mercredi, 30 décembre 2009

Sequences scalability

Sequences are central part of most computer systems. They are used both in technical part (generate unique identifier for object) and in business part (count the remaining free tickets which can be sold).

When the load on the application goes higher, sequences become quickly central bottlenecks of systems. So the goal here is to first categorize different types of sequences and then gives some keys to see how sequences be distributed to increase their scalability, and hence the global scalability of the whole system.

Characteristics of sequences :

Order : the number generate should be used in a given order (ascendant or descendant), or the order do not matter. I will talk about ordered or unordered sequence.

Missing values : some values can be missing in the sequence. For example the sequence miss the value 4 should not alter the behavior of the application. I will talk about continued or discontinued sequence.

Finite sequence : all the values of the sequence are know and countable. Countable sequences are particular case of finite sequence where the sequence has a reasonable amount of values, and thus could be represented by one object per values. I will talk about finite or infinite sequence, and countable sequence.

Key to distribute sequences

Discontinued sequences are easy to scale : the application can pick numbers n by n in the sequence, reducing the amount of calls to the sequence by n. Good example for of discontinued sequences are objects identifier generators.

Unordered sequences are also easy to scale : a counter can be decomposed in n subcounter, picking at random one of the subcounter to increment or decrement will reduce the contention on one counter by n. A typical example is a quantity of object to sell. N subcounters are initialized with the sum is the total amount to sell. The object cannot be sold anymore when every subcounters are at 0.

Countable sequences can be scaled by representing every value of the sequence as a single object in the database. Updating an unused object is a easy task.

Finaly infinite, continued ordered sequence are very hard to scale because synchronization between all callers is required, so a single point of contention is mandatory.


Sequences are choke point in most systems, but as we saw above they can scale relatively simply with two simple tricks : read number n by n instead of 1 by 1, or decompose a counter in n subcounters.