Q:

Global sequential number generator without using a relational database

I have an application. Suppose it's an invoice service. Each time a user creates an invoice I need to assign the next sequential number (I.e: ISequentialNumberGeneratorRepository.Next(); So essentially the invoice number must be unique despite having several instances of my application running (horizontal scalability is likely in the future).
In other words, I need a global sequential number generator.
Traditionally this problem is resolved by using a relational database such as SQL server, PostgreSQL, MySQL, etc. because these systems have the capability to generate sequential unique IDs on inserting a record and returning the generated id as part of the same atomic operation, so they're a perfect fit for a centralised sequential number generator.
But I don't have a relational database and I don't need one, so it's a bit brutal having to use one just for this tiny functionality.
I have, however, an EventStore available (EventStore.org) but I couldn't find out whether it has sequential number generation capability.
So my question is: Is there any available product out there which I could use to generate unique sequential numbers so that I can implement my Next(); repository's method with, and which would work well independently of how many instances of my client invoice application I have?
Note: Alternatively, if someone can think of a way to use EventStore for this purpose or how did they achieve this in a DDD/CQRS/ES environment it'd also be great.

A:

You have not stated the reasons(or presented any code) as to why you want this capability.  I will assume the term sequential should be taken as monotonically increasing(sorting not looping).
I tend to agree with A.Chiesa, I would add timestamps to the list, although not applicable here.
Since your post does not indicate how the data is to be consumed, I purpose two solutions, the second preferred over the first, if possible; and for all later visitors, use a database solution instead.
The only way to guarantee numerical order across a horizontally scaled application without aggregation, is to utilize a central server to assign the numbers(using REST or RPCs or custom network code; not to mention an SQL server, as a side note).  Due to concurrency, the application must wait it's turn for the next number and including network usage and delay, this delay limits the scalability of the application, and provides a single point of failure. These risks can be minimized by creating multiple instances of the central server and multiple application pools(You will lose the global sorting ability).
As an alternative, I would recommend the HI/LO Assigning method, combined with batch aggregation.  Each instance has a four? digit identifier prefixed to an incrementing number per instance.  Schedule an aggregation task on a central(or more than one, for redundancy) server(s) to pickup the data and assign a sequential unique id during aggregation. This process localizes the data(until pickup, which could be scheduled for (100, 500, 1000)? millisecond intervals if needed for coherence; minutes or more ,if not), and provides almost perfect horizontal scaling, with the drawback of increased vertical scaling requirements at the aggregation server(s).  
Distributed computing is a balancing act, between processing, memory, and communication overhead. Where your computing/memory/network capacity boundaries lie cannot be determined from your post.
There is no single correct answer.  I have provided you with two possibilities, but without specific requirements of the task at hand, I can go no further.

