Integrating Legacy Systems

Recently we tried to integrate a legacy system via exposing a few mainframe programs via web services.

Exposing data as a web service does not make a web service.
Lets examine why:
a) Lack of re-usability - The mainframe programs return course grained data specific to a particular need it was developed for. This leads to having more data and more specific data than any other app would need to reuse the data. For example instead of a code or status returned it returns a message intended to be used by the consuming application. This data is virtually unusable to another application because most of the time you need the deciding code or status to perform logic later on.
b) Lack of performance - Instead of returning a code or status very verbose text is being returned. Now instead of a low overhead code or status you have a large message being transferred over the wire which may not be used. If this web service is consumed by another web service layer then transporting this around is going to be slow.
c) Design constraints - The mainframe program was designed for a particular use which they didn't take into account limiting the repetitive sections. They would return all 100 instances of a section of data. If only the first 5 had data they would return all 100. This adds great overhead on a web service. Also the data that comes from a mainframe program is not trimmed and has filler fields filled with whitespace.


If you don't have somebody regulating or asking the right questions then anything will become a web service which shouldn't be. I recommend a group of architects and/or tech leads making the decision of whether something should be a service.

Here are some guidelines/questions to ask yourself for web services:
1) Ask yourself or group why are we making this a web service? What are we hoping to accomplish?
a) When you go to a web service you are giving up performance for agility.
b) Make sure you're willing to deal or accept the trade off.
2) Does it meet any of this criteria?
a) Is this going to reusable by other applications? Which ones?
b) Is this something that crosses boundaries or something which I can't control? For example if you are exposing something to another business unit or exposing it for public use.
c) Is this enabling system integration for systems that were not able to integrate before.
d) Is this coarse grained enough for general use?
e) Is this data going to be suitable for multiple hops?