+1 on the new config. Just one comment. Currently, when initiating a config (e.g. ProducerConfig), we log those overridden property values and unused property keys (likely due to mis-spelling). This has been very useful for config verification. It would be good to add similar support in the new config.
Thanks, Jun On Tue, Feb 4, 2014 at 9:34 AM, Jay Kreps <jay.kr...@gmail.com> wrote: > We touched on this a bit in previous discussions, but I wanted to draw out > the approach to config specifically as an item of discussion. > > The new producer and consumer use a similar key-value config approach as > the existing scala clients but have different implementation code to help > define these configs. The plan is to use the same approach on the server, > once the new clients are complete; so if we agree on this approach it will > be the new default across the board. > > Let me split this into two parts. First I will try to motivate the use of > key-value pairs as a configuration api. Then let me discuss the mechanics > of specifying and parsing these. If we agree on the public api then the > public api then the implementation details are interesting as this will be > shared across producer, consumer, and broker and potentially some tools; > but if we disagree about the api then there is no point in discussing the > implementation. > > Let me explain the rationale for this. In a sense a key-value map of > configs is the worst possible API to the programmer using the clients. Let > me contrast the pros and cons versus a POJO and motivate why I think it is > still superior overall. > > Pro: An application can externalize the configuration of its kafka clients > into its own configuration. Whatever config management system the client > application is using will likely support key-value pairs, so the client > should be able to directly pull whatever configurations are present and use > them in its client. This means that any configuration the client supports > can be added to any application at runtime. With the pojo approach the > client application has to expose each pojo getter as some config parameter. > The result of many applications doing this is that the config is different > for each and it is very hard to have a standard client config shared > across. Moving config into config files allows the usual tooling (version > control, review, audit, config deployments separate from code pushes, > etc.). > > Pro: Backwards and forwards compatibility. Provided we stick to our java > api many internals can evolve and expose new configs. The application can > support both the new and old client by just specifying a config that will > be unused in the older version (and of course the reverse--we can remove > obsolete configs). > > Pro: We can use a similar mechanism for both the client and the server. > Since most people run the server as a stand-alone process it needs a config > file. > > Pro: Systems like Samza that need to ship configs across the network can > easily do so as configs have a natural serialized form. This can be done > with pojos using java serialization but it is ugly and has bizare failure > cases. > > Con: The IDE gives nice auto-completion for pojos. > > Con: There are some advantages to javadoc as a documentation mechanism for > java people. > > Basically to me this is about operability versus niceness of api and I > think operability is more important. > > Let me now give some details of the config support classes in > kafka.common.config and how they are intended to be used. > > The goal of this code is the following: > 1. Make specifying configs, their expected type (string, numbers, lists, > etc) simple and declarative > 2. Allow for validating simple checks (numeric range checks, etc) > 3. Make the config "self-documenting". I.e. we should be able to write code > that generates the configuration documentation off the config def. > 4. Specify default values. > 5. Track which configs actually get used. > 6. Make it easy to get config values. > > There are two classes there: ConfigDef and AbstractConfig. ConfigDef > defines the specification of the accepted configurations and AbstractConfig > is a helper class for implementing the configuration class. The difference > is kind of like the difference between a "class" and an "object": ConfigDef > is for specifying the configurations that are accepted, AbstractConfig is > the base class for an instance of these configs. > > You can see this in action here: > > https://git-wip-us.apache.org/repos/asf?p=kafka.git;a=blob_plain;f=clients/src/main/java/kafka/clients/producer/ProducerConfig.java;hb=HEAD > > (Ignore the static config names in there for now...I'm not actually sure > that is the best approach). > > So the way this works is that the config specification is defined as: > > config = new ConfigDef().define("bootstrap.brokers", Type.LIST, > "documentation") > > .define("metadata.timeout.ms", Type.LONG, > 60 * 1000, atLeast(0), "documentation") > .define("max.partition.size", Type.INT, > 16384, atLeast(0), "documentation") > > > This is used in a ProducerConfig class which extends AbstractConfig to get > access to some helper methods as well as the logic for tracking which > configs get accessed. > > Currently I have included static String variables for each of the config > names in that class. However I actually think that is not very helpful as > the javadoc for them doesn't give the constant value and requires > duplicating the documentation. To understand this point look at the javadoc > and note that the doc on the string is not the same as what we define in > the ConfigDef. We could just have the javadoc for the config string be the > source of truth but it is actually pretty inconvient for that as it doesn't > show you the value of the constant, just the variable name (unless you > discover how to unhide it). That is fine for the clients, but for the > server would be very weird especially for non-java people. We could attempt > to duplicate documentation between the javadoc and the ConfigDef but given > our struggle to get well-documented config in a single place this seems > unwise. > > So I recommend we have a single source for documentation of these and that > that source be the website documentation on configuration that covers > clients and server and that that be generated off the config defs. The > javadoc on KafkaProducer will link to this table so it should be quite > convenient to discover. This makes things a little more typo prone, but > that should be easily caught by the key detection. This will also make it > possible for us to retire configs in the future without causing compile > failures and add configs without having use of them break backwards > compatibility. This is useful during upgrades where you want to be > compatible with the old and new version so you can roll forwards and > backwards. > > -Jay >
