My team recently completed a rewrite of a major system. It was approached with the following goals.
- Port from Java to C#. The Java teams no longer maintained it beyond critical fixes. Many original experts were gone. My team had more vested interest and expertise. We wanted to be in charge of this system, and we work in C#.
- Switch from horizontal processing to vertical processing. The original system would query one record, query one associated record, query yet another associated record, etc. to form the complete picture of one entity. The new system reads and writes batches of homogeneous data. (Kind of a leap from OOP to DOD in terms of database accesses.) This optimization was largely necessary as the old system could not keep up with load spikes.
- Along with the previous change, we completely separated the data processing into two giant phases: one read-only step and one write-only step. This allows the read-only step to target replication databases. This reduces the stress on the primary database, which is a huge win.
- Detach the system so that it can be run, tested, and released in isolation. The original system was part of a much larger whole, so any fixes or enhancements had to wait for weekly or biweekly monolithic releases.
Everything you expect from a rewrite happened but not to a devastating degree. It went overbudget, missed key bug fixes, etc. but we worked through them and came out the other side with all the wins we hoped for. Being able to deploy multiple times within a single day is amazing. Our iteration time on new bugs or features is very small. The overall architecture is simpler and easier to navigate. I have confidence that a member of my team could enter the code base cold and fix a bug within a day.
I don't think it had a visible cost in our case. For starters, my team understood the system adequately to not need to reference the original source code so closely. (Much of the work happened before even beginning to read the original source code.) Secondly, we all had enough Java exposure to be comfortable porting behavior over when it was needed.
- Port from Java to C#. The Java teams no longer maintained it beyond critical fixes. Many original experts were gone. My team had more vested interest and expertise. We wanted to be in charge of this system, and we work in C#.
- Switch from horizontal processing to vertical processing. The original system would query one record, query one associated record, query yet another associated record, etc. to form the complete picture of one entity. The new system reads and writes batches of homogeneous data. (Kind of a leap from OOP to DOD in terms of database accesses.) This optimization was largely necessary as the old system could not keep up with load spikes.
- Along with the previous change, we completely separated the data processing into two giant phases: one read-only step and one write-only step. This allows the read-only step to target replication databases. This reduces the stress on the primary database, which is a huge win.
- Detach the system so that it can be run, tested, and released in isolation. The original system was part of a much larger whole, so any fixes or enhancements had to wait for weekly or biweekly monolithic releases.
Everything you expect from a rewrite happened but not to a devastating degree. It went overbudget, missed key bug fixes, etc. but we worked through them and came out the other side with all the wins we hoped for. Being able to deploy multiple times within a single day is amazing. Our iteration time on new bugs or features is very small. The overall architecture is simpler and easier to navigate. I have confidence that a member of my team could enter the code base cold and fix a bug within a day.
So I dunno. AMA?