2.2 · Sequential vs Direct Access

Goal: explain the difference, list pros and cons, and pick the right method for a scenario.

What "access method" means

When data is stored, there are two fundamentally different ways to read a specific record:

Access method	Strategy	Analogy
Sequential access	Read records in order, starting from the beginning	Listening to a song on cassette tape — fast-forward through the others first
Direct (random) access	Jump straight to the record using an index/address	Selecting a track on Spotify — click and it plays instantly

Sequential access

How it works

Records are stored one after another in a fixed order (usually sorted by a key like student ID). To find record N, the system reads records 1, 2, … N.

Devices best suited to sequential access

• Magnetic tape — moving the tape head is slow, so reading in order is the only realistic option.
• Optical disks when streaming sequential data (rare today).
• Streaming log files.

Pros and cons

Pros	Cons
Simple, cheap storage	Slow to find a specific record
Efficient when processing every record in order (e.g. monthly payroll)	Inefficient for random lookups
Good for backup because tapes are cheap per GB	Adding a new record requires re-writing the whole file (in strict sequential layout)

Typical use cases

• Backups — write once, restore in order if needed.
• Batch processing — overnight payroll, monthly billing.
• Log files — append-only, read in order during debugging.

Direct (random) access

How it works

Each record is at a specific address (a disk block, a hash position, or pointed to by an index). To find record N, the system computes the address and reads it directly — no scanning required.

Devices best suited to direct access

• Hard disk drives (HDD) — read/write head seeks to a track and sector.
• Solid-state drives (SSD) — even faster, no moving parts.
• RAM — basically all random access.
• Flash memory, optical disks.

Pros and cons

Pros	Cons
Fast lookup of any single record	More expensive per GB (especially SSDs)
Easy to insert/update individual records	Requires more complex software (indexes, hash tables)
Supports interactive applications (web apps, games)	If keys collide, performance suffers

Typical use cases

• Online banking — instant balance check.
• Database queries — SELECT … WHERE id = 1001.
• Operating system files — locate any file by path immediately.

Side-by-side comparison

Aspect	Sequential	Direct
Order of reading	First to last	Any order
Time to find a specific record	O(N)	O(1) on average
Time to read all records	O(N) — efficient	O(N) — fine but no advantage
Storage hardware	Tape, sequential files	Disk, SSD, RAM
Update cost	Re-write whole file (often)	Update in place
Cost per GB (typical)	Low (tape)	Higher (SSD)
Insert/delete	Hard	Easy
Suitable for	Backup, batch jobs	Real-time / interactive

Choosing between the two

Ask three questions:

1. Do I need to access individual records quickly? → Direct.
2. Do I process every record every time? → Sequential is fine and cheaper.
3. Do I need cheap, archival storage? → Tape (sequential) wins.

Many real systems use both:

• A bank stores transactions on disk (direct) for daily use.
• A nightly job copies transactions to tape (sequential) for off-site backup.

Real-life example · Salary system

A 10,000-employee company runs payroll once a month and lets each employee check their pay slip online instantly.

Activity	Access method	Reason
Monthly payroll	Sequential	Process every employee in order — efficient
Employee check pay slip	Direct	Need that specific employee's record now
Archive last year's records	Sequential (tape)	Cold storage, rarely needed

The same data is organised differently for different needs.

Common misconceptions

• "Sequential is always slower." False — it is the fastest way to read every record in order.
• "Direct access means random data." No — random access, not random data. The data itself is structured.
• "Modern systems only use direct access." False — tape backups are still standard in enterprises.

Practice activity

For each scenario, choose sequential or direct and justify:

1. A government tax office processing 7 million tax returns in one batch overnight.
2. An MTR gate looking up your Octopus card balance.
3. A school printing 1,200 report cards once per term.
4. A flight booking system checking seat availability in real time.
5. A surveillance camera DVR storing 30 days of footage and only watched after an incident.

Answers

1. Sequential — batch processing of every record.
2. Direct — must look up one card now.
3. Sequential — every student processed once.
4. Direct — single seat query.
5. Sequential write, mostly sequential read; could be either for review.

Exam-style question

Q (4 marks): State two advantages of direct access compared with sequential access, and one situation where sequential access is more suitable.

Sample answer:

Advantages of direct (any 2):

• Faster to retrieve a specific record (O(1) vs O(N)).
• Suitable for interactive applications (e.g. ATM, online booking).
• Records can be inserted or updated in place without rewriting the whole file.

Situation where sequential is better:

• A monthly payroll job that reads every employee record in order — sequential access on cheap tape is fast and cost-effective for that workload.

Key takeaways

• Two access methods: sequential and direct.
• Sequential = read in order; direct = jump anywhere.
• Match the method to the workload. Real systems often use both.

➡️ Next: 2.3 Validation, Verification & Parity