Signed-off-by: Ben Pfaff <[email protected]> Suggested-by: Numan Siddique <[email protected]> --- Documentation/tutorials/ddlog-new-feature.rst | 31 +++++++++++++++++++ northd/lswitch.dl | 12 +++++++ 2 files changed, 43 insertions(+)
diff --git a/Documentation/tutorials/ddlog-new-feature.rst b/Documentation/tutorials/ddlog-new-feature.rst index 02876db66d74..de66ca5ada5c 100644 --- a/Documentation/tutorials/ddlog-new-feature.rst +++ b/Documentation/tutorials/ddlog-new-feature.rst @@ -252,6 +252,37 @@ corresponding DDlog operations that need to be performed are: 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 +<https://github.com/vmware/differential-datalog/blob/master/doc/tutorial/tutorial.md#avoiding-weight-overflow>`_ +in the DDlog tutorial. + Write rules to update output relations -------------------------------------- diff --git a/northd/lswitch.dl b/northd/lswitch.dl index 8fbb313b9666..b98a9ea04c3c 100644 --- a/northd/lswitch.dl +++ b/northd/lswitch.dl @@ -70,6 +70,8 @@ LogicalSwitchPortWithUnknownAddress(ls_uuid, lsp_uuid) :- lsp in nb::Logical_Switch_Port(._uuid = lsp_uuid), lsp.is_enabled() and lsp.addresses.contains("unknown"). +# "Pitfalls of projections" in ddlog-new-feature.rst explains why this +# is an output relation: output relation LogicalSwitchHasUnknownPorts(ls: uuid, has_unknown: bool) LogicalSwitchHasUnknownPorts(ls, true) :- LogicalSwitchPortWithUnknownAddress(ls, _). LogicalSwitchHasUnknownPorts(ls, false) :- @@ -116,6 +118,8 @@ LogicalSwitchStatefulACL(ls, acl) :- LogicalSwitchACL(ls, acl), nb::ACL(._uuid = acl, .action = "allow-related"). +# "Pitfalls of projections" in ddlog-new-feature.rst explains why this +# is an output relation: output relation LogicalSwitchHasStatefulACL(ls: uuid, has_stateful_acl: bool) LogicalSwitchHasStatefulACL(ls, true) :- @@ -125,6 +129,8 @@ LogicalSwitchHasStatefulACL(ls, false) :- nb::Logical_Switch(._uuid = ls), not LogicalSwitchStatefulACL(ls, _). +# "Pitfalls of projections" in ddlog-new-feature.rst explains why this +# is an output relation: output relation LogicalSwitchHasACLs(ls: uuid, has_acls: bool) LogicalSwitchHasACLs(ls, true) :- @@ -170,6 +176,8 @@ LogicalSwitchWithDNSRecords(ls) :- nb::DNS(._uuid = dns_uuid, .records = records), not records.is_empty(). +# "Pitfalls of projections" in ddlog-new-feature.rst explains why this +# is an output relation: output relation LogicalSwitchHasDNSRecords(ls: uuid, has_dns_records: bool) LogicalSwitchHasDNSRecords(ls, true) :- @@ -186,6 +194,8 @@ LogicalSwitchHasNonRouterPort0(ls_uuid) :- lsp in nb::Logical_Switch_Port(._uuid = lsp_uuid), lsp.__type != "router". +# "Pitfalls of projections" in ddlog-new-feature.rst explains why this +# is an output relation: output relation LogicalSwitchHasNonRouterPort(ls: uuid, has_non_router_port: bool) LogicalSwitchHasNonRouterPort(ls, true) :- LogicalSwitchHasNonRouterPort0(ls). @@ -285,6 +295,8 @@ SwitchLBVIP(sw_uuid, lb, vip, backends) :- var kv = FlatMap(vips), (var vip, var backends) = kv. +# "Pitfalls of projections" in ddlog-new-feature.rst explains why this +# is an output relation: output relation LogicalSwitchHasLBVIP(sw_uuid: uuid, has_lb_vip: bool) LogicalSwitchHasLBVIP(sw_uuid, true) :- SwitchLBVIP(.sw_uuid = sw_uuid). -- 2.31.1 _______________________________________________ dev mailing list [email protected] https://mail.openvswitch.org/mailman/listinfo/ovs-dev
