gszadovszky commented on a change in pull request #164: URL: https://github.com/apache/parquet-format/pull/164#discussion_r544375834
########## File path: CoreFeatures.md ########## @@ -0,0 +1,178 @@ +<!-- + - Licensed to the Apache Software Foundation (ASF) under one + - or more contributor license agreements. See the NOTICE file + - distributed with this work for additional information + - regarding copyright ownership. The ASF licenses this file + - to you under the Apache License, Version 2.0 (the + - "License"); you may not use this file except in compliance + - with the License. You may obtain a copy of the License at + - + - http://www.apache.org/licenses/LICENSE-2.0 + - + - Unless required by applicable law or agreed to in writing, + - software distributed under the License is distributed on an + - "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY + - KIND, either express or implied. See the License for the + - specific language governing permissions and limitations + - under the License. + --> + +# Parquet Core Features + +This document lists the core features for each parquet-format release. This +list is a subset of the features which parquet-format makes available. + +## Purpose + +The list of core features for a certain release makes a compliance level for +implementations. If a writer implementation claims that it is at a certain +compliance level then it must use only features from the *core feature list* of +that parquet-format release. If a reader implementation claims the same if must +implement all of the listed features. This way it is easier to ensure +compatibility between the different parquet implementations. + +We cannot and don't want to stop our clients to use any features that are not +on this list but it shall be highlighted that using these features might make +the written parquet files unreadable by other implementations. We can say that +the features available in a parquet-format release (and one of the +implementations of it) and not on the *core feature list* are experimental. + +## Versioning + +This document is versioned by the parquet-format releases which follows the +scheme of semantic versioning. It means that no feature will be deleted from +this document under the same major version. (We might deprecate some, though.) +Because of the semantic versioning if one implementation supports the core +features of the parquet-format release `a.b.x` it must be able to read any +parquet files written by implementations supporting the release `a.d.y` where +`b >= d`. + +If a parquet file is written according to a released version of this document +it might be a good idea to write this version into the field `compliance_level` +in the thrift object `FileMetaData`. + +## Adding new features + +The idea is to only include features which are specified correctly and proven +to be useful for everyone. Because of that we require to have at least two +different implementations that are released and widely tested. We also require +to implement interoperability tests for that feature to prove one +implementation can read the data written by the other one and vice versa. + +## Core feature list + +This list is based on the [parquet thrift file](src/main/thrift/parquet.thrift) +where all the data structures we might use in a parquet file are defined. + +### File structure + +All of the required fields in the structure (and sub-structures) of +`FileMetaData` must be set according to the specification. +The following page types are supported: +* Data page V1 (see `DataPageHeader`) +* Dictionary page (see `DictionaryPageHeader`) + +**TODO**: list optional fields that must be filled properly. + +### Types + +#### Primitive types + +The following [primitive types](README.md#types) are supported +* `BOOLEAN` +* `INT32` +* `INT64` +* `FLOAT` +* `DOUBLE` +* `BYTE\_ARRAY` +* `FIXED\_LEN\_BYTE\_ARRAY` + +NOTE: The primitive type `INT96` is deprecated so it is intentionally not listed +here. + +#### Logical types + +The [logical type](LogicalTypes.md)s are practically annotations helping to +understand the related primitive type (or structure). Originally we have had +the `ConvertedType` enum in the thrift file representing all the possible +logical types. After a while we realized it is hard to extend and so introduced +the `LogicalType` union. For backward compatibility reasons we allow to use the +old `ConvertedType` values according to the specified rules but we expect that +the logical types in the file schema are defined with `LogicalType` objects. + +The following LogicalTypes are supported: +* `STRING` +* `MAP` +* `LIST` +* `ENUM` +* `DECIMAL` (for which primitives?) +* `DATE` +* `TIME`: **(Which unit, utc?)** +* `TIMESTAMP`: **(Which unit, utc?)** +* `INTEGER`: (all bitwidth 8, 16, 32, 64) **(unsigned?)** +* `UNKNOWN` **(?)** +* `JSON` **(?)** +* `BSON` **(?)** +* `UUID` **(?)** Review comment: My JSON example tried to point out that you have to recognize the logical type JSON to know it is a UTF-8 encoded string and not an arbitrary BINARY value. So in this case you at least partly support the logical type JSON. As per the specification a value with the logical type JSON has to be a UTF-8 encoded json document (json spec referenced). What the parquet implementation does with it is not part of the parquet spec. So, from write point of view you shall only use this annotation if it is really a json and is in UTF-8. From read point of view it is up to the implementation I guess. I agree it is not always clear what to specify for supporting a logical type from reading. At least it shall respect the semantics of the related type. For example in case of TIMESTAMP if isAdjustedToUTC is true then the timestamp value shall be represented according to the *instant* semantics otherwise to the *local* semantics. (See details [here](https://github.com/apache/parquet-format/blob/master/LogicalTypes.md#timestamp).) Similarly even though UUID is represented in a binary format it is highly suggested to be represented in the UUID string format when displayed (if UUID logical type is supported). ---------------------------------------------------------------- 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. For queries about this service, please contact Infrastructure at: [email protected]
