Sunday, January 29, 2017

Handling data with changing schema on Hadoop

User data often is unpredictable, even when we can predict a change coming our way we need to prepare for that. Make changes in our environment to accept the incoming change, accommodate and absorb. 

With that being a fact of life, our design should allow for those changes to happen.  With the structured data warehouses gone (well, mostly), Hadoop based data stores are the norm these days, and would be default in future not so distant.  By definition, Hadoop allows all kinds of data to be stored on the cluster and happily provides various tools to process such data.

However, with different types of data out there on cluster, any given tool would need to "know" what data that is? How to read it, interpret it and process it. That would have been relatively straightforward with the data schema being static, as in, a file comes in from source A, with a certain format, and that’s about it. You can define programs/structures to work with it. 

What if the source system decides to alter the data set schema, add a few columns, and change some column ordering, perhaps data type too, what then? Would our program reading that dataset on Hadoop cluster still work as it? No.. It won’t .. Because it would also require change to reflect the same changes sent from source.

Assume that you did those changes too. So, now our program is version 2, able to handle the dataset sent by source (v2 as well). What about the older data lying around on Hadoop cluster (v1, with old schema)? How is that dataset going to used/usable?  What if that data was used by hive tables, would you have two (or more) set of tables? 

What if that data was exposed to some analytical programs, through R or similar? I am sure we won’t want to rewrite/test the whole solutions just because source system decided to change something?

And this scenario is not purely hypothetical, it happens around us, more often than not, just that we deal with it in our own patchwork styles, or solutions varying on a case to case basis.

Here I propose a somewhat generic solution, one that might work for all, well, nearly all use cases.

Avro is a data serialization system, wherein the schema is built into the file itself. As a result, programs reading the data-file doesn’t have to bother with interpreting the data file through external schema.

Avro was designed by Doug Cutting for similar reasons, as a tool that is language and schema independent and therefore unrelated tools/technologies can deal with data files.

That being said, is not a silver bullet in itself, but it handles the schema change fairly nicely.

Basically, you store the data on hdfs in Avro formatted files. For that, Avro schema needs to be created, which is fairly straightforward json structured file, indicating the record structure. Take a look at an example here - 

{"namespace": "myExample.avro",  "type": "record",  "name": "User",  "fields": [      {"name": "name", "type": "string"},      {"name": "age",  "type": ["int", "null"]},      {"name": "address", "type": ["string", "null"]}  ] }
create external  table ex_users (name     String ,  age     int , address     String) stored as AVRO location '/apps/data/hive/ex_users.tbl';




When files are saved as Avro formatted files, information from the schema is built into them, in plain text, and is parse-able for programs like hive and others to interpret the rest of the file.

For this to work, the tool has to rely on Avro. Look at this hive table definition - 

With the clause "Stored as Avro" we are basically telling hive to match the columns defined in hive with columns in Avro fi
les in that hdfs location and use whatever columns map to hive definition.
  
Avro is compressible, split-able and stores data in binary format, providing some additional features.

When this Avro formatted file is exposed to programs like hive, they don’t have to worry about changes in schema, since they rely on the schema information in the file header. If the hive / R schema asks for 5 columns, but the file only has 3 of them (with names as matching criteria and not order/position of columns), those matching columns are project to the consuming tool, and the rest is ignored. 

Similarly, if the tool asks for data that is not present in a certain data file (remember our example v2 file with additional column as against v1), the column that is not present is reported as null.

Now, look at the following data file (new avro schema - after source system decided to change)

{"namespace": "myExample.avro", "type": "record", "name": "User", "fields": [     {"name": "name", "type": "string"},     {"name": "age",  "type": ["int", "null"]},       {"name": "salary",  "type": ["float", "null"]},     {"name": "address", "type": ["string", "null"]} ]}

And the corresponding hive table definition - 

create external  table ex_users (name     String ,  age     int , address     String, salary float)stored as AVROlocation '/apps/data/hive/ex_users.tbl';


For this table definition, its not a problem to query and report a data file in old schema (v1) wherein the column salary is not present. it would simply know from the avro schema definition in that header that the file doenst have that column, and it would gladly report that as null.

On the other hand, in the new structure has a different order of columns, which is also fine for hive/avro combination since the schema definition helps match up the columns and report the right content in right columns. 

Similar analogies can be derived for other data consumers who need to interpret the schemas for data files on hdfs.

In comparison to some other storage formats, we do lose some points on the performance scale, but for the use case, where schema is changing and we want to be able to handle the changes with little or no effort, this is a better fit than many others.

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