AlexanderAshitkin commented on code in PR #40: URL: https://github.com/apache/maven-build-cache-extension/pull/40#discussion_r1125518369
########## src/site/markdown/index.md: ########## @@ -19,102 +19,110 @@ Build cache is an extension targeted to simplify and make more efficient work with large builds in Maven. -That is achieved by a combination of features: +A combination of features achieves that: -* Incremental builds over the changed project graph part only -* Subtree support in multimodule projects (caches discovered from the larger project) +* Incremental builds works on the modified part of the project graph part only +* Subtree support for multimodule projects to work on the part of the codebase in isolation * Version normalization to support project version agnostic caches * Project state restoration (partial) to avoid expensive tasks (code generation and similar) -Large projects usually pose scalability challenges and work with such projects require build tool which scales. Cache -extension addresses that with incremental build execution and ability to efficiently work on sub-parts of a larger +Large projects usually pose scalability challenges, and working with such projects requires a build tool that scales. +The cache +extension addresses that with incremental build execution and the ability to efficiently work on sub-parts of a larger project without building and installing dependencies from the larger project. Though, features implemented in Maven -should give noticeable benefits in medium and small sized projects as well. +should also give noticeable benefits in medium and small-sized projects. ### Cache concepts -The idea of the build cache is to calculate key from module inputs, store outputs in cache and restore them later -transparently to the standard Maven core. In order to calculate the key cache engine analyzes source code, build flow, -plugins and their parameters. This allows to deterministically associate each project state with unique key and restore -up-to-date (not changed) projects from cache and rebuild out-of-date(changed) ones. Restoring artifacts associated with -a particular project state improves build times by avoiding re-building unnecessary modules. Cache does not make any -interventions to actual build execution process and fully delegates build work to Maven core. This ensures that -artifacts produced in presence of cache are equivalent to result produced by a standard Maven build. -To achieve accurate key calculation incremental Maven combines automatic introspection -of [project object model](https://maven.apache.org/pom.html#What_is_the_POM) and allows fine-grained tuning by means of -configuration file and xml attributes. Source code content fingerprinting is digests based which is more reliable over -widely used file timestamps in tools like Make or Apache Ant. Deterministic build state allows reliably cache outputs -even of the build in progress and share them between teams using remote cache. Deterministic inputs calculation allows -distributed and parallel builds running in heterogeneous environments (like cloud of build agents) -could efficiently reuse cached build artifacts. Therefore, incremental Maven is particularly well-suited for large Maven -projects that have significant number of small modules. Remote cache in conjunction with relocatable inputs -identification effectively enables "change once - build once" approach across all environments. +The idea of the build cache is to calculate a key from module inputs, store outputs in the cache, and restore them later +transparently to the standard Maven core. The cache deterministically associates each project state with a unique key +and restores it in subsequent builds. It analyzes source code, project model, +plugins, and their parameters. Projects with the same key are up-to-date (not changed) and could be safely restored from +the cache. Projects producing different keys are out-of-date (changed), and the cache fully rebuilds them. In the latter +case, the cache does not make any +interventions to the build execution logic and delegates build work to the standard maven Maven core. This approach +ensures that +artifacts produced in the presence of a cache are equivalent to the result produced by a standard Maven build. +To achieve an accurate key calculation, incremental Maven combines automatic introspection +of [project object model](https://maven.apache.org/pom.html#What_is_the_POM) and fine-grained tuning using +a configuration file. Source code content fingerprinting is digests based, which is more reliable over +widely used file timestamps in tools like Make or Apache Ant. Cache outputs could be shared using a remote cache. +Deterministic inputs calculation allows distributed and parallel builds running in heterogeneous environments (like a +cloud of build agents) efficiently reuse cached build artifacts as soon as they are published. Therefore, incremental +Maven is particularly well-suited for large Maven +projects that have a significant number of small modules. Remote cache, combined with relocatable inputs +identification, effectively enables the "change once - build once" approach across all environments. ### Maven insights -The challenge of implementing build cache in Maven is that domain model is overly generic and doesn't have dedicated api -for build inputs. Because of that, even 2 identically looking builds from the same source code could normally produce 2 -different results. The question here is tolerance level - can you accept particular discrepancies or not. For most of -teams artifacts produced in the same build environment from the same source code will be considered equivalent and -technical differences between them (like different timestamps in jar manifests) could be ignored. Now consider scenario -when artifact is first produced with compiler X and cached. Later, without touching source code, compiler changes to Y -and build yields significantly different outcomes of compilation. Should the produced artifacts be considered as -equivalent? Both Yes and No answers are possible and could be even desirable in different scenarios. When productivity -and performance are the primary concerns it could be desirable to tolerate insignificant discrepancies and maximise -reuse of cached builds. As long as correctness is in focus there could be demand to comply with the exact release -process. In the same way as with classic Maven, correctness is ensured by proper build configuration and controllable -build environments. In the same way as with classic Maven the previous build is just an approximation of today build -with some tolerance (implementation, configuration and environment driven). +Maven is a proven tool with a long history and core design established many years ago. Historically, Maven's core was +designed with generic stable interfaces that don't have a concept of inputs and outputs. It just runs as configured, but +the core does not control the inputs and effects of the run. Most commonly, artifacts produced in the same build +environment from the same source code will be considered equivalent. But even two identically looking builds from the +same source code could have two different results. The question here is tolerance level - can you accept particular +discrepancies? Though technical differences between artifacts like timestamps in manifests are largely ignored, when +compilers used are of different levels, it is likely a critical difference. Should the produced artifacts be considered +equivalents? Yes and No answers are possible and could be desirable in different scenarios. When productivity +and performance are the primary concerns, it could be beneficial to tolerate insignificant discrepancies and maximize +the reuse. As long as correctness is in focus, there could be a demand to comply with the exact release requirements. In +the same way as Maven, the cache correctness is ensured by proper build configuration and control over the build +environment. As Maven itself, the cached result is just an approximation of another build with some tolerance level ( +implementation, configuration, and environment driven) that must be configured to meet your needs. ### Implementation insights -At very simple form, the build cache Maven is essentially a hash function which takes Maven project and produces cache -key for a project. Then the key is used to store and restore build results. Because of different factors there could be -collisions and instabilities in the produced key. Collision could happen if the same key produced from the semantically -different build states and will result in unintended reuse. Instability means that same input yields different key in -different runs resulting in cache misses. The ultimate target is to find tradeoff between correctness and performance by -means of configuration. In current implementation this is achieved by configuring cache processing rules in xml file. +Simply put, the build cache is a hash function that takes a Maven project and produces a unique key. Then the key is +used to store and restore build results. Because of different factors, there could be +collisions and instabilities in the produced key. A collision happens when the semantically different builds have the +same key and will result in unintended reuse. Instability means that the same input yields different keys resulting in +cache misses. The ultimate target is to find a tradeoff between correctness and performance by configuring cache +processing rules in an xml file. -In order to achieve better correctness need to: +To maximize correctness: -* Verify that every relevant file is selected as input to engine -* Add critical plugin parameters to reconciliation (because they could be overridden from command line) +* Select every relevant file as input to the engine +* Add critical plugin parameters to the reconciliation -In order to achieve better reuse need to: +To maximize reuse need to: -* ensure that non-critical files (test logs, readme and similar) are filtered out from build inputs. -* non-critical plugin parameters (like number of threads in build) are filtered out from build inputs -* Source code is relocatable and build parameters are relocatable (not environment specific) +* Filter out non-essential files (logs, documentation, and similar) +* Filter out non-critical plugin parameters (like the number of threads or log level) +* Make source code relocatable (environment agnostic) -Essentially cache setup is a process of inspecting build, taking these decision and reflect them in the cache +Effectively, cache setup involves inspecting the build, taking these decisions, and reflecting them in the cache configuration. -Please notice though idea of perfectly matching builds might be tempting, but it is not practical with regard to -caching. Perfect correctness could lead to prevailing hit misses and render caching useless when applied to real -projects. In practice, configuring sufficient(good enough) correctness might yield the best outcomes. Incremental Maven -provides flexible and transparent control over caching policy and allows achieving desired outcomes - maximize usability -or maximize equivalence(correctness) between pre-cached candidates and requested builds. +Though strict, comprehensive cache rules aiming for 100% coverage of all parameters and files might be tempting, it is +rarely the optimal decision. When applied to real projects, perfect correctness could lead to prevailing hit misses and +render caching useless. Configuring sufficient (good enough) correctness might yield the best outcomes. Incremental +Maven +provides flexible and transparent control over caching policy and allows achieving desired results - maximizes usability +or maximize equivalency between pre-cached candidates and requested builds. ## Usage -Cache extension is an opt-in feature. It is delivered as is and though the tool went through careful verification it's -still build owner's responsibility to verify build outcomes. +Cache extension is an opt-in feature. It is delivered as is, and though the tool went through careful verification, it's +still the build owner's responsibility to verify build outcomes. ### Recommended Scenarios Given all the information above, the Incremental Maven is recommended to use in scenarios when productivity and Review Comment: fixed -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. 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