Java Sequence Generation without repetition

Question

I have a specific requirement to generate a unique sequence number for the day. The utility should be able to generate the sequence without repeating even if there's a JVM restart.

Prerequisites:

should not use database sequence.
should not store anything in filesystem.
Sequence can be repeated across the day.
Sequence should not get repeated within the day, even if there's a JVM restart (this is already ensured with different attribute).
No of sequence per second is min requirement 99.

Sequence format: ######## (8 digits max)

Note: this will be running in different instance of nodes and hence first digits of sequence is reserved for identifying the node.

The only way I can see to do this is to have one method generate sequence numbers based on the system clock, and every application has to get the sequence number from this one method. — Gilbert Le Blanc
– Gilbert Le Blanc, Commented Oct 30, 2020 at 10:43

Nowhere Man · Accepted Answer · 2020-10-30 17:13:24Z

2

A simple clock-based solution may look like this:

static int seq(int nodeId) {
    int val = nodeId * 100_000_000 + (int) (System.currentTimeMillis() % 100_000_000);
    try {
        Thread.sleep(1); // introduce delay to ensure the generated values are unique
    } catch (InterruptedException e) {}
    return  val;
}

The delay may be randomized additionally (up to 5 millis):

static Random random = new SecureRandom();
static int seq(int nodeId) {
    int val = nodeId * 100_000_000 + (int) (System.currentTimeMillis() % 100_000_000);
    try {
        Thread.sleep(1 + random.nextInt(4));
    } catch (InterruptedException e) {}
    return  val;
}

answered Oct 30, 2020 at 17:13

Nowhere Man

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