Simulation methods

In DataFakeR: Generate Fake Data for Relational Databases

knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>",
  eval = TRUE,        
  echo = TRUE,         # echo code?
  message = TRUE,     # Show messages
  warning = TRUE,     # Show warnings
  fig.width = 8,       # Default plot width
  fig.height = 6,      # .... height
  dpi = 200,           # Plot resolution
  fig.align = "center"
)
knitr::opts_chunk$set()  # Figure alignment   
library(DataFakeR)
set.seed(123)
options(tibble.width = Inf)

DataFakeR offers various methods for defining how exactly each column should be simulated.

We can highlight the below four simulation method types:

1. Deterministic (formula or constraint-based) simulation.
2. Special method simulation.
3. Restricted simulation.
4. Default simulation.

The package tries to simulate the column with each method, with preserving the order of the above list.

That means, the package will try to simulate the column using special method first and when such method cannot be executed (the conditions for running such method are not met) then, the next method is applied. More to that when one of the methods successfully generated the data, the following ones are skipped.

The goal of this document is to describe each method providing required details and, if applied, provide an information how each method can be customized.

Deterministic (formula or constraint-based) simulation.

Let's imagine we want to generate tables that reflects standard database used in the library. One of the tables used there, can be the one describing all the books owned by the library.

The YAML configuration of such table can have a form:

# schema-books.yml
public:
  tables:
    books:
      columns:
        book_id:
          type: char(12)
        author:
          type: varchar
        title:
          type: varchar
        genre:
          type: varchar
        bought:
          type: date
        amount:
          type: smallint
        purchase_id:
          type: varchar

While running the standard simulation process we get the following result:

set.seed(123)
sch <- schema_source(system.file("extdata", "schema-books.yml", package = "DataFakeR"))
sch <- schema_simulate(sch)
schema_get_table(sch, "books")

Even though many columns don't look realistic let's take care of book_id and purchase_id columns. From the rules followed in the library, we know that:

• book_id is the concatenation of first 4 letters from author, title and bought,
• purchase_id is concatenation of word 'purchase_' and column bought (such rule is checked by SQL purchase_id = 'purchase_' || bought check constraint).

If we want to preserve such rules, we have two options:

• store the rules as check_constraints rewritten in R code,
• define book_id and purchase_id using formula.

Let's describe purchase_id using the first method:

# schema-books_2.yml
public:
  tables:
    books:
      columns:
        book_id:
          type: char(12)
        author:
          type: varchar
        title:
          type: varchar
        genre:
          type: varchar
        bought:
          type: date
        amount:
          type: smallint
        purchase_id:
          type: varchar
      check_constraints:
        purchase_id_check:
          column: purchase_id
          expression: !expr purchase_id == paste0('purchase_', bought)

let's update the source and plot column dependencies:

sch <- schema_update_source(sch, file = system.file("extdata", "schema-books_2.yml", package = "DataFakeR"))
schema_plot_deps(sch, "books")

As you can see DataFakeR detected dependency between purchase_id and bought column. More to that the package will assure bought column will be simulated before purchase_id.

Let's take a quick look at the data:

sch <- schema_simulate(sch)
schema_get_table(sch, "books")

As we can see check constraint rule was applied to correctly create purchase_id column.

Note: Check constraint expressions are used to detect column dependency and to create the checked column, only when the ones are equality expressions (that means the expression contains == operator). Such expression assures the column definition is deterministic. In the future releases (depending on the user's needs) the check expression rule can be extended to the cases where expression constains < and > operators.

Now let's take care to assure that book_id is the first 8 letters of concatenation of author, title and bought-year. Such example can be also described using check constraint, but we'll use a different method that opens various options for defining column rules.

If you want to create a column using custom expression, you pass it as a formula parameter in yaml configuration file. In our case, we want book_id be created with expression paste0(substr(author, 1, 4), substr(title, 1, 4), substr(bought, 1, 4)). Let's put it in configuration as formula parameter:

# schema-books_3.yml
public:
  tables:
    books:
      columns:
        book_id:
          type: char(12)
          formula: !expr paste0(substr(author, 1, 4), substr(title, 1, 4), substr(bought, 1, 4))
        author:
          type: varchar
        title:
          type: varchar
        genre:
          type: varchar
        bought:
          type: date
        amount:
          type: smallint
        purchase_id:
          type: varchar
      check_constraints:
        purchase_id_check:
          column: purchase_id
          expression: !expr purchase_id == paste0('purchase_', bought)

Again, update the source and plot column dependencies:

sch <- schema_update_source(sch, file = system.file("extdata", "schema-books_3.yml", package = "DataFakeR"))
schema_plot_deps(sch, "books")

The column dependencies are detected correctly. Let's move then to simulation step:

sch <- schema_simulate(sch)
schema_get_table(sch, "books")

As we can see the result is again as expected.

Note The formula expression is passed to dplyr::mutate in the implementation, which means you may use in the formula any dplyr-specific functions, such as n().

Note Formula-based column definition was classified as a 'Deterministic simulation' method, but you may also define formulas with random sampling inside. For example assuring that column end_date have values larger than start_date you may define: formula: start_date + sample(1:10, dplyr::n(), replace = TRUE).

Special method simulation

Let's take a look at the last simulated data:

schema_get_table(sch, "books")

As we mentioned in the previous section, we're not happy with the result of multiple column values. In this section we'll take of the result of author and title columns.

In case of author column, we'd like the values to be random, human readable names. Across R packages, there are many that offer such functionality. In case of DataFakeR, it's just enough to define spec: name for the column definition:

# schema-books_4.yml
public:
  tables:
    books:
      columns:
        book_id:
          type: char(12)
          formula: !expr paste0(substr(author, 1, 4), substr(title, 1, 4), substr(bought, 1, 4))
        author:
          type: varchar
          spec: name
        title:
          type: varchar
        genre:
          type: varchar
        bought:
          type: date
        amount:
          type: smallint
        purchase_id:
          type: varchar
      check_constraints:
        purchase_id_check:
          column: purchase_id
          expression: !expr purchase_id == paste0('purchase_', bought)

Again, update the source and plot column dependencies:

sch <- schema_update_source(sch, file = system.file("extdata", "schema-books_4.yml", package = "DataFakeR"))
sch <- schema_simulate(sch)
schema_get_table(sch, "books")

Voila!

How does it work?

Whenever you precise spec parameter to the column, DataFakeR will look for the simulation options if the special method with such name was defined. For the specific column type, such option is defined at options$opt_simul_spec_<column-type>.

So for the default options and character column type, we have:

default_faker_opts$opt_simul_spec_character

The name method was defined and we were able to use it in the simulation. Looking at the function body, we may see it uses charlatan::ch_name function to simulate human-readable names.

What else can we spot regarding the method definition? The below points are worth notice:

• the method have n parameter. This is obligatory parameter for each simulation method.
• the method parameters also contain standard column ones from YAML file. Whenever you need access to column parameters, just define them while creating the method. DataFakeR passes all the column parameters (not only the default ones) to each method.
• if the parameter is not directly defined in the function it's still possible to access all of them by parsing ellipsis (...). Please make sure ellipsis is always added as a function parameter.
• by defining spec_params yaml configuration parameter you may access and parse them directly in the method definition. Such parameter allows to resolve conflicts with standard parameters names when duplication exist.

Now, let's take care of preparing human-readable title. For this case we'll create our custom function. The titles will consist of combination of four words from predefined values:

books <- function(n) {
  first <- c("Learning", "Amusing", "Hiding", "Symbols", "Hunting", "Smile")
  second <- c("Of", "On", "With", "From", "In", "Before")
  third <- c("My", "Your", "The", "Common", "Mysterious", "A")
  fourth <- c("Future", "South", "Technology", "Forest", "Storm", "Dreams")
  paste(sample(first, n), sample(second, n), sample(third, n), sample(fourth, n))
}

Let's check a few possible results:

books(3)

Perfect!

In order to present how spec_params can be used let's add an option to skip the second word in the result:

books <- function(n, add_second = FALSE) {
  first <- c("Learning", "Amusing", "Hiding", "Symbols", "Hunting", "Smile")
  second <- c("Of", "On", "With", "From", "In", "Before")
  third <- c("My", "Your", "The", "Common", "Mysterious", "A")
  fourth <- c("Future", "South", "Technology", "Forest", "Storm", "Dreams")
  second_res <- NULL
  if (add_second) {
    second_res <- sample(second, n, replace = TRUE)
  }
  paste(
    sample(first, n, replace = TRUE), second_res, 
    sample(third, n, replace = TRUE), sample(fourth, n, replace = TRUE)
  )
}

Now, let's create the final method to use in the workflow.

The function needs to:

• take n parameter,
• optional standard parameter unique (we'll use it to assure unique results are returned),
• spec_params that allows us to modify special method parameters

simul_spec_character_book <- function(n, unique, spec_params, ...) {
  spec_params$n <- n

  DataFakeR::unique_sample(
    do.call(books, spec_params), 
    spec_params = spec_params, unique = unique
  )
}

Before we run the example, let's explain code blocks in function definition.

We're evaluating books using do.call. That's why we need to store all the parameters in the list passed to do.call. The only missing one is n, so:

spec_params$n <- n

We allowed our method to respect unique parameter, that's why we want to assure the returned sample is unique. We may achieve this using DataFakeR::unique_sample.

The function evaluates sampling expression multiple times, replacing duplicated values with the new ones. It's worth to mention some of the function parameters:

• sim_expr - simulation expression to be evaluated,
• ... - parameters values used in sim_expr,
• unique - if TRUE, the function will try to generate unique values,
• n_iter - number of iteration to try generate unique value.

So it's enough to define:

DataFakeR::unique_sample(
  sim_expr = do.call(books, spec_params), 
  spec_params = spec_params, unique = unique
)

Let's modify configuration file with the book method (with using spec_params to simulate full title):

schema-books_5.yml
public:
  tables:
    books:
      columns:
        book_id:
          type: char(8)
          formula: !expr paste0(substr(author, 1, 4), substr(title, 1, 4), substr(bought, 1, 4))
        author:
          type: varchar
          spec: name
        title:
          type: varchar
          spec: book
          spec_params:
            add_second: true
        genre:
          type: varchar
        bought:
          type: date
        amount:
          type: smallint
        purchase_id:
          type: varchar
      check_constraints:
        purchase_id_check:
          column: purchase_id
          expression: !expr purchase_id == paste0('purchase_', bought)

and define the new method in the package options:

my_opts <- set_faker_opts(
  opt_simul_spec_character = opt_simul_spec_character(book = simul_spec_character_book)
)
sch <- schema_source(
  system.file("extdata", "schema-books_5.yml", package = "DataFakeR"), 
  faker_opts = my_opts
)
sch <- schema_simulate(sch)
schema_get_table(sch, "books")

Great! We've managed to use our custom special method to simulate human-readable titles.

For the last part of this section it's worth to mention the remaining special methods for each column type. For numeric, integer, logical and Date column classes, DataFakeR offer spec: distr method that allow to simulate column from the selected distribution. The method requires to provide:

spec_params:
  method: <method-name>

where method name is the name of simulation function such as rnorm, rbinom etc.

For example, in order to simulate the column from normal distribution with mean = 10 and sd = 5 we should define:

spec: distr
spec_params:
  method: rnorm
  mean: 10
  sd: 5

Restricted simulation

Restricted simulation methods allow to simulate data considering exceptional parameters defined for each column.

Such parameters can be for example:

• values - possible column values for the target column
• range - possible column values range for the target column

All the restricted methods offered by DataFakeR for column type are defined in default_faker_opts$opt_simul_restricted_<column-type>.

Let's take a look what methods are offered for integer columns:

default_faker_opts$opt_simul_restricted_integer

We can see there are three methods defined:

• f_key - restricted method responsible for simulating foreign keys (we'll come back to this one in the last part of the section),
• range - method taking range parameter. When range is missing NULL is returned, otherwise integers between range[1] and range[2] are returned,
• in_set- method taking values parameter. When values is missing NULL is returned, otherwise sampled values from values are returned.

Let's highlight the general rule for restricted methods used by the package (excluding f_key one):

• each method has n as obligatory parameter that determines number of returned values,
• standard column parameters (not_null, unique, type) are optional but respected,
• the method usually have defined 'restriction' parameter that determines whether the method should be applied or not,
• when restriction condition is not met (for example 'restriction' parameter is missing) the method should return NULL.

The last point allows DataFakeR to scan across all the restriction methods. When a method returns NULL result, the package moves to simulate from the next method defined in opt_simul_restricted_<column-type> setting.

Similar to special methods, you may also define a set of your custom restricted methods. You may achieve this by setting up new methods with:

set_faker_opts(
  opt_simul_restricted_<column-type> = opt_simul_restricted_<column-type>(my_method = method, ...)
)

Note: DataFakeR allow also to rewrite currently existing method. So whenever you need to rewrite for example in_set method, just specify it as above.

Having the knowledge about restricted methods, let's use them to generate amount column from values between 1 and 99. We'll do it by using range method for amount column:

# schema-books_6.yml
public:
  tables:
    books:
      columns:
        book_id:
          type: char(8)
          formula: !expr paste0(substr(author, 1, 4), substr(title, 1, 4), substr(bought, 1, 4))
        author:
          type: varchar
          spec: name
        title:
          type: varchar
          spec: book
          spec_params:
            add_second: true
        genre:
          type: varchar
        bought:
          type: date
        amount:
          type: smallint
          range: [1, 99]
        purchase_id:
          type: varchar
      check_constraints:
        purchase_id_check:
          column: purchase_id
          expression: !expr purchase_id == paste0('purchase_', bought)

sch <- schema_update_source(sch, system.file("extdata", "schema-books_6.yml", package = "DataFakeR"))
sch <- schema_simulate(sch)
schema_get_table(sch, "books")

Awesome! Let's simulate now human-readable book genre from a set of defined values. We can achieve this using in_set method for character column type:

default_faker_opts$opt_simul_restricted_character$in_set

So let's assume we want the genre to be simulated from set: Fantasy, Adventure, Horror, Romance.

Let's define such set as possible values for genre column:

schema-books_7.yml
public:
  tables:
    books:
      columns:
        book_id:
          type: char(8)
          formula: !expr paste0(substr(author, 1, 4), substr(title, 1, 4), substr(bought, 1, 4))
        author:
          type: varchar
          spec: name
        title:
          type: varchar
          spec: book
          spec_params:
            add_second: true
        genre:
          type: varchar
          values: [Fantasy, Adventure, Horror, Romance]
        bought:
          type: date
        amount:
          type: smallint
          range: [1, 99]
        purchase_id:
          type: varchar
      check_constraints:
        purchase_id_check:
          column: purchase_id
          expression: !expr purchase_id == paste0('purchase_', bought)

sch <- schema_update_source(sch, system.file("extdata", "schema-books_7.yml", package = "DataFakeR"))
sch <- schema_simulate(sch)
schema_get_table(sch, "books")

Let's add the last improvement by specifying date range for books:

schema-books_8.yml
public:
  tables:
    books:
      columns:
        book_id:
          type: char(8)
          formula: !expr paste0(substr(author, 1, 4), substr(title, 1, 4), substr(bought, 1, 4))
        author:
          type: varchar
          spec: name
        title:
          type: varchar
          spec: book
          spec_params:
            add_second: true
        genre:
          type: varchar
          values: [Fantasy, Adventure, Horror, Romance]
        bought:
          type: date
          range: ['2020-01-02', '2021-06-01']
        amount:
          type: smallint
          range: [1, 99]
        purchase_id:
          type: varchar
      check_constraints:
        purchase_id_check:
          column: purchase_id
          expression: !expr purchase_id == paste0('purchase_', bought)

sch <- schema_update_source(sch, system.file("extdata", "schema-books_8.yml", package = "DataFakeR"))
sch <- schema_simulate(sch)
schema_get_table(sch, "books")

Here we are.

For the last part of restricted methods, let's add a few words about f_key restricted method.

As mentioned the method is responsible for simulating foreign key columns. When the column is defined in schema as a foreign key, DataFakeR will source possible values from the parent table and pass such values set as a values parameter to f_key method.

Simulating foreign key will skip execution of the remaining restricted methods defined in options.

To see it in action, let's extend our schema definition by adding a new borrowed table (let's also precise nrows for each table).

# schema-books_9.yml
public:
  tables:
    books:
      nrows: 10
      columns:
        book_id:
          type: char(8)
          formula: !expr paste0(substr(author, 1, 4), substr(title, 1, 4), substr(bought, 1, 4))
        author:
          type: varchar
          spec: name
        title:
          type: varchar
          spec: book
          spec_params:
            add_second: true
        genre:
          type: varchar
          values: [Fantasy, Adventure, Horror, Romance]
        bought:
          type: date
          range: ['2020-01-02', '2021-06-01']
        amount:
          type: smallint
          range: [1, 99]
        purchase_id:
          type: varchar
      check_constraints:
        purchase_id_check:
          column: purchase_id
          expression: !expr purchase_id == paste0('purchase_', bought)
    borrowed:
      nrows: 30
      columns:
        book_id:
          type: char(8)
        user_id:
          type: char(10)
      foreign_keys:
        book_id_fkey:
          columns: book_id
          references:
            columns: book_id
            table: books

Let's update the file and check table dependencies:

sch <- schema_update_source(sch, system.file("extdata", "schema-books_9.yml", package = "DataFakeR"))
schema_plot_deps(sch)

As shown, DataFakeR detected dependency between books and borrowed, and will generate books table first to get possible values for foreign key column.

Let's simulate the data and compare simumlated book ids:

sch <- schema_simulate(sch)
schema_get_table(sch, "books")

unique(schema_get_table(sch, "borrowed")$book_id)

So the values were correctly inherited from parent table.

Default simulation

When none of the above methods were applied DataFakeR will simulate the column using the default method. For each column type you may find the default methods defined at default_faker_opts$default_faker_opts$opt_simul_default_fun_<column-type>.

In order to overwrite such method it's just enough to:

set_faker_opts(
  opt_simul_default_fun_<column-type> = my_custom_method
)

DataFakeR documentation built on Feb. 16, 2023, 7:38 p.m.