Re: [DISCUSS] KIP-649: Dynamic Client Configuration

[Ryan Dielher <di...@gmail.com>]

Hi Jason,

Thank you again for all of your feedback, it is greatly appreciated.
Here are some more changes to the KIP: https://cwiki.apache.org/confluence/pages/diffpagesbyversion.action?pageId=158869615&selectedPageVersions=21&selectedPageVersions=17

> 1. I wonder if we need to bother with `enable.dynamic.config`, especially
> if the default is going to be true anyway.

I removed this / static / config in favor of a / dynamic / config `supported.configs`. This config is explained in more detail below.

> 3. I'm trying to understand the contract between brokers and clients to
> support dynamic configurations. I imagine that once this is available,
> users will have a hard time telling which applications support the
> capability and which do not. Also, we would likely add new dynamic config
> support over time which would make this even harder since we cannot
> retroactively change clients to add support for new dynamic configs. I'm
> wondering if there is anything we can do to make it easier for users to
> tell which dynamic configs are available for each application.


I added the / dynamic / config `supported.configs` to the KIP. This is a map of client software name and version to a list of supported configs registered with that software name and version. 

e.g.
{
'ClientInformation(softwareName=apache-kafka-java, softwareVersion=x.y.a-SNAPSHOT)': 'acks',
'ClientInformation(softwareName=apache-kafka-java, softwareVersion=x.y.b-SNAPSHOT)': 'acks, enable.idempotence'
}  

The changes propose that `supported.configs` be serialized as a json and stored in an entity config alongside dynamic configs and quotas for an entity. This config is updated for an entity when producers and consumers register the dynamic configurations that they support with the entity. This registration happens every time that they request configs.

This makes the information refreshable since `supported.configs` can be deleted from an entity. Clients register configs every time they request configs, so the compatibility information will eventually be added again and continuously updated as new clients associate themselves with the entity.

> 2. Tying dynamic configurations to clientId has some downsides.

The new protocols in the changes to the KIP are based on the protocols for client quotas. 
They allow dynamic configs to be tied to a user principal and optionally to a client-id without trying to fit multiple entity names and types into the {Describe, IncrementalAlter}Configs APIs. They also provide a more expressive and extensible interface for dynamic client config entity names and types.

> It is common for users to use a different clientId for every application in a
> consumer group so that it is easier to tie group members back to where
> the client is running. This makes setting configurations at an application
> level cumbersome. The alternative is to use the default, but that means
> hitting /all/ applications which I think is probably not a good idea. A
> convenient alternative for consumers would be to use group.id, but we don't
> have anything similar for the producer. I am wondering if we need to give
> the clients a separate config label of some kind so that there is a
> convenient way to group configurations. For example `config.group`. Note
> that this would be another way to opt into dynamic config support.

Would it be reasonable to treat consumer entities slightly differently than producer entities since they are different types of clients with different needs?
Since the design pattern of this new protocol is built in part for extensibility of entity names and types, could consumers be associated with an additional entity name and type? 

e.g. Consumer configs could be set at the user, group-id and client-id levels. They could be resolved with the following order of precedence from most precedent to least precedent.

/config/users/<user>/groups/<group-id>/clients/<client-id>
/config/users/<user>/groups/<group-id>
/config/users/<user>

This doesn’t introduce a new label but it would instead allow dynamic consumer configs to be set at the group-id level.

> 4. In the case of `session.timeout.ms`, even if the config is updated, the
> group will need to be rebalanced for it to take effect. This is because the
> session timeout is sent to the group coordinator in the JoinGroup request.
> I'm wondering if we need to change the JoinGroup behavior so that it can be
> used to update the session timeout without triggering a rebalance.

I updated the KIP and PR so that a JoinGroupRequest will not rebalance the group if the session timeout in the JoinGroupRequest is different from the session timeout in the GroupCoordinator’s metadata and if the group member that is rejoining is in a stable state. In practice this just changes the behavior of the leader of the group when the timeout is updated since the leader is the only one that will trigger a rebalance when rejoining while in a stable state. 

I would greatly appreciate any more feedback from you about these ideas.

Best,
Ryan

On 2020/08/06 00:23:28, Jason Gustafson <ja...@confluent.io> wrote: 
> Hi Ryan,
> 
> Thanks for the proposal. Just a few quick questions:
> 
> 1. I wonder if we need to bother with `enable.dynamic.config`, especially
> if the default is going to be true anyway. I think users who don't want to
> use this capability can just not set dynamic configs. The only case I can
> see an explicit opt-out being useful is when users are trying to avoid
> getting affected by dynamic defaults. And on that note, is there a strong
> case for supporting default overrides? Many client configs are tied closely
> to application behavior, so it feels a bit dangerous to give users the
> ability to override the configuration for all applications.
> 
> 2. Tying dynamic configurations to clientId has some downsides. It is
> common for users to use a different clientId for every application in a
> consumer group so that it is easier to tie group members back to where
> the client is running. This makes setting configurations at an application
> level cumbersome. The alternative is to use the default, but that means
> hitting /all/ applications which I think is probably not a good idea. A
> convenient alternative for consumers would be to use group.id, but we don't
> have anything similar for the producer. I am wondering if we need to give
> the clients a separate config label of some kind so that there is a
> convenient way to group configurations. For example `config.group`. Note
> that this would be another way to opt into dynamic config support.
> 
> 3. I'm trying to understand the contract between brokers and clients to
> support dynamic configurations. I imagine that once this is available,
> users will have a hard time telling which applications support the
> capability and which do not. Also, we would likely add new dynamic config
> support over time which would make this even harder since we cannot
> retroactively change clients to add support for new dynamic configs. I'm
> wondering if there is anything we can do to make it easier for users to
> tell which dynamic configs are available for each application.
> 
> 4. In the case of `session.timeout.ms`, even if the config is updated, the
> group will need to be rebalanced for it to take effect. This is because the
> session timeout is sent to the group coordinator in the JoinGroup request.
> I'm wondering if we need to change the JoinGroup behavior so that it can be
> used to update the session timeout without triggering a rebalance.
> 
> Thanks,
> Jason
> 
> 
> 
> 
> On Mon, Aug 3, 2020 at 3:10 PM Ryan Dielhenn <rd...@confluent.io> wrote:
> 
> > Hi David,
> >
> > Here are some additional thoughts...
> >
> > > 1. Once dynamic configs have been loaded and resolved, how can a client
> > > know what values are selected?
> >
> > A copy of the original user-provided configs is kept by the client.
> > Currently these are used to revert to the user-provided config if a dynamic
> > config is deleted. However, they can also be used to distinguish between
> > dynamic and user-provided configs.
> >
> > > 3. Are there other configs we'd like to allow the broker to push up to
> > the
> > > clients? Did we consider making this mechanism generic so the broker
> > could
> > > push any consumer/producer config up to the clients via dynamic configs?
> >
> > Rephrasing my answer to this question:
> >
> > The mechanism for sending and altering configs is rather generic. However,
> > the client-side handling of these configs is not. The reason for this is
> > that configs affect the behavior of the clients in specific ways, so the
> > client must reconfigure itself in a specific way for each different config.
> >
> > An example of this is that when session.timeout.ms is dynamically
> > configured, the consumer must rejoin the group by sending a
> > JoinGroupRequest. This is because the session timeout is sent in the
> > initial JoinGroupRequest to the coordinator and stored with the rest of the
> > group member's metadata. To reconfigure the client, the value in the
> > coordinator must also be changed. This does not need to be done for
> > heartbeat.interval.ms.
> >
> >
> > On 2020/08/03 17:47:19, David Arthur <mu...@gmail.com> wrote:
> > > Hey Ryan, thanks for the KIP. This will be a really useful feature. Few
> > > questions
> > >
> > > 1. Once dynamic configs have been loaded and resolved, how can a client
> > > know what values are selected? Will we log the actual resolved values
> > once
> > > they are loaded?
> > >
> > > 2. Do we want to support the case that we load dynamic configs but also
> > > allow client overrides?
> > >
> > > 3. Are there other configs we'd like to allow the broker to push up to
> > the
> > > clients? Did we consider making this mechanism generic so the broker
> > could
> > > push any consumer/producer config up to the clients via dynamic configs?
> > >
> > > Also, minor note, in ConfigResource#Type, I think the byte values are
> > used
> > > for bit masking somewhere, so 9 won't work -- you'd need to bump up to 16
> > >
> > > -David
> > >
> > > On Fri, Jul 31, 2020 at 6:06 PM Ryan Dielhenn <rd...@confluent.io>
> > > wrote:
> > >
> > > > Thanks Jose,
> > > > Here are the changes to the KIP:
> > > >
> > https://cwiki.apache.org/confluence/pages/diffpagesbyversion.action?pageId=158869615&selectedPageVersions=10&selectedPageVersions=9
> > > >
> > > > 1. Done.
> > > > 2. Done.
> > > > 3. Done.
> > > > 4. Yes metadata.max.age.ms will be use, I updated the KIP with this.
> > > >
> > > > > 5.
> > > > > The Consumer Changes section mentions that the consumer would ask for
> > > > > the dynamic configuration from the broker before joining the group
> > > > > coordinator. This makes sense to me. How about the producer? Should
> > > > > the producer also describe the dynamic configuration before sending
> > > > > acks for the "produce" messages?
> > > >
> > > >
> > > > I do think that the producer should do one synchronous
> > > > DescribeConfigsRequest before producing messages, just for different
> > > > reasons than the consumer. The decision to do this in the consumer was
> > to
> > > > avoid an expensive rebalance operation associated with sending an extra
> > > > join group request.
> > > >
> > > > If this were done for the producer it would be to maintain durability
> > > > semantics by ensuring that the correct dynamic acks value is being used
> > > > before producing messages, however it would not improve performance
> > like it
> > > > does for consumer groups.
> > > >
> > > > > 6.
> > > > > For the Admin Client Changes section, how are DescribeConfigs and
> > > > > IncrementalAlterConfig requests going to get routed by the client to
> > > > > the different brokers in the cluster?
> > > >
> > > > They will all be routed to the least loaded node for the CLIENT
> > resource
> > > > type. I update the KIP with this.
> > > >
> > > > > 7.
> > > > > You mentioned that the producer and the consumer will validate the
> > > > > keys and values received from the broker through DescribeConfigs.
> > Will
> > > > > the ConfigCommand validate any of the keys or values specified in
> > > > > --add-config and --delete-config? Will the broker validate any of the
> > > > > keys or values received in the IncrementalAlterConfigs?
> > > >
> > > > Yes, ConfigCommand will only allow keys that we are providing support
> > for
> > > > to be sent to the broker. The broker will validate the keys as well as
> > the
> > > > values and return an InvalidRequest error code if any key-value pairs
> > are
> > > > invalid.
> > > >
> > > > 8. I removed this line.
> > > >
> > > > On 2020/07/31 18:27:38, Jose Garcia Sancio <js...@confluent.io>
> > wrote:
> > > > > Thanks for the KIP Ryan. Here are some of my observations.
> > > > >
> > > > > 1.
> > > > > > We will also be adding a new client configuration
> > > > enable.dynamic.config to both the producer and consumer that will be
> > true
> > > > by default so that the user has the option to disable this feature.
> > > > >
> > > > > How about?
> > > > > "The Java producer and consumer clients will have a new configuration
> > > > > property `enable.dynamic.config` with a default value of `true`. When
> > > > > this configuration property is true the proposed producer and
> > consumer
> > > > > changes in this KIP are enabled."
> > > > >
> > > > > You can go into more details in the producer and consumer changes
> > > > section.
> > > > >
> > > > > 2.
> > > > > > ... the following order of precedence:
> > > > >
> > > > > I would be a bit more specific here. For example, when the broker
> > > > > handles `DescribeConfigRequest`, it will first use the client config
> > > > > key-values store in `/config/clients/<default>`. If there is a
> > > > > matching `config/clients/<client-id>` then any client config
> > key-value
> > > > > in the matching znode will override the key-values found in
> > > > > `config/clients/<default>`.
> > > > >
> > > > > 3.
> > > > > > These dynamic configs will be stored in zookeeper as the children
> > of
> > > > the ZNode /config/clients
> > > > >
> > > > > How are the client dynamic config key-values stored here? I assume
> > > > > that they are updated based on the content of
> > > > > `IncrementalAlterConfigsRequest`. When is `.../<default>` updated?
> > > > > When is `.../<client-id>` updated?
> > > > >
> > > > > 4.
> > > > > > The interval on which dynamic configs are fetched will be the same
> > > > amount of time as the interval for MetadataRequest which is currently
> > five
> > > > minutes.
> > > > >
> > > > > Will this be hard-coded to 5 minutes? Or is this KIP going to use the
> > > > > same frequency as the producer config `metadata.max.age.ms`? Same
> > > > > question for the "Consumer Changes" section.
> > > > >
> > > > > 5.
> > > > > The Consumer Changes section mentions that the consumer would ask for
> > > > > the dynamic configuration from the broker before joining the group
> > > > > coordinator. This makes sense to me. How about the producer? Should
> > > > > the producer also describe the dynamic configuration before sending
> > > > > acks for the "produce" messages?
> > > > >
> > > > > 6.
> > > > > For the Admin Client Changes section, how are DescribeConfigs and
> > > > > IncrementalAlterConfig requests going to get routed by the client to
> > > > > the different brokers in the cluster?
> > > > >
> > > > > 7.
> > > > > You mentioned that the producer and the consumer will validate the
> > > > > keys and values received from the broker through DescribeConfigs.
> > Will
> > > > > the ConfigCommand validate any of the keys or values specified in
> > > > > --add-config and --delete-config? Will the broker validate any of the
> > > > > keys or values received in the IncrementalAlterConfigs?
> > > > >
> > > > > 8.
> > > > > In rejected ideas the KIP says:
> > > > > > This might make sense for certain configurations such as acks, but
> > > > does not for others such as timeouts.
> > > > >
> > > > > I don't think it makes sense even for acks since the clients of the
> > > > > Java Producer assume that all of the produce messages are sent with
> > > > > the same ack value.
> > > > >
> > > > > --
> > > > > -Jose
> > > > >
> > > >
> > >
> > >
> > > --
> > > David Arthur
> > >
> >
>