Dec7

If You Care About Performance, Employ User Defined Types

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What is a User Defined Type? (UDT)?

User Defined Types (UDTs) allow a definition of struct that includes multiple typed named fields (including other UDTs). Once a UDT is defined, it can be used as a column type in a table definition. In Scylla, you can define a Column as a frozen<UDT>.

One of the hidden advantages of using UDTs is better performance. The example in this blog post will show that you can get more than a 225% increase in write throughput, 150% increase in read throughput, and ~35% better (lower) 99.9% latency in read. To understand why you can get better performance, let’s review what CQL allows for each Column/Cell in a Row:

  • Insert/Update each Cell independently
  • Delete each Cell independently
  • Set a different TTL for each Cell (CQL: INSERT INTO … USING TTL X; )
  • Set a different Writetime for each Cell (CQL: INSERT INTO… USING WRITETIME X;)

When a Column is defined as of type frozen, the Cell stored UDT fields share the same metadata (same TTL, Writetime).

So how much does it matter? Let’s test it out!

In the tests we compared the following:

  • “Regular Columns” table with X columns of blob type. The schema for a 10 column regular table tested:

  • “User Defined Types” table with a single column using a UDT of X fields of blob type. The schema for a table with a 10 column UDT field tested:

The tests were run multiple times with an increasing number of columns vs fields:

  • The selected X values 10,20,30,40,50 (e.g in the last case, a table with 50 blob columns was compared to a table with a UDT table of 50 blob fields)
  • In the write tests, all blob fields/columns have been set with 10-byte values
  • In the read case, all blob fields/columns have been retrieved
  • All tests were run with Scylla 2.0.0

Test 1: Write Throughput

UDT Throughput Write

# of fields/columns 10 20 30 40 50
User Defined Types 26870.19698 26151.79679 25352.78645 25179.01465 24562.727
Regular Columns 23161.82146 20403.72706 17759.65957 11979.37982 10738.29647
Gain 116.01% 128.17% 142.75% 210.19% 228.74%

Test 2: Read Throughput

# of fields/columns 10 20 30 40 50
User Defined Types 21924.22546 21858.83075 20520.39159 20827.56757 20448.30044
Regular Columns 20114.99601 17521.71543 16212.19758 13664.00421 12595.18025
Gain 108.99%  124.75%  126.57%  152.43%  162.35%

Test 3: Read Latency

# of fields/columns 10 20 30 40 50
User Defined Types 1.08134 1.01581 1.08134 1.24518 1.14688
Regular Columns 1.14688 1.27795 1.34349 1.57286 1.76947
Gain 5.71% 20.51% 19.51% 20.83% 35.19%

With regard to reads, there are two things worth mentioning:

  1. In CQL, you can return a subset of some UDT fields so there is no need to return all of the fields.
  2. The gains results shown above would not have changed significantly if a single column/field had been selected instead of all columns/fields that were requested in the test. Scylla’s cache is a data row cache—when a query is run to return part of a row Scylla will search for the complete row and insert that into the cache and only then create the query result.

Please note:

  • UDTs require additional processing on the client side (serialize/deserialize the structure)
  • Very limited test results show that not all drivers support UDTs as best as they can:
    • The tests above have been done using GoCQL driver—which seems to work well. You can find the scylla-bench fork with extensions for these tests here.
    • Previous tests with the Java driver were not as shiny.

To conclude, if you care about performance, consider using UDTs. Scylla currently does not support updating a subset of the fields – the enhancement is enqueued for a future release (vote for it if your use case may need it).

This a recap of an Ignite Talk I gave at Scylla Summit 2017 – you can find a recording of the talk and the slides on our Techtalk page.

Shlomi LivneAbout Shlomi Livne

Shlomi Livne, VP of R&D at ScyllaDB, has 15 years of experience in software development of large-scale systems. Previously he has led the research and development team at Convergin, which was acquired by Oracle. Shlomi holds a BA and MSc in Computer Science from the Technion-Machon Technologi Le' Israel.


Tags: features, internals, tutorial, udt