Thanks Bowen for bringing up this discussion ~ I think the JDBC catalog is a useful feature.
Just one question about the "Flink-Postgres Metaspace Mapping” part: Since the PostgreSQL does not have catalog but schema under database, why not mapping the PG-database to Flink catalog and PG-schema to Flink database Flink Catalog Metaspace Structure Postgres Metaspace Structure catalog name (defined in Flink only) database name database name schema name table name table name Because the “catalog/database/table” are just the categories to manage the tables, and they are not standard, this is just a suggestion from my side I’m a little worried about the extensibility of the current categories of Flink, i.e. how about there are JDBC vendors that have something other than “schema”(for example “namespace” or something like that) ? For example, here is what I found for MySQL[1] >As defined in the MySQL Glossary: In MySQL, physically, a schema is synonymous with a database. You can substitute the keyword SCHEMA instead of DATABASE in MySQL SQL syntax, for example using CREATE SCHEMA instead of CREATE DATABASE. [1] https://stackoverflow.com/questions/11618277/difference-between-schema-database-in-mysql Best, Danny Chan 在 2020年1月10日 +0800 AM10:34，Jark Wu <imj...@gmail.com>，写道： > Thanks Bowen for the reply, > > A user-facing JDBCCatalog and 'catalog.type' = 'jdbc' sounds good to me. > > I have some other minor comments when I went through the updated > documentation: > > 1) 'base_url' configuration: We are following the configuration format > guideline [1] which suggest to use dash (-) instead of underline (_). > And I'm a little confused the meaning of "base_url" at the first > glance, another idea is split it into several configurations: 'driver', > 'hostname', 'port'. > > 2) 'default-database' is optional, then which database will be used or what > is the behavior when the default database is not selected. > > 3) a builder for jdbc catalogs: I agree with Jingsong to provide a builder. > Because there is optional configuration here (the default database), > and providind Builder as the API will be easier for evolution, I'm not > sure we won't add/modify parameters in the future. > > [1]: > https://flink.apache.org/contributing/code-style-and-quality-components.html#configuration-changes > > On Fri, 10 Jan 2020 at 04:52, Bowen Li <bowenl...@gmail.com> wrote: > > > Hi Jark and Jingsong, > > > > Thanks for your review. Please see my reply in line. > > > > > why introducing a `PostgresJDBCCatalog`, not a generic `JDBCCatalog` > > (catalog.type = 'postgres' vs 'jdbc') ? > > > > Thanks for the reminding and I looked at JDBCDialect. A generic, > > user-facing JDBCCatalog with catalog.type = jdbc and find specific db > > implementations (pg v.s. mysql v.s. ...) is more aligned with how jdbc > > sink/source is handled, indeed. However, the catalogs would also need to > > execute the query and parse query results in a db-dependent way. E.g. jdbc > > catalog needs to establish connections to different databases within a db > > instance on demand. So just having JDBCDialect won't be enough. > > > > I think we can do the following: > > - provide a user-facing JDBCCatalog, composing a db-specific impl like > > PostgresJDBCCatalog and MySQLJDBCCatalog. Users still specify "jdbc" as > > type in both Table API and SQL CLI, internally it will create a db-specific > > impl depending on jdbc base url. > > - some statements can reside in JDBCDialect. Query execution and result > > parsing logic would be located in db-specific impls. > > > > - We can provide a Builder for Catalog, In my opinion, defaultDatabase, > > username, pwd can be included in JDBC DB url. > > > > I don't see much value in providing a builder for jdbc catalogs, as they > > only have 4 or 5 required params, no optional ones. I prefer users just > > provide a base url without default db, usrname, pwd so we don't need to > > parse url all around, as I mentioned jdbc catalog may need to establish > > connections to different databases in a db instance, > > > > - About timestamp and time, write down the specific Flink precision of > > Postgres? > > > > I've documented that. It's 0-6 > > > > - I think there is a part missing in your document, that is how to use this > > catalog. If you can write a complete example, I think it will be much > > clearer. > > > > I added some examples in both table api and SQL Cli. It will be no > > different from existing catalogs. > > > > - So a thing is what TableFactory will this catalog use? For example, > > JDBCTableSourceSinkFactory has different parameters for source or sink? How > > do you think about it? > > > > This catalog will directly call JDBCTableSourceSinkFactory without going > > thru service discovery because we are sure it's a jdbc table. I added it to > > the doc. > > > > For the different params besides schema, as we discussed offline, > > unfortunately we can't do anything right now until Flink DDL/DML are able > > to distinguish 3 types of params - external data's metada, source/sink > > runtime params, and Flink semantics params. The latter two can't be > > provided by catalogs. The problem is actually general to all catalogs, not > > just JDBCCatalog. I'm pushing for such an effort to solve it. At this > > moment we can only use some default params for some cases, and the other > > cases cannot take advantage of the JDBC catalog and users still have to > > write DDL manually. > > > > Thanks, > > Bowen > > > > On Wed, Jan 8, 2020 at 7:46 PM Jingsong Li <jingsongl...@gmail.com> wrote: > > > > > Thanks Bowen for driving this, > > > > > > +1 for this, The DDL schema definition is a headache for users, and > > catalog > > > is a solution to this problem. > > > > > > I have some questions and suggestions: > > > > > > - We can provide a Builder for Catalog, In my opinion, defaultDatabase, > > > username, pwd can be included in JDBC DB url. > > > > > > - About timestamp and time, write down the specific Flink precision of > > > Postgres? > > > > > > - I think there is a part missing in your document, that is how to use > > this > > > catalog. If you can write a complete example, I think it will be much > > > clearer. > > > > > > - So a thing is what TableFactory will this catalog use? For example, > > > JDBCTableSourceSinkFactory has different parameters for source or sink? > > How > > > do you think about it? > > > > > > Best, > > > Jingsong Lee > > > > > > On Thu, Jan 9, 2020 at 11:33 AM Jark Wu <imj...@gmail.com> wrote: > > > > > > > Thanks Bowen for driving this. > > > > > > > > +1 to this feature. > > > > > > > > My concern is that why introducing a `PostgresJDBCCatalog`, not a > > generic > > > > `JDBCCatalog` (catalog.type = 'postgres' vs 'jdbc') ? > > > > From my understanding, JDBC catalog is similar to JDBC source/sink. For > > > > JDBC source/sink, we have a generic > > > > implementation for JDBC and delegate operations to JDBCDialect. > > Different > > > > driver may have different implementation of > > > > JDBCDialect, e.g `quoteIdentifier()`. > > > > > > > > For JDBC catalog, I guess maybe we can do it in the same way, i.e. a > > > > generic JDBCCatalog implementation and delegate > > > > operations to JDBCDialect, and we will have `listDataBase()`, > > > > `listTables()` interfaces in JDBCDialect. The benefit is that: > > > > 0) reuse the existing `JDBCDialect`, I guess JDBCCatalog also need to > > > quote > > > > identifiers. > > > > 1) we can easily to support a new database catalog (e.g. mysql) by > > > > implementing new dialects (e.g. MySQLDialect). > > > > 2) this can keep the same behavior as JDBC source/sink, i.e. > > > > connector.type=jdbc, catalog.type=jdbc > > > > > > > > Best, > > > > Jark > > > > > > > > > > > > On Thu, 9 Jan 2020 at 08:33, Bowen Li <bowenl...@gmail.com> wrote: > > > > > > > > > Hi dev, > > > > > > > > > > I'd like to kick off a discussion on adding JDBC catalogs, > > specifically > > > > > Postgres catalog in Flink [1]. > > > > > > > > > > Currently users have to manually create schemas in Flink source/sink > > > > > mirroring tables in their relational databases in use cases like JDBC > > > > > read/write and consuming CDC. Many users have complaint about the > > > > > unnecessary, redundant, manual work. Any mismatch can lead to a > > failing > > > > > Flink job at runtime instead of compile time. All these have been > > quite > > > > > unpleasant, resulting in a broken user experience. > > > > > > > > > > We want to provide a JDBC catalog interface and a Postgres > > > implementation > > > > > for Flink as a start to connect to all kinds of relational databases, > > > > > enabling Flink SQL to 1) retrieve table schema automatically without > > > > > requiring user writes duped DDL 2) check at compile time for schema > > > > errors. > > > > > It will greatly streamline user experiences when using Flink to deal > > > with > > > > > popular relational databases like Postgres, MySQL, MariaDB, AWS > > Aurora, > > > > > etc. > > > > > > > > > > Note that the problem and solution are actually very general to Flink > > > > when > > > > > connecting to all kinds of external systems. We just focus on solving > > > > that > > > > > for relational databases in this FLIP. > > > > > > > > > > Thanks, > > > > > Bowen > > > > > > > > > > [1] > > > > > > > > > > > > > > > > > > > https://cwiki.apache.org/confluence/display/FLINK/FLIP-92%3A+JDBC+catalog+and+Postgres+catalog > > > > > > > > > > > > > > > > > > -- > > > Best, Jingsong Lee > > > > >
