Adding a new OVN feature to the DDlog version of ovn-northd

This document describes the usual steps an OVN developer should go through when adding a new feature to ovn-northd-ddlog. In order to make things less abstract we will use the IP Multicast ovn-northd-ddlog implementation as an example. Even though the document is structured as a tutorial there might still exist feature-specific aspects that are not covered here.


DDlog is a dataflow system: it receives data from a data source (a set of “input relations”), processes it through “intermediate relations” according to the rules specified in the DDlog program, and sends the processed “output relations” to a data sink. In OVN, the input relations primarily come from the OVN Northbound database and the output relations primarily go to the OVN Southbound database. The process looks like this:

from NBDB  +----------+   +-----------------+   +-----------+  to SBDB
---------->|Input rels|-->|Intermediate rels|-->|Output rels|---------->
           +----------+   +-----------------+   +-----------+

Adding a new feature to ovn-northd-ddlog usually involves the following steps:

  1. Update northbound and/or southbound OVSDB schemas.

  2. Configure DDlog/OVSDB bindings.

  3. Define intermediate DDlog relations and rules to compute them.

  4. Write rules to update output relations.

  5. Generate Logical_Flow``s and/or other forwarding records (e.g., ``Multicast_Group) that will control the dataplane operations.

Update NB and/or SB OVSDB schemas

This step is no different from the normal development flow in C.

Most of the times a developer chooses between two ways of configuring a new feature:

  1. Adding a set of columns to tables in the NB and/or SB database (or adding key-value pairs to existing columns).

  2. Adding new tables to the NB and/or SB database.

Looking at IP Multicast, there are two OVN Northbound tables where configuration information is stored:

  • Logical_Switch, column other_config, keys mcast_*.

  • Logical_Router, column options, keys mcast_*.

These tables become inputs to the DDlog pipeline.

In addition we add a new table IP_Multicast to the SB database. DDlog will update this table, that is, IP_Multicast receives output from the above pipeline.

Configuring DDlog/OVSDB bindings

Configuring northd/

The OVN build process uses DDlog’s ovsdb2ddlog utility to parse ovn-nb.ovsschema and ovn-sb.ovsschema and then automatically populate OVN_Northbound.dl and OVN_Southbound.dl. For each OVN Northbound and Southbound table, it generates one or more corresponding DDlog relations.

We need to supply ovsdb2ddlog with some information that it can’t infer from the OVSDB schemas. This information must be specified as ovsdb2ddlog arguments, which are read from northd/ovn-nb.dlopts and northd/ovn-sb.dlopts.

The main choice for each new table is whether it is used for output. Output tables can also be used for input, but the converse is not true. If the table is used for output at all, we add -o <table> to the option file. Our new table IP_Multicast is an output table, so we add -o IP_Multicast to ovn-sb.dlopts.

For input-only tables, ovsdb2ddlog generates a DDlog input relation with the same name. For output tables, it generates this table plus an output relation named Out_<table>. Thus, OVN_Southbound.dl has two relations for IP_Multicast:

input relation IP_Multicast (
    _uuid: uuid,
    datapath: string,
    enabled: Set<bool>,
    querier: Set<bool>
output relation Out_IP_Multicast (
    _uuid: uuid,
    datapath: string,
    enabled: Set<bool>,
    querier: Set<bool>

For an output table, consider whether only some of the columns are used for output, that is, some of the columns are effectively input-only. This is common in OVN for OVSDB columns that are managed externally (e.g. by a CMS). For each input-only column, we add --ro <table>.<column>. Alternatively, if most of the columns are input-only but a few are output columns, add --rw <table>.<column> for each of the output columns. In our case, all of the columns are used for output, so we do not need to add anything.

Finally, in some cases ovn-northd-ddlog shouldn’t change values in . One such case is the seq_no column in the IP_Multicast table. To do that we need to instruct ovsdb2ddlog to treat the column as read-only by using the --ro switch.

ovsdb2ddlog generates a number of additional DDlog relations, for use by auto-generated OVSDB adapter logic. These are irrelevant to most DDLog developers, although sometimes they can be handy for debugging. See the appendix for details.

Define intermediate DDlog relations and rules to compute them.

Obviously there will be a one-to-one relationship between logical switches/routers and IP multicast configuration. One way to represent this relationship is to create multicast configuration DDlog relations to be referenced by &Switch and &Router DDlog records:

/* IP Multicast per switch configuration. */
relation &McastSwitchCfg(
    datapath      : uuid,
    enabled       : bool,
    querier       : bool

        .datapath = ls_uuid,
        .enabled  = map_get_bool_def(other_config, "mcast_snoop", false),
        .querier  = map_get_bool_def(other_config, "mcast_querier", true)) :-
    nb.Logical_Switch(._uuid        = ls_uuid,
                      .other_config = other_config).

Then reference these relations in &Switch and &Router. For example, in lswitch.dl, the &Switch relation definition now contains:

relation &Switch(
    ls:                nb.Logical_Switch,
    mcast_cfg:         Ref<McastSwitchCfg>

And is populated by the following rule which references the correct McastSwitchCfg based on the logical switch uuid:

&Switch(.ls        = ls,
        .mcast_cfg = mcast_cfg) :-
    mcast_cfg in &McastSwitchCfg(.datapath = ls._uuid).

Build state based on information dynamically updated by ovn-controller

Some OVN features rely on information learned by ovn-controller to generate Logical_Flow or other records that control the dataplane. In case of IP Multicast, ovn-controller uses IGMP to learn multicast groups that are joined by hosts.

Each ovn-controller maintains its own set of records to avoid ownership and concurrency with other controllers. If two hosts that are connected to the same logical switch but reside on different hypervisors (different ovn-controller processes) join the same multicast group G, each of the controllers will create an IGMP_Group record in the OVN Southbound database which will contain a set of ports to which the interested hosts are connected.

At this point ovn-northd-ddlog needs to aggregate the per-chassis IGMP records to generate a single Logical_Flow for group G. Moreover, the ports on which the hosts are connected are represented as references to Port_Binding records in the database. These also need to be translated to &SwitchPort DDlog relations. The corresponding DDlog operations that need to be performed are:

  • Flatten the <IGMP group, ports> mapping in order to be able to do the translation from Port_Binding to &SwitchPort. For each IGMP_Group record in the OVN Southbound database generate an individual record of type IgmpSwitchGroupPort for each Port_Binding in the set of ports that joined the group. Also, translate the Port_Binding uuid to the corresponding Logical_Switch_Port uuid:

    relation IgmpSwitchGroupPort(
        address: string,
        switch : Ref<Switch>,
        port   : uuid
    IgmpSwitchGroupPort(address, switch, lsp_uuid) :-
        sb::IGMP_Group(.address = address, .datapath = igmp_dp_set,
                        .ports = pb_ports),
        var pb_port_uuid = FlatMap(pb_ports),
        sb::Port_Binding(._uuid = pb_port_uuid, .logical_port = lsp_name),
            .lsp = nb.Logical_Switch_Port{._uuid = lsp_uuid, .name = lsp_name},
            .sw = switch).
  • Aggregate the flattened IgmpSwitchGroupPort (implicitly from all ovn-controller instances) grouping by adress and logical switch:

    relation IgmpSwitchMulticastGroup(
        address: string,
        switch : Ref<Switch>,
        ports  : Set<uuid>
    IgmpSwitchMulticastGroup(address, switch, ports) :-
        IgmpSwitchGroupPort(address, switch, port),
        var ports = port.group_by((address, switch)).to_set().

At this point we have all the feature configuration relevant information stored in DDlog relations in ovn-northd-ddlog memory.

Pitfalls of projections

A projection is a join that uses only some of the data in a record. When the fields that are used have duplicates, the result can be many “copies” of a record, which DDlog represents internally with an integer “weight” that counts the number of copies. We don’t have a projection with duplicates in this example, but lswitch.dl has many of them, such as this one:

relation LogicalSwitchHasACLs(ls: uuid, has_acls: bool)

LogicalSwitchHasACLs(ls, true) :-
    LogicalSwitchACL(ls, _).

LogicalSwitchHasACLs(ls, false) :-
    nb::Logical_Switch(._uuid = ls),
    not LogicalSwitchACL(ls, _).

When multiple projections get joined together, the weights can overflow, which causes DDlog to malfunction. The solution is to make the relation an output relation, which causes DDlog to filter it through a “distinct” operator that reduces the weights to 1. Thus, LogicalSwitchHasACLs is actually implemented this way:

output relation LogicalSwitchHasACLs(ls: uuid, has_acls: bool)

For more information, see Avoiding weight overflow in the DDlog tutorial.

Write rules to update output relations

The developer updates output tables by writing rules that generate Out_* relations. For IP Multicast this means:

/* IP_Multicast table (only applicable for Switches). */
sb::Out_IP_Multicast(._uuid = hash128(cfg.datapath),
                     .datapath = cfg.datapath,
                     .enabled = set_singleton(cfg.enabled),
                     .querier = set_singleton(cfg.querier)) :-


OVN_Southbound.dl also contains an IP_Multicast relation with input qualifier. This relation stores the current snapshot of the OVSDB table and cannot be written to.

Generate Logical_Flow and/or other forwarding records

At this point we have defined all DDlog relations required to generate Logical_Flow``s.  All we have to do is write the rules to do so. For each ``IgmpSwitchMulticastGroup we generate a Flow that has as action "outport = <Multicast_Group>; output;":

/* Ingress table 17: Add IP multicast flows learnt from IGMP (priority 90). */
for (IgmpSwitchMulticastGroup(.address = address, .switch = &sw)) {
    Flow(.logical_datapath = sw.dpname,
         .stage            = switch_stage(IN, L2_LKUP),
         .priority         = 90,
         .__match          = "eth.mcast && ip4 && ip4.dst == ${address}",
         .actions          = "outport = \"${address}\"; output;",
         .external_ids     = map_empty())

In some cases generating a logical flow is not enough. For IGMP we also need to maintain OVN southbound Multicast_Group records, one per IGMP group storing the corresponding Port_Binding uuids of ports where multicast traffic should be sent. This is also relatively straightforward:

/* Create a multicast group for each IGMP group learned by a Switch.
 * 'tunnel_key' == 0 triggers an ID allocation later.
sb::Out_Multicast_Group (.datapath   = switch.dpname,
                         .name       = address,
                         .tunnel_key = 0,
                         .ports      = set_map_uuid2name(port_ids)) :-
    IgmpSwitchMulticastGroup(address, &switch, port_ids).

We must also define DDlog relations that will allocate tunnel_key values. There are two cases: tunnel keys for records that already existed in the database are preserved to implement stable id allocation; new multicast groups need new keys. This kind of allocation can be tricky, especially to new users of DDlog. OVN contains multiple instances of allocation, so it’s probably worth reading through the existing cases and following their pattern, and, if it’s still tricky, asking for assistance.

Appendix A. Additional relations generated by ovsdb2ddlog

ovsdb2ddlog generates some extra relations to manage communication with the OVSDB server. It generates records in the following relations when rows in OVSDB output tables need to be added or deleted or updated.

In the steady state, when everything is working well, a given record stays in any one of these relations only briefly: just long enough for ovn-northd-ddlog to send a transaction to the OVSDB server. When the OVSDB server applies the update and sends an acknowledgement, this ordinarily means that these relations become empty, because there are no longer any further changes to send.

Thus, records that persist in one of these relations is a sign of a problem. One example of such a problem is the database server rejecting the transactions sent by ovn-northd-ddlog, which might happen if, for example, a bug in a .dl file would cause some OVSDB constraint or relational integrity rule to be violated. (Such a problem can often be diagnosed by looking in the OVSDB server’s log.)

  • DeltaPlus_IP_Multicast used by the DDlog program to track new records that are not yet added to the database:

    output relation DeltaPlus_IP_Multicast (
        datapath: uuid_or_string_t,
        enabled: Set<bool>,
        querier: Set<bool>
  • DeltaMinus_IP_Multicast used by the DDlog program to track records that are no longer needed in the database and need to be removed:

    output relation DeltaMinus_IP_Multicast (
        _uuid: uuid
  • Update_IP_Multicast used by the DDlog program to track records whose fields need to be updated in the database:

    output relation Update_IP_Multicast (
        _uuid: uuid,
        enabled: Set<bool>,
        querier: Set<bool>