write apache iceberg table to azure ADLS / S3 without using external catalog

Question

I'm trying to create an iceberg table format on cloud object storage.

In the below image we can see that iceberg table format needs a catalog. This catalog stores current metadata pointer, which points to the latest metadata. The Iceberg quick start doc lists JDBC, Hive MetaStore, AWS Glue, Nessie and HDFS as list of catalogs that can be used.

My goal is to store the current metadata pointer(version-hint.text) along with rest of the table data(metadata, manifest lists, manifest, parquet data files) in the object store itself.

With HDFS as the catalog, there’s a file called version-hint.text in the table’s metadata folder whose contents is the version number of the current metadata file.

Looking at HDFS as one of the possible catalogs, I should be able to use ADLS or S3 to store the current metadata pointer along with rest of the data. For example: spark connecting to ADLS using ABFSS interface and creating iceberg table along with catalog.

My question is

• Is it safe to use version hint file as current metadata pointer in ADLS/S3? Will I lose any of the iceberg features if I do this? Looking at this comment from one of the contributors suggests that its not ideal for production.

The version hint file is used for Hadoop tables, which are named that way because they are intended for HDFS. We also use them for local FS tests, but they can't be safely used concurrently with S3. For S3, you'll need a metastore to enforce atomicity when swapping table metadata locations. You can use the one in iceberg-hive to use the Hive metastore.

• Looking at comments on this thread, Is version-hint.text file optional?

we iterate through on the possible metadata locations and stop only if there is not new snapshot is available

Could someone please clarify?

I'm trying to do a POC with Iceberg. At this point the requirement is to be able to write new data from data bricks to the table at least every 10 mins. This frequency might increase in the future.

The data once written will be read by databricks and dremio.

Answer 1

I would definitely try to use a catalog other than the HadoopCatalog / hdfs type for production workloads.

As somebody who works on Iceberg regularly (I work at Tabular), I can say that we do think of the hadoop catalog as being more for testing.

The major reason for that, as mentioned in your threads, is that the catalog provides an atomic locking compare-and-swap operation for the current top level metadata.json file. This compare and swap operation allows for the query that's updating the table to grab a lock for the table after doing its work (optimistic locking), write out the new metadata file, update the state in the catalog to point to the new metadata file, and then release that lock.

The lock isn't something that really works out of the box with HDFS / hadoop type catalog. And then it becomes possible for two concurrent actions to write out a metadata file, and then one sets it and the other's work gets erased or undefined behavior occurs as ACID compliance is lost.

If you have an RDS instance or some sort of JDBC database, I would suggest that you consider using that temporarily. There's also the DynamoDB catalog, and if you're using Dremio then nessie can be used as your catalog as well

In the next version of Iceberg -- the next major version after 0.14, which will likely be 1.0.0, there is a procedure to register tables into a catalog, which makes it easy to move a table from one catalog to another in a very efficient metadata only operation, such as CALL catalog.system.register_table('$new_table_name', '$metadata_file_location');

So you're not locked into one catalog if you start with something simple like the JDBC catalog and then move onto something else. If you're just working out a POC, you could start with the Hadoop catalog and then move to something like the JDBC catalog once you're more familiar, but it's important to be aware of the potential pitfalls of the hadoop type catalog which does not have the atomic compare-and-swap locking operation for the metadata file that represents the current table state.

There's also an option to provide a locking mechanism to the hadoop catalog, such as zookeeper or etcd, but that's a somewhat advanced feature and would require that you write your own custom lock implementation.

So I still stand by the JDBC catalog as the easiest to get started with as most people can get an RDBMS from their cloud provider or spin one up pretty easily -- especially now that you will be able to efficiently move your tables to a new catalog with the code in the current master branch or in the next major Iceberg release, it's not something to worry about too much.

Looking at comments on this thread, Is version-hint.text file optional?

Yes, the version-hint.txt file is used by the hadoop type catalog to attempt to provide an authoritative location where the table's current top-level metadata file is located. So version-hint.txt is only found with hadoop catalog, as other catalogs store it in their own specific mechanism. A table in an RDBMS instance is used to store all of the catalogs "version hints" when using the JDBC catalog or even the Hive catalog, which is backed by Hive Metastore (and very typically an RDBMS). Other catalogs include the DynamoDB catalog.

If you have more questions, the Apache Iceberg slack is very active.

Feel free to check out the docker-spark-iceberg getting started tutorial (which I helped create), which includes Jupyter notebooks and a docker-compose setup.

It uses the JDBC catalog backed by Postgres. With that, you can get a feel for what the catalog is doing by ssh'ing into the containers and running psql commands, as well as looking at table data on your local machine. There's also some nice tutorials with sample data! https://github.com/tabular-io/docker-spark-iceberg