the ovsdb-server/compact command. Each server in an active-backup or clustered
database maintains its database file independently, so to compact all of them, issue this command
separately on each server.

Viewing History
The ovsdb-tool utility’s show-log command displays the transaction records in an OVSDB database file in a
human-readable format. By default, it shows minimal detail, but adding the option -m once or twice
increases the level of detail. In addition to the transaction data, it shows the time and date of each
transaction and any “comment” added to the transaction by the client. The comments can be helpful for
quickly understanding a transaction; for example, ovs-vsctl adds its command line to the transactions
that it makes.

The show-log command works with both OVSDB file formats, but the details of the output format differ.
For active-backup and clustered databases, the sequence of transactions in each server’s log will differ,
even at points when they reflect the same data.

Truncating History
It may occasionally be useful to “roll back” a database file to an earlier point. Because of the
organization of OVSDB records, this is easy to do. Start by noting the record number <i> of the first
record to delete in ovsdb-tool show-log output. Each record is two lines of plain text, so trimming the
log is as simple as running head -n <j>, where <j> = 2 * <i>.

Corruption
When ovsdb-server opens an OVSDB database file, of any kind, it reads as many transaction records as it
can from the file until it reaches the end of the file or it encounters a corrupted record. At that
point it stops reading and regards the data that it has read to this point as the full contents of the
database file, effectively rolling the database back to an earlier point.

Each transaction record contains an embedded SHA-1 checksum, which the server verifies as it reads a
database file. It detects corruption when a checksum fails to verify. Even though SHA-1 is no longer
considered secure for use in cryptography, it is acceptable for this purpose because it is not used to
defend against malicious attackers.

The first record in a standalone or active-backup database file specifies the schema. ovsdb-server will
refuse to work with a database where this record is corrupted, or with a clustered database file with
corruption in the first few records. Delete and recreate such a database, or restore it from a backup.

When ovsdb-server adds records to a database file in which it detected corruption, it first truncates the
file just after the last good record.

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