Folks,
I wanted to make sure that my promoting CDI, Camel Java DSL, & static profiles didn't obscure the point I was trying to make. Whatever mechanics we choose I'd really like us to be unified behind a common paradigm so that our documentation, exemplars, archetypes, blogs, libraries, and so on are all organized the same and use the same mechanics and layouts for projects. We should promote an idiomatic way to develop software using Karaf Boot. That's one problem I hear from a lot of clients. There are such cross-currents of information about how to develop OSGi-based software that it gets confusing. Best or preferred practices are lost in the noise. I won't get into all that since I'm sure most of you have dealt this problem. Not to pick on it but a good example is that the Camel in Action book recommends using Pojos instead of using Processors/Exchanges. It is on somewhere near the back of the book in a few pages. I don't know how many examples on the web site actually use the Processor/Exchange but it is a lot. Then there are examples with Spring, Blueprint, Java DSL, Scala, etc. There are annotations that only work in one environment but not in all of them. By selecting an idiomatic and "opinionated" way of creating Karaf Boot microcontainers we could make sure that sort of confusion isn't continued forward. It would require a lot less documentation to cover the same ground and make editing and updating easier. It would make creating sample and example projects a lot easier. It would simplify what Karaf Boot appliances have to support and make sure there aren't concerns that work in one environment and not in another or that might work differently in a different environment. I'm personally interested in Karaf Appliances with standard Maven structures, standard bundle structures, and reference implementations that have a good chunk of the basic functionality. I'd say we take a page from the "convention over configuration" book or, at least, a "conventional configuration" and likely a bit of both. Because the appliances are focused on microservices we should get out ahead of the Gartner hype cycle. Right now we are at the Peak of Inflated Expectations and in a couple of years we'll be at the Trough of Disillusionment. That disillusionment will come for a number of reasons. Flying Spaghetti Monster topology will be one of them but, more importantly for a Karaf Appliance, is the consistent problem of "network fallacies". Every Karaf Kontainer should have standard OSGi service interfaces and basic implementations that address each of the fallacies that apply to a uService. The Kontainers should insist on it and not make it optional. If the user doesn't want that functionality they would then need to disable via configuration. But the Kontainer will get stuck in a grace period and then fail if an expected, standard service isn't available. All of the standard OSGi service APIs would have basic implementations to start but as more specific Kontainers. But, because they are standard services new ones can be developed by the community or by the end developer. As developers, we've all had to implement functionality and then come back and deal with error handling, security, etc. I say we simply cut those services in to the Kontainer right from the get-go. The Kontainer doesn't run if it doesn't find the service. That isn't to say these become a fundamental part of Karaf but a fundamental part of the Kontainer service that runs in Karaf. The standard bundles would only implement basic functionality and not do anything sophisticated. New bundles and libraries for more sophisticated implementations could be added later. All of the bundles would likely have disable flags if the developer found the particular concern irrelevant. For example, security might not be relevant. The following aren't meant to be comprehensive. Just addressing key concerns. Other standards like LoggingService might be included by default as well. The intent here isn't to define the exact mechanics but the standard OSGi service interface that would be _required_ in any implementation of a Kontainer, even if the implemented bundle is simply a passthrough or can be disabled, it forces the developer to explicitly deal with the problems or choose to ignore them altogether. Because these service interfaces and the bundles that implement them are standard, the set can be specified by the dependencies specified in the Maven build, features and/or profiles. 1. The network is reliable. A standard "Error Handler" OSGi service. The default bundle would simply capture errors/exceptions and log them. Perhaps it would specify retries. Drop in solutions might include errors going to dead letter queues and so on. The OSGi service interface is required for Kontainer bootstrap so use the default or use a standard one or create one of your own. If they want to change configuration of this bundle or put in a new one, they know exactly what it is, where it exists, how it is specified to the build, and what configuration file is associated with it. No rummaging around through code. When the inevitable error, exceptions and problems arise, the developer isn't left wondering where and how they should add the functionality to handle it. A standard "Circuit Breaker" service API and basic implemented bundle should be provided. Perhaps the standard bundle would simply count errors over a time frame and shut down if that limit is hit and allow those values to be configured. Default would be a rather unsophisticated implementation but provide the convention and automated wiring of a circuit breaker OSGi service. Other implementations might fire off emails to Sys Admins or be combinations. And if it is really undesirable, set a disable flag. 2. Latency is zero. A standard OSGi Throttling service interface and bundle implementation would be included. If you want different behavior, change it. If you want to disable it, set the flag. However, there are bigger issues here that I'll address a bit more down below. 3. Bandwidth is infinite. Throttling OSGi service again. Ditto to comment 2. 4. The network is secure. Standard OSGi service to plug in in various authentication/authorization mechanisms. By default it might be pass through but also have a different implementation that uses a simple username/password. Obviously LDAP, JAAS, and other bundles could be created and dropped into place. 5. Topology doesn't change. Back to the Circuit Breaker, logging and perhaps notification mechanism. Also the transport issue below where I'll mention some configuration. 6. There is one administrator. //No particular plugin for this but standardized configuration and expected bundles help and this also relates to the transport discussion. 7. Transport cost is zero. //Probably not a concern here directly but will be a big issue of uServices. 8. The network is homogeneous. //I think this issue can be dealt with in our context with many of the standard libraries but can be abstracted a bit more. Obviously a big issue we'll see, and I've seen in the past, is chained request/response calls. Service 1 making a REST call to service 2 making a REST call to service 3.etc. And all of a sudden the latency is a killer. ServiceMix/Karaf/Camel can already abstract away some of that via property substitution. I'd suggest we take that one step further and put _all_ transport/protocol information in configuration and create a standardized URI. As a developer or a senior developer over a group of developers, I don't want them to be concerned with the fiddly bits of the transport in the code and routes and I certainly don't want to recompile just to make such changes. Akka, for example, uses local URIs like akka://. But a similar Karaf/Camel URI could be used and mapped via the configuration files. So the developer would always use karaf:// in their routes and configuration mapping would use the URI specified. karaf://myserviceName. In the configuration file might be mapped a transport.configuration.cfg file. I believe that is important for a lot of reasons. A mid-level or junior-level developer shouldn't be involved in configuration like: "ftp://foo@myserver? <ftp://foo@myserver/?> password=secret& recursive=true& ftpClient.dataTimeout=30000& ftpClientConfig.serverLanguageCode=fr" So the cfg file might look like this: clientService="ftp://foo@myserver?password=secret&; recursive=true& ftpClient.dataTimeout=30000& ftpClientConfig.serverLanguageCode=fr" (At least properties get rid of the gawdaful escaped ampersands). The code would then say "karaf://clientService" One can do much of that via configuration right now but I think it is critical to move it completely to configuration so that admins know exactly what to change and where to find it when topologies change. It also means that when the backlash from microservice calling microservice calling microservice being slow happens, that simple mapping would permit things like going to JMS asynchronous request/response (or other fast, async mechanisms) that don't swamp the virtual machine's or Karaf instance resources. It would also allow for easy stubbing or mock testing of the Kontainer as it will be deployed without using PAX exam or other mechanism. Creating standard OSGi service APIs in an anticipation of these problems would permit for an evolutionary approach to these problems in the future and specific solutions when a standard Kontainer is developed. Even standard error handler service implementations can be created. Once such a basic, standard Kontainer exists, then uKontainers that implement basic functionality commonly used could be created. There are JPA examples already. But the average developer is going to be given a task to receive some canonical data model via a REST service and poke it into a database. That database model probably won't look like what they are receiving. So a uKontainer that has a REST front end they can modify, a Dozer object mapping file in the middle with a transform, and a call to the database will be used repeatedly. It may be that Oracle, MySQL, BerklyDB, and so on each endup with different error handler plugin implementations which are used with the same REST, mapping, JPA container. Just change the Maven dependency or profile. There are a large number of examples like that. In the case of that uKontainer there would like be a JPAErrorService for catching common errors and another for Dozer errors and for unmarshaling errors. As a developer looking to solve very specific problems, I just download the uContainer and do the Dozer mapping, change some configuration and then test it. That also means, that much like Camel EIPs, open source developers can focus on hardening these containers, fixing bugs, putting in performance enhancements and the like. If a new error is coming from JPA that a user finds and isn't being handled in a coherent fashion, then a new block or delegate code is added and released. Just as we'd do with a Camel endpoint or component. Having standard error handlers built into uKontainers would also help make coherent messages from the large and unwieldy stack traces full of reflection that we commonly see. The error handler OSGi plugin for a given problem would be highly focused on identifying and reporting problems with a specific technology or set of technologies. https://en.wikipedia.org/wiki/Fallacies_of_distributed_computing
