1.9. Internal data structure in CamCOPS¶
The CamCOPS client uses a simple, standard relational database design.
The server tables mirror the client device tables closely.
On top of this, a layer of complexity is grafted: (a) to allow devices to operate independently, (b) to allow an audit trail, and (c) to allow devices to wipe their data whilst preserving it on the server.
To overcome this complexity for most practical purposes, the server provides methods of viewing or downloading task data that hides out-of-date versions and so forth.
The thought process behind the design runs as follows.
Tablet devices (“clients”) must be able to operate offline, without internet connectivity. Therefore, they must store their data locally, at least for a time; and they must periodically communicate with their server.
Patient details should not be retrievable from a central server to the tablet device, because that isn’t necessary (the web viewer offers an alternative route) and therefore represents an unnecessary security vulnerability. Additionally, requiring different tablets to be synchronized is potentially unreliable (e.g. if someone adds the same patient on two tablets, both currently disconnected from the network). Consequently, patient details on different tablet devices are not necessarily synchronized. Therefore, the server must maintain records for each of its client tablet devices and keep them independent.
All client tables have a primary key called id; this primary key is unique in that table on that device.
The server mirrors the tables found on the client device.
In addition, a _device field on the server discriminates records by the unique device ID of the uploading device.
The server maintains its own primary key in the _pk field; this field is unique in that table on the server (across all devices).
If someone edits a record on the server, the server shouldn’t rewrite its own records; it should maintain an audit trail. Therefore, the server must keep “old” records and distinguish them from “current” records.
The server maintains a _current field, to mark current records.
Records that are not current have additional information. (Is the record not current because it was deleted or because it was modified? Who deleted or modified it? Which is its predecessor or successor record?)
Inclusion of fields for “when added” and “when removed” allows a historical snapshot for any moment in time to be created.
It should be possible to wipe a tablet device, yet keep its data actively accessible on the server. Therefore, the server must keep “era” information for each device.
The server has an _era field for each table.
The _era field begins life with the value ‘NOW’, representing the “current” era. (The use of ‘NOW’ rather than NULL allows string comparison at all times; see below regarding date/time storage.)
When a device “preserves” its data on the server and wipes its database, the _era field is set to the date/time of the preservation process, for all records in all tables for that device that were in the “current” era. A new era is therefore begun for that device.
The combination of the _device, _era, and id fields is unique for all _current fields.
The upload process is transactional: it either succeeds as a whole, or fails as a whole. (This is necessary because there may be arbitrary relationships between tables on the tablet, e.g. between the main table of a task and its sub-tables.)
Databases like MySQL discard fractions of a second, and CamCOPS needs to store millisecond-accuracy timing information. Moreover, it is often helpful to work in local time zones. Few databases handle this well, and there are no consistent standards for database timezone handling.
For consistency, therefore, all date and date/time fields are stored as TEXT fields in ISO 8601 format 1, specifically YYYY-MM-DDTHH:mm:ss.SSS+ZZ:ZZ. An example is 2013-04-22T14:35:07.381+01:00 – this means 22 April 2013 at 7.381 seconds after 2:35pm in a time zone 1 hour ahead of Coordinated Universal Time (UTC, GMT), such as British Summer Time (+01:00). Greater (e.g. microsecond) accuracy is permitted but not generally used.
There are a few fields that are an exception to this rule and use DATETIME format, but these are only used by the server and are not generally accessible to users.
Full details are available via the server’s option to view device definition language (DDL, a subset of SQL) for all tables. These are fully commented, for DDL dialects that support comments (try MySQL).
Note in particular the following conventions
Tables beginning with an underscore (_) are private to the server, i.e. data is not uploaded into them.
Fields (columns) beginning with an underscore are added by the server.
Columns with “(TASK)” in their comments are generic task fields.
Columns with “(CLINICIAN)” in their comments are generic fields for tasks having a clinician whose details are recorded.
Columns with “(RESPONDENT)” in their comments are generic fields for tasks having a respondent (i.e. someone answering the questions who’s not the patient/subject and who’s not the clinician – such as a carer).
Columns with “(SERVER)” in their comments are added by the server.
Columns with “(GENERIC)” in their comments are generic summary fields. Summary fields are not present on the server, but are created dynamically. See summary fields.