I've got 100G text files coming in daily, and I wish to create an efficient "database" accessible from Spark. By "database" I mean the ability to execute fast queries on the data (going back about a year), and incrementally add data each day, preferably without read locks.
Assuming I want to use Spark SQL and parquet, what's the best way to achieve this?
give up on concurrent reads/writes and append new data to the existing parquet file.
create a new parquet file for each day of data, and use the fact that Spark can load multiple parquet files to allow me to load e.g. an entire year. This effectively gives me "concurrency".
something else?
Feel free to suggest other options, but let's assume I'm using parquet for now, as from what I've read this will be helpful to many others.
My Level 0 design of this
Use partitioning by date/time (if your queries are based on date/time to avoid scanning of all data)
Use Append SaveMode where required
Run SparkSQL distributed SQL engine so that
You enable querying of the data from multiple clients/applications/users
cache the data only once across all clients/applications/users
Use just HDFS if you can to store all your Parquet files
I have very similar requirement in my system. I would say if load the whole year's data -for 100g one day that will be 36T data ,if you need to load 36TB daily ,that couldn't be fast anyway. better to save the processed daily data somewhere(such as count ,sum, distinct result) and use that to go back for whole year .
Related
I'm trying to convert large parquet files to delta format for performance optimization and a faster job run.
I'm trying to research the best practices to migrate huge parquet files to delta format on Databricks.
There are two general approaches to that, but it's really depends on your requirements:
Do in-place upgrade using the CONVERT TO DELTA (SQL Command) or corresponding Python/Scala/Java APIs (doc). You need to take into account following consideration - if you have a huge table, then default CONVERT TO DELTA command may take too long as it will need to collect statistics for your data. You can avoid this by adding NO STATISTICS to the command, and then it will run faster. With it, you won't be able to get benefits of data skipping, and other optimizations, but these statistics could be collected later when executing OPTIMIZE command.
Create a copy of your original table by reading original Parquet data & writing as a Delta table. After you check that everything is correct, you may remove original table. This approach have following benefits:
You can change partitioning schema if you have too many levels of partitioning in your original table
You can change the order of columns in the table to take advantage of data skipping for numeric & date/time data types - it should improve the query performance.
I have to read in N parquet files, sort all the data by a particular column, and then write out the sorted data in N parquet files. While I'm processing this data, I also have to produce an index that will later be used to optimize the access to the data in these files. The index will also be written as a parquet file.
For the sake of example, let's say that the data represents grocery store transactions and we want to create an index by product to transaction so that we can quickly know which transactions have cottage cheese, for example, without having to scan all N parquet files.
I'm pretty sure I know how to do the first part, but I'm struggling with how to extract and tally the data for the index while reading in the N parquet files.
For the moment, I'm using PySpark locally on my box, but this solution will eventually run on AWS, probably in AWS Glue.
Any suggestions on how to create the index would be greatly appreciated.
This is already built into spark SQL. In SQL use "distribute by" or pyspark: paritionBy before writing and it will group the data as you wish on your behalf. Even if you don't use a partitioning strategy Parquet has predicate pushdown that does lower level filtering. (Actually if you are using AWS, you likely don't want to use partitioning and should stick with large files that use predicate pushdown. Specifically because s3 scanning of directories is slow and should be avoided.)
Basically, great idea, but this is already in place.
Help me please to understand how can I write data to the place that is also being read from without any issue, using EMR and S3.
So I need to read partitioned data, find old data, delete it, write new data back and I'm thinking about 2 ways here:
Read all data, apply a filter, write data back with save option SaveMode.Overwrite. I see here one major issue - before writing it will delete files in S3, so if EMR cluster goes down by some reason after deletion but before writing - all data will be lost. I can use dynamic partition but that would mean that in such situation I'm gonna lost data from 1 partition.
Same as above but write to the temp directory, then delete original, move everything from temp to original. But as this is S3 storage it doesn't have move operation and all files will be copied, which can be a bit pricy(I'm going to work with 200GB of data).
Is there any other way or am I'm wrong in how spark works?
You are not wrong. The process of deleting a record from a table on EMR/Hadoop is painful in the ways you describe and more. It gets messier with failed jobs, small files, partition swapping, slow metadata operations...
There are several formats, and file protocols that add transactional capability on top of a table stored S3. The open Delta Lake (https://delta.io/) format, supports transactional deletes, updates, merge/upsert and does so very well. You can read & delete (say for GDPR purposes) like you're describing. You'll have a transaction log to track what you've done.
On point 2, as long as you have a reasonable # of files, your costs should be modest, with data charges at ~$23/TB/mo. However, if you end with too many small files, then the API costs of listing the files, fetching files can add up quickly. Managed Delta (from Databricks) will help speed of many of the operations on your tables through compaction, data caching, data skipping, z-ordering
Disclaimer, I work for Databricks....
I'm exploring ways to store a high volume of data from sensors (time series data), in a way that's scalable and cost-effective.
Currently, I'm writing a CSV file for each sensor, partitioned by date, so my filesystem hierarchy looks like this:
client_id/sensor_id/year/month/day.csv
My goal is to be able to perform SQL queries on this data, (typically fetching time ranges for a specific client/sensor, performing aggregations, etc) I've tried loading it to Postgres and timescaledb, but the volume is just too large and the queries are unreasonably slow.
I am now experimenting with using Spark and Parquet files to perform these queries, but I have some questions I haven't been able to answer from my research on this topic, namely:
I am converting this data to parquet files, so I now have something like this:
client_id/sensor_id/year/month/day.parquet
But my concern is that when Spark loads the top folder containing the many Parquet files, the metadata for the rowgroup information is not as optimized as if I used one single parquet file containing all the data, partitioned by client/sensor/year/month/day. Is this true? Or is it the same to have many parquet files or a single partitioned Parquet file? I know that internally the parquet file is stored in a folder hierarchy like the one I am using, but I'm not clear on how that affects the metadata for the file.
The reason I am not able to do this is that I am continuously receiving new data, and from my understanding, I cannot append to a parquet file due to the nature that the footer metadata works. Is this correct? Right now, I simply convert the previous day's data to parquet and create a new file for each sensor of each client.
Thank you.
You can use Structured Streaming with kafka(as you are already using it) for real time processing of your data and store data in parquet format. And, yes you can append data to parquet files. Use SaveMode.Append for that such as
df.write.mode('append').parquet(path)
You can even partition your data on hourly basis.
client/sensor/year/month/day/hour which will further provide you performance improvement while querying.
You can create hour partition based on system time or timestamp column based on type of query you want to run on your data.
You can use watermaking for handling late records if you choose to partition based on timestamp column.
Hope this helps!
I could share my experience and technology stack that being used at AppsFlyer.
We have a lot of data, about 70 billion events per day.
Our time-series data for near-real-time analytics are stored in Druid and Clickhouse. Clickhouse is used to hold real-time data for the last two days; Druid (0.9) wasn't able to manage it. Druid holds the rest of our data, which populated daily via Hadoop.
Druid is a right candidate in case you don't need a row data but pre-aggregated one, on a daily or hourly basis.
I would suggest you let a chance to the Clickhouse, it lacks documentation and examples but works robust and fast.
Also, you might take a look at Apache Hudi.
I have a quite hefty parquet file where I need to change values for one of the column. One way to do this would be to update those values in source text files and recreate parquet file but I'm wondering if there is less expensive and overall easier solution to this.
Lets start with basics:
Parquet is a file format that needs to be saved in a file system.
Key questions:
Does parquet support append operations?
Does the file system (namely, HDFS) allow append on files?
Can the job framework (Spark) implement append operations?
Answers:
parquet.hadoop.ParquetFileWriter only supports CREATE and OVERWRITE; there is no append mode. (Not sure but this could potentially change in other implementations -- parquet design does support append)
HDFS allows append on files using the dfs.support.append property
Spark framework does not support append to existing parquet files, and with no plans to; see this JIRA
It is not a good idea to append to an existing file in distributed systems, especially given we might have two writers at the same time.
More details are here:
http://bytepadding.com/big-data/spark/read-write-parquet-files-using-spark/
http://bytepadding.com/linux/understanding-basics-of-filesystem/
There are workarounds, but you need to create your parquet file in a certain way to make it easier to update.
Best practices:
A. Use row groups to create parquet files. You need to optimize how many rows of data can go into a row group before features like data compression and dictionary encoding stop kicking in.
B. Scan row groups one at a time and figure out which row groups need to be updated. Generate new parquet files with amended data for each modified row group. It is more memory efficient to work with one row group's worth of data at a time instead of everything in the file.
C. Rebuild the original parquet file by appending unmodified row groups and with modified row groups generated by reading in one parquet file per row group.
it's surprisingly fast to reassemble a parquet file using row groups.
In theory it should be easy to append to existing parquet file if you just strip the footer (stats info), append new row groups and add new footer with update stats, but there isn't an API / Library that supports it..
Look at this nice blog which can answer your question and provide a method to perform updates using Spark(Scala):
http://aseigneurin.github.io/2017/03/14/incrementally-loaded-parquet-files.html
Copy & Paste from the blog:
when we need to edit the data, in our data structures (Parquet), that are immutable.
You can add partitions to Parquet files, but you can’t edit the data in place.
But ultimately we can mutate the data, we just need to accept that we won’t be doing it in place. We will need to recreate the Parquet files using a combination of schemas and UDFs to correct the bad data.
If you want to incrementally append the data in Parquet (you did n't ask this question, still it would be useful for other readers)
Refer this well written blog:
http://aseigneurin.github.io/2017/03/14/incrementally-loaded-parquet-files.html
Disclaimer: I have n't written those blogs, I just read it and found it might be useful for others.
You must re-create the file, this is the Hadoop way. Especially if the file is compressed.
Another approach, (very common in Big-data), is to do the update on another Parquet (or ORC) file, then JOIN / UNION at query time.
Well, in 2022, I strongly recommend to use a lake house solution, like deltaLake or Apache Iceberg. They will care about that for you.