Google Cloud Data Engineer Exam Preparation

Google Cloud Data Engineer Exam

This is a little text with all the stuff that helped me prepare for the Google Cloud Data Engineer Exam. There are a lot of courses and resources, that help you in preparing for this. The following links helped me in preparation for my Google Data Engineer Exam.

On Coursera there is are several courses provided by Google to prepare for this exam, including a Specialization for the Google Cloud Data Engineer Exam.

And to complete your preparation for the exam Coursera also offers a final preparation course Preparing for the Google Cloud Professional Data Engineer Exam

Other resources I used included these:

And Google also offers a test exam for the Google Cloud Data Engineer.

And now to a little deeper description of the topics I struggled a bit with.

Big Query


When you use streaming to insert data into BigQuery, it gets saved into the streaming buffer and then is loaded into the persistent table from there. It can take up to 90 minutes until the data is persistent. But all data is queryable together, buffer and persistent table. So your results update immediately.

During streaming inserts each row gets an insertID which Bigquery uses to try, find and remove duplicates, if these occur in a time window of one minute.

If this method is not secure enough, Google Cloud Datastore is an option, since it supports transactions.

You can also stream into partitioned tables, if you add a partition decorator $YYYYMMDD to the table. Streamed data gets saved to the UNPARTITIONED partition. To check this query the system column _PARTITIONDATE IS NULL.

Typical use cases include

  • high volume input, but not transactional data
  • analysis on aggregated data and not for single row queries


  • max row size: 1 MB
  • HTTP limit: 10 MB
  • max 10,000 rows / second per project
  • max 10,000 rows / query; 500 rows recommended
  • max 100 MB / s and table inserts in streaming
  • API limit of 100 queries / s per user
  • max 300 concurrent users
  • max 500 tablesets / s

Partitioned tables

Partitioned tables are separated into smaller units to optimize query performance. There are several ways to to partition tables in BigQuery:

  • partitioned by loading time (automatically)
  • partitioned by field with the data type TIMESTAMP or DATE

You can correct loading time by updating the suffix or pseudo columns. Partitioned tables contain a system column _PARTITION_TIME, you can use to filter. If you used an explizit, not automatic partitioning, the column _PARTITION_TIME is not exiting.

There are two columns for data, that do not belong to partitions:

  • __NULL for rows with a NULL value in the date field
  • __UNPARTITIONED for rows with a date outside of the valid timerange#

Another way to optimize tables, is sharding, where you use different tables to split data, instead of a partitioning column. Join these tables using a UNION. In this case keep in mind, that there is a limit of 1,000 tables per query.

Access rights

  • viewer: right to read all tables in a dataset
  • editor: viewer plus creating, deleting and changing of tables in a dataset
  • owner: editor plus creating and deleting datasets
  • admin: everything


There are some restrictions you need to keep in mind, when using BigQuery:

  • max 100 concurrent interactive queries possible
  • max 4 concurrent interactive queries on external sources
  • max 6 UDFs in a single interactive query
  • max 1,000 updates per table per day
  • max query duration is 6 hours
  • max 1,000 tables per query
  • SQL command can be max 1 MB in size
  • max 10,000 query parameters
  • max 100 MB / row
  • max 1,000 columns per table / query / view
  • max 16 levels of nested fields

Best Practices

ETL from relational database to BigQuery

There are several ways to import data from a relational database into BigQuery. Do this by using the WebUI, in which case the schema is upload able as as JSON file. You can export data to Google Cloud Storage in JSON format and import from there.

Another way is using Google Cloud Dataflow. Automatic setup is possible, but you can customize the process and maybe join data already in Dataflow, to denormalize it by using lookup tables as side input.

Backup / Snapshots

BigQuery supports a feature for “Point in Time Snapshots”. If you you a query decorator @<time> in Legacy SQL you can get an image of the table during the specified timestamp. This decorator can contain relative or absolute values.

BiqQuery ML

BigQuery supports machine learning in SQL for structured data. This is easy to learn, as it is based on SQL and not some other programming language. Right now it does provide linear regression, binary logistic regression for classification, multi class logistic regression and k-means clustering. The advantage of using this is, that you only have to use one tool, SQL, for everything. This means, data need not be moved from one place to another.


BigQuery does support user defined functions (UDF) in the languages SQL or JavaScript. Load external libraries by storing them in GCS.

Denormalize data

BigQuery does not need normalized data for fast query speed, but rather denormalized data. This means you should use nested and repeated fields in tables. Disk usage is not an issue here and denormalized data saves you the need to join.

You can reuse nested and repeated fields by using formats like AVRO, Parquet, ORC, JSON, Datastore or Firestore.

BigQuery Transfer Service

There is an option to repeatedly load data into BigQuery from Campaign Manager, GCS, S3, AdManager, Google Ads, Google Play and Youtube. You can also load Stackdriver logs into BigQuery.


BigTable is Googles solution for a real time key / value datastore. It handles sparse tables, meaning tables, where a lot of columns in rows are null. Empty columns do not use any space. This system is designed for billions of rows, thousands of columns and data the size of TB or PB.

BigTable only has one key per row, called rowkey. It has small latency and is HBase compatible. Also it scales linearly corresponding to the number of nodes in a cluster. There is an option for replication over two clusters, in which sets up one as cold standby. It is possible to change cluster sizes while the system is running, but after that BigTable takes a few minutes to rebalance cluster load.

To make the most out of it, its use case is for high scalability, key / value data and streaming.

BigTable orders columns belonging together into groups or families. Empty columns do not use any space. To make the most of it, divide key values equally on all nodes. Data is not stored on the cluster nodes, but on Colossus. The nodes only contain references to the data. This makes it easy to recover data after a system crash and no data is lost if nodes fail.

The master node distributes data node to optimize performance. Data locality is important. It can be influenced by key design. Group data that is queried together by designing key in that way. But BigTable does not support SQL or joins and is only really recommended with over 1TB of data.


SSDread 10,000 rows in 6 ms
write 10,000 rows in 6 ms
HDDread 5,000 rows in 200 ms
write 10,000 rows in 50 ms

Reasons for underperformance

  • incorrect schema design: key does not distribute data equally, only one key is possible
  • columns are sorted by family and then alphabetical
  • reads / writes are scewed
  • all operations need to be atomic on a row
  • information is distributed over several rows
  • row is to big (only 10MB per cell and 100MB per row)
  • timestamp is beginning of rowkey
  • not using combined keys
  • key consists of domains, hash or sequential ids
  • too little data (< 300GB)
  • test was to short (needs to run at least 10 minutes)
  • cluster too small (>70% CPU; 70% space)
  • HDD is used
  • still is a development instance
  • data is distributed automatically due to usage pattern, make sure to give system time to distribute to your needs

Key Visualizer

The Key Visualizer is a tool for supporting you in finding BigTable usage patterns. It provides graphic reports to help you recognize performance problems, like spotting hotspots (rows with a lot of actions) or rows with too much data. It helps you in getting evenly distributed accesses over all rows.

The reports are generated automatically (daily / hourly) if a table has more than 300GB or at least 10,000 row read or writes per second. The reports also recognize hierarchical keys and provides them as this hierarchy. Also connected keys are summarized.

Scale BigTable

You can scale BigTable programmatically. This is helpful if you, for example get monitoring data from Stackdriver and then react on this databy scaling up or down. After scaling a cluster it can take up to 20 minutes until you can see better performance, as data needs to be redistributed.

This automatic process does not help in cases where performance peaks for a short period of time. In this case, you need to scale manually before the event. If scaling does not help performance, then probably the key design leads to imbalanced load distribution.

You can scale a running cluster. It is possible to change the following things:

  • nodes in cluster
  • cluster per instance
  • usage profiles
  • label / names
  • upgrade from development to production instance

Schema design

Schema design in BigTable is crucial for performance. There is only one index per row, called rowkey. All rows are automatically sorted due to the rowkey, in alphabetical order. And as all operations are atomical per row, rows should not be dependent on one another.

Use key design to make sure data queried together is lying close to each other. And keep in mind, that one cell should not be bigger than 10MB and one row no larger than 100MB. The max values for this is 100MB per cell and 256MB per row.

There are up to 100 column families possible. If all dependent values are in one family, it is possible to only query this family. Less columns is better and column names should be short.

Key design

There are several best pratices for key design:

  • use inverted domain names, like
  • string ids should not contain hashes, due to readability
  • integrate timestamp if time is queried often, but not only timestamp
  • use inverted timestamp if newest data is queried more often
  • always use grouping field first in key
  • try to avoid a lot of updates on a row, try to write several rows instead


Dataflow is a system to develop and run data transformation pipelines in. It has the following key concepts:

  • Pipeline: complete process in a DAG
  • PCollection: distributed dataset for a pipeline
  • PTransform: data processing operation, that takes a PCollection and performs processing. It returns again a PCollection
  • ParDo: Parallel processing runs a function on a PCollection
  • SideInputs: additional inputs for ParDo / DoFns

It is a best practice to write invalid input to its own sink and not to throw it away.


Messaging queue provided by Google. It keeps data up to seven days. All logs are automatically pushed to Stackdriver for analysis. It has the following attributes:

  • messages might not be delivered in the order they were received in, because the system is distributed.
  • if message delivery is not acknowledged after a defined time, message will be redelivered. This timespan is configurable

Data Migrations

Data Transfer Appliance

The Data Transfer Appliance is an encrypted offline data transfer up to PB in size. You get the appliance and ship it back to Google where it will be uploaded into the cloud. This is faster than internet upload and can be handled like a NAS. Only applicable if the data is not changed in between.

Storage Transfer Service

Storage Transfer Service is a transfer service online to GCS or from bucket to bucket. It can be used to backup online data, or as a sync from source to sink, including deletion of source once it is copied. If you need to decide between gsutil and Storage Transfer Service it is:

  • local data -> gsutil; this also supports rsync
  • other cloud providers -> Storage Transfer

Dedicated Interconnect

Dedicated Interconnect is a physical connection between on premise and Google Cloud. You provide your own routing equipment. It is possible at 10 or 100Gbit.


Dataproc is a managed Hadoop / Spark cluster in Google Cloud. It offers some specials:

  • Cloud Storage Connector: Hadoop or Spark jobs access GCS directly
    • data is not transferred to HDFS
    • GCS is HDFS compatible, just use gs:// instead of hdfs://
    • data still available if cluster is gone
    • high availability is possible
  • Preemptible workers:
    • same machine type is others
    • own instance group
    • lost instances will be re-added once there are free resources
    • save no data
    • no clusters with just preemptible nodes
    • local disk is not available in hdfs, it is only used for local caching
  • Scale cluster:
    • scaling is always possible, even with running jobs

Machine Learning

Machine Learning Engine

The Machine Learning Engine provides predefined scaling values, but there is also a custom tier. There you can change:

  • masterType: machine type for the master
  • workerCount: number of workers in cluster
  • parameterServerCount: number of parameter servers

Data Loss Prevention API

This API finds sensible data (PPI) and can remove it, e.g. personal data, payment data, device identification, also custom and country specific. You can define word lists and regexes. It also provides data masking, removing and encryption. Use it in streams, text, files, BigQuery, pictures

Natural Language API

Textanalysis, Sentiment Analysis, Topic Mining, People and Location identification

Vision API

Picture analysis, object recognition, etc


Translation, Speech to Text, Text to Speech, Dialogflow as Chatbot, Video API


Datastore is a NoSQL DB, that is ACID compliant and has a SQL like querying language. There is an index for rows, combined indexes are possible too. It also provides a Firestore export to a GCS bucket.


Reporting solution provided by Google. It provides caching to speed up reports or reduce costs. There are two different kind of caches:

  • responsive cache: remembers results of a query and if data is queried again, cache supplies it
  • predictive cache:
    • analyses dimensions, metrics and filters and tries to predict possible queries.
    • queries are executed in background and response is stored in predictable cache
    • if responsive cache has no data, predictive cache is asked
    • if no data in predictive cache, data is queried from source
    • works only with owner credentials

Cache is automatically refreshed, but it can be done manually. The default for refreshing is 15 minutes, but also 4 and 12 hours. You should turn of predictive cache in case of:

  • data changes frequently and data freshness is more important than speed
  • reduce costs by turning it off

After the Google Cloud Data Engineer Exam

After taking the exam, you get a preliminary result on the spot. You can see it after submission or later in your Webassessor account. Once Google checked the exam you get an email with a voucher and a link to their perk shop, where you can chose between a backpack and a hoodie. I chose the backpack.

Google Cloud Data Engineer Exam Perk
Google Cloud Data Engineer Exam Perk

Plumber: Getting R ready for production environments?

R Project and Production

Running R Project in production is a controversially discussed topic, as is everything concerning R vs Python. Lately there have been some additions to the R Project, that made me look into this again. Researching R and its usage in production environments I came across several packages / project, that can be used as a solution for this:

There are several more, but those I found the most interesting.


For reasons of ease of use and because it was not a hosted version, I took a deeper look into Plumber. This felt quite natural as it uses function decorators for defining endpoints and parameters. This is similar to Spring Boot, which I normally use for programming REST APIs.
So using Plumber is really straight forward, as the example below shows:

#' return text "Hello"
#' @get /hello
function() {
  list(msg = "hello")

The #’ @get defines the endpoint for this request. In this case /hello, so the full url on localhost is To pass in one or more parameters you can use the decorator #’ @param parameter_name parameter_description. A more complicated example using Plumber is hosted on our Gitlab. This example was built with the help of Tidy Textmining.

Production ready?

Plumber comes with Swagger, so the webserver is automatically available. As the R instance is already running, processing the R code does not take long. If your model is complicated, then, of course, this is reflected in the processing time. But as R is a single thread programming language, Plumber can only process one request at a time.
There are ways to tweak this of course. You can run several instances of the service, using a Docker image. This is decribed here. There is also the option of using a webserver to fork the request to serveral instances of R. Depending on the need of the API, single thread processing can be fast enough. If the service has to be highly available the Docker solutions seems like a good choice, as it comes with a load balancer.


After testing Plumber I am surprised by the easy of use. This package makes deploying an REST API in R really easy. Depending on your business needs, it might even be enough for a productive scenario, especially when used in combination with Docker.

Apache Spark 2.0

Apache Spark has release version 2.0, which is a major step forward in usability for Spark users and mostly for people, who refrained from using it, due to the costs of learning a new programming language or tool. This is in the past now, as Spark 2.0 supports improved SQL functionalities with SQL2003 support. It can now run all 99 TPC-DS. The new SQL parser supports ANSI-SQL and HiveQL and sub queries.
Another new features is native csv data source support, based on the already existing Databricks spark csv module. I personally used this module as well as the spark avro module before and they make working with data in those formats really easy.
Also there were some new features added to MLlib:

  • PySpark includes new algorithms like LDA, Gaussian Mixture Model, Generalized Linear Regression
  • SparkR now includes generalized linear models, naive Bayes, k-means clustering, and survival regression.

Spark increased its performance with the release of 2.0. The goal was to make Spark 2.0 10x faster and Databricks shows this performance tuning in a notebook.

All of these improvements make Spark a more complete tool for data processing and analysing. The added SQL2003 support even makes it available for a larger user base and more importantly makes it easier to migrate existing applications from databases to Spark.

Python vs. R for Data Science

In Data Science there are two languages that compete for users. On one side there is R, on the other Python. Both have a huge userbase, but there is some discussion, which is better to use in a Data Science context. Lets explore both a bit:

R is a language and programming environment especially developed for statistical computing and grahics. It has been around some time and several thousand packages to tackle statistical problems. With RStudio it also provides an interactive programming environment, that makes analysing data pretty easy.

Python is a full range programming language, that makes it easy to integrate into a company wide system. With the packages Numpy, Pandas and Scikit-learn, Mathplotlib in combination with IPython, it also provides a full range suite for statistical computing and interactive programming environment.

R was developed solely for the purpose of statistical computing, so it has some advantages there, since it is specialized and has been around some years. Python is coming from a programming language and moves now into the data analysis field. In combination with all the other stuff it can do, websites and easy integrations into Hadoop Streaming or Apache Spark.
And for people who want to use the best of both sides can always use the R Python integration Rpy2.

I personally am recently working with Python for my ETL processes, including MapReduce, and anlysing data, which works awesome in combination with IPython as interactive development tool.

Apache Spark: The Next Big (Data) Thing?

Since Apache Spark became a Top Level Project at Apache almost a year ago, it has seen some wide coverage and adoption in the industry. Due to its promise of being faster than Hadoop MapReduce, about 100x in memory and 10x on disk, it seems like a real alternative to doing pure MapReduce.
Written in Scala, it provides the ability to write applications fast in Java, Python and Scala, and the syntax isn’t that hard to learn. There are even tools available for using SQL (Spark SQL), Machine Learning (MLib) interoperating with Pythons Numpy, graphics and streaming. This makes Spark to a real good alternative for big data processing.
Another feature of Apache Spark is, that it runs everywhere. On top of Hadoop, standalone, in the cloud and can easily access diverse data stores, such as HDFS, Amazon S3, Cassandra, HBase.

The easy integration into Amazon Web Services is what makes it attractive to me, since I am using this already. I also like the Python integration, because latelly, that became my favourite language for data manipulation and machine learning.

Besides the official parts of Spark mentioned above, there are also some really nice external packages, that for example integrate Spark with tools such as PIG, Amazon Redshift, some machine learning algorithms, and so on.

Given the promised speed gain, the ease of use and the full range of tools available, and the integration in third party programms, such as Tableau or MicroStrategy, Spark seems to look into a bright future.

The inventors of Apache Spark also founded a company called databricks, which offers professional services around Spark.

SQL on Hadoop: Facebook’s Presto

Earlier this month Facebook open sourced its own product for using SQL on Hadoop. It is called Presto and is something like Facebook’s answer to Cloudera’s Impala or Hortonwork’s Stinger already presented in an earlier post called SQL and Hadoop on this site.
Presto is unlike Hive and more like Impala, since it doesn’t rely on MapReduce for its queries. This makes it about 10 times faster than Hive on large datasets, or so Facebook claims in a blog post.
This product may have a huge impact on the further development of SQL on Hadoop tools, if it’s taken up by enough companies. But since there is no commercial goal linked to it right now, it seems more like Facebook will develop it as their needs increase. So they will not be hurried along.
Like Impala it does support a huge subset of ANSI SQL contrary to Hive’s SQL like HiveQL. So it again aims on making Hadoop more accessible for a broader audience of analyst, that already are familiar with SQL.
Analysis on Big Data sets have been strengthened by this release even more and the entry level investments for more companies to use Hadoop as data storage system are decreasing with every development in this direction.

Big Data in Learning

There are many fields in which big data can improve results. One of these being (e-)learning. Until recently the focus on analysing learning lay on analysing results of exams but with big data and analytics there are new possibilities to enhance the experience of learning as a whole. For example there is the possibility to personalize learning and helping students to achieve better results. Big Data makes this possible in nearly real time. There is the possibility to help students in the process of learning, as soon as the programm realizes a problem and providing a solution in the workflow, instead of the student having to stop his learning process for his problem to be solved and then continue. This also applies for working environments.
Not only inside a process of learning or work analysing data can come in handy. Even after a course is finished analysing the data produced during the course by all students can help optimize the course and resulting exam. Identifying where users got stuck or what was to easy will improve the learning experience for everyone.
There are already efforts to integrate this into the learning experience like Predictive Analytics Reporting (PAR) Framework.
PAR is trying to integrate several data sources and base their studies on this data instead of the studies that are based on individual programms. This approach broadens the base and this may make it able to find other (better) insights into the educational system of the U.S.

Data Science and Machine Learning

Machine Learning is acknowlegded as a part of Data Science, but will it be able to replace a Data Scientist?
There have been several articles around that topic in the last few years and months. It’s true there has been some major progress in the field of machine learning and there are already articles about the beginning of automated science like Lipson and Schmidt.
During the SXSWeek there will even be a Panel concerning this topic The Data Scientist Will Be Replaced By Tools.
The main question is, will machine results replace human expertise? There are several startups that provide data science as a service, like Prior Knowledge or Platfora. These companies help to discover knowledge hidden in a company’s data. PK looks in the provided data for correlations and helps to build predictive models. Pltfora on the other hand wants to make Hadoop usable for everyone.
These companies can help discover information, but only combined with human expertise from inside the company it is possible to make the most of the uncovered information. So, in my opinion, machine learning helps making the job of a data scientist easier, because he can concentrate more on his expertise with the context the data was created in.
This may even help in broadening the access to data science to more people.

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