Flavors for L3 Service Plugin


This blueprint covers the introduction of the flavor framework into the in-tree L3 service plugin. By adding support for flavors, operators will be able to run multiple L3 drivers in the same deployment.

RFE: https://bugs.launchpad.net/neutron/+bug/1461133

Problem Description

The in-tree L3 service plugin only has support for the Neutron software routers (legacy/ha/dvr/ha+dvr). If an operator wants to use another type of router, the service plugin must be changed which will preclude the use of the reference plugin routers. This current model fails to address any use cases where a specific routing backend should be used for only a subset of all of the logical routers in a Neutron deployment.

More use cases can be found in the following etherpad: https://etherpad.openstack.org/p/neutron-modular-l3-router-plugin-use-cases

Proposed Change

Add flavor support to the in-tree L3 service plugin so requests can be dispatched to different drivers depending on the flavor associated with a given router. This will have no impact on other existing L3 service plugins that exist outside of Neutron.

block diagram:

|   Neutron Server                                              |
|                                                               |
|  +----------------------------------------------------------+ |
|  | L3 router service plugin                                 | |
|  |                                                          | |
|  | +------------------------------------------------------+ | |
|  | | router driver controller                             | | |
|  | +------------------------------------------------------+ | |
|  |                                                          | |
|  | +-----------+  +----+  +---+  +------+  +----------+     | |
|  | |single_node|  |dvr |  |ha |  |ha+dvr|  | vendor X | ... | |
|  | |           |  |    |  |   |  |      |  | driver   |     | |
|  | +--+--------+  ++---+  ++--+  ++-----+  +----------+     | |
        |            |       |      |         |
        |RPC         |       |      |         | vendor
        |            |       |      |         | specific
        |            |       |      |         | communication
        |            |       |      |         |
        |            |       |      |         V
        V            |       |      |      +------------------+
    +---------+      |       |      |      | Proprietron 9000 |
    |l3 agent |<-----+-------+------+      +------------------+

Within the flavor framework there are flavors and service providers. Flavors are user-facing via the flavors API and they allow users to choose the type of router they would like (e.g. ‘slow and cheap’). Each flavor is associated to one or more service profiles. These service profiles define a driver and some metadata to pass to that driver when it is used.

When a user creates a router with a specific flavor the flavor framework will look up the service profiles for that flavor and create the router using one of the associated drivers. Once a driver is selected for a router, the router/driver association is stored in the DB so any future operations on the router can lookup the driver without going through the flavor framework.

If a user does not choose a flavor, their router will have a NULL flavor. The chosen driver will then depend on the operator configured service providers, or in the backward compat case, it will be determined based on the HA and distributed configuration flags.

Currently, the flavor framework only supports one service profile per flavor because it lacks the logic to schedule between profiles [1] So initially we will only support one service profile per flavor but this will not prevent future support for scheduling among multiple providers.

The scheduling among multiple providers is not be confused with router scheduling: the one referenced in the flavors framework is actually used to determine which service provider to use for a given flavor if it has multiple service profiles associated with it. Router scheduling is used in agent-based implementations of L3 where multiple nodes can host the routing function and thus the server needs to determine which one to choose based on a specified policy (e.g. random vs load-based policy).

For the sake of this effort, routing scheduling is to be considered implementation specific and left to the driver. In other words, the L3 plugin should be made unaware of routing scheduling issues as a driver may or may not need to schedule the routing function to a node depending on how the L3 function is actually implemented.

This means that it is within the scope of this blueprint to move the scheduling implementation details within the default drivers and leave other drivers in charge of scheduling if they need it.

Responsibility of DB operations

The L3 service plugin will remain responsible for the CRUD operations on the router DB objects. This will ensure consistency across common fields between different flavors/drivers. If a driver needs to perform validation on input before the record is created in the DB, it can subscribe a validation callback to the PRECOMMIT events for the router. Additionally, it is the responsibility of the driver to adjust its own records based on the PRECOMMIT events if the driver requires storing extra information about the router.

Each driver may potentially require distributed coordination in order to dispatch and implement router operations, and as such transactionality of these operations is left as a driver specific issue. This aspect may be revised in the future according to findings/experiments developed in the context of [3]

Comparison to ML2

This differs from ML2 in an important aspect. All L3 drivers will not be called for each operation. Only a single driver will be responsible for all operations regarding a given router. The driver that is called is determined by the driver associated to the router when it is created.

Associating a Router to a Driver

This association is performed one of two ways: either via an explicit flavor request from the user, or via the distributed/ha attributes and distributed/ha configuration.

The driver manager will load up four default drivers to represent the single node, ha, distributed, and HA+distributed router types. The driver manager will then select the appropriate one in the absense of a flavor request from the user. In other words, the manager chooses a driver at creation time based on the user selected flavor, or it will fall back on HA/DVR attributes, if presented. If everything is ommitted, the API behavior will remain unchanged and the centralized software router is created. In other words if a custom driver is loaded up next to the default providers, a user must specify the flavor in order to select the requested behavior.

In the event a user requests flags incompatible with the flavor (e.g. neutron router-create –flavor-id=<ha-id> –distributed=True), an error will be returned (e.g. invalid input). This can be potentially solved either client side or server side; however the server side is to be preferred as it makes the behavior consistent for API users too.

Operators will be able to override the default drivers by explicitly defining other service_providers in their configuration.

It is up to the operator if he/she wants to actually expose any of the drivers as flavors to the end user to pick from. The flavor_id associated with a router is nullable, so an operator can maintain backwards compatiblilty with the current model by not defining any flavors (a.k.a ‘a tasteless deployment’).

Data Model Impact

A flavor_id will be added to the Router table with a foreign key constraint to the flavors table.[2] It will be nullable to preserve compatibility with the current behavior.

No other modifications to the existing tables are required since the service type framework already exists and allows a driver to be associated with a resource_id.


All router objects will now have a nullable ‘flavor_id’ attribute that indicates the router’s flavor. This attribute can also be used in create/update calls to request a specific flavor. From an API extension standpoint, the proposed framework will not include any capability to allow drivers to bring their own extensions to the L3 models, not now and not in the future. In other words, there will not be a supported programmatic API for extension pluggability. Having said that, the existing mechanisms to plug into the Neutron frameworks can still be exploited as they are available at the time of the driver development.

RPC API Impact

The L3 agent based solutions rely on a crucial RPC that allows agents to sync their state with the Server. This API (sync_state) is heavily specialized across the hierarchy l3->dvr->ha->ha+dvr. Resolving the mess is a long term objective of this effort, however, for now the default software drivers will share this RPC detail and therefore will continue to rely on the existing behavior of the sync_state operation.

As soon as the first iteration of the framework is complete, strategies to address this aspect will be explored, so that each driver that needs to share this RPC call can do so by means of composition rather than inheritance.

Backward compatibility

Flavors and service providers are defined by operators. However, we want the L3 reference plugin to continue to work without action by the operator when they upgrade. To that effect, the current L3 plugin driver will automatically register the single_node, ha, dvr, and dvr+ha drivers as service providers so the driver manager can work the same as the current system.

Work Items

  • add flavor framework support into the existing L3 plugin

  • create separate drivers for single_node, HA, DVR, and HA+DVR routers

  • decompose the giant mixin containing logic for all types in the main plugin and move type-specific logic into each driver

  • add API tests to exercise flavors


  1. https://github.com/openstack/neutron/blob/33eec87d7822c0915bd45f2c9d2de0b6dc455771/neutron/db/flavors_db.py#L263-L273

  2. https://specs.openstack.org/openstack/neutron-specs/specs/liberty/neutron-flavor-framework.html

  3. https://bugs.launchpad.net/neutron/+bug/1552680