Data Model

Underlying the Questionmark system is a data model that describes the data stored about quizzes, tests and exams (content); about the delivery of the content and about results.

Internally, that data is organized into a number of databases containing tables. Users of Questionmark OnDemand access these tables indirectly through API calls. Users of Questionmark OnPremise (or Perception 5) are encouraged to use APIs too but, in some cases, they may wish to access the database tables directly. In either case, the tables define the underlying data model for all Questionmark data.

In many cases the same data fields are exposed through multiple APIs. The data model is documented to provide a convenient central point for the detailed explanation of these data elements.

This part of the guide is in preparation and is not yet complete. For information about tables not documented here please refer to the older Database Schema Reference Guide. Where tables are documented both here and in the older guide the documentation here should be used in preference as it is more up-to-date. The older guide is no longer being maintained.

Misc Topics

Fields defined with varchar

Users are strongly advised to use API calls where possible. OnPremise or Perception users who must manipulate data in the database tables directly should be aware of the way text fields are encoded.

For legacy reasons, many fields in the data model are defined to have the SQL server type varchar. This represents a non-Unicode string of data in the database but in the Questionmark data model these fields are frequently used to contain Unicode data by applying a special transformation to the data when reading and writing to the underlying database field.

It is important to understand that varchar fields in the database are strings of characters drawn from the character set defined by the collation in operation. For Questionmark databases this collation will always be one of: SQL_Latin1_General_CP1_CI_AS or Latin1_General_CI_AS. In both cases, the character set is the same and is the ‘Latin1’ character set defined by Microsoft. This should not be confused with the ISO Latin-1 character set defined by ISO-8859-1 as it is different. Microsoft’s Latin1 character set is also often referred to as CP1252.

To interpret data drawn directly from a Questionmark database it is necessary to understand the transformation applied to enable Unicode data to be stored in varchar fields.

varchar encoding:

  1. Convert a string of Unicode characters into bytes by applying the UTF-8 encoding.
  2. Transform this string of bytes back into a string of characters by replacing each byte with its corresponding character in CP1252.
  3. Store the resulting string of characters in the varchar data field.

varchar decoding:

  1. Retrieve the string of characters from the varchar data field
  2. Encode these characters into a string of bytes by replacing each character with the byte that represents it in CP1252.
  3. Decode the resulting string of bytes using UTF-8 to obtain the original Unicode data.

The following example may help. Suppose we have the Unicode string:

© 2016

To convert this into its varchar representation for storage in a varchar data field we follow the encoding procedure defined above.

  1. Encode with UTF-8 to obtain (in hex):

    C2 A9 20 32 30 31 36

  2. Replace bytes with their CP1252 characters:

    © 2016

The above string is then saved to the varchar field. It may appear odd at first that the copyright sign is itself part of the encoded string but this is just a quirk of the encoding algorithm (and similar behaviour is observed for all the characters with Unicode code points 0x80 through to 0xBF).

Warning

The UTF-8 encoding may create a sequence of bytes that is longer than the corresponding sequence of characters. Non-ASCII characters will result in 2 or 3 bytes each. As a result, the length of a varchar field given in the data model represents a restriction on the length of the UTF-8 byte sequence and not the maximum number of characters.

In some cases, APIs are conservative in their implementation and may truncate or reject character strings that might exceed the underlying data model storage.

Note

Due to the nature of UTF-8 and the way the western European characters are encoded you are highly likely to see byte values C0-C5 in the encoded string. These bytes are all represented as upper-case ‘A’ (with various accents) in CP1252 and if unexpectedly seen in Unicode data are diagnostic of a failure to decode data correctly (or evidence of earlier double-encoding).

Warning

Unfortunately, the use of control characters can cause problems as CP1252 does not define mappings for all bytes. The values 0x81, 0x8D, 0x8F, 0x90 and 0x9D are left undefined. As a result, these values can cause issues during step 2 of the encoding/decoding algorithm described above if they appear in the UTF-8 encoded byte sequence. Windows-based technologies tend to ignore this inconsistency and allow such sequences to be decoded using CP1252 as if it contained the corresponding C1 control at that point (i.e., byte 0x81 decodes to the Unicode character with codepoint 0x81 when using CP1252). Other technologies may not be so generous and may decode such sequences as the replacement character (Unicode 0xFFFD) or simply raise exceptions.

This is only a problem if you are attempting to read/write directly from/to a Questionmark database as API calls always manage the encoding for you.

Most low-level database drivers will have a ‘raw’ mode in which varchar data can be read as a string of bytes (instead of characters) and it may be necessary to use this mode, in which case, you can ignore CP1252 completely and treat the raw data as if it were the UTF-8 encoded version of the original Unicode data string.

Timezones

Datetime values in the data model may be stored in one of three timezones.

UTC
Typically indicated by a _UTC suffix to the name of the field in the data model.
Repository Time

In Questionmark’s OnDemand platform repositories have their own default timezone. This is set when the repository is first created and does not usually change. Historically this was the main timezone used for datetime values and is the timezone likely to be experienced in the user interface.

For Perception users the repository time cannot be set and is treated as the same as the platform time.

Platform Time
In some cases, datetime values are stored using the timezone of the platform. This is the default timezone of the server running the database. In Questionmark OnDemand this will vary depending on the region in which the repository is hosted.

MID and LID

For legacy reasons, some 64-bit integer IDs are split into two values in the data model. The two values are typically indicated with a _MID and _LID suffix.

Calculation of the 64-bit ID from a MID and LID uses a decimal algorithm:

ID = MID * 100000000 + LID

Likewise, you need to use decimal arithmetic to split an ID into the two separate parts:

MID = ID / 100000000
LID = ID % 100000000

For example:

MID = 80317034
LID = 6550612
ID = 8031703406550612

Values of MID and LID are limited to the range [1, 99999999]. If you are creating 64-bit ID values yourself you must ensure that they satisfy the following constraints:

ID % 10 != 0
ID > 100000000
ID <= 9999999999999999

As a result, IDs contains approximately 50 bits of information.