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I am looking at rewriting a VB based on-premise (locally installed) application (invoicing+inventory) as a web based Clojure application for small enterprise customers. I am intending this to be offered as a SaaS application for customers in similar trade.

I was looking at database options: My choice was an RDBMS: Postgresql/ MySQL. I might scale up to 400 users in the first year, with typically a 20-40 page views/ per day per user - mostly for transactions not static views. Each view will involve fetch data and update data. ACID compliance is necessary(or so I think). So the transaction volume is not huge.

It would have been a no-brainer to pick either of these based on my preference, but for this one requirement, which I believe is typical of a SaaS app: The Schema will be changing as I add more customers/users and for each customer's changing business requirement (I will be offering some limited flexibility only to start with). As I am not a DB expert, based on what I can think of and has read, I can handle that in a number of ways:

  1. Have a traditional RDBMS schema design in MySQl/Postgresql with a single DB hosting multiple tenants. And add enough "free-floating" columns in each table to allow for future changes as I add more customers or changes for an existing customer. This might have a downside of propagating the changes to the DB every time a small change is made to the Schema. I remember reading that in Postgresql schema updates can be done real time without locking. But not sure, how painful or how practical is it in this use case. And also, as the schema changes might also introduce new/ minor SQL changes as well.
  2. Have an RDBMS, but design the database schema in a flexible manner: with a close to entity-attribute-value or just as a key-value store. (Workday, FriendFeed for example)
  3. Have the entire thing in-memory as objects and store them in log files periodically.(e.g., edval, lmax)
  4. Go for a NoSQL DB like MongoDB or Redis. But based on what I can gather, they are not suitable for this use-case and not fully ACID compliant.
  5. Go for some NewSQL Dbs like VoltDb or JustoneDb(cloud based) which retain the SQL and ACID compliant behaviour and are "new-gen" RDBMS.
  6. I looked at neo4j(graphdb), but not sure if that will fit this use-case

In my use case, more than scalability or distributed computing, I am looking at a better way to achieve "Flexibility in Schema + ACID + some reasonable Performance". Most of the articles I could find on the net speak of flexibility in schema as a cause leading to performance(in the case of NoSQL DBs) and scalability while leaving out the ACID/Transactions side.

Is this an "either or" case of 'Schema flexibility vs ACID' transactions or Is there a better way out?

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Check out the hstore module in PostgreSQL. That's "NoSQL" inside a SQL database: postgresql.org/docs/current/static/hstore.html –  a_horse_with_no_name Jan 7 '12 at 17:32
    
@horse: Thanks...It's a good pointer. I've heard NoSQL plugins for MySQL . I was looking out similar for Postgres. –  tmbsundar Jan 8 '12 at 6:47
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2 Answers

Option 1

There are several reasons for this, which I'll explain below. First, here's how to do it.

  • Use your choice of standard RDBMS platform.

  • Set up your schema with several user-configurable fields, and make your application facilitate the configuration on a per-tenant basis.

  • From the per-tenant metadata, you can create a per-tenant view of their data, which has the filters built in, and the columns named from your metadata. Any reports provided can also inherit the metadata. If they want to do M.I. off the data then provide them an extract of the transactional data, or maybe some additional MIS application on a different server if they will pay for that.

  • Don't try to provide more customisation than this (i.e. no radical changes to the schema) unless the client is prepared to pay for their own private instance and maintaing a custom build.

The reasons behind this are:

  • These database systems will handle the sort of volumes you describe on fairly ordinary hardware. You don't really have the sort of transaction volume that merits a NoSQL database. Unless you have some other architectural reason to want one, there's not much point in going bleeding-edge.

  • They're mature, well-understood technologies.

  • System management, backup/restore, replication, reporting and disaster recovery are all well sorted on RDBMS platforms.

  • You can get client libraries including JDBC for all major RDBMS platforms.

  • Views can be used for the per-user customisation and generatedfrom your application metadata.

  • It's substantially more efficient than XML fields or EAV structures.

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@COTW: Thanks for the detailed answer. One major thing I was concerned with was the "anticipated" change of schema, which I guess I have to think through and make it as much "pre-configurable" as possible upfront and avoid drastic schema changes later. –  tmbsundar Jan 8 '12 at 6:57
    
Disaster recovery for a single tenant is not simple if they're sharing tables. (If each row has a tenant ID number.) –  Mike Sherrill 'Cat Recall' Dec 21 '12 at 1:40
    
Do this, but use a JSON column: gist.github.com/tobyhede/2715918 –  mwhite Jan 11 at 1:17
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With PostgreSQL you have the option of using separate databases, separate schemas or views to deal with multi-tenancy.

Using multiple databases (within the same database server) makes administration more complex because each database must be managed individually. Therefore, this is only advisable if security between tenants is of the utmost concern.

Separate schemas offer a lot of flexibility and security but makes upgrades more complex because they must be applied individually and is probably only necessary if your tenants use completely different table structures; which is unlikely if they are using the same application.

Views allow tenants to see different parts of a common table structure and allows you to control which tables, which columns and which rows they have access to. The only caveat is that your application must ensure it only uses those views and not the base tables otherwise there is potential for accidental data leaks between tenants because of software defects.

You do not really need to create columns in advance of application requirements. Columns can be added to tables dynamically (without any noticeable impact on users) and views can be updated dynamically too. You only need to think about the order of making changes - ie. change tables, then views then application code.

Your only potential concern is if you need to add a new column that needs to be added to an existing index or requires a new index. That is when the table can get locked from usage while the index is being built - but PostgreSQL supports the ability to build indexes concurrently without locking the table. This works fine unless the new index needs to be unique and finds a uniqueness violation.

You probably don't need a NoSQL database as they effectively remove the schema from the database and require the application to manage it instead. It doesn't sound like your volumes demand that kind of sacrifice.

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With 9.1 you can even replace a unique constraint or primary key without locking the table. See here: depesz.com/index.php/2011/02/19/… –  a_horse_with_no_name Jan 8 '12 at 11:26
    
Agreed. I was attempting to say that a problem arises when a unique index is created but the constraint is violated - then you have to resolve the uniqueness problem. This more a problem of adding columns rather than adding indexes per se. –  Duncan Pauly Jan 8 '12 at 12:34
    
@DuncanPauly: Thanks for the insight. I understand from your answer that Postgresql allows 'online/live schema change'. But, when I google, I get mostly 'facebook online schema change' or 'pt-online...' etc., which pertain to MySQL. Would you know of a link or material which helps me understand live schema change for Postgresql? Appreciate your help. Thanks. –  tmbsundar Jan 8 '12 at 16:41
    
This link describes how you can change tables postgresql.org/docs/8.1/static/ddl-alter.html. The important principle to remember is that creating, altering and dropping tables or views is virtually instantaneous; whereas creating and altering indexes is anything but. –  Duncan Pauly Jan 8 '12 at 17:32
    
@DuncanPauly: Thanks for the pointers. –  tmbsundar Jan 8 '12 at 17:50
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