Filed under: Big Data — @ 8th of February 2021, 08:25
I recently wrote a post about the concept of the Data Lakehouse, which in some ways, brings components of what I outlined in the first post around my desires for a new database system to life. In this post, I am going to make an attempt to describe a roll-up of some recent big data developments that you should be aware of.
Let’s start with the lowest layer in the database or big data stack, which in many cases is Apache Spark as the processing engine powering a lot of the big data components. The component itself is obviously not new, but there is an interesting feature that was added in Spark 3.0, which is the Adaptive Query Execution (AQE). This features allows Spark to optimize and adjust query plans based on runtime statistics collected while the query is running. Make sure to turn it on for SparkSQL (spark.sql.adaptive.enabled) as it’s off by default.
The next component of interest is Apache Kudu. You are probably familiar with parquet. Unfortunately, parquet has some significant drawbacks, like it’s innate batch approach (you have to commit written data before it’s available for read). Specifically when it comes to real-time applications. Kudu’s on-disk data format closely resembles parquet, with a few differences to support efficient random access as well as updates. Also notable is that Kudu can’t use cloud object storage due to it’s use of Ext4 or XFS and the reliance on a consensus algorithm which isn’t supported in cloud object storage (RAFT).
At the same layer in the stack as Kudu and parquet, we have to mention Apache Hudi. Apache Hudi, like Kudu, brings stream processing to big data by providing fresh data. Like Kudu it allows for updates and deletes. Unlike Kudu though, Hudi doesn’t provide a storage layer and therefore you generally want to use parquet as its storage format. That’s probably one of the main differences, Kudu tries to be a storage layer for OLTP whereas Hudi is strictly OLAP. Another powerful feature of Hudi is that it makes a ‘change stream’ available, which allows for incremental pulling. With that it supports three types of queries:
Snapshot Queries : Queries see the latest snapshot of the table as of a given commit or compaction action. Here the concepts of ‘copy on write’ and ‘merge on read’ become important. The latter being useful for near real-time querying.
Incremental Queries : Queries only see new data written to the table, since a given commit/compaction.
Read Optimized Queries : Queries see the latest snapshot of table as of a given commit/compaction action. This is mostly used for high speed querying.
What then is Apache Iceberg and the Delta Lake then? These two projects yet another way of organizing your data. They can be backed by parquet, and each differ slightly in the exact use-cases and how they handle data changes. And just like Hudi, they both can be used with Spark and Presto or Hive. For a more detailed discussion on the differences, have a look here and this blog walks you through an example of using Hudi and Delta Lake.
Enough about tables and storage formats. While they are important when you have to deal with large amounts of data, I am much more interested in the query layer.
The project to look at here is Apache Calcite which is a ‘data management framework’ or I’d call it a SQL engine. It’s not a full database mainly due to omitting the storage layer. But it supports multiple storage engines. Another cool feature is the support for streaming and graph SQL. Generally you don’t have to bother with the project as it’s built into a number of the existing engines like Hive, Drill, Solr, etc.
As a quick summary and a slightly different way of looking at why all these projects mentioned so far have come into existence, it might make sense to roll up the data pipeline challenge from a different perspective. Remember the days when we deployed Lambda architectures? You had two separate data paths; one for real-time and one for batch ingest. Apache Flink can help unify these two paths. Others, instead of rewriting their pipelines, let developers write the batch layer and then used Calcite to automatically translate that into the real-time processing code and to merge the real-time and batch outputs, used Apache Pinot.
The nice thing is that there is a Presto to Pinot connector, allowing you to stay in your favorite query engine. Sidenote: don’t worry about Apache Samza too much here. It’s another distributed processing engine like Flink or Spark.
Enough of the geekery. I am sure your head hurts just as much as mine, trying to keep track of all of these crazy projects and how they hang together. Maybe another interesting lens would be to check out what AWS has to offer around databases. To start with, there is PartiQL. In short, it’s a SQL-compatible query language that enables querying data regardless of where or in what format it is stored; structured, unstructured, columnar, row-based, you name it. You can use PartiQL within DynamoDB or the project’s REPL. Glue Elastic views also support PartiQL at this point.
Well, I get it, a general purpose data store that just does the right thing, meaning it’s fast, it has the correct data integrity properties, etc, is a hard problem. Hence the sprawl of all of these data stores (search, graph, columnar, row) and processing and storage projects (from hudi to parquet and impala back to presto and csv files). But eventually, what I really want is a database that just does all these things for me. I don’t want to learn about all these projects and nuances. Just give me a system that lets me dump data into it and answers my SQL queries (real-time and batch) quickly.
Filed under: Big Data — @ 8th of February 2021, 08:22
In my previous blog post, I ranted a little about database technologies and threw a few thoughts out there on what I think a better data system would be able to do. In this post, I am going to talk a bit about the concept of the Data Lakehouse.
The term ‘data lakehouse‘ has been making the rounds in the data and analytics space for a couple of years. It describes an environment combining data structure and data management features of a data warehouse with the low-cost scalable storage of a data lake. Data lakes have advanced the separation of storage from compute, but do not solve problems of data management (what data is stored, where it is, etc). These challenges often turn a data lake into a data swamp. Said a different way, the data lakehouse maintains the cost and flexibility advantages of storing data in a lake while enabling schemas to be enforced for subsets of the data.
Let’s dive a bit deeper into the Lakehouse concept. We are looking at the Lakehouse as an evolution of the data lake. And here are the features it adds on top:
Data mutation – Data lakes are often built on top of Hadoop or AWS and both HDFS and S3 are immutable. This means that data cannot be corrected. With this also comes the problem of schema evolution. There are two approaches here: copy on write and merge on read – we’ll probably explore this some more in the next blog post.
Transactions (ACID) / Concurrent read and write – One of the main features of relational databases that help us with read/write concurrency and therefore data integrity.
Time-travel – This can feature is sort of provided through the transaction capability. The lakehouse keeps track of versions and therefore allows for going back in time on a data record.
Data quality / Schema enforcement – Data quality has multiple facets, but mainly is about schema enforcement at ingest. For example, ingested data cannot contain any additional columns that are not present in the target table’s schema and the data types of the columns have to match.
Storage format independence is important when we want to support different file formats from parquet to kudu to CSV or JSON.
Support batch and streaming (real-time) – There are many challenges with streaming data. For example the problem of out-of order data, which is solved by the data lakehouse through watermarking. Other challenges are inherent in some of the storage layers, like parquet, which only works in batches. You have to commit your batch before you can read it. That’s where Kudu could come in to help as well, but more about that in the next blog post.
If you are interested in a practitioners view of how increased data loads create challenges and how a large organization solved them, read about Uber’s journey that ended up in the development of Hudi, a data layer that supports most of the above features of a Lakehouse. We’ll talk more about Hudi in our next blog post.
Filed under: Big Data — @ 2nd of February 2021, 15:18
In 2015, I wrote a book about the Security Data Lake. At the time, the big data space was not as mature as today and especially the intersection of big data and security wasn’t a well understood area. Fast forward to today, people are talking about to the “Data Lakehouse“. A new concept that has been made possible by new database technologies, projects, and companies pushing the envelope. All of which are trying to solve our modern data management and analytics challenges. Or said differently, they are all trying to make our data actionable at the lowest possible cost. In this first of three blog post, I am going to look at what happened in the big data world during the past few years. In the second blog post, we’ll explore what a data lakehouse is and we will look around to understand some of the latest big data projects and tools that promise to uncover the secrets hidden in our data.
Let me start with a bit of a rant about database technologies. Back in the day, we had relational databases; the MySQL’s and Oracle’s of the world. And the world was good. Then we realized that not all data and not all access patterns were suited for these databases, so we invented the document stores, the search engines, the graph databases, the key value stores, the columnar databases, etc. And that’s when life got complicated. What database do you use for what purposes? Often it seemed like we’d need multiple ones. But that would have meant we’d needed to duplicate data, pick the right database for the task at hand, synchronize the data, etc. A nightmare. What happened then was that we just started using the technology that seemed to cover most of our needs and abused it for the other tasks. I have seen one too many document stores used to serve complex analytical questions (i.e., asking Lucene to return aggregate metrics and ad-hoc summaries).
Alongside the database technologies themselves, there is a notable secondary trend: increased requirements from a regulatory, privacy, and data locality perspective. Regulations like GDPR are imposing restrictions and requirements on how data can be stored and give individuals the right to see their data and even modify or delete it upon request. Some data stores have come up with privacy features, which are often in harsh contradiction to the insights we are looking for in the data. Finally, with increasingly going global, it matters where we collect and process our data. Not just for privacy purposes, but rather for processing speed and storage requirements. How, for example, do you compute global summaries over your data? Do you bring the data into one data center? Or do you compute local aggregates to then summarize them? Latency and storage costs are important factors to consider.
Wouldn’t it be nice if we had a data system that took care of all the above mentioned requirements automatically? It ingests the data we send to it – structured, unstructured, sensitive, non sensitive, anything. And on the other side, we formulate queries (I think we should keep SQL as the lingua franca for this) to answer the questions we have. Of course, we can add nice visualization layers on top, but that’s icing on the cake. I’d love a self-adjusting system. Don’t make me choose whether I wanted a graph database or not. Don’t make me configure data localities or privacy parameters. Let the system determine the necessary parameters – maybe bring me in the loop for things that the system cannot figure out itself, but make it easy on me. Definitely don’t ask me to create indexes or views. Let the system figure out those properties on the fly, while observing my access patterns. Move the data to where it is needed, create summary tables and materialized views transparently, while keeping storage cost and regulatory constraints in mind.
Now that we talked about storage and access, what about ETL? The challenge with translating data on ingest is that the translation often means loss of information. On the flip side, it makes analytics tasks easier and it helps clean the data. Take security logs (syslog), for example. We could store them in their original form as an unstructured string, or we could parse out every element to store the individual fields in a structured way. The challenge is the parser. If we get things wrong, we will loose entire log records. If, however, we stored the logs in their original form, we could do the transformation (parsing) at the time of analytics. The drawback then being that we will parse the same data multiple times over; every time we query or run any analytics on it. What to do? Again, wouldn’t it be nice if the data system took care of this decision for us? Keep the original data around if necessary, parse where needed, re-parse on error, etc.
Let’s look at one final piece of the data system puzzle, analytics. With the advent of cloud, there has been a big push to centralize analytics. That means all the data has to be shipped to a single, central location. That in itself is not always cheap, nor fast. We need an approach that allows us to keep some data completely decentralized. Leave the data at the place of generation and use the compute there to derive partial answer. Only send around the data that is needed. Again, with all the constraints and requirements we might have, such as compute availability and cost, hybrid data storage, considerations of fail over, redundancy, backups, etc. And again, I don’t want to configure these things. I’d like the system to take care of them after I told it some guiding parameters.
In a future post I will explore what has happened in the last couple of years in the big data ecosystem and what the lakehouse is about. Is there maybe a solution out there that sufficiently satisfies the above requirements?