Definition “Sequential data storage” What is sequential access?
Sequentially stored data is available in exactly the order in which it was originally stored. Due to the storage medium, access is also only sequential. Despite this limitation, sequential data storage is still used today.
Data access to magnetic tapes – like data storage – takes place in sequential order.
Sequential data records or data are stored immediately one after the other. The data stored in this way has a fixed order, which can no longer be changed (without overwriting). This order also applies to the processing of the data; in this context, one speaks of sequential access.
With this storage method, access must always take place in a predefined, fixed order. Of course, you can jump back or forward to the desired location, but the information contained must always be read in the order in which it was stored.
Example: A radio play cassette tells a story that is always the same in an unchanging sequence. If you want to start at a certain point in the history, you have to let the magnetic tape of the cassette run by fast-forward (or rewind) until you have reached the corresponding position on the storage medium.
A direct consequence of this mechanism is that the access time for a sequential storage medium can depend on the position at which the desired information is located. This means that an information at the beginning of the storage medium can be available faster than one that is located towards the end of the recording. However, this circumstance is not relevant in all scenarios, e.g. if the data is also to be processed according to plan in the sequence in which it was stored.
On the other hand, there is the concept of random access, in which any sections of the memory can be accessed freely as needed at any time. This is also referred to as random or direct access. Here, the user can freely access exactly the information he needs at the moment, without having to actively skip the unwanted data first.
Storage media and technologies for sequential data storage
Magnetic tapes were invented in the 1940s and have long been a widespread technology. They were appreciated as a reliable storage medium and used for versatile purposes. In many areas, they have long been replaced by other options (e.g. hard disk drives, SSDs, flash memory, optical data storage such as CD / DVD), which allow more flexible working.
However, magnetic tapes are still used today. The storage medium offers a high storage density, is durable for a long time and is comparatively inexpensive. This makes it particularly suitable as a medium for archives, for example. The use of magnetic tape as a storage medium is also still conceivable for certain batch processes.
Other examples of magnetic tapes:
- Compact cassettes (audio cassettes)
- Micro-cassettes (e.g. for dictaphones)
A common file format for storing sequential data is CSV (Comma-separated values). With this file format, the data is stored in simple text form. Individual data fields are delimited from each other by a predetermined separator (usually a comma, sometimes also a semicolon, space, colon or tabulator). Similarly, data records can also be separated from each other (e.g. by line break).
This format is often used to export data from one program in order to be able to use it in another program. Common example: The data from a spreadsheet is converted into a .csv file, because they are to be further processed in a third-party software that cannot handle the available file formats of the source software. Simply put, CSV is a simple way to export data into a generally usable form.